General mechanisms of drought response and their application in drought resistance improvement in plants

RRS1 shapes robust root system to enhance drought resistance in rice

A strong root system facilitates the absorption of water and nutrients from the soil, to improve the growth of crops. However, to date, there are still very few root development regulatory genes that can be used in crop breeding for agriculture. In this study, we cloned a negative regulator gene of root development, Robust Root System 1 (RRS1), which encodes an R2R3-type MYB family transcription factor. RRS1 knockout plants showed enhanced root growth, including longer root length, longer lateral root length, and larger lateral root density. RRS1 represses root development by directly activating the expression of OsIAA3 which is involved in the auxin signaling pathway. A natural variation in the coding region of RRS1 changes the transcriptional activity of its protein. RRS1^T allele, originating from wild rice, possibly increases root length by means of weakening regulation of OsIAA3. Knockout of RRS1 enhances drought resistance by promoting water absorption and improving water use efficiency. This study provides a new gene resource for improving root systems and cultivating drought-resistant rice varieties with important values in agricultural applications.

Overexpression of a WRKY transcription factor McWRKY57-like from Mentha canadensis L. enhances drought tolerance in transgenic Arabidopsis

BACKGROUND

Drought has become a major environmental problem affecting crop production. Members of the WRKY family play important roles in plant development and stress responses. However, their roles in mint have been barely explored.

RESULTS

In this study, we isolated a drought-inducible gene McWRKY57-like from mint and investigated its function. The gene encodes a group IIc WRKY transcription factor, McWRKY57-like, which is a nuclear protein with a highly conserved WRKY domain and a C2H2 zinc-finger structure, and has transcription factor activity. Its expression levels were examined in different tissues of mint and under the treatment of mannitol, NaCl, abscisic acid, and methyl jasmonate. We found that McWRKY57-like overexpression in Arabidopsis significantly increased drought tolerance. Further studies showed that under drought stress, McWRKY57-like-overexpressing plants had higher chlorophyll, soluble sugar, soluble protein, and proline contents but lower water loss rate and malondialdehyde content than wild-type plants. Moreover, the activities of antioxidant enzymes catalase, superoxide dismutase, and peroxidase were enhanced in McWRKY57-like transgenic plants. Furthermore, qRT-PCR analysis revealed that the drought-related genes AtRD29A, AtRD29B, AtRD20, AtRAB18, AtCOR15A, AtCOR15B, AtKIN2, and AtDREB1A were upregulated in McWRKY57-like transgenic plants than in wild-type Arabidopsis under simulated drought conditions.

CONCLUSION

These data demonstrated that McWRKY57-like conferred drought tolerance in transgenic Arabidopsis by regulating plant growth, osmolyte accumulation and antioxidant enzyme activities, and the expression of stress-related genes. The study indicates that McWRKY57-like plays a positive role in drought response in plants.

Exploring the quantitative genetics of traits associated with drought tolerance and yield in Pimpinella anisum L. under different water regimes

MAIN CONCLUSION

Genetic improvement of seed yield and drought resistance could be simultaneously gained in anise when breeding for drought resistance. Improving the water use efficiency of anise is a primary objective of anise breeding programs aimed at mitigating the impacts of drought stress. This study aimed to determine the predominant mechanisms involved in drought tolerance and investigate the genetic control of associated traits with drought tolerance and higher grain yield. According to these aims, 10 half-diallel hybrids and their five parents were evaluated in both field and greenhouse lysimetric experiments under well-watered and water deficit stress conditions. The results indicated that the inheritance of grain yield is complex and affected by water deficit stress. Similar heritability and genetic architecture were detected for flowering time and percentages of photosynthate partitioned to grain (PPPG) in both well-watered and water deficit stress treatments. Significant negative genetic correlations were observed between grain yield and flowering time, root dry mass, root diameter, root volume, root number, percentages of photosynthate partitioned to shoot, and percentages of photosynthate partitioned to root. Therefore, the selection of low values of these attributes can be used to improve grain yield under drought conditions. In contrast, a positive significant genetic linkage between grain yield and PPPG, chlorophyll content, cell membrane stability, and leaf relative water content reveal selection for high values of these attributes is favored. These attributes could be used as surrogate selection criteria in the early segregating generations. The P₁ parent (early ripening parent) contained key genes associated with PPPG and drought escape. It was concluded that improvement of drought tolerance and grain yield could be simultaneously achieved in anise breeding programs.

Population growth of Tetranychus truncatus (Acari: Tetranychidae) on different drought-tolerant potato cultivars

Tetranychus truncatus Ehara (Acari: Tetranychidae) has become one of the major phytophagous pests in China in recent years, and is found on a wide range of host plants. However, little information is available on the population performance of this arthropod pest on potatoes. In this study, we explored the population growth of T. truncatus on two drought-tolerant potato (Solanum tuberosum L.) cultivars under laboratory conditions using the age-stage, two-sex life table. Tetranychus truncatus completed its entire life history on both potato cultivars tested, Holland 15 and Longshu 10. There was no significant difference between two potato cultivars in developmental duration. Tetranychus truncatus had shorter adult longevity (20.61 days), adult female longevity (20.41 days), and total female longevity (33.66 days) on Longshu 10 than Holland 15 (21.16 days, 21.19 days, and 34.38 days, respectively). However, it exhibited a higher preadult survival rate, higher fecundity (F = 88.32 eggs per female), and relatively higher population parameters when reared on Longshu 10 than on Holland 15 (F = 75.70 eggs per female). Growth projection also showed that the population size of T. truncatus on Longshu 10 (expand 750-fold) was larger than that on Holland 15 (expand 273-fold) after 60 days. Our results demonstrate that the drought-sensitive potato variety, Holland 15, is relatively resistant to T. truncatus compared with the drought-tolerant variety, Longshu 10, and suggest that T. truncatus exhibited a trade-off between longevity and reproduction on both potato cultivars. Our findings provide information on population prediction, which may aid the management of this pest mite species of potatoes.

The SsWRKY1 transcription factor of Saccharum spontaneum enhances drought tolerance in transgenic Arabidopsis thaliana and interacts with 21 potential proteins to regulate drought tolerance in S. spontaneum

In this study, we characterized a WRKY family member gene, SsWRKY1, which is located in the nucleus and contains multiple stress-related cis-acting elements. In addition, constructed SsWRKY1-overexpressing Arabidopsis thaliana had higher antioxidant enzyme activity and proline content under drought stress conditions, with lower malondialdehyde content and reactive oxygen species (ROS) accumulation, and the expression levels of six stress-related genes were significantly upregulated. This indicates that the overexpression of SsWRKY1 in Arabidopsis thaliana improves resistance to drought stress. SsWRKY1 does not have transcriptional autoactivation activity in yeast cells. The yeast two-hybrid (Y2H) system and the S. spontaneum cDNA library were used to screen 21 potential proteins that interact with SsWRKY1, and the interaction between SsWRKY1 and ATAF2 was verified by GST pull-down assay. In summary, our results indicate that SsWRKY1 plays an important role in the response to drought stress and provide initial insights into the molecular mechanism of SsWRKY1 in response to drought stress.

Intra-individual variation in Galium odoratum is affected by experimental drought and shading

BACKGROUND AND AIMS

Climate-change induced warmer spring temperatures advance tree leaf-out and result in earlier shading of the forest floor. Climate change also leads to more frequent droughts. Forest understorey herbs may respond to these environmental changes by varying traits at different hierarchical levels of organization. While trait mean variation at the inter-individual level in response to environmental changes is well-studied, little is known about how variation at the intra-individual level responds.

METHODS

We sampled genets of the forest understorey herb Galium odoratum from 21 populations in three regions in Germany, varying in microclimatic conditions. The genets were transplanted into a common garden, where we applied shading and drought treatments. We measured plant height and leaf length and width, and calculated the coefficient of variation (CV) at different hierarchical levels: intra-population, intra-genet, intra-ramet and intra-shoot.

KEY RESULTS

Variance partitioning showed that intra-shoot CV represented most of the total variation, followed by intra-ramet CV. We found significant variation in CV of plant height and leaf width among populations of origin, indicating that CV is at least partly genetically based. The soil temperature at populations' origins correlated negatively with CV in plant height, suggesting adaptation to local conditions. Furthermore, we observed that early shade led to increased intra-ramet CV in leaf length, while drought reduced intra-shoot CV in leaf width. Finally, intra-shoot leaf width mean and CV were independent under control conditions but correlated under drought.

CONCLUSIONS

Our experimental results reveal correlations of intra-individual variation with soil temperature, indicating that intra-individual variation can evolve and may be adaptive. Intra-individual variation responded plastically to drought and shading, suggesting functional changes to improve light capture and reduce evapotranspiration. In conclusion, intra-individual variation makes up the majority of total trait variation in this species and can play a key role in plant adaptation to climatic change.

Abscisic acid collaborates with lignin and flavonoid to improve pre-silking drought tolerance by tuning stem elongation and ear development in maize (Zea mays L.)

Drought is a major abiotic stress reducing maize (Zea mays) yield worldwide especially before and during silking. The mechanism underlying drought tolerance in maize and the roles of different organs have not been elucidated. Hence, we conducted field trials under pre-silking drought conditions using two maize genotypes: FM985 (drought-tolerant) and ZD958 (drought-sensitive). The two genotypes did not differ in plant height, grain number, and yield under control conditions. However, the grain number per ear and the yield of FM985 were 38.1 and 35.1% higher and plants were 17.6% shorter than ZD958 under drought conditions. More ¹³ C photosynthates were transported to the ear in FM985 than in ZD958, which increased floret fertility and grain number. The number of differentially expressed genes was much higher in stem than in other organs. Stem-ear interactions are key determinants of drought tolerance, in which expression of genes related to abscisic acid, lignin, and flavonoid biosynthesis and carbon metabolism in the stem was induced by drought, which inhibited stem elongation and promoted assimilate allocation to the ear in FM985. In comparison with ZD958, the activities of trehalose 6-phosphate phosphatase and sucrose non-fermentation-associated kinase 1 were higher in the stem and lower in the kernel of FM985, which facilitated kernel formation. These results reveal that, beyond the ear response, stem elongation is involved in the whole process of drought tolerance before silking. Abscisic acid together with trehalose 6-phosphate, lignin, and flavonoid suppresses stem elongation and allocates assimilates into the ear, providing a novel and systematic regulatory pathway for drought tolerance in maize.

A novel TF molecular switch-mechanism found in two contrasting ecotypes of a psammophyte, Agriophyllum squarrosum, in regulating transcriptional drought memory

BACKGROUND

Prior drought stress may change plants response patterns and subsequently increase their tolerance to the same condition, which can be referred to as "drought memory" and proved essential for plants well-being. However, the mechanism of transcriptional drought memory in psammophytes remains unclear. Agriophyllum squarrosum, a pioneer species on mobile dunes, is widely spread in Northern China's vast desert areas with outstanding ability of water use efficiency. Here we conducted dehydration-rehydration treatment on A. squarrosum semi-arid land ecotype AEX and arid land ecotype WW to dissect the drought memory mechanism of A. squarrosum, and to determine the discrepancy in drought memory of two contrasting ecotypes that had long adapted to water heterogeneity.

RESULT

Physiological traits monitoring unveiled the stronger ability and longer duration in drought memory of WW than that of AEX. A total of 1,642 and 1,339 drought memory genes (DMGs) were identified in ecotype AEX and WW, respectively. Furthermore, shared DMGs among A. squarrosum and the previously studied species depicted that drought memory commonalities in higher plants embraced pathways like primary and secondary metabolisms; while drought memory characteristics in A. squarrosum were mainly related to response to heat, high light intensity, hydrogen peroxide, and dehydration, which might be due to local adaptation to desert circumstances. Heat shock proteins (HSPs) occupied the center of the protein-protein interaction (PPI) network in drought memory transcription factors (TF), thus playing a key regulatory role in A. squarrosum drought memory. Co-expression analysis of drought memory TFs and DMGs uncovered a novel regulating module, whereby pairs of TFs might function as molecular switches in regulating DMG transforming between high and low expression levels, thus promoting drought memory reset.

CONCLUSION

Based on the co-expression analysis, protein-protein interaction prediction, and drought memory metabolic network construction, a novel regulatory module of transcriptional drought memory in A. squarrosum was hypothesized here, whereby recurrent drought signal is activated by primary TF switches, then amplified by secondary amplifiers, and thus regulates downstream complicated metabolic networks. The present research provided valuable molecular resources on plants' stress-resistance basis and shed light on drought memory in A. squarrosum.

Microbial Drivers of Plant Performance during Drought Depend upon Community Composition and the Greater Soil Environment

The increasing occurrence of drought is a global challenge that threatens food security through direct impacts to both plants and their interacting soil microorganisms. Plant growth promoting microbes are increasingly being harnessed to improve plant performance under stress. However, the magnitude of microbiome impacts on both structural and physiological plant traits under water limited and water replete conditions are not well-characterized. Using two microbiomes sourced from a ponderosa pine forest and an agricultural field, we performed a greenhouse experiment that used a crossed design to test the individual and combined effects of the water availability and the soil microbiome composition on plant performance. Specifically, we studied the structural and leaf functional traits of maize that are relevant to drought tolerance. We further examined how microbial relationships with plant phenotypes varied under different combinations of microbial composition and water availability. We found that water availability and microbial composition affected plant structural traits. Surprisingly, they did not alter leaf function. Maize grown in the forest-soil microbiome produced larger plants under well-watered and water-limited conditions, compared to an agricultural soil community. Although leaf functional traits were not significantly different between the watering and microbiome treatments, the bacterial composition and abundance explained significant variability in both plant structure and leaf function within individual treatments, especially water-limited plants. Our results suggest that bacteria-plant interactions that promote plant performance under stress depend upon the greater community composition and the abiotic environment. IMPORTANCE Globally, drought is an increasingly common and severe stress that causes significant damage to agricultural and wild plants, thereby threatening food security. Despite growing evidence of the potential benefits of soil microorganisms on plant performance under stress, decoupling the effects of the microbiome composition versus the water availability on plant growth and performance remains a challenge. We used a highly controlled and replicated greenhouse experiment to understand the impacts of microbial community composition and water limitation on corn growth and drought-relevant functions. We found that both factors affected corn growth, and, interestingly, that individual microbial relationships with corn growth and leaf function were unique to specific watering/microbiome treatment combinations. This finding may help explain the inconsistent success of previously identified microbial inocula in improving plant performance in the face of drought, outside controlled environments.

Physiological responses of three field-grown species (Ceratonia siliqua, Eucalyptus camaldulensis, and Moringa oleifera) to water deficits in a Mediterranean semi-arid climate

Reforestation of degraded drylands calls for the selection of species with the capacity to withstand water scarcity. In this current study we have assessed, the physiological responses of three field-grown species (Ceratonia siliqua, Eucalyptus camaldulensis and Moringa oleifera) to water deficits in semi-arid regions in order to suggest a potential species for rehabilitation programs. The physiological behavior of the given species was studied in three irrigation schemes: subsurface drip irrigation (applied weekly), tank irrigation (applied monthly), and unirrigated plants. In a stressed state, an assessment of relative water content (RWC), water potential (pre-dawn water potential PWP and midday water potential MWP) and stomatal conductance revealed three contrasting physiological responses. First, C. siliqua stomata remained open with a high RWC at low water potentials. Consequently, this species tolerated water deficits by decreasing its leaf water potential, primarily associated with osmotic adjustment. On the other hand, E. camaldulensis was found to be a drought-avoider species, mutated to a water-saving strategy by complete stomatal closure. Finally, for the extreme case, M. oleifera showed leaf shedding under water deficit conditions. These different physiological responses allowed these species to survive water deficits, and consequently, could be considered suitable candidates for rehabilitating degraded semi-arid areas.

Multi-omics joint analysis reveals how Streptomyces albidoflavus OsiLf-2 assists Camellia oleifera to resist drought stress and improve fruit quality

Camellia oleifera (C. oleifera) is a unique edible oil crop in China cultivated in the hilly southern mountains. Although C. oleifera is classified as a drought-tolerant tree species, drought remains the main factor limiting the growth of C. oleifera in summer and autumn. Using endophytes to improve crop drought tolerance is one effective strategy to meet our growing food crop demand. In this study, we showed that endophyte Streptomyces albidoflavus OsiLf-2 could mitigate the negative impact of drought stress on C. oleifera, thus improving seed, oil, and fruit quality. Microbiome analysis revealed that OsiLf-2 treatment significantly affected the microbial community structure in the rhizosphere soil of C. oleifera, decreasing both the diversity and abundance of the soil microbe. Likewise, transcriptome and metabolome analyses found that OsiLf-2 protected plant cells from drought stress by reducing root cell water loss and synthesizing osmoregulatory substances, polysaccharides, and sugar alcohols in roots. Moreover, we observed that OsiLf-2 could induce the host to resist drought stress by increasing its peroxidase activity and synthesizing antioxidants such as cysteine. A multi-omics joint analysis of microbiomes, transcriptomes, and metabolomes revealed OsiLf-2 assists C. oleifera in resisting drought stress. This study provides theoretical and technical support for future research on endophytes application to enhance the drought resistance, yield, and quality of C. oleifera.

Bacilli Rhizobacteria as Biostimulants of Growth and Production of Sesame Cultivars under Water Deficit

A strategy using bacilli was adopted aiming to investigate the mitigation of the effects of water deficit in sesame. An experiment was carried out in a greenhouse with 2 sesame cultivars (BRS Seda and BRS Anahí) and 4 inoculants (pant001, ESA 13, ESA 402, and ESA 441). On the 30th day of the cycle, irrigation was suspended for eight days, and the plants were subjected to physiological analysis using an infrared gas analyzer (IRGA). On the 8th day of water suspension, leaves were collected for analysis: superoxide dismutase, catalase, ascorbate peroxidase, proline, nitrogen, chlorophyll, and carotenoids. At the end of the crop cycle, data on biomass and vegetative growth characteristics were collected. Data were submitted for variance analysis and comparison of means by the Tukey and Shapiro-Wilk tests. A positive effect of inoculants was observed for all characteristics evaluated, contributing to improvements in plant physiology, induction of biochemical responses, vegetative development, and productivity. ESA 13 established better interaction with the BRS Anahí cultivar and ESA 402 with BRS Seda, with an increase of 49% and 34%, respectively, for the mass of one thousand seeds. Thus, biological indicators are identified regarding the potential of inoculants for application in sesame cultivation.

Plant root plasticity during drought and recovery: What do we know and where to go?

AIMS

Drought stress is one of the most limiting factors for agriculture and ecosystem productivity. Climate change exacerbates this threat by inducing increasingly intense and frequent drought events. Root plasticity during both drought and post-drought recovery is regarded as fundamental to understanding plant climate resilience and maximizing production. We mapped the different research areas and trends that focus on the role of roots in plant response to drought and rewatering and asked if important topics were overlooked.

METHODS

We performed a comprehensive bibliometric analysis based on journal articles indexed in the Web of Science platform from 1900-2022. We evaluated a) research areas and temporal evolution of keyword frequencies, b) temporal evolution and scientific mapping of the outputs over time, c) trends in the research topics analysis, d) marked journals and citation analysis, and e) competitive countries and dominant institutions to understand the temporal trends of root plasticity during both drought and recovery in the past 120 years.

RESULTS

Plant physiological factors, especially in the aboveground part (such as "photosynthesis", "gas-exchange", "abscisic-acid") in model plants Arabidopsis, crops such as wheat and maize, and trees were found to be the most popular study areas; they were also combined with other abiotic factors such as salinity, nitrogen, and climate change, while dynamic root growth and root system architecture responses received less attention. Co-occurrence network analysis showed that three clusters were classified for the keywords including 1) photosynthesis response; 2) physiological traits tolerance (e.g. abscisic acid); 3) root hydraulic transport. Thematically, themes evolved from classical agricultural and ecological research via molecular physiology to root plasticity during drought and recovery. The most productive (number of publications) and cited countries and institutions were situated on drylands in the USA, China, and Australia. In the past decades, scientists approached the topic mostly from a soil-plant hydraulic perspective and strongly focused on aboveground physiological regulation, whereas the actual belowground processes seemed to have been the elephant in the room. There is a strong need for better investigation into root and rhizosphere traits during drought and recovery using novel root phenotyping methods and mathematical modeling.

Drought stress in 'Shine Muscat' grapevine: Consequences and a novel mitigation strategy-5-aminolevulinic acid

Drought is a common and serious abiotic stress in viticulture, and it is urgent to select effective measures to alleviate it. The new plant growth regulator 5-aminolevulinic acid (ALA) has been utilized to alleviate abiotic stresses in agriculture in recent years, which provided a novel idea to mitigate drought stress in viticulture. The leaves of 'Shine Muscat' grapevine (Vitis vinifera L.) seedlings were treated with drought (Dro), drought plus 5-aminolevulinic acid (ALA, 50 mg/L) (Dro_ALA) and normal watering (Control) to clarify the regulatory network used by ALA to alleviate drought stress in grapevine. Physiological indicators showed that ALA could effectively reduce the accumulation of malondialdehyde (MDA) and increase the activities of peroxidase (POD) and superoxide dismutase (SOD) in grapevine leaves under drought stress. At the end of treatment (day 16), the MDA content in Dro_ALA was reduced by 27.63% compared with that in Dro, while the activities of POD and SOD reached 2.97- and 5.09-fold of those in Dro, respectively. Furthermore, ALA reduces abscisic acid by upregulating CYP707A1, thus, relieving the closure of stomata under drought. The chlorophyll metabolic pathway and photosynthetic system are the major pathways affected by ALA to alleviate drought. Changes in the genes of chlorophyll synthesis, including CHLH, CHLD, POR, and DVR; genes related to degradation, such as CLH, SGR, PPH and PAO; the RCA gene that is related to Rubisco; and the genes AGT1 and GDCSP related to photorespiration form the basis of these pathways. In addition, the antioxidant system and osmotic regulation play important roles that enable ALA to maintain cell homeostasis under drought. The reduction of glutathione, ascorbic acid and betaine after the application of ALA confirmed the alleviation of drought. In summary, this study revealed the mechanism of effects of drought stress on grapevine, and the alleviating effect of ALA, which provides a new concept to alleviate drought stress in grapevine and other plants.

Comprehensive Genome-Wide Analyses of Poplar R2R3-MYB Transcription Factors and Tissue-Specific Expression Patterns under Drought Stress

R2R3-type MYB transcription factors are implicated in drought stress, which is a primary factor limiting the growth and development of woody plants. The identification of R2R3-MYB genes in the Populus trichocarpa genome has been previously reported. Nevertheless, the diversity and complexity of the conserved domain of the MYB gene caused inconsistencies in these identification results. There is still a lack of drought-responsive expression patterns and functional studies of R2R3-MYB transcription factors in Populus species. In this study, we identified a total of 210 R2R3-MYB genes in the P. trichocarpa genome, of which 207 genes were unevenly distributed across all 19 chromosomes. These poplar R2R3-MYB genes were phylogenetically divided into 23 subgroups. Collinear analysis demonstrated that the poplar R2R3-MYB genes underwent rapid expansion and that whole-genome duplication events were a dominant factor in the process of rapid gene expansion. Subcellular localization assays indicated that poplar R2R3-MYB TFs mainly played a transcriptional regulatory role in the nucleus. Ten R2R3-MYB genes were cloned from P. deltoides × P. euramericana cv. Nanlin895, and their expression patterns were tissue-specific. A majority of the genes showed similar drought-responsive expression patterns in two out of three tissues. This study provides a valid cue for further functional characterization of drought-responsive R2R3-MYB genes in poplar and provides support for the development of new poplar genotypes with elevated drought tolerance.

The Prospects of gene introgression from crop wild relatives into cultivated lentil for climate change mitigation

Crop wild relatives (CWRs), landraces and exotic germplasm are important sources of genetic variability, alien alleles, and useful crop traits that can help mitigate a plethora of abiotic and biotic stresses and crop yield reduction arising due to global climatic changes. In the pulse crop genus Lens, the cultivated varieties have a narrow genetic base due to recurrent selections, genetic bottleneck and linkage drag. The collection and characterization of wild Lens germplasm resources have offered new avenues for the genetic improvement and development of stress-tolerant, climate-resilient lentil varieties with sustainable yield gains to meet future food and nutritional requirements. Most of the lentil breeding traits such as high-yield, adaptation to abiotic stresses and resistance to diseases are quantitative and require the identification of quantitative trait loci (QTLs) for marker assisted selection and breeding. Advances in genetic diversity studies, genome mapping and advanced high-throughput sequencing technologies have helped identify many stress-responsive adaptive genes, quantitative trait loci (QTLs) and other useful crop traits in the CWRs. The recent integration of genomics technologies with plant breeding has resulted in the generation of dense genomic linkage maps, massive global genotyping, large transcriptomic datasets, single nucleotide polymorphisms (SNPs), expressed sequence tags (ESTs) that have advanced lentil genomic research substantially and allowed for the identification of QTLs for marker-assisted selection (MAS) and breeding. Assembly of lentil and its wild species genomes (~4Gbp) opens up newer possibilities for understanding genomic architecture and evolution of this important legume crop. This review highlights the recent strides in the characterization of wild genetic resources for useful alleles, development of high-density genetic maps, high-resolution QTL mapping, genome-wide studies, MAS, genomic selections, new databases and genome assemblies in traditionally bred genus Lens for future crop improvement amidst the impending global climate change.

Genetic enhancement of climate-resilient traits in small millets: A review

Agriculture is facing the challenge of feeding the ever-growing population that is projected to reach ten billion by 2050. While improving crop yield and productivity can address this challenge, the increasing effects of global warming and climate change seriously threaten agricultural productivity. Thus, genomics and genome modification technologies are crucial to improving climate-resilient traits to enable sustained yield and productivity; however, significant research focuses on staple crops such as rice, wheat, and maize. Crops that are naturally climate-resilient and nutritionally superior to staple cereals, such as small millets, remain neglected and underutilized by mainstream research. The ability of small millets to grow in marginal regions having limited irrigation and poor soil fertility makes these crops a better choice for cultivation in arid and semi-arid areas. Hence, mainstreaming small millets for cultivation and using omics technologies to dissect the climate-resilient traits to identify the molecular determinants underlying these traits are imperative for addressing food and nutritional security. In this context, the review discusses the genomics and genome modification approaches for dissecting key traits in small millets and their application for improving these traits in cultivated germplasm. The review also discusses biofortification for nutritional security and machine-learning approaches for trait improvement in small millets. Altogether, the review provides a roadmap for the effective use of next-generation approaches for trait improvement in small millets. This will lead to the development of improved varieties for addressing multiple insecurities prevailing in the present climate change scenario.

Global Responses of Autopolyploid Sugarcane Badila (Saccharum officinarum L.) to Drought Stress Based on Comparative Transcriptome and Metabolome Profiling

Sugarcane (Saccharum spp. hybrid) is frequently affected by seasonal drought, which causes substantial declines in quality and yield. To understand the drought resistance mechanisms of S. officinarum, the main species of modern sugarcane, at a molecular level, we carried out a comparative analysis of transcriptome and metabolome profiling of the sugarcane variety Badila under drought stress (DS). Compared with control group (CG) plants, plants exposed to DS had 13,744 (6663 up-regulated and 7081 down-regulated) differentially expressed genes (DEGs). GO and KEGG analysis showed that the DEGs were enriched in photosynthesis-related pathways and most DEGs had down-regulated expression. Moreover, the chlorophyll content, photosynthesis (Photo), stomatal conductance (Cond), intercellular carbon dioxide concentration (Ci) and transpiration rate (Trmmol) were sharply decreased under DS. These results indicate that DS has a significant negative influence on photosynthesis in sugarcane. Metabolome analysis identified 166 (37 down-regulated and 129 up-regulated) significantly regulated metabolites (SRMs). Over 50% of SRMs were alkaloids, amino acids and their derivatives, and lipids. The five most significantly enriched KEGG pathways among SRMs were Aminoacyl-tRNA biosynthesis, 2-Oxocarboxylic acid metabolism, Biosynthesis of amino acids, Phenylalanine metabolism, and Arginine and proline metabolism (p < 0.05). Comparing CG with DS for transcriptome and metabolome profiling (T_CG/DS and M_CG/DS, respectively), we found three of the same KEGG-enriched pathways, namely Biosynthesis of amino acids, Phenylalanine metabolism and Arginine and proline metabolism. The potential importance of Phenylalanine metabolism and Arginine and proline metabolism was further analyzed for response to DS in sugarcane. Seven SRMs (five up-regulated and two down-regulated) and 60 DEGs (17 up-regulated and 43 down-regulated) were enriched in Phenylalanine metabolism under DS, of which novel.31261, Sspon.04G0008060-1A, Sspon.04G0008060-2B and Sspon.04G0008060-3C were significantly correlated with 7 SRMs. In Arginine and proline metabolism, eight SRMs (seven up-regulated and one down-regulated) and 63 DEGs (32 up-regulated and 31 down-regulated) were enriched, of which Sspon.01G0026110-1A (OAT) and Sspon.03G0002750-3D (P5CS) were strongly associated with proline (r > 0.99). These findings present the dynamic changes and possible molecular mechanisms of Phenylalanine metabolism as well as Arginine and proline metabolism under DS and provide a foundation for future research and sugarcane improvement.

Changes in physio-biochemical parameters and expression of metallothioneins in Avena sativa L. in response to drought

Drought is one of the major threats to food security. Among several mechanisms involved in plant stress tolerance, one protein family-the plant metallothioneins (MTs)-shows great promise for enhancing drought resistance. Plant metallothioneins in oat (Avena sativa L.) have not yet been deeply analysed, and the literature lacks a comprehensive study of the whole family of plant MTs in response to drought. In this study, we showed that the number and nature of cis-elements linked with stress response in promoters of AsMTs1-3 differed depending on the MT type. Drought stress in oat plants caused an increase in the expression of AsMT2 and AsMT3 and a decrease in the expression of AsMT1 compared to well-watered plants. Moreover, the low values of relative water content, water use efficiency, net photosynthesis (P_N), transpiration (E), stomatal conductance (g_s), chlorophyll a, and carotenoid were accompanied by high levels of electrolyte leakage, internal CO₂ concentration (C_i) and abscisic acid content, and high activity of antioxidants enzymes in plants under drought stress. The present study puts forward the idea that AsMTs are crucial for oat response to drought stress not only by regulating antioxidant activity but also by changing the plant water regime and photosynthesis. Our results support the hypothesis that structural differences among types of plant MTs reflect their diversified physiological roles.

SUPPRESSOR of MAX2 1 (SMAX1) and SMAX1-LIKE2 (SMXL2) Negatively Regulate Drought Resistance in Arabidopsis thaliana

Recent investigations in Arabidopsis thaliana suggest that SUPPRESSOR of MORE AXILLARY GROWTH 2 1 (SMAX1) and SMAX1-LIKE2 (SMXL2) are negative regulators of karrikin (KAR) and strigolactone (SL) signaling during plant growth and development, but their functions in drought resistance and related mechanisms of action remain unclear. To understand the roles and mechanisms of SMAX1 and SMXL2 in drought resistance, we investigated the drought-resistance phenotypes and transcriptome profiles of smax1 smxl2 (s1,2) double-mutant plants in response to drought stress. The s1,2 mutant plants showed enhanced drought-resistance and lower leaf water loss when compared with wild-type (WT) plants. Transcriptome comparison of rosette leaves from the s1,2 mutant and the WT under normal and dehydration conditions suggested that the mechanism related to cuticle formation was involved in drought resistance. This possibility was supported by enhanced cuticle formation in the rosette leaves of the s1,2 mutant. We also found that the s1,2 mutant plants were more sensitive to abscisic acid in assays of stomatal closure, cotyledon opening, chlorophyll degradation and growth inhibition, and they showed a higher reactive oxygen species detoxification capacity than WT plants. In addition, the s1,2 mutant plants had longer root hairs and a higher root-to-shoot ratio than the WT plants, suggesting that the mutant had a greater capacity for water absorption than the WT. Taken together, our results indicate that SMAX1 and SMXL2 negatively regulate drought resistance, and disruption of these KAR- and SL-signaling-related genes may therefore provide a novel means for improving crop drought resistance.

Cultivation Using Coir Substrate and P or K Enriched Fertilizer Provides Higher Resistance to Drought in Ecologically Diverse Quercus Species

Nursery cultivation practices can be modified to increase resistance to water stress in forest seedlings following field establishment, which may be increasingly important under climate change. We evaluated the morphological (survival, growth) and physiological (chlorophyll fluorescence, leaf water potential) responses to water stress for three ecologically diverse Quercus species (Q. robur, Q. pubescens, and Q. ilex) with varying traits resulting from the combination of growing media (peat, coir) and fertilization (standard, P-enriched, K-enriched). For all species under water stress, seedlings grown in coir had generally higher growth than those grown in peat. Seedlings fertilized with P performed better, particularly for survival; conversely, K fertilization resulted in inconsistent findings. Such results could be explained by a combination of factors. P fertilization resulted in higher P accumulation in seedlings, while no K accumulation was observed in K fertilized seedlings. As expected, the more drought-sensitive species, Q. robur, showed the worst response, while Q. pubescens had a drought resistance equal or better to Q. ilex despite being classified as intermediate in drought resistance in Mediterranean environments.

Silencing of GhORP_A02 enhances drought tolerance in Gossypium hirsutum

BACKGROUND

ORP (Oxysterol-binding protein-related proteins) genes play a role in lipid metabolism, vesicular transferring and signaling, and non-vesicular sterol transport. However, no systematic identification and analysis of ORP genes have been reported in cotton.

RESULT

In this study, we identified 14, 14, 7, and 7 ORP genes in G. hirsutum, G. barbadense, G. arboreum, and G. raimondii, respectively. Phylogenetic analysis showed that all ORP genes could be classified into four groups. Gene structure and conserved motif analysis suggest that the function of this gene family was conserved. The Ka/Ks analysis showed that this gene family was exposed to purifying selection during evolution. Transcriptome data showed that four ORP genes, especially GhORP_A02, were induced by abiotic stress treatment. The cis-acting elements in the ORP promoters were responsive to phytohormones and various abiotic stresses. The silenced plants of GhORP_A02 were more sensitive to drought stress when compared to control.

CONCLUSION

The major finding of this study shed light on the potential role of ORP genes in abiotic stress and provided a fundamental resource for further analysis in cotton.

Comparative Transcriptome Analysis of Tolerant and Sensitive Genotypes of Common Bean (Phaseolus vulgaris L.) in Response to Terminal Drought Stress

We conducted a genome-wide transcriptomic analysis of three drought tolerant and sensitive genotypes of common bean to examine their transcriptional responses to terminal drought stress. We then conducted pairwise comparisons between the root and leaf transcriptomes from the resulting tissue based on combined transcriptomic data from the tolerant and sensitive genotypes. Our transcriptomic data revealed that 491 (6.4%) DEGs (differentially expressed genes) were upregulated in tolerant genotypes, whereas they were downregulated in sensitive genotypes; likewise, 396 (5.1%) DEGs upregulated in sensitive genotypes were downregulated in tolerant genotypes. Several transcription factors, heat shock proteins, and chaperones were identified in the study. Several DEGs in drought DB (data Base) overlapped between genotypes. The GO (gene ontology) terms for biological processes showed upregulation of DEGs in tolerant genotypes for sulfate and drug transmembrane transport when compared to sensitive genotypes. A GO term for cellular components enriched with upregulated DEGs for the apoplast in tolerant genotypes. These results substantiated the temporal pattern of root growth (elongation and initiation of root growth), and ABA-mediated drought response in tolerant genotypes. KEGG (kyoto encyclopedia of genes and genomes) analysis revealed an upregulation of MAPK (mitogen activated protein kinase) signaling pathways and plant hormone signaling pathways in tolerant genotypes. As a result of this study, it will be possible to uncover the molecular mechanisms of drought tolerance in response to terminal drought stress in the field. Further, genome-wide transcriptomic analysis of both tolerant and sensitive genotypes will assist us in identifying potential genes that may contribute to improving drought tolerance in the common bean.

Burning questions for a warming and changing world: 15 unknowns in plant abiotic stress

We present unresolved questions in plant abiotic stress biology as posed by 15 research groups with expertise spanning eco-physiology to cell and molecular biology. Common themes of these questions include the need to better understand how plants detect water availability, temperature, salinity, and rising carbon dioxide (CO2) levels; how environmental signals interface with endogenous signaling and development (e.g. circadian clock and flowering time); and how this integrated signaling controls downstream responses (e.g. stomatal regulation, proline metabolism, and growth versus defense balance). The plasma membrane comes up frequently as a site of key signaling and transport events (e.g. mechanosensing and lipid-derived signaling, aquaporins). Adaptation to water extremes and rising CO2 affects hydraulic architecture and transpiration, as well as root and shoot growth and morphology, in ways not fully understood. Environmental adaptation involves tradeoffs that limit ecological distribution and crop resilience in the face of changing and increasingly unpredictable environments. Exploration of plant diversity within and among species can help us know which of these tradeoffs represent fundamental limits and which ones can be circumvented by bringing new trait combinations together. Better defining what constitutes beneficial stress resistance in different contexts and making connections between genes and phenotypes, and between laboratory and field observations, are overarching challenges.

Metabolism of crown tissue is crucial for drought tolerance and recovery after stress cessation in Lolium/Festuca forage grasses

A process of plant recovery after drought cessation is a complex trait which has not been fully recognized. The most important organ associated with this phenomenon in monocots, including forage grasses, is the crown tissue located between shoots and roots. The crown tissue is a meristematic crossroads for metabolites and other compounds between these two plant organs. Here, for the first time, we present a metabolomic and lipidomic study focused on the crown tissue under drought and recovery in forage grasses, important for agriculture in European temperate regions. The plant materials involve high (HDT) and low drought-tolerant (LDT) genotypes of Festuca arundinacea, and Lolium multiflorum/F. arundinacea introgression forms. The obtained results clearly demonstrated that remodeling patterns of the primary metabolome and lipidome in the crown under drought and recovery were different between HDT and LDT plants. Furthermore, HDT plants accumulated higher contents of primary metabolites under drought in the crown tissue, especially carbohydrates which could function as osmoprotectants and storage materials. On the other hand, LDT plants characterized by higher membranes damage under drought, simultaneously accumulated membrane phospholipids in the crown and possessed the capacity to recover their metabolic functions after stress cessation to the levels observed in HDT plants.

Application of CRISPR/Cas9-mediated gene editing for abiotic stress management in crop plants

Abiotic stresses, including drought, salinity, cold, heat, and heavy metals, extensively reducing global agricultural production. Traditional breeding approaches and transgenic technology have been widely used to mitigate the risks of these environmental stresses. The discovery of engineered nucleases as genetic scissors to carry out precise manipulation in crop stress-responsive genes and associated molecular network has paved the way for sustainable management of abiotic stress conditions. In this context, the clustered regularly interspaced short palindromic repeat-Cas (CRISPR/Cas)-based gene-editing tool has revolutionized due to its simplicity, accessibility, adaptability, flexibility, and wide applicability. This system has great potential to build up crop varieties with enhanced tolerance against abiotic stresses. In this review, we summarize the latest findings on understanding the mechanism of abiotic stress response in plants and the application of CRISPR/Cas-mediated gene-editing system towards enhanced tolerance to a multitude of stresses including drought, salinity, cold, heat, and heavy metals. We provide mechanistic insights on the CRISPR/Cas9-based genome editing technology. We also discuss applications of evolving genome editing techniques such as prime editing and base editing, mutant library production, transgene free and multiplexing to rapidly deliver modern crop cultivars adapted to abiotic stress conditions.

Medicago sativa and M. tunetana reveal contrasting physiological and metabolic responses to drought

Alfalfa production is frequently constrained by drought, indicating the importance of assessing species biodiversity in endemic close relatives to enhance forage production under future global change conditions. In the present study, plants of two ecotypes of M. tunetana, native to Tunisia, and four commercial cultivars of M. sativa were subjected to two water regimes (control vs drought [15% field capacity]). Physiological, isotopic and metabolic analyses were used to characterize leaf and nodule profiles of the plants. Biomass, gas exchange and the maximum carboxylation rate (Vc_max) indicated significant decreases in photosynthetic capacity under drought in M. sativa cultivars. However, M. tunetana ecotypes maintained photosynthetic performance and aboveground biomass under drought conditions. Furthermore, nitrogen isotope composition (δ¹⁵N) in nodules and leaves was significantly decreased, which reveals a reduction in the N₂ fixing activity of nodules under drought conditions that was not translated into lower leaf N content but was probably due to lower N demand. Analyses of starch, soluble sugar, and amino acid content in leaves and nodules have clearly proven the ability of Medicago spp. cultivars to increase the accumulation of osmo-protectors under drought. This study demonstrated the genetic variability of the strategy adopted among the studied cultivars in response to drought. In this sense, M. tunetana, and in part the M. sativa cultivar adapted to Mediterranean conditions, seem capable of maintaining adequate biomass, photosynthesis and biological N₂ fixation in comparison to the other M. sativa cultivars.

Characterization of fennel germplasm for physiological persistence and drought recovery: Association with biochemical properties

In the perennial medicinal plant fennel (Foeniculum vulgare), persistence over years, production stability, and successful post-drought recovery are as important as plant productivity. Characterization of productivity, post-drought recovery, persistency and their association with phytochemical properties has not yet been performed in fennel. In this study, 64 fennel genotypes (from 23 different countries) from four subspecies/varieties including F. vulgare ssp. piperitum, F. vulgare var. vulgare and F. vulgare var. dulce and F. vulgare var. azoricum were evaluated in the field over four years (2015-2018), then was assessed for post-drought recovery over the next two years (2019-2020). High genotypic variation was observed among genotypes and subspecies. Based on the GC-MS analysis, trans-anethole (22.4-90.6%), estragole (2.1-25.8%), fenchone (4.9-19.8%), and limonene (0.5-11.9%) are major components in the essential oils of the studied germplasm. The highest persistence and the lowest average post-drought recovery belonged to ssp. piperitum. This subspecies also had the highest amount of limonene, fenchone, and estragole and the lowest amount of trans-anethole compared to other subspecies. The highest essential oil content and stability was observed in var. dulce. Seed yield and persistence were found to be negatively associated, suggesting that selection for more productivity may indirectly promote less persistent genotypes. Post-drought recovery was negatively associated with two main phytochemical compounds of essential oil, fenchone and estragole, but positively associated with trans-anethole. Persistence had negative correlation with estragole content. Results indicated that indirect selection for post-drought recovery and persistence may be possible through selection for phytochemical properties. The negative correlation between some essential oil components may indicate that some are isomers, which can limit the ability to select for certain combinations. Superior genotypes identified in this study can be used to construct populations for future studies and as parents of crosses to develop new varieties.

Impact of wild solanaceae rootstocks on morphological and physiological response, yield, and fruit quality of tomato (Solanum lycopersicum L.) grown under deficit irrigation conditions

It has been established that climate change has a direct impact on water availability, an essential resource for agricultural development. As a result, controlling, mitigating, and adapting to water deficit requires the advancement of research on promising wild flora species. As recent studies have shown, wild relatives of certain cultivars are tolerant to adverse factors, enabling the development of sustainable and resilient agriculture. The present study evaluated the morpho-physiology and productivity of tomato scions grafted on wild Solanaceae (Datura stramonium, Solanum sisymbriifolium, Solanum quitoense, and Cyphomandra betacea) grown under water deficit conditions (100% ETc - high level, 75% ETc - moderate level, 50% ETc - medium level, and 25% ETc - low level). The results showed that tomato plants grafted on Datura stramonium rootstocks performed better morpho-physiologically under deficit irrigation. The improved osmoregulation caused by a higher relative water content (98.49%) allowed the scion to be more tolerant to water stress. In addition, these scions showed high water potential during their phenological stages (vegetative -0.47 MPa, flowering -0.59 MPa, and production -0.64 MPa), as well as improved photosynthetic efficiency. The overall tolerance of the scion resulted in better yield (8.14 kg/plant) with higher number of commercially valuable fruits. The D. stramonium rootstock allowed better management and use of irrigation water, increasing productivity (54.95 kg/m³); that is, it is presented as a species with potential for establishing tomato production areas in scenarios of water scarcity or cultivation under deficit irrigation.

Rubber Genotypes with Contrasting Drought Factor Index Revealed Different Mechanisms for Drought Resistance in Hevea brasiliensis

It is predicted that drought will be more frequent and sustained in the future, which may affect the decline of rubber tree production. Therefore, it is critical to research some of the variables related to the drought-resistance mechanism of the rubber tree. As a result, it can be used to guide the selection of new rubber drought-resistance clones. The goal of this study was to identify drought-resistance mechanisms in rubber clones from the high drought factor index (DFI) group using ecophysiological and biochemical variables. The treatments consist of two factors, namely water deficit and contrasting clones based on the DFI variable. The first factor consisted of three levels, namely normal (fraction of transpirable soil water (FTSW) > 0.75), severe water deficit (0.1 < FTSW < 0.20), and recovery condition (FTSW > 0.75 after rewatering). The second factor consisted of seven clones, namely clones G239, GT1 (low DFI), G127, SP 217, PB 260 (moderate DFI), as well as G206 and RRIM 600 (high DFI). RRIM 600 had the highest DFI among the other clones as a drought-tolerance mechanism characteristic. Furthermore, clones RRIM 600, GT1, and G127 had lower stomatal conductance and transpiration rate than drought-sensitive clone PB 260. As a result, as drought avoidance mechanisms, clones RRIM 600, GT1, and G127 consume less water than clone PB 260. These findings indicated that clone RRIM 600 was a drought-resistant clone with drought tolerance and avoidance mechanisms.

Association analysis revealed loci linked to post-drought recovery and traits related to persistence of smooth bromegrass (Bromus inermis)

Association analysis has been proven as a powerful tool for the genetic dissection of complex traits. This study was conducted to identify association of recovery, persistence, and summer dormancy with sequence related amplified polymorphism (SRAP) markers in 36 smooth bromegrass genotypes under two moisture conditions and find stable associations. In this study, a diverse panel of polycross-derived progenies of smooth bromegrass was phenotyped under normal and water deficit regimes for three consecutive years. Under water deficit, dry matter yield of cut 1 was approximately reduced by 36, 39, and 37% during 2013, 2014, and 2015, respectively, compared with the normal regime. For dry matter yield of cut 2, these reductions were approximately 38, 60, and 56% in the same three consecutive years relative to normal regime. Moreover, water deficit decreased the RY and PER of the genotypes by 35 and 28%, respectively. Thirty primer combinations were screened by polymerase chain reaction (PCR). From these, 541 polymorphic bands were developed and subjected to association analysis using the mixed linear model (MLM). Population structure analysis identified five main subpopulations possessing significant genetic differences. Association analysis identified 69 and 46 marker-trait associations under normal and water deficit regimes, respectively. Some of these markers were associated with more than one trait; which can be attributed to pleiotropic effects or tightly linked genes affecting several traits. In normal and water-deficit regimes, these markers could potentially be incorporated into marker-assisted selection and targeted trait introgression for the improvement of drought tolerance of smooth bromegrass.

A Natural Bio-Stimulant Consisting of a Mixture of Fish Protein Hydrolysates and Kelp Extract Enhances the Physiological, Biochemical and Growth Responses of Spinach under Different Water Levels

Spinach (Spinacia oleracea L.) is a highly nutritious, desirable green leafy vegetable, which is less tolerant to drought. This study was conducted to establish the impact of a natural bio-stimulant consisting of a mixture of fish protein hydrolysates and kelp extract (trade name, Xcell Boost) on the physiological and biochemical responses as well as vegetative growth of spinach (Spinacia oleracea L.) under different water levels (100% (full irrigation), 50% (mild drought stress) and 30% (severe drought stress) water holding capacity). Bio-stimulant application at any strength (single, BX1 or double, BX2) had no effect on the photochemical reactions. The application of bio-stimulant at double strength concentration (BX2) increased the chlorophyll and carotenoid contents, as well as the activities of antioxidative enzymes, ascorbate peroxidase (APX) and guaiacol peroxidase (GPX), under drought stress. Application at single strength (BX1) increased the normalised difference vegetation index (NDVI), stomatal conductance, accumulation of osmoprotectants (proline and total soluble sugars) and reduced electrolyte leakage under drought stress. Furthermore, bio-stimulant applications at either concentration induced remarkable increases in plant height, leaf area, stem dry weight, root length and root moisture. Under BX2, APX and stomatal conductance positively correlated with stem dry weight, while root length positively correlated with total chlorophyll content. These results show that Xcell Boost is a highly advantageous bio-stimulant for increasing the tolerance of spinach to drought stress, which can most likely benefit other crops grown in semi-arid and arid areas.

Leaf transcriptome analysis of Medicago ruthenica revealed its response and adaptive strategy to drought and drought recovery

BACKGROUND

Drought is one of the main causes of losses in forage crop yield and animal production. Medicago ruthenica (L.) cv. Zhilixing is a high-yielding alfalfa cultivar also known for its high tolerance to drought. We analyzed the transcriptome profile of this cultivar throughout drought stress and recovery and we were able to describe its phased response through the expression profiles of overlapping gene networks and drought-specific genes.

RESULTS

The ABA and auxin signal transduction pathways are overlapping pathways in response to drought and drought recovery in forage crops. Medicago ruthenica (L.) cv. Zhilixing adopts different strategies at different degrees of drought stress. On the 9th day of drought, transcriptional regulations related to osmoregulation are enhanced mainly through increased activities of carbohydrate and amino acid metabolism, while photosynthetic activities were reduced to slow down growth. With drought prolonging, on the 12th day of drought, the synthesis of proline and other stored organic substances was suppressed in general. After recovery, Medicago ruthenica synthesizes flavonoids through the flavonoid biosynthesis pathway to remove accumulated ROS and repair the oxidative damage from water stress. In addition, the regulation of circadian rhythm seems to accelerate the drought recovery process.

CONCLUSIONS

Medicago ruthenica adapts to drought by regulating the osmoregulatory system and photosynthesis, which appears to involve the ABA and auxin signaling pathways as key regulators. Furthermore, the synthesis of flavonoids and the regulation of the circadian rhythm can accelerate the recovery process. These results enriched our knowledge of molecular responses to drought and drought recovery in Medicago ruthenica and provide useful information for the development of new legume forage grass varieties with improved adaptability to drought stress.

GhCDPK60 positively regulates drought stress tolerance in both transgenic Arabidopsis and cotton by regulating proline content and ROS level

Calcium-Dependent Protein Kinases (CDPKs) involved in regulating downstream components of calcium signaling pathways play a role in tolerance to abiotic stresses and seed development in plants. However, functions of only a few cotton CDPKs have been clarified at present. In this study, 80 conserved CDPKs in Gossypium hirsutum L. were identified and characterized, which was divided into four subgroups. Among them, the transcript level of GhCDPK60 was significantly upregulated under drought and several hormone treatments. And we found that the expression levels of several stress-inducible genes down-regulated in GhCDPK60-silence cotton and up-regulated in GhCDPK60-overexpressing Arabidopsis. In addition, physiological analyses demonstrated that GhCDPK60 improved drought stress tolerance by improving the osmotic adjustment ability and reducing the accumulation of reactive oxygen species (ROS) in plants. These findings broaden our understanding of the biological roles of GhCDPK60 and mechanisms underlying drought stress tolerance in cotton.

Melatonin and arbuscular mycorrhizal fungi synergistically improve drought toleration in kiwifruit seedlings by increasing mycorrhizal colonization and nutrient uptake

Kiwifruit is a vine fruit tree that is vulnerable to water deficiency due to its shallow root system and large leaves. Although mycorrhizal inoculation and melatonin application has been proved to improve plants drought tolerance, their interaction effects are still unclear. In this study, arbuscular mycorrhizal (AM) fungi incubation and melatonin (MT) irrigation were applied to kiwifruit seedlings alone or in combination to investigate their effect on drought tolerance. The results revealed that AM had more effect on promoting root biomass, water use efficiency, and uptake of nitrogen, phosphorus and iron. While MT was more effective in promoting shoot biomass and antioxidant enzyme activities to remove reactive oxygen species accumulation. Moreover, MT supplementary significantly increased the AM colonization, spore density and hyphal length density in roots. Therefore, combined application of AM fungi and MT had additive effects on improvement biomass accumulation, increasing chlorophyll content, photosynthetic efficiency, catalase activity, and decreasing malondialdehyde accumulation under drought stress, thus promoting plant growth and alleviating the drought damage to plant. These results provide guidance for AM and MT combined application to improve abiotic resistance in plants.

A Novel bHLH Transcription Factor PtrbHLH66 from Trifoliate Orange Positively Regulates Plant Drought Tolerance by Mediating Root Growth and ROS Scavenging

Drought limits citrus yield and fruit quality worldwide. The basic helix-loop-helix (bHLH) transcription factors (TFs) are involved in plant response to drought stress. However, few bHLH TFs related to drought response have been functionally characterized in citrus. In this study, a bHLH family gene, named PtrbHLH66, was cloned from trifoliate orange. PtrbHLH66 contained a highly conserved bHLH domain and was clustered closely with bHLH66 homologs from other plant species. PtrbHLH66 was localized to the nucleus and had transcriptional activation activity. The expression of PtrbHLH66 was significantly induced by polyethylene glycol 6000 (PEG6000) and abscisic acid (ABA) treatments. Ectopic expression of PtrbHLH66 promoted the seed germination and root growth, increased the proline and ABA contents and the activities of antioxidant enzymes, but reduced the accumulation of malondialdehyde (MDA) and reactive oxygen species (ROS) under drought stress, resulting in enhanced drought tolerance of transgenic Arabidopsis. In contrast, silencing the PtrbHLH66 homolog in lemon plants showed the opposite effects. Furthermore, under drought stress, the transcript levels of 15 genes involved in ABA biosynthesis, proline biosynthesis, ROS scavenging and drought response were obviously upregulated in PtrbHLH66 ectopic-expressing Arabidopsis but downregulated in PtrbHLH66 homolog silencing lemon. Thus, our results suggested that PtrbHLH66 acted as a positive regulator of plant drought resistance by regulating root growth and ROS scavenging.

Combined Metabolomic and Transcriptomic Analysis Reveals Allantoin Enhances Drought Tolerance in Rice

Drought is a misfortune for agriculture and human beings. The annual crop yield reduction caused by drought exceeds the sum of all pathogens. As one of the gatekeepers of China's "granary", rice is the most important to reveal the key drought tolerance factors in rice. Rice seedlings of Nipponbare (Oryza sativa L. ssp. Japonica) were subjected to simulated drought stress, and their root systems were analyzed for the non-targeted metabolome and strand-specific transcriptome. We found that both DEGs and metabolites were enriched in purine metabolism, and allantoin accumulated significantly in roots under drought stress. However, few studies on drought tolerance of exogenous allantoin in rice have been reported. We aimed to further determine whether allantoin can improve the drought tolerance of rice. Under the treatment of exogenous allantoin at different concentrations, the drought resistant metabolites of plants accumulated significantly, including proline and soluble sugar, and reactive oxygen species (ROS) decreased and reached a significant level in 100 μmol L^-1. To this end, a follow-up study was identified in 100 μmol L^-1 exogenous allantoin and found that exogenous allantoin improved the drought resistance of rice. At the gene level, under allantoin drought treatment, we found that genes of scavenge reactive oxygen species were significantly expressed, including peroxidase (POD), catalase (CATA), ascorbate peroxidase 8 (APX8) and respiratory burst oxidase homolog protein F (RbohF). This indicates that plants treated by allantoin have better ability to scavenge reactive oxygen species to resist drought. Alternative splicing analysis revealed a total of 427 differentially expressed alternative splicing events across 320 genes. The analysis of splicing factors showed that gene alternative splicing could be divided into many different subgroups and play a regulatory role in many aspects. Through further analysis, we restated the key genes and enzymes in the allantoin synthesis and catabolism pathway, and found that the expression of synthetase and hydrolase showed a downward trend. The pathway of uric acid to allantoin is completed by uric acid oxidase (UOX). To find out the key transcription factors that regulate the expression of this gene, we identified two highly related transcription factors OsERF059 and ONAC007 through correlation analysis. They may be the key for allantoin to enhance the drought resistance of rice.

Co-expression of stress-responsive regulatory genes, MuNAC4, MuWRKY3 and MuMYB96 associated with resistant-traits improves drought adaptation in transgenic groundnut (Arachis hypogaea l.) plants

Groundnut, cultivated under rain-fed conditions is prone to yield losses due to intermittent drought stress. Drought tolerance is a complex phenomenon and multiple gene expression required to maintain the cellular tolerance. Transcription factors (TFs) regulate many functional genes involved in tolerance mechanisms. In this study, three stress-responsive regulatory TFs cloned from horse gram, (Macrotyloma uniflorum (Lam) Verdc.), MuMYB96, involved in cuticular wax biosynthesis; MuWRKY3, associated with anti-oxidant defense mechanism and MuNAC4, tangled with lateral root development were simultaneously expressed to enhance drought stress resistance in groundnut (Arachis hypogaea L.). The multigene transgenic groundnut lines showed reduced ROS production, membrane damage, and increased superoxide dismutase (SOD) and ascorbate peroxidase (APX) enzyme activity, evidencing improved antioxidative defense mechanisms under drought stress. Multigene transgenic plants showed lower proline content, increased soluble sugars, epicuticular wax content and higher relative water content suggesting higher maintenance of tissue water status compared to wildype and mock plants. The scanning electron microscopy (SEM) analysis showed a substantial increase in deposition of cuticular waxes and variation in stomatal number in multigene transgenic lines compared to wild type and mock plants. The multigene transgenic plants showed increased growth of lateral roots, chlorophyll content, and stay-green nature in drought stress compared to wild type and mock plants. Expression analysis of transgenes, MuMYB96, MuWRKY3, and MuNAC4 and their downstream target genes, KCS6, KCR1, APX3, CSD1, LBD16 and DBP using qRT-PCR showed a two- to four-fold increase in transcript levels in multigene transgenic groundnut plants over wild type and mock plants under drought stress. Our study demonstrate that introducing multiple genes with simultaneous expression of genes is a viable option to improve stress tolerance and productivity under drought stress.

Shift in the migration trajectory of the green biomass loss barycenter in Central Asia

Revealing the vegetation response law under drought stress has become a hot issue in global climate change research. Against the background of human beings actively responding to climate change, quantitatively revealing the change and migration laws of green biomass loss (GBL) caused by drought in historical and future periods is insufficient. In this regard, we innovatively constructed a joint kNDVI-SPEI (kernel normalized difference vegetation index and standardized precipitation evapotranspiration index) distribution based on copula theory to accurately capture GBL dynamic under various drought scenarios unlike previous studies conducted in a deterministic way. Taking the drought-sensitive and ecologically vulnerable Central Asia (CA) as a typical region, we verified that an average 94.4 % of region showed greater vegetation vulnerability in times of water shortage from May to October, which exhibited the greatest probability of GBL under different drought scenarios, mainly in Kazakhstan and Uzbekistan. Significantly intensified drought due to high emissions will cause an 18.16 percentage-point increase in GBL probability in the far future (FFP, 2061-2100) compared to the near future (NFP, 2019-2060), which is much higher than in the lower-emission (0.38 %) and moderate-emission scenarios (9.82 %). In the NFP, the GBL barycenter will shift from Kazakhstan to Xinjiang, China; in the FFP, it will shift back to Kazakhstan due to the measures taken by the Chinese government to conserve energy and reduce emissions. Results illustrate that against the background of worsening drought, active climate change coping strategies can reverse the migration trajectory of the GBL barycenter caused by drought, which provides a new idea for vegetation protection research in response to global climate change.

Engineering drought and salinity tolerance traits in crops through CRISPR-mediated genome editing: Targets, tools, challenges, and perspectives

Prolonged periods of drought triggered by climate change hamper plant growth and cause substantial agricultural yield losses every year. In addition to drought, salinity is one of the major abiotic stresses that severely affect crop health and agricultural production. Plant responses to drought and salinity involve multiple processes that operate in a spatiotemporal manner, such as stress sensing, perception, epigenetic modifications, transcription, post-transcriptional processing, translation, and post-translational changes. Consequently, drought and salinity stress tolerance are polygenic traits influenced by genome-environment interactions. One of the ideal solutions to these challenges is the development of high-yielding crop varieties with enhanced stress tolerance, together with improved agricultural practices. Recently, genome-editing technologies, especially clustered regularly interspaced short palindromic repeats (CRISPR) tools, have been effectively applied to elucidate how plants deal with drought and saline environments. In this work, we aim to portray that the combined use of CRISPR-based genome engineering tools and modern genomic-assisted breeding approaches are gaining momentum in identifying genetic determinants of complex traits for crop improvement. This review provides a synopsis of plant responses to drought and salinity stresses at the morphological, physiological, and molecular levels. We also highlight recent advances in CRISPR-based tools and their use in understanding the multi-level nature of plant adaptations to drought and salinity stress. Integrating CRISPR tools with modern breeding approaches is ideal for identifying genetic factors that regulate plant stress-response pathways and for the introgression of beneficial traits to develop stress-resilient crops.

Biochar as a soil amendment in the tree establishment phase: What are the consequences for tree physiology, soil quality and carbon sequestration?

Trees play a pivotal role in the urban environment alleviating the negative impacts of urbanization, and for this reason, local governments have promoted strongly tree planting policies. However, poor soil quality and neglect tree maintenance (e.g., irrigation and fertilization) can seriously mine the plant health status during the tree establishment phase. The use of biochar to provide long-lasting C to the soil and, at the same time, improving soil properties (e.g., improved water holding capacity), soil enzymes activities and NPK concentrations, is a promising research field. Therefore, with a two-step experiment, the study aimed to assay the physiological responses of a commonly used urban tree species (Tilia × europaea L.) to 1.5 % (w/w) biochar amendment (B), and secondly, to assess the ability of trees, grown in biochar amended soil, to tolerate a period of drought. Biochar amendment increased P and K availability in the soil, resulting in higher P and K concentrations in B than control leaves, according to the leaf stage. This induced B trees, higher values in both total biomass than controls (+22 %) in well-watered plants. Moreover, the higher water availability in soil amended with biochar helped B trees to tolerate water stress, with better leaf photosynthetic performances and a faster recovery than stressed controls after the re-watering. This study highlights the dual function of the biochar, improving CO₂ sequestration and soil properties, and at the same time, enhancing plant physiological responses to environmental constraints. The use of biochar at the tree planting, especially in an urban environment, is a feasible and environmentally sustainable strategy to improve the success during the tree establishment phase.

Dissection of a grain yield QTL from wild emmer wheat reveals sub-intervals associated with culm length and kernel number

Genetic diversity in wheat has been depleted due to domestication and modern breeding. Wild relatives are a valuable source for improving drought tolerance in domesticated wheat. A QTL region on chromosome 2BS of wild emmer wheat (Triticum turgidum ssp. dicoccoides), conferring high grain yield under well-watered and water-limited conditions, was transferred to the elite durum wheat cultivar Uzan (T. turgidum ssp. durum) by a marker-assisted backcross breeding approach. The 2B introgression line turned out to be higher yielding but also exhibited negative traits that likely result from trans-, cis-, or linkage drag effects from the wild emmer parent. In this study, the respective 2BS QTL was subjected to fine-mapping, and a set of 17 homozygote recombinants were phenotyped at BC4F5 generation under water-limited and well-watered conditions at an experimental farm in Israel and at a high-throughput phenotyping platform (LemnaTec-129) in Germany. In general, both experimental setups allowed the identification of sub-QTL intervals related to culm length, kernel number, thousand kernel weight, and harvest index. Sub-QTLs for kernel number and harvest index were detected specifically under either drought stress or well-watered conditions, while QTLs for culm length and thousand-kernel weight were detected in both conditions. Although no direct QTL for grain yield was identified, plants with the sub-QTL for kernel number showed a higher grain yield than the recurrent durum cultivar Uzan under well-watered and mild drought stress conditions. We, therefore, suggest that this sub-QTL might be of interest for future breeding purposes.

Molecular and Physiological Mechanisms to Mitigate Abiotic Stress Conditions in Plants

Agriculture production faces many abiotic stresses, mainly drought, salinity, low and high temperature. These abiotic stresses inhibit plants' genetic potential, which is the cause of huge reduction in crop productivity, decrease potent yields for important crop plants by more than 50% and imbalance agriculture's sustainability. They lead to changes in the physio-morphological, molecular, and biochemical nature of the plants and change plants' regular metabolism, which makes them a leading cause of losses in crop productivity. These changes in plant systems also help to mitigate abiotic stress conditions. To initiate the signal during stress conditions, sensor molecules of the plant perceive the stress signal from the outside and commence a signaling cascade to send a message and stimulate nuclear transcription factors to provoke specific gene expression. To mitigate the abiotic stress, plants contain several methods of avoidance, adaption, and acclimation. In addition to these, to manage stress conditions, plants possess several tolerance mechanisms which involve ion transporters, osmoprotectants, proteins, and other factors associated with transcriptional control, and signaling cascades are stimulated to offset abiotic stress-associated biochemical and molecular changes. Plant growth and survival depends on the ability to respond to the stress stimulus, produce the signal, and start suitable biochemical and physiological changes. Various important factors, such as the biochemical, physiological, and molecular mechanisms of plants, including the use of microbiomes and nanotechnology to combat abiotic stresses, are highlighted in this article.

Arbuscular Mycorrhizal Fungi Promote Physiological and Biochemical Advantages in Handroanthus serratifolius Seedlings Submitted to Different Water Deficits

Climate change causes increasingly longer periods of drought, often causing the death of plants, especially when they are in the early stages of development. Studying the benefits provided by arbuscular mycorrhizal (AM) fungi to plants in different water regimes is an efficient and sustainable strategy to face climate change. Thus, this study investigated the influence of AM fungi on Handroanthus serratifolius seedlings under different water regimes, based on biochemical, and nutritional growth parameters. The experiment was carried out in H. serratifolius seedlings cultivated with mycorrhizas (+AMF) and without mycorrhizas (-AMF) in three water regimes; a severe water deficit (SD), a moderate water deficit (MD), and a well-watered (WW) condition. AM fungi provided greater osmoregulation under water deficit conditions through the accumulation of soluble sugars, total free amino acids, and proline, as well as by reducing sugar. The increase in the absorption of phosphorus and nitrate was observed only in the presence of fungi in the well-watered regimen. A higher percentage of colonization was found in plants submitted to the well-watered regimen. Ultimately, AM fungi promoted biochemical, nutritional, and growth benefits for H. serratifolius seedlings under the water deficit and well-hydrated conditions, proving that AMF can be used to increase the tolerance of H. serratifolius plants, and help them to survive climate change.

Meta-Analysis Reveals Challenges and Gaps for Genome-to-Phenome Research Underpinning Plant Drought Response

Severe drought conditions and extreme weather events are increasing worldwide with climate change, threatening the persistence of native plant communities and ecosystems. Many studies have investigated the genomic basis of plant responses to drought. However, the extent of this research throughout the plant kingdom is unclear, particularly among species critical for the sustainability of natural ecosystems. This study aimed to broaden our understanding of genome-to-phenome (G2P) connections in drought-stressed plants and identify focal taxa for future research. Bioinformatics pipelines were developed to mine and link information from databases and abstracts from 7730 publications. This approach identified 1634 genes involved in drought responses among 497 plant taxa. Most (83.30%) of these species have been classified for human use, and most G2P interactions have been described within model organisms or crop species. Our analysis identifies several gaps in G2P research literature and database connectivity, with 21% of abstracts being linked to gene and taxonomy data in NCBI. Abstract text mining was more successful at identifying potential G2P pathways, with 34% of abstracts containing gene, taxa, and phenotype information. Expanding G2P studies to include non-model plants, especially those that are adapted to drought stress, will help advance our understanding of drought responsive G2P pathways.

Non-coding RNAs fine-tune the balance between plant growth and abiotic stress tolerance

To survive in adverse environmental conditions, plants have evolved sophisticated genetic and epigenetic regulatory mechanisms to balance their growth and abiotic stress tolerance. An increasing number of non-coding RNAs (ncRNAs), including small RNAs (sRNAs) and long non-coding RNAs (lncRNAs) have been identified as essential regulators which enable plants to coordinate multiple aspects of growth and responses to environmental stresses through modulating the expression of target genes at both the transcriptional and posttranscriptional levels. In this review, we summarize recent advances in understanding ncRNAs-mediated prioritization towards plant growth or tolerance to abiotic stresses, especially to cold, heat, drought and salt stresses. We highlight the diverse roles of evolutionally conserved microRNAs (miRNAs) and small interfering RNAs (siRNAs), and the underlying phytohormone-based signaling crosstalk in regulating the balance between plant growth and abiotic stress tolerance. We also review current discoveries regarding the potential roles of ncRNAs in stress memory in plants, which offer their descendants the potential for better fitness. Future ncRNAs-based breeding strategies are proposed to optimize the balance between growth and stress tolerance to maximize crop yield under the changing climate.

Acoustic Tweezer-Modulated Biomimetic Patterned Particle-Polymer Composite for Water Vapor Harvesting

With the recent threat of climate change and global warming, ensuring access to safe drinking water is a great challenge in many areas worldwide. Designing functional materials for capturing water from natural resources like fog and mist has become one of the key research areas to maximize the production of clean water. From this aspect, nature is a great source for designing bioinspired functional materials as some of the plant leaves and animal exoskeletons can harness water and then store it to save themselves from arid, xeric conditions. Inspired by the Stenocara beetle, we have designed a composite surface structure with periodic islands made of aluminum microparticles surrounded by poly(dimethylenesiloxane) (PDMS). An acoustic tweezer-based method was used to fabricate the bioinspired composite structures, where surface acoustic waves at specific frequencies and amplitudes are applied to align the microparticles as islands in the polymer matrix. An oxygen plasma etching step was applied to expose the microparticles on the PDMS surface. The average water harvesting efficiencies for structures made with 120 and 80 kHz acoustic frequencies and 1 hour etching time were found to be 9.41 and 8.84 g cm^-2 h^-1, respectively. The acoustically patterned biomimetic composite surface showed higher water harvesting efficiency compared with completely hydrophobic PDMS and hydrophilic aluminum surfaces, demonstrating the advantages of the bioinspired composite material design and acoustic-assisted manufacturing technique. The biomimetic fog water harvesting material is a promising avenue to fulfill the demand for a cost-effective, sustainable, and energy-efficient solution to safe drinking water.

Comparative physiological and root transcriptome analysis of two annual ryegrass cultivars under drought stress

Annual ryegrass is a widely cultivated forage grass with rapid growth and high productivity. However, drought is one of the abiotic stresses affecting ryegrass growth and quality. In this study, we compared the physiological and transcriptome responses of Chuansi No.1 (drought-tolerant, DT) and Double Barrel (drought-sensitive, DS) under drought stress simulated by PEG-6000 for 7 days. The results showed that Chuansi No. 1 had stronger physiological and biochemical parameters such as root properties, water content, osmotic adjustment ability and antioxidant ability. In addition, RNA-seq was used to elucidate the molecular mechanism of root drought resistance. We identified 8588 differentially expressed genes related to drought tolerance in root, which were mainly enriched in oxidation-reduction process, carbohydrate metabolic process, apoplast, arginine and proline metabolism, and phenylpropanoid biosynthesis pathways. The expression levels of DEGs were consistent with physiological changes of ryegrass under drought stress. We found that genes related to sucrose and starch synthesis, root development, osmotic adjustment, ABA signal regulation and specifically up-regulated transcription factors such as WRKY41, WRKY51, ERF7, ERF109, ERF110, NAC43, NAC68, bHLH162 and bHLH148 in Chuansi No. 1 may be the reason for its higher drought tolerance. This study revealed the underlying physiological and molecular mechanisms of root response to drought stress in ryegrass and provided some new candidate genes for breeding rye drought tolerant varieties.

Comparative physiological and coexpression network analyses reveal the potential drought tolerance mechanism of peanut

BACKGROUND

Drought stress has negative effects on plant growth and productivity. In this study, a comprehensive analysis of physiological responses and gene expression was performed. The responses and expressions were compared between drought-tolerant (DT) and drought-sensitive (DS) peanut varieties to investigate the regulatory mechanisms and hub genes involved in the impact of drought stress on culture.

RESULTS

The drought-tolerant variety had robust antioxidative capacities with higher total antioxidant capacity and flavonoid contents, and it enhanced osmotic adjustment substance accumulation to adapt to drought conditions. KEGG analysis of differentially expressed genes demonstrated that photosynthesis was strongly affected by drought stress, especially in the drought-sensitive variety, which was consistent with the more severe suppression of photosynthesis. The hub genes in the key modules related to the drought response, including genes encoding protein kinase, E3 ubiquitin-protein ligase, potassium transporter, pentatricopeptide repeat-containing protein, and aspartic proteinase, were identified through a comprehensive combined analysis of genes and physiological traits using weighted gene co-expression network analysis. There were notably differentially expressed genes between the two varieties, suggesting the positive roles of these genes in peanut drought tolerance.

CONCLUSION

A comprehensive analysis of physiological traits and relevant genes was conducted on peanuts with different drought tolerances. The findings revealed diverse drought-response mechanisms and identified candidate genes for further research.