Role of abscisic acid (ABA) and Arabidopsis thaliana ABA-insensitive loci in low water potential-induced ABA and proline accumulation

Transcriptomic dynamics of ABA response in Brassica napus guard cells

Drought has a significant, negative impact on crop production; and these effects are poised to increase with climate change. Plants acclimate to drought and water stress through diverse physiological responses, primarily mediated by the hormone abscisic acid (ABA). Because plants lose the majority of their water through stomatal pores on aerial surfaces of plants, stomatal closure is one of the rapid responses mediated by ABA to reduce transpirational water loss. The dynamic changes in the transcriptome of stomatal guard cells in response to ABA have been investigated in the model plant Arabidopsis thaliana. However, guard cell transcriptomes have not been analyzed in agronomically valuable crops such as a major oilseed crop, rapeseed. In this study, we investigated the dynamics of ABA-regulated transcriptomes in stomatal guard cells of Brassica napus and conducted comparison analysis with the transcriptomes of A. thaliana. We discovered changes in gene expression indicating alterations in a host of physiological processes, including stomatal movement, metabolic reprogramming, and light responses. Our results suggest the existence of both immediate and delayed responses to ABA in Brassica guard cells. Furthermore, the transcription factors and regulatory networks mediating these responses are compared to those identified in Arabidopsis. Our results imply the continuing evolution of ABA responses in Brassica since its divergence from a common ancestor, involving both protein-coding and non-coding nucleotide sequences. Together, our results will provide a basis for developing strategies for molecular manipulation of drought tolerance in crop plants.

ABSCISIC ACID INSENSITIVE 3 promotes auxin signalling by regulating SHY2 expression to control primary root growth in response to dehydration stress

Plants combat dehydration stress through different strategies including root architectural changes. Here we show that when exposed to varying levels of dehydration stress, primary root growth in Arabidopsis is modulated by regulating root meristem activity. Abscisic acid (ABA) in concert with auxin signalling adjust primary root growth according to stress levels. ABSCISIC ACID INSENSITIVE 3 (ABI3), an ABA-responsive transcription factor, stands at the intersection of ABA and auxin signalling and fine-tunes primary root growth in response to dehydration stress. Under low ABA or dehydration stress, induction of ABI3 expression promotes auxin signalling by decreasing expression of SHY2, a negative regulator of auxin response. This further enhances the expression of auxin transporter gene PIN1 and cell cycle gene CYCB1;1, resulting in an increase in primary root meristem size and root length. Higher levels of dehydration stress or ABA repress ABI3 expression and promote ABSCISIC ACID INSENSITIVE 5 (ABI5) expression. This elevates SHY2 expression, thereby impairing primary root meristem activity and retarding root growth. Notably, ABI5 can promote SHY2 expression only in the absence of ABI3. Such ABA concentration-dependent expression of ABI3 therefore functions as a regulatory sensor of dehydration stress levels and orchestrates primary root growth by coordinating its downstream regulation.

Crosstalk between Ethylene, Jasmonate and ABA in Response to Salt Stress during Germination and Early Plant Growth in Cucurbita pepo

The crosstalk of phytohormones in the regulation of growth and development and the response of plants to environmental stresses is a cutting-edge research topic, especially in crop species. In this paper, we study the role and crosstalk between abscisic acid (ABA), ethylene (ET), and jasmonate (JA) in the control of germination and seedling growth in water or in standard nutrient solution and under salt stress (supplemented with 100-200 mM NaCl). The roles of ET and JA were studied using squash ET- and JA-deficient mutants aco1a and lox3a, respectively, while the crosstalk between ET, JA, and ABA was determined by comparing the expression of the key ABA, JA, and ET genes in wild-type (WT) and mutant genotypes under standard conditions and salt stress. Data showed that ET and JA are positive regulators of squash germination, a function that was found to be mediated by downregulating the ABA biosynthesis and signaling pathways. Under salt stress, aco1a germinated earlier than WT, while lox3a showed the same germination rate as WT, indicating that ET, but not JA, restricts squash germination under unfavorable salinity conditions, a function that was also mediated by upregulation of ABA. ET and JA were found to be negative regulators of plant growth during seedling establishment, although ET inhibits both the aerial part and the root, while JA inhibits only the root. Both aco1a and lox3a mutant roots showed increased tolerance to salt stress, a phenotype that was found to be mainly mediated by JA, although we cannot exclude that it is also mediated by ABA.

Introgression of Δ1-pyrroline-5-carboxylate synthetase (PgP5CS) confers enhanced resistance to abiotic stresses in transgenic tobacco

Δ1-pyrroline-5-carboxylate synthetase (P5CS) is one of the key regulatory enzymes involved in the proline biosynthetic pathway. Proline acts as an osmoprotectant, molecular chaperone, antioxidant, and regulator of redox homeostasis. The accumulation of proline during stress is believed to confer tolerance in plants. In this study, we cloned the complete CDS of the P5CS from pearl millet (Pennisetum glaucum (L.) R.Br. and transformed into tobacco. Three transgenic tobacco plants with single-copy insertion were analyzed for drought and heat stress tolerance. No difference was observed between transgenic and wild-type (WT) plants when both were grown in normal conditions. However, under heat and drought, transgenic plants have been found to have higher chlorophyll, relative water, and proline content, and lower malondialdehyde (MDA) levels than WT plants. The photosynthetic parameters (stomatal conductance, intracellular CO2 concentration, and transpiration rate) were also observed to be high in transgenic plants under abiotic stress conditions. qRT-PCR analysis revealed that the expression of the transgene in drought and heat conditions was 2-10 and 2-7.5 fold higher than in normal conditions, respectively. Surprisingly, only P5CS was increased under heat stress conditions, indicating the possibility of feedback inhibition. Our results demonstrate the positive role of PgP5CS in enhancing abiotic stress tolerance in tobacco, suggesting its possible use to increase abiotic stress-tolerance in crops for sustained yield under adverse climatic conditions.

ARGONAUTE10 controls cell fate specification and formative cell divisions in the Arabidopsis root

A key question in plant biology is how oriented cell divisions are integrated with patterning mechanisms to generate organs with adequate cell type allocation. In the root vasculature, a gradient of miRNA165/6 controls the abundance of HD-ZIP III transcription factors, which in turn control cell fate and spatially restrict vascular cell proliferation to specific cells. Here, we show that vascular development requires the presence of ARGONAUTE10, which is thought to sequester miRNA165/6 and protect HD-ZIP III transcripts from degradation. Our results suggest that the miR165/6-AGO10-HDZIP III module acts by buffering cytokinin responses and restricting xylem differentiation. Mutants of AGO10 show faster growth rates and strongly enhanced survival under severe drought conditions. However, this superior performance is offset by markedly increased variation and phenotypic plasticity in sub-optimal carbon supply conditions. Thus, AGO10 is required for the control of formative cell division and coordination of robust cell fate specification of the vasculature, while altering its expression provides a means to adjust phenotypic plasticity.

A mutation in the brassinosteroid biosynthesis gene CpDWF5 disrupts vegetative and reproductive development and the salt stress response in squash (Cucurbita pepo)

A Cucurbita pepo mutant with multiple defects in growth and development has been identified and characterized. The mutant dwfcp displayed a dwarf phenotype with dark green and shrinking leaves, shortened internodes and petioles, shorter but thicker roots and greater root biomass, and reduced fertility. The causal mutation of the phenotype was found to disrupt gene Cp4.1LG17g04540, the squash orthologue of the Arabidopsis brassinosteroid (BR) biosynthesis gene DWF5, encoding for 7-dehydrocholesterol reductase. A single nucleotide transition (G > A) causes a splicing defect in intron 6 that leads to a premature stop codon and a truncated CpDWF5 protein. The mutation co-segregated with the dwarf phenotype in a large BC1S1 segregating population. The reduced expression of CpDWF5 and brassinolide (BL) content in most mutant organs, and partial rescue of the mutant phenotype by exogenous application of BL, showed that the primary cause of the dwarfism in dwfcp is a BR deficiency. The results showed that in C. pepo, CpDWF5 is not only a positive growth regulator of different plant organs but also a negative regulator of salt tolerance. During germination and the early stages of seedling development, the dwarf mutant was less affected by salt stress than the wild type, concomitantly with a greater upregulation of genes associated with salt tolerance, including those involved in abscisic acid (ABA) biosynthesis, ABA and Ca2+ signaling, and those coding for cation exchangers and transporters.

Comparative transcriptome analysis reveals the adaptive mechanisms of halophyte Suaeda dendroides encountering high saline environment

Suaeda dendroides, a succulent euhalophyte of the Chenopodiaceae family, intermittently spread around northern Xinjiang, China, has the ability to grow and develop in saline and alkali environments. The objective of this study was therefore to investigate the underlying molecular mechanisms of S. dendroides response to high salt conditions. 27 sequencing libraries prepared from low salt (200 mM NaCl) and high salt (800 mM NaCl) treated plants at 5 different stages were sequenced using Illumina Hiseq 2000. A total of 133,107 unigenes were obtained, of which 4,758 were DEGs. The number of DEGs in the high salt group (3,189) was more than the low salt treatment group (733) compared with the control. GO and KEGG analysis of the DEGs at different time points of the high salt treatment group showed that the genes related to cell wall biosynthesis and modification, plant hormone signal transduction, ion homeostasis, organic osmolyte accumulation, and reactive oxygen species (ROS) detoxification were significantly expressed, which indicated that these could be the main mechanisms of S. dendroides acclimate to high salt stress. The study provides a new perspective for understanding the molecular mechanisms of halophytes adapting to high salinity. It also provides a basis for future investigations of key salt-responsive genes in S. dendroides.

GIGANTEA-ENHANCED EM LEVEL complex initiates drought escape response via dual function of ABA synthesis and flowering promotion

Plants use the regulation of their circadian clock to adapt to daily environmental challenges, particularly water scarcity. During drought, plants accelerate flowering through a process called drought escape (DE) response, which is promoted by the circadian clock component GIGANTEA (GI). GI up-regulates the flowering inducer gene FLOWERING LOCUS T (FT). Phytohormone Abscisic acid (ABA) is also required for drought escape, and both GIGANTEA and Abscisic acid are interdependent in the transition. Recent research has revealed a new mechanism by which GIGANTEA and the protein ENHANCED EM LEVEL form a heterodimer complex that turns on ABA biosynthesis during drought stress by regulating the transcription of 9-CIS-EPOXYCAROTENOID DIOXYGENASE 3 (NCED3). This highlights the close connection between the circadian clock and ABA regulation and reveals a new adaptive strategy for plants to cope with drought and initiates the DE response.

Exogenous preculture with sucrose and abscisic acid improves post-cryopreservation survival of eastern bracken fern gametophytes

Cryopreservation is an important technique used in the conservation of various plant tissues. This study proposes a cryopreservation method for the long-term conservation of eastern bracken fern gametophytes (Pteridium aquilinum var. latiusculum). Encapsulation-dehydration of the gametophytes was performed, and the exogenous sucrose and abscisic acid (ABA) preculture conditions were investigated. Gametophytes are sensitive to dehydration and drying, and the following treatment conditions were applied: encapsulation by alginate containing 0.75 M sucrose, 18-h loading treatment with 0.75 M sucrose, and 6-h drying treatment. The survival rate following cryopreservation was determined. The water content of < 27.5% in the alginate beads after dehydration and drying was found to be appropriate for ensuring survival. Additionally, performing an exogenous sucrose and ABA preculture was essential before encapsulation to achieve a survival of ≥ 90%. The high stress induced by cryopreservation and exogenous preculture regulated the expression of PaSuSy, PaLEA14, and PaABI1b and the endogenous ABA content. In eastern bracken gametophytes, ABI1 appears to be a negative regulator of ABA signaling. These results indicate that the encapsulation-dehydration method is effective for the long-term conservation of eastern bracken fern gametophytes, and exogenous preculture alleviates abiotic stress and increases the survival rate.

Genome-Wide Analysis of Stress-Responsive Genes and Alternative Splice Variants in Arabidopsis Roots under Osmotic Stresses

Plant roots show distinct gene-expression profiles from those of shoots under abiotic stress conditions. In this study, we performed mRNA sequencing (mRNA-Seq) to analyze the transcriptional profiling of Arabidopsis roots under osmotic stress conditions-high salinity (NaCl) and drought (mannitol). The roots demonstrated significantly distinct gene-expression changes from those of the aerial parts under both the NaCl and the mannitol treatment. We identified 68 closely connected transcription-factor genes involved in osmotic stress-signal transduction in roots. Well-known abscisic acid (ABA)-dependent and/or ABA-independent osmotic stress-responsive genes were not considerably upregulated in the roots compared to those in the aerial parts, indicating that the osmotic stress response in the roots may be regulated by other uncharacterized stress pathways. Moreover, we identified 26 osmotic-stress-responsive genes with distinct expressions of alternative splice variants in the roots. The quantitative reverse-transcription polymerase chain reaction further confirmed that alternative splice variants, such as those for ANNAT4, MAGL6, TRM19, and CAD9, were differentially expressed in the roots, suggesting that alternative splicing is an important regulatory mechanism in the osmotic stress response in roots. Altogether, our results suggest that tightly connected transcription-factor families, as well as alternative splicing and the resulting splice variants, are involved in the osmotic stress response in roots.

D, Olivo M, Urano D. Classification of Plant Endogenous States Using Machine Learning-Derived Agricultural Indices

Leaf color patterns vary depending on leaf age, pathogen infection, and environmental and nutritional stresses; thus, they are widely used to diagnose plant health statuses in agricultural fields. The visible-near infrared-shortwave infrared (VIS-NIR-SWIR) sensor measures the leaf color pattern from a wide spectral range with high spectral resolution. However, spectral information has only been employed to understand general plant health statuses (e.g., vegetation index) or phytopigment contents, rather than pinpointing defects of specific metabolic or signaling pathways in plants. Here, we report feature engineering and machine learning methods that utilize VIS-NIR-SWIR leaf reflectance for robust plant health diagnostics, pinpointing physiological alterations associated with the stress hormone, abscisic acid (ABA). Leaf reflectance spectra of wild-type, ABA2-overexpression, and deficient plants were collected under watered and drought conditions. Drought- and ABA-associated normalized reflectance indices (NRIs) were screened from all possible pairs of wavelength bands. Drought associated NRIs showed only a partial overlap with those related to ABA deficiency, but more NRIs were associated with drought due to additional spectral changes within the NIR wavelength range. Interpretable support vector machine classifiers built with 20 NRIs predicted treatment or genotype groups with an accuracy greater than those with conventional vegetation indices. Major selected NRIs were independent from leaf water content and chlorophyll content, 2 well-characterized physiological changes under drought. The screening of NRIs, streamlined with the development of simple classifiers, serves as the most efficient means of detecting reflectance bands that are highly relevant to characteristics of interest.

Assessment of proline function in higher plants under extreme temperatures

Climate change and abiotic stress factors are key players in crop losses worldwide. Among which, extreme temperatures (heat and cold) disturb plant growth and development, reduce productivity and, in severe cases, lead to plant death. Plants have developed numerous strategies to mitigate the detrimental impact of temperature stress. Exposure to stress leads to the accumulation of various metabolites, e.g. sugars, sugar alcohols, organic acids and amino acids. Plants accumulate the amino acid 'proline' in response to several abiotic stresses, including temperature stress. Proline abundance may result from de novo synthesis, hydrolysis of proteins, reduced utilization or degradation. Proline also leads to stress tolerance by maintaining the osmotic balance (still controversial), cell turgidity and indirectly modulating metabolism of reactive oxygen species. Furthermore, the crosstalk of proline with other osmoprotectants and signalling molecules, e.g. glycine betaine, abscisic acid, nitric oxide, hydrogen sulfide, soluble sugars, helps to strengthen protective mechanisms in stressful environments. Development of less temperature-responsive cultivars can be achieved by manipulating the biosynthesis of proline through genetic engineering. This review presents an overview of plant responses to extreme temperatures and an outline of proline metabolism under such temperatures. The exogenous application of proline as a protective molecule under extreme temperatures is also presented. Proline crosstalk and interaction with other molecules is also discussed. Finally, the potential of genetic engineering of proline-related genes is explained to develop 'temperature-smart' plants. In short, exogenous application of proline and genetic engineering of proline genes promise ways forward for developing 'temperature-smart' future crop plants.

Transcriptome-based analysis of key pathways relating to yield formation stage of foxtail millet under different drought stress conditions

Although foxtail millet, as small Panicoid crop, is of drought resilient, drought stress has a significant effect on panicle of foxtail millet at the yield formation stage. In this study, the changes of panicle morphology, photosynthesis, antioxidant protective enzyme system, reactive oxygen species (ROS) system, and osmotic regulatory substance and RNA-seq of functional leaves under light drought stress (LD), heavy drought stress (HD), light drought control (LDCK) and heavy drought control (HDCK) were studied to get a snap-shot of specific panicle morphological changes, physiological responses and related molecular mechanisms. The results showed that the length and weight of panicle had decreased, but with increased empty abortive rate, and then yield dropped off 14.9% and 36.9%, respectively. The photosynthesis of millet was significantly decreased, like net photosynthesis rate, stomatal conductance and transpiration rate, especially under HD treatment with reluctant recovery from rehydration. Under LD and HD treatment, the peroxidase (POD) was increased by 34% and 14% and the same as H₂O₂ by 34.7% and 17.2% compared with LDCK and HDCK. The ability to produce and inhibit O^2- free radicals under LD treatment was higher than HD. The content of soluble sugar was higher under LD treatment but the proline was higher under HD treatment. Through RNA-seq analysis, there were 2,393 and 3,078 different genes expressed under LD and HD treatment. According to the correlation analysis between weighted gene coexpression network analysis (WGCNA) and physiological traits, the co-expression network of several modules with high correlation was constructed, and some hub genes of millet in response to drought stress were found. The expression changes relating to carbon fixation, sucrose and starch synthesis, lignin synthesis, gibberellin synthesis, and proline synthesis of millet were specifically analyzed. These findings provide a full perspective on how drought affects the yield formation of foxtail millet by constructing one work model thereby providing theoretical foundation for hub genes exploration and drought resistance breeding of foxtail millet.

FtbZIP12 Positively Regulates Responses to Osmotic Stress in Tartary Buckwheat

ABFs play a key role in regulating plant osmotic stress. However, in Tartary buckwheat, data on the role of ABF genes in osmotic stress remain limited and its associated mechanism in osmoregulation remain nebulous. Herein, a novel ABF family in Tartary buckwheat, FtbZIP12, was cloned and characterized. FtbZIP12 is a transcriptional activator located in the nucleus; its expression is induced by NaCl, mannitol, and abscisic acid (ABA). Atopic expression of FtbZIP12 in Arabidopsis promoted seed germination, reduced damage to primary roots, and improved the tolerance of seedlings to osmotic stress. The quantitative realtime polymerase chain reaction (RT-qPCR) results showed that the expressions of the typical genes related to stress, the SOS pathway, and the proline synthesis pathway in Arabidopsis were significantly (p < 0.05) upregulated under osmotic stress. FtbZIP12 improved the osmotic pressure resistance by reducing the damage caused by reactive oxygen species to plants and maintained plant homeostasis by upregulating the expression of genes related to stress, osmotic regulation, and ion homeostasis. This study identified a key candidate gene for understanding the mechanism underlying osmotic-stress-regulated function in Tartary buckwheat, thereby providing a theoretical basis for improving its yield and quality.

The roles of HD-ZIP proteins in plant abiotic stress tolerance

Homeodomain leucine zipper (HD-ZIP) proteins are plant-specific transcription factors that contain a homeodomain (HD) and a leucine zipper (LZ) domain. The highly conserved HD binds specifically to DNA and the LZ mediates homodimer or heterodimer formation. HD-ZIP transcription factors control plant growth, development, and responses to abiotic stress by regulating downstream target genes and hormone regulatory pathways. HD-ZIP proteins are divided into four subclasses (I-IV) according to their sequence conservation and function. The genome-wide identification and expression profile analysis of HD-ZIP proteins in model plants such as Arabidopsis (Arabidopsis thaliana) and rice (Oryza sativa) have improved our understanding of the functions of the different subclasses. In this review, we mainly summarize and discuss the roles of HD-ZIP proteins in plant response to abiotic stresses such as drought, salinity, low temperature, and harmful metals. HD-ZIP proteins mainly mediate plant stress tolerance by regulating the expression of downstream stress-related genes through abscisic acid (ABA) mediated signaling pathways, and also by regulating plant growth and development. This review provides a basis for understanding the roles of HD-ZIP proteins and potential targets for breeding abiotic stress tolerance in plants.

Salt stress resilience in plants mediated through osmolyte accumulation and its crosstalk mechanism with phytohormones

Salinity stress is one of the significant abiotic stresses that influence critical metabolic processes in the plant. Salinity stress limits plant growth and development by adversely affecting various physiological and biochemical processes. Enhanced generation of reactive oxygen species (ROS) induced via salinity stress subsequently alters macromolecules such as lipids, proteins, and nucleic acids, and thus constrains crop productivity. Due to which, a decreasing trend in cultivable land and a rising world population raises a question of global food security. In response to salt stress signals, plants adapt defensive mechanisms by orchestrating the synthesis, signaling, and regulation of various osmolytes and phytohormones. Under salinity stress, osmolytes have been investigated to stabilize the osmotic differences between the surrounding of cells and cytosol. They also help in the regulation of protein folding to facilitate protein functioning and stress signaling. Phytohormones play critical roles in eliciting a salinity stress adaptation response in plants. These responses enable the plants to acclimatize to adverse soil conditions. Phytohormones and osmolytes are helpful in minimizing salinity stress-related detrimental effects on plants. These phytohormones modulate the level of osmolytes through alteration in the gene expression pattern of key biosynthetic enzymes and antioxidative enzymes along with their role as signaling molecules. Thus, it becomes vital to understand the roles of these phytohormones on osmolyte accumulation and regulation to conclude the adaptive roles played by plants to avoid salinity stress.

Leafamine®, a Free Amino Acid-Rich Biostimulant, Promotes Growth Performance of Deficit-Irrigated Lettuce

Water deficit causes substantial yield losses that climate change is going to make even more problematic. Sustainable agricultural practices are increasingly developed to improve plant tolerance to abiotic stresses. One innovative solution amongst others is the integration of plant biostimulants in agriculture. In this work, we investigate for the first time the effects of the biostimulant -Leafamine®-a protein hydrolysate on greenhouse lettuce (Lactuca sativa L.) grown under well-watered and water-deficit conditions. We examined the physiological and metabolomic water deficit responses of lettuce treated with Leafamine® (0.585 g/pot) or not. Root application of Leafamine® increased the shoot fresh biomass of both well-watered (+40%) and deficit-irrigated (+20%) lettuce plants because the projected leaf area increased. Our results also indicate that Leafamine® application could adjust the nitrogen metabolism by enhancing the total nitrogen content, amino acid (proline) contents and the total protein level in lettuce leaves, irrespective of the water condition. Osmolytes such as soluble sugars and polyols, also increased in Leafamine®-treated lettuce. Our findings suggest that the protective effect of Leafamine is a widespread change in plant metabolism and could involve ABA, putrescine and raffinose.

Nitrogen represses haustoria formation through abscisic acid in the parasitic plant Phtheirospermum japonicum

Parasitic plants are globally prevalent pathogens that withdraw nutrients from their host plants using an organ known as the haustorium. The external environment including nutrient availability affects the extent of parasitism and to understand this phenomenon, we investigated the role of nutrients and found that nitrogen is sufficient to repress haustoria formation in the root parasite Phtheirospermum japonicum. Nitrogen increases levels of abscisic acid (ABA) in P. japonicum and prevents the activation of hundreds of genes including cell cycle and xylem development genes. Blocking ABA signaling overcomes nitrogen’s inhibitory effects indicating that nitrogen represses haustoria formation by increasing ABA. The effect of nitrogen appears more widespread since nitrogen also inhibits haustoria in the obligate root parasite Striga hermonthica. Together, our data show that nitrogen acts as a haustoria repressing factor and suggests a mechanism whereby parasitic plants use nitrogen availability in the external environment to regulate the extent of parasitism.

Parasitic plants obtain nutrients from their hosts. Here the authors show that nitrogen sufficiency suppresses parasitism in the root parasite Phtheirospermum japonicum by increasing levels of the phytohormone ABA suggesting that the degree of parasitism is regulated by nutrient availability.

Integrate Small RNA and Degradome Sequencing to Reveal Drought Memory Response in Wheat (Triticum aestivum L.)

Drought has gradually become one of the most severe abiotic stresses on plants. Plants that experience stress training can exhibit enhanced stress tolerance. According to MicroRNA (miRNA) sequencing data, this study identified 195 candidate drought memory-related miRNAs in wheat, and targets of 64 (32.8%) candidate miRNAs were validated by degradome sequencing. Several drought memory-related miRNAs such as tae-miR9676-5p, tae-MIR9676-p3_1ss21GA, tae-miR171a, tae-miR531_L-2, tae-miR408_L-1, PC-3p-5049_3565, tae-miR396c-5p, tae-miR9778, tae-miR164a-5p, and tae-miR9662a-3p were validated as having a strong response to drought memory by regulating the expression of their target genes. In addition, overexpression of drought memory-related miRNA, tae-miR531_L-2, can remarkably improve the drought tolerance of transgenic Arabidopsisthaliana. Drought memory can regulate plant cellular signal transduction, plant biosynthetic processes, and other biological processes to cope with drought via transcriptional memory. In addition, drought memory-related miRNAs can promote starch and sucrose catabolism and soluble sugar accumulation and regulate proline homeostasis to improve plant drought resistance. Our results could contribute to an understanding of drought memory in wheat seedlings and may provide a new strategy for drought-resistant breeding.

Abscisic acid alleviates harmful effect of saline-alkaline stress on tomato seedlings

Saline-alkaline stress inhibits plant growth and reduces yield. Abscisic acid (ABA) is an important plant hormone in response to plant stress. However, the role of ABA under saline-alkaline stress is poorly understood. Therefore, the mechanisms of ABA accumulation and resistance improvement in tomato seedlings were studied under saline-alkaline stress. We investigated whether ABA accumulation improved the saline-alkaline stress resistance ability of tomato. Here, wild-type (Solanum lycopersicum cv. Ailsa Craig) and ABA-deficient mutant (notabilis) seedlings were used to determine the membrane lipid peroxidation, osmotic substance and chlorophyll contents. ABA synthesis and signal transduction changes and ABA roles regulating the antioxidation in tomato seedlings subject to saline-alkaline stress were further explored. Results showed that ABA synthesis and signal transduction were induced by saline-alkaline stress. Under saline-alkaline stress, tomato seedlings had decreased relative water content, increased relative electrical conductivity and malondialdehyde content, and these changes were alleviated by exogenous ABA treatment. Exogenous ABA alleviated the degradation of chlorophyll in the leaves of tomato seedlings caused by saline-alkaline stress, further promoted the accumulation of proline and soluble sugar, reduced the content of ROS and improved the ability of the antioxidant enzyme system. Moreover, notabilis appeared to be sensitive to saline-alkaline stress. Overall, ABA is involved in the resistance of tomato seedlings to saline-alkaline stress, and exogenous ABA improves the saline-alkaline tolerance of tomato seedlings.

Cytidine Triphosphate Synthase Four From Arabidopsis thaliana Attenuates Drought Stress Effects

Cytidine triphosphate synthase (CTPS) catalyzes the final step in pyrimidine de novo synthesis. In Arabidopsis, this protein family consists of five members (CTPS1-5), and all of them localize to the cytosol. Specifically, CTPS4 showed a massive upregulation of transcript levels during abiotic stress, in line with increased staining of CTPS4 promoter:GUS lines in hypocotyl, root and to lesser extend leaf tissues. In a setup to study progressive drought stress, CTPS4 knockout mutants accumulated less fresh and dry weight at days 5-7 and showed impaired ability to recover from this stress after 3 days of rewatering. Surprisingly, a thorough physiological characterization of corresponding plants only revealed alterations in assimilation and accumulation of soluble sugars including those related to drought stress in the mutant. Bimolecular fluorescence complementation (BiFC) studies indicated the interaction of CTPS4 with other isoforms, possibly affecting cytoophidia (filaments formed by CTPS formation. Although the function of these structures has not been thoroughly investigated in plants, altered enzyme activity and effects on cell structure are reported in other organisms. CTPS activity is required for cell cycle progression and growth. Furthermore, drought can lead to the accumulation of reactive oxygen species (ROS) and by this, to DNA damage. We hypothesize that effects on the cell cycle or DNA repair might be relevant for the observed impaired reduced drought stress tolerance of CTPS4 mutants.

An alternative, zeaxanthin epoxidase-independent abscisic acid biosynthetic pathway in plants

Abscisic acid (ABA) is an important carotenoid-derived phytohormone that plays essential roles in plant response to biotic and abiotic stresses as well as in various physiological and developmental processes. In Arabidopsis, ABA biosynthesis starts with the epoxidation of zeaxanthin by the ABA DEFICIENT 1 (ABA1) enzyme, leading to epoxycarotenoids; e.g., violaxanthin. The oxidative cleavage of 9-cis-epoxycarotenoids, a key regulatory step catalyzed by 9-CIS-EPOXYCAROTENOID DIOXYGENASE, forms xanthoxin, which is converted in further reactions mediated by ABA DEFICIENT 2 (ABA2), ABA DEFICIENT 3 (ABA3), and ABSCISIC ALDEHYDE OXIDASE 3 (AAO3) into ABA. By combining genetic and biochemical approaches, we unravel here an ABA1-independent ABA biosynthetic pathway starting upstream of zeaxanthin. We identified the carotenoid cleavage products (i.e., apocarotenoids, β-apo-11-carotenal, 9-cis-β-apo-11-carotenal, 3-OH-β-apo-11-carotenal, and 9-cis-3-OH-β-apo-11-carotenal) as intermediates of this ABA1-independent ABA biosynthetic pathway. Using labeled compounds, we showed that β-apo-11-carotenal, 9-cis-β-apo-11-carotenal, and 3-OH-β-apo-11-carotenal are successively converted into 9-cis-3-OH-β-apo-11-carotenal, xanthoxin, and finally into ABA in both Arabidopsis and rice. When applied to Arabidopsis, these β-apo-11-carotenoids exert ABA biological functions, such as maintaining seed dormancy and inducing the expression of ABA-responsive genes. Moreover, the transcriptomic analysis revealed a high overlap of differentially expressed genes regulated by β-apo-11-carotenoids and ABA, suggesting that β-apo-11-carotenoids exert ABA-independent regulatory activities. Taken together, our study identifies a biological function for the common plant metabolites, β-apo-11-carotenoids, extends our knowledge about ABA biosynthesis, and provides new insights into plant apocarotenoid metabolic networks.

A type-2C protein phosphatase (GhDRP1) participates in cotton (Gossypium hirsutum) response to drought stress

GhDRP1 acts as a negatively regulator to participate in response to drought stress possibly by modulating ABA signaling pathway and flavonoid biosynthesis pathway which affects stomata movement and thus water loss, ROS scavenging enzymes, and proline accumulation in cotton. Type-2C protein phosphatases (PP2C) may play important roles in plant stress signal transduction. Here, we show the evidence that a cotton PP2C protein GhDRP1 participates in plant response to drought stress. GhDRP1 gene encodes an active type-2C protein phosphatase (PP2C) and its expression is significantly induced in cotton by drought stress. Compared with wild type, the GhDRP1 overexpression (OE) transgenic cotton and Arabidopsis displayed reduced drought tolerance, whereas GhDRP1-silenced (RNAi) cotton showed enhanced drought tolerance. Under drought stress, malondialdehyde content was lower, whereas superoxide dismutase and peroxidase activities, proline content, stomata closure and relative water content were higher in GhDRP1 RNAi plants compared with those in wild type. In contrast, GhDRP1 OE plants showed the opposite phenotype under the same conditions. Expression levels of some stress-related and flavonoid biosynthesis-related genes were altered in GhDRP1 transgenic plants under drought stress. Additionally, GhDRP1 protein could interact with other proteins such as PYLs, SNF1-related protein kinase and GLK1-like protein. Collectively, these data suggest that GhDRP1 participates in plant response to drought stress possibly by modulating ABA signaling pathway and flavonoid biosynthesis pathway which affects stomata movement and thus water loss, ROS scavenging enzymes, and proline accumulation in cotton.

Differential Response of Two Tomato Genotypes, Wild Type cv. Ailsa Craig and Its ABA-Deficient Mutant flacca to Short-Termed Drought Cycles

Two tomato genotypes with constitutively different ABA level, flacca mutant and wild type of Ailsa Craig cv. (WT), were subjected to three repeated drought cycles, with the aim to reveal the role of the abscisic acid (ABA) threshold in developing drought tolerance. Differential responses to drought of two genotypes were obtained: more pronounced stomatal closure, ABA biosynthesis and proline accumulation in WT compared to the mutant were compensated by dry weight accumulation accompanied by transient redox disbalance in flacca. Fourier-transform infrared (FTIR) spectra analysis of isolated cell wall material and morphological parameter measurements on tomato leaves indicated changes in dry weight accumulation and carbon re-allocation to cell wall constituents in flacca, but not in WT. A higher proportion of cellulose, pectin and lignin in isolated cell walls from flacca leaves further increased with repeated drought cycles. Different ABA-dependent stomatal closure between drought cycles implies that acquisition of stomatal sensitivity may be a part of stress memory mechanism developed under given conditions. The regulatory role of ABA in the cell wall restructuring and growth regulation under low leaf potential was discussed with emphasis on the beneficial effects of drought priming in developing differential defense strategies against drought.

Cold Stress in Citrus: A Molecular, Physiological and Biochemical Perspective

Due to climate change, we are forced to face new abiotic stress challenges like cold and heat waves that currently result from global warming. Losses due to frost and low temperatures force us to better understand the physiological, hormonal, and molecular mechanisms of response to such stress to face losses, especially in tropical and subtropical crops like citrus fruit, which are well adapted to certain weather conditions. Many of the responses to cold stress that are found are also conserved in citrus. Hence, this review also intends to show the latest work on citrus. In addition to basic research, there is a great need to employ and cultivate new citrus rootstocks to better adapt to environmental conditions.

Abscisic acid signaling activates distinct VND transcription factors to promote xylem differentiation in Arabidopsis

Plants display remarkable abilities to adjust growth and development to environmental conditions, such as the amount of available water. This developmental plasticity is apparent not only in root and shoot growth rates, but also in tissue patterning and cell morphology.1,2 We have previously shown that in response to limited water availability, Arabidopsis thaliana root displays changes in xylem morphology, mediated by the non-cell-autonomous action of abscisic acid, ABA.2 Here, we show, through analyses of ABA response reporters and tissue-specific suppression of ABA signaling, that xylem cells themselves act as primary signaling centers governing both xylem cell fate and xylem differentiation rate, revealing the cell-autonomous control of multiple aspects of xylem development by ABA. ABA rapidly activates the expression of genes encoding VASCULAR-RELATED NAC DOMAIN (VND) transcription factors. Molecular and genetic analyses revealed that the two ABA-mediated xylem developmental changes are regulated by distinct members of this transcription factor family, with VND2 and VND3 promoting differentiation rate of metaxylem cells, while VND7 promotes the conversion of metaxylem toward protoxylem morphology. This phenomenon shows how different aspects of developmental plasticity can be interlinked, yet genetically separable. Moreover, similarities in phenotypic and molecular responses to ABA in diverse species indicate evolutionary conservation of the ABA-xylem development regulatory network among eudicots. Hence, this study gives molecular insights into how environmental stress modifies plant vascular anatomy and has potential relevance for water use optimization and adaptation to drought conditions.

Loss of Arabidopsis matrix metalloproteinase-5 affects root development and root bacterial communities during drought stress

Matrix metalloproteinases (MMPs) are zinc-dependent endo-peptidases that in mammals are known to be involved in remodeling the extracellular matrix (ECM) in developmental and pathological processes. In this study, we report At5-MMP of Arabidopsis thaliana to be important for root development and root bacterial communities. At5-MMP is mainly localized in the root vasculature and lateral root, an At5-MMP T-DNA insertion mutant (mmp5 KO) showed reduced root growth and a lower number of root apexes, causing reduced water uptake from the soil. Subsequently, mmp5 KO is sensitive to drought stress. Inhibited auxin transport was accompanied with resistance to indole-3-acetic acid (IAA), 2, 4-dichlorophenoxyacetic acid (2, 4-D), and 1-naphthaleneacetic acid (NAA). The content of endogenous abscisic acid (ABA) was lower in roots of mmp5 KO than in wild type. Genes responsive to ABA as well as genes encoding enzymes of the proline biosynthesis were expressed to a lower extent in mmp5 KO than in wild type. Moreover, drought stress modulated root-associated bacterial communities of mmp5 KO: the number of Actinobacteria increased. Therefore, At5-MMP modulates auxin/ABA signaling rendering the plant sensitive to drought stress and recruiting differential root bacterial communities.

Abscisic acid-responsive element binding transcription factors contribute to proline synthesis and stress adaptation in Arabidopsis

Proline accumulation is one of the most common adaptive responses of higher plants against abiotic stresses like drought. It plays multiple roles in osmotic adjustment, cell homeostasis and stress recovery. Genetic regulation of proline accumulation under drought is complex, and transcriptional cascades modulating proline is poorly understood. Here, we employed quadruple mutant (abf1 abf2 abf3 abf4) to dissect the role of ABA-responsive elements (ABREs) binding transcription factors (ABFs) in modulating proline accumulation across varying stress scenarios. ABREs are present across the promoter of the P5CS1 gene, whose upregulation is considered a hallmark for drought inducible proline accumulation. Upon ABA treatment, P5CS1 mRNA expression and proline content in the shoot were significantly higher in Col-0 compared to the quadruple mutant. Similar results were found at 2 h and 3 h after acute dehydration. We quantified proline at different time points after drought stress treatment. The proline content was higher in wild type (Col-0) than the quadruple mutant at the early stage of drought. Notably, the proline accumulation in wild type increased at a slower rate than the quadruple mutant 7 d after drought stress. Besides, the quadruple mutant displayed significant oxidative damage, low tissue turgidity and higher membrane damage under terminal drought stress. Both terminal drought stress and long-term constant water stress revealed substantial differences in growth rate between wild type and quadruple mutant. The study provides evidence that ABFs are involved in drought stress response, such as proline biosynthesis in Arabidopsis.

Proline Concentration and Its Metabolism Are Regulated in a Leaf Age Dependent Manner But Not by Abscisic Acid in Pea Plants Exposed to Cadmium Stress

The accumulation of proline is one of the defense mechanisms of plants against the harmful effects of adverse environmental conditions; however, when pea plants were treated for 12 h with CdCl2, the proline concentration decreased in the youngest A (not expanded) and B1 (expanded) leaves, and did not change significantly in the B2 (mature, expanded) or C (the oldest) leaves. After 24 h of cadmium (Cd) stress, the proline concentration remained low in A and B1 leaves, while in B2 and C leaves, it increased, and after 48 h, an increase in the proline concentration in the leaves at each stage of development was observed. The role of proline in the different phases of plant response to the Cd treatment is discussed. Changes in proline accumulation corresponded closely with changes in the transcript levels of PsP5CS2, a gene encoding D1-pyrroline-5-carboxylate synthetase involved in proline synthesis, and PsPDH1, a gene encoding proline dehydrogenase engaged in proline degradation. CdCl2 application induced the expression of PsProT1 and PsProT2, genes encoding proline transporters, especially during the first 12 h of treatment in A and B1 leaves. When the time courses of abscisic acid (ABA) and proline accumulation were compared, it was concluded that an increase in the proline concentration in the leaves of Cd-treated pea plants was more related to a decrease in chlorophyll concentration (leaves B2 and C) and an increase in the malondialdehyde level (A and B1 leaves) than with an increase in ABA concentration alone. Exogenous application of ABA (0.5, 5, 50 µM) significantly increased the proline concentration in the A leaves of pea plants only, and was accompanied by an elevated and repressed expression of PsP5CS2 and PsPDH1 in these leaves, respectively. The presented results suggest that under Cd stress, the accumulation of proline in leaves of pea plants may take place independently of the ABA signaling.

Effects of low molecular weight polysaccharides from Ulva prolifera on the tolerance of Triticum aestivum to osmotic stress

Polysaccharides derived from seaweeds can be used as biostimulants to enhance plant resistance to different stressors. In this study, we investigated the effects of applying low molecular weight polysaccharides (LPU) derived from Ulva prolifera with 14.2 kDa on the responses of wheat (Triticum aestivum) to osmotic stress. The results showed that osmotic stress simulated using polyethylene glycol inhibited seedling growth, whereas we observed increases in the fresh weights and shoot lengths of seedlings treated with polysaccharide for 120 h. Furthermore, we observed enhanced activities of antioxidant enzymes, and significant reductions in malondialdehyde content of 23.13%, 19.82%, and 20.04% in response treatment for 120 h with 0.01%, 0.03%, and 0.05% LPU, respectively, relative to those in the group treated with polyethylene glycol alone. In all treatments, expression of the P5CS gene was upregulated to promote proline accumulation. Moreover, after 120 h, exogenously applied LPU induced the expression of stress-related genes, including SnRK2, Wabi5, Wrab18, and Wdhn13. Collectively, these findings indicate that LPU might have the effect of regulating the abscisic acid-dependent pathway in wheat, thereby increasing seedling antioxidant capacity and growth. Application of LPU may accordingly represent an effective approach for enhancing the resistance to osmotic stress in wheat.

Involvement of ethylene receptors in the salt tolerance response of Cucurbita pepo

Abiotic stresses have a negative effect on crop production, affecting both vegetative and reproductive development. Ethylene plays a relevant role in plant response to environmental stresses, but the specific contribution of ethylene biosynthesis and signalling components in the salt stress response differs between Arabidopsis and rice, the two most studied model plants. In this paper, we study the effect of three gain-of-function mutations affecting the ethylene receptors CpETR1B, CpETR1A, and CpETR2B of Cucurbita pepo on salt stress response during germination, seedling establishment, and subsequent vegetative growth of plants. The mutations all reduced ethylene sensitivity, but enhanced salt tolerance, during both germination and vegetative growth, demonstrating that the three ethylene receptors play a positive role in salt tolerance. Under salt stress, etr1b, etr1a, and etr2b germinate earlier than WT, and the root and shoot growth rates of both seedlings and plants were less affected in mutant than in WT. The enhanced salt tolerance response of the etr2b plants was associated with a reduced accumulation of Na⁺ in shoots and leaves, as well as with a higher accumulation of compatible solutes, including proline and total carbohydrates, and antioxidant compounds, such as anthocyanin. Many membrane monovalent cation transporters, including Na⁺/H⁺ and K⁺/H⁺ exchangers (NHXs), K⁺ efflux antiporters (KEAs), high-affinity K⁺ transporters (HKTs), and K⁺ uptake transporters (KUPs) were also highly upregulated by salt in etr2b in comparison with WT. In aggregate, these data indicate that the enhanced salt tolerance of the mutant is led by the induction of genes that exclude Na⁺ in photosynthetic organs, while maintaining K⁺/Na⁺ homoeostasis and osmotic adjustment. If the salt response of etr mutants occurs via the ethylene signalling pathway, our data show that ethylene is a negative regulator of salt tolerance during germination and vegetative growth. Nevertheless, the higher upregulation of genes involved in Ca²⁺ signalling (CpCRCK2A and CpCRCK2B) and ABA biosynthesis (CpNCED3A and CpNCED3B) in etr2b leaves under salt stress likely indicates that the function of ethylene receptors in salt stress response in C. pepo can be mediated by Ca²⁺ and ABA signalling pathways.

Short-term drought triggers defence mechanisms faster than ABA accumulation in the epiphytic bromeliad Acanthostachys strobilacea

Epiphytic bromeliads might experience drought after a few hours without water, which is especially critical during early life stages. Consequently, juvenile epiphytic bromeliads probably rely on short-term activation of drought tolerance strategies, although the biochemical processes involved are still poorly understood. In this study, we aimed to evaluate the short-term drought response of juvenile plants of the epiphytic bromeliad Acanthostachys strobilacea (Schult. & Schult. f.) Klotzsch. We hypothesized that short-term drought would induce the accumulation of abscisic acid (ABA) and secondary messengers such as reactive oxygen and nitrogen species (ROS and RNS, respectively) before the activation of defence mechanisms. Three-month-old plants were transferred from well-watered to dry substrates and stress markers were assessed at 0, 2, 5, 10, 24, 48, and 72 h. Drought caused a 27.3% decrease in relative water content compared to the well-watered control at 72 h. A nearly 5-fold increment in the ABA content occurred at 72 h of stress, which was about two days after the first detection of peaks in RNS levels and defence mechanisms activation. Indeed, ascorbate peroxidase (EC 1.11.1.11) activities and proline content increased after 10 h, whereas after 24 h a higher catalase (EC 1.11.1.6) activity and osmotic adjustment occurred. Oxidative stress markers and photochemical efficiency of photosystem II indicated no significant damage induced by drought. We concluded that defence mechanisms activation during early drought in juvenile A. strobilacea might be regulated initially by ABA-independent pathways and RNS, while ABA-induced responses are triggered at subsequent stages of stress.

ADP-ribosylation factors improve biomass yield and salinity tolerance in transgenic switchgrass (Panicum virgatum L.)

PvArf regulate proline biosynthesis by physically interacting with PvP5CS1 to improve salt tolerance in switchgrass. The genetic factors that contribute to stress resiliency are yet to be determined. Here, we identified three ADP-ribosylation factors, PvArf1, PvArf-B1C, and PvArf-related, which contribute to salinity tolerance in transgenic switchgrass (Panicum virgatum L.). Switchgrass overexpressing each of these genes produced approximately twofold more biomass than wild type (WT) under normal growth conditions. Transgenic plants accumulated modestly higher levels of proline under normal conditions, but this level was significantly increased under salt stress providing better protection to transgenic plants compared to WT. We found that PvArf genes induce proline biosynthesis genes under salt stress to positively regulate proline accumulation, and further demonstrated that PvArf physically interact with PvP5CS1. Moreover, the transcript levels of two key ROS-scavenging enzyme genes were significantly increased in the transgenic plants compared to WT, leading to reduced H₂O₂ accumulation under salt stress conditions. PvArf genes also protect cells against stress-induced changes in Na⁺ and K⁺ ion concentrations. Our findings uncover that ADP-ribosylation factors are key determinants of biomass yield in switchgrass, and play pivotal roles in salinity tolerance by regulating genes involved in proline biosynthesis.

Exogenous proline enhances antioxidant enzyme activities but does not mitigate growth inhibition by selenate stress in tobacco BY-2 cells

Selenium (Se) causes oxidative damage to plants. Proline is accumulated as a compatible solute in plants under stress conditions and mitigates stresses. Selenate at 250 µM increased cell death and inhibited the growth of tobacco BY-2 cells while exogenous proline at 10 mM did not mitigate the inhibition by selenate. Selenate increased accumulation of Se and ROS and activities of antioxidant enzymes but not lipid peroxidation in the BY-2 cells. Proline increased Se accumulation and antioxidant enzyme activities but not either ROS accumulation or lipid peroxidation in the selenate-stressed cells. Glutathione (GSH) rather than ascorbic acid (AsA) mitigated the growth inhibition although both reduced the accumulation of ROS induced by selenate. These results indicate that proline increases both antioxidant enzyme activities and Se accumulation, which overall fails to ameliorate the growth inhibition by selenate and that the growth inhibition is not accounted for only by ROS accumulation. Abbreviations: APX: ascorbate peroxidase; AsA: ascorbic acid; BY-2: Bright Yellow-2; CAT: catalase; DAI: days after inoculation; DW: dry weight; FW: fresh weight; GSH: glutathione; ROS: reactive oxygen species.

Silicon confers cucumber resistance to salinity stress through regulation of proline and cytokinins

Salt stress is a continuous threat to global crop production. Here, we studied the alleviation role of exogenous silicon (Si) in NaCl-stressed cucumber, with special emphasis on plant growth, proline (Pro) and hormone metabolisms. The results showed that Si supplementation ameliorated the adverse effects of NaCl on plants growth, biomass, and oxidative stress. Salt stress greatly increased the content of Pro throughout the experiment, while Si regulated Pro content in two distinct ways. Si promoted the salt-induced Pro levels after 3 and 6 days of treatment, but decreased it after 9 and 12 days of treatment. Moreover, P5CS and ProDH activities and P5CS gene play important roles in Si and salt-regulated Pro levels in different stress phase. Under stress condition, Si addition tend to revert the content of ABA, IAA, cytokinin and SA to the control levels in most cases. Further correlation analysis revealed a negative correlation between the root cytokinin and Pro content after 3 days of treatment, suggesting the interaction between cytokinin and Pro metabolism. Exogenous application of Pro and ProDH competitive inhibitor D-Lactate confirmed the possible interplay between Pro and cytokinin metabolism. Further study identified several CKX (Csa4G647490 and Csa1G589070) and IPT (Csa7G392940 and Csa3G150100) genes that may be responsible for the regulation of cytokinin accumulation by Si and/or Pro after short-term of treatment. The results suggested that Pro is a key factor in Si-induced salt tolerance, and Si-increased Pro content may participate in the regulation of cytokinin metabolism under short-term of salt stress.

Abscisic acid and hydrogen peroxide are involved in drought priming-induced drought tolerance in wheat (Triticum aestivum L.)

Drought is one of the major stress factors in wheat production on a global scale. Drought priming during the early growth stage can enhance drought tolerance in wheat (Triticum aestivum L.). Abscisic acid (ABA) and hydrogen peroxide (H₂ O₂ ) are important signal molecules in the adaptation of plants to drought stress. However, the roles of ABA and H₂ O₂ in drought priming-induced drought tolerance are not clear. In the present study, we evaluated the responses of wheat to an ABA inhibitor, H₂ O₂ scavenger and an inhibitor to investigate the (i) relationship between ABA and H₂ O₂ in osmotic adjustment after drought priming in the vegetative stage and (ii) responses to drought stress during grain filling. In the drought priming alone treatments, chemical application resulted in the scavenging of ABA and H₂ O₂ , weakening the alleviation effects of drought priming on drought stress, as demonstrated by the lower leaf water potential and grain yield. The ABA inhibitor completely inhibited accumulation of ABA and H₂ O₂ ; the ABA inhibitor inhibited respiratory burst oxidase homologue expression, whereas the H₂ O₂ inhibitor resulted in higher 9-cis-epoxycarotenoid dioxygenase expression and ABA concentration in primed plants, indicating that ABA scavenging inhibited H₂ O₂ biosynthesis while H₂ O₂ scavenging did not inhibit ABA biosynthesis. The results further demonstrated that NADPH oxidase-mediated H₂ O₂ production functions downstream of ABA, which induces osmolyte transcript expression and accumulation, and thus contributes to drought priming-induced stress tolerance. These results provide a theoretical basis for a better understanding of the mechanisms involved in drought priming-induced tolerance in wheat plants.

Abscisic Acid Biosynthesis and Signaling in Plants: Key Targets to Improve Water Use Efficiency and Drought Tolerance

The observation of a much-improved fitness of wild-type plants over abscisic acid (ABA)-deficient mutants during drought has led researchers from all over to world to perform experiments aiming at a better understanding of how this hormone modulates the physiology of plants under water-limited conditions. More recently, several promising approaches manipulating ABA biosynthesis and signaling have been explored to improve water use efficiency and confer drought tolerance to major crop species. Here, we review recent progress made in the last decade on (i) ABA biosynthesis, (ii) the roles of ABA on plant-water relations and on primary and secondary metabolisms during drought, and (iii) the regulation of ABA levels and perception to improve water use efficiency and drought tolerance in crop species.

A Chloroplast COR413 Protein From Physcomitrella patens Is Required for Growth Regulation Under High Light and ABA Responses

COR413 genes belong to a poorly characterized group of plant-specific cold-regulated genes initially identified as part of the transcriptional activation machinery of plants during cold acclimation. They encode multispanning transmembrane proteins predicted to target the plasma membrane or the chloroplast inner membrane. Despite being ubiquitous throughout the plant kingdom, little is known about their biological function. In this study, we used reverse genetics to investigate the relevance of a predicted chloroplast localized COR413 protein (PpCOR413im) from the moss Physcomitrella patens in developmental and abiotic stress responses. Expression of PpCOR413im was strongly induced by abscisic acid (ABA) and by various environmental stimuli, including low temperature, hyperosmosis, salinity and high light. In vivo subcellular localization of PpCOR413im-GFP fusion protein revealed that this protein is localized in chloroplasts, confirming the in silico predictions. Loss-of-function mutants of PpCOR413im exhibited growth and developmental alterations such as growth retardation, reduced caulonema formation and hypersensitivity to ABA. Mutants also displayed altered photochemistry under various abiotic stresses, including dehydration and low temperature, and exhibited a dramatic growth inhibition upon exposure to high light. Disruption of PpCOR413im also caused altered chloroplast ultrastructure, increased ROS accumulation, and enhanced starch and sucrose levels under high light or after ABA treatment. In addition, loss of PpCOR413im affected both nuclear and chloroplast gene expression in response to ABA and high light, suggesting a role for this gene downstream of ABA in the regulation of growth and environmental stress responses. Developmental alterations exhibited by PpCOR413im knockout mutants had remarkable similarities to those exhibited by hxk1, a mutant lacking a major chloroplastic hexokinase, an enzyme involved in energy homeostasis. Based on these findings, we propose that PpCOR413im is involved in coordinating energy metabolism with ABA-mediated growth and developmental responses.

Arabidopsis CK2 family gene CKB3 involved in abscisic acid signaling

CKB3 is a regulatory (beta) subunit of CK2. In this study Arabidopsis thaliana homozygous T-DNA mutant ckb3 was studied to understand the role of CKB3 in abscisic acid (ABA) signaling. The results shown: CKB3 was expressed in all organs and the highest expression in the seeds, followed by the root. During seed germination and root growth the ckb3 mutant showed reduced sensitivity to ABA. The ckb3 mutant had more stomatal opening and increased proline accumulation and leaf water loss. The expression levels of number of genes in the ABA regulatory network had changed. This study demonstrates that CKB3 is an ABA signaling-related gene and may play a positive role in ABA signaling.

The cloning and characterization of hypersensitive to salt stress mutant, affected in quinolinate synthase, highlights the involvement of NAD in stress-induced accumulation of ABA and proline

Nicotinamide adenine dinucleotide (NAD), a ubiquitous coenzyme, is required for many physiological reactions and processes. However, it remains largely unknown how NAD affects plant response to salt stress. We isolated a salt-sensitive mutant named hypersensitive to salt stress (hss) from an ethyl methanesulfonate-induced mutation population. A point mutation was identified by MutMap in the encoding region of Quinolinate Synthase (QS) gene required for the de novo synthesis of NAD. This point mutation caused a substitution of amino acid in the highly-conserved NadA domain of QS, resulting in an impairment of NAD biosynthesis in the mutant. Molecular and chemical complementation have restored the response of the hss mutant to salt stress, indicating that the decreased NAD contents in the mutant were responsible for its hypersensitivity to salt stress. Furthermore, the endogenous levels of abscisic acid (ABA) and proline were also reduced in stress-treated hss mutant. The application of ABA or proline could alleviate stress-induced oxidative damage of the mutant and partially rescue its hypersensitivity to salt stress, but not affect NAD concentration. Taken together, our results demonstrated that the NadA domain of QS is important for NAD biosynthesis, and NAD participates in plant response to salt stress by affecting stress-induced accumulation of ABA and proline.

Genotypic difference in secondary metabolism-related enzyme activities and their relative gene expression patterns, osmolyte and plant hormones in wheat

Salinity and drought are the two most important and frequently co-occurring abiotic factors. A greenhouse pot experiment was carried out on two contrasting wheat genotypes (Jimai22, salt tolerant; Yangmai20, salt sensitive) to analyze the effect of drought (4% soil moisture content, D) and salinity (100 mM NaCl, S) either individually or combined on secondary metabolism-related enzyme activities and osmolytes. Results showed that drought, salinity and their combination (D + S) caused increases in phenylalanine ammonialyase (PAL, EC 4.3.1.24) activities compared with controls with a greater enhancement in Jimai22 than Yangmai20. Polyphenol peroxidase (PPO, EC 1.14.18.1) and shikimate dehydrogenase (SKDH, EC 1.1.1.25) activities increased more in Jimai22 both under salinity alone and D + S stresses. The D + S combination increased cinnamyl alcohol dehydrogenase (CAD, EC 1.1.1.195) activity and glycine betaine (GB) under both 10 and 4% soil moisture contents (SMC), and elevated abscisic acid (ABA), indole-3-acetic acid (IAA) and flavonoid contents at 4% SMC in Jimai22, contents of the compounds remained unchanged in Yangmai20. The treatment with salinity alone at both SMCs significantly increased callose and flavonoid contents in Jimai22 more than in Yangmai20, as compared to controls. In addition, the total phenol content at 4% SMC increased in the salt-tolerant genotype more. Moreover, total tocopherol under salinity alone and D + S at 4% SMC and chitinase activity under salinity at both SMC remarkably increased in Jimai22 while non-significant change observed in Yangmai20. Also, the expression of genes related to secondary metabolism (PAL, PPO, CAD, SKDH, and GB) was more induced in Jimai22 than Yangmai20 under D + S, and lower accumulation of H₂ O₂ and O₂ ^- also occurred. Our findings suggest that high tolerance to D + S stress in Jimai22 was closely related to enhanced secondary metabolism-related enzyme activities and osmolytes such as PAL, CAD, PPO, SKDH, GB, total tocopherol, callose, plant hormones and their transcript level, which may beneficial to lower the reactive oxygen species (ROS) accumulation.

Promoted ABA Hydroxylation by Capsicum annuum CYP707As Overexpression Suppresses Pollen Maturation in Nicotiana tabacum

Abscisic acid (ABA) is a key signaling molecule that mediates plant response to stress. Increasing evidence indicates that ABA also regulates many aspects of plant development, such as seed germination, leaf development, and ripening. ABA metabolism, including ABA biosynthesis and degradation, is an essential aspect of ABA response in plants. In this study, we identified four cytochrome P450 genes (CaCYP707A1, 2, 3, and 4) that mediate ABA hydroxylation, which is required for ABA degradation in Capsicum annuum. We observed that CaCYP707A-mediated ABA hydroxylation promotes ABA degradation, leading to low levels of ABA and a dehydration phenotype in 35S:CaCYP707A plants. Importantly, seed formation was strongly inhibited in 35S:CaCYP707A plants, and a cross-pollination test suggested that the defect in seed formation is caused by improper pollen development. Phenotypic analysis showed that pollen maturation is suppressed in 35S:CaCYP707A1 plants. Consequently, most 35S:CaCYP707A1 pollen grains degenerated, unlike non-transgenic (NT) pollen, which developed into mature pollen grains. Together our results indicate that CaCYP707A mediates ABA hydroxylation and thereby influences pollen development, helping to elucidate the mechanism underlying ABA-regulated pollen development.

Regulation of L-proline biosynthesis, signal transduction, transport, accumulation and its vital role in plants during variable environmental conditions

Background

In response to various environmental stresses, many plant species synthesize L-proline in the cytosol and accumulates in the chloroplasts. L-Proline accumulation in plants is a well-recognized physiological reaction to osmotic stress prompted by salinity, drought and other abiotic stresses. L-Proline plays several protective functions such as osmoprotectant, stabilizing cellular structures, enzymes, and scavenging reactive oxygen species (ROS), and keeps up redox balance in adverse situations. In addition, ample-studied osmoprotective capacity, L-proline has been also ensnared in the regulation of plant improvement, including flowering, pollen, embryo, and leaf enlargement.

Scope and conclusions

Albeit, ample is now well-known about L-proline metabolism, but certain characteristics of its biological roles are still indistinct. In the present review, we discuss the L-proline accumulation, metabolism, signaling, transport and regulation in the plants. We also discuss the effects of exogenous L-proline during different environmental conditions. L-Proline biosynthesis and catabolism are controlled by several cellular mechanisms, of which we identify only very fewer mechanisms. So, in the future, there is a requirement to identify such types of cellular mechanisms.

Overexpression of ABA Receptor PYL10 Gene Confers Drought and Cold Tolerance to Indica Rice

Abscisic acid (ABA) plays versatile functions in regulating plant development and tolerance to various biotic and abiotic stresses. Towards elucidating the functions of one of the ABA receptors (ABARs) in rice, OsPYL10 was cloned from drought tolerant rice cv. Nagina 22 and was overexpressed under stress inducible RD29A promoter in a mega rice variety MTU1010 by using Agrobacterium mediated genetic transformation. Four single copy transgenic lines selected based on Southern blot analysis were used for physiological and molecular analysis. PYL10 receptor appears to regulate its ligand ABA accumulation as PYL10 overexpressing transgenics accumulated 2-3.3-fold higher levels of ABA than that of WT in flag leaf at anthesis under non-stress conditions. The enhanced accumulation of ABA was associated with enhanced expression of genes for ABA biosynthesis viz., ZEP1, NCED1, NCED2, NCED3, and NCED4 in transgenics than in WT plants. At seedling stage, PYL10 transgenics showed significantly higher survival rate under cold stress as compared with WT plants. qRT-PCR analysis showed that expression levels of cold responsive genes viz., DREB1F, MYB3R2, TPP1, COR410, DEHYDRIN, and LEA3 were significantly higher in PYL10 overexpressing transgenic lines as compared to WT plants under cold stress. PYL10 transgenic and WT plants grown in the same pot were subjected to -80 kPa drought stress and recovery treatments at vegetative and reproductive stages. At vegetative stage drought stress, three overexpressing lines showed significantly higher grain yield (40-58%) and at reproductive stage drought stress one of these overexpression lines showed two-fold higher grain yield than that of WT plants. Excised leaf water loss analysis showed that PYL10 transgenic lost about 20% less water than WT plants. At reproductive stage, OsPYL10 transgenic maintained higher RWC, membrane stability index, chlorophyll content, and accumulated lower amount of MDA and H2O2 as compared with WT plants. qRT-PCR analysis showed that expression levels of RAB16, Dehydrin, LEA3, and ABA45 were higher in PYL10 transgenics as compared with WT plants under drought stress. Thus, overall results showed that OsPYL10 overexpression has potential to improve both drought and cold stress tolerance of indica rice.

Elevated carbon dioxide decreases the adverse effects of higher temperature and drought stress by mitigating oxidative stress and improving water status in Arabidopsis thaliana

This study revealed that elevated carbon dioxide increases Arabidopsis tolerance to higher temperature and drought stress by mitigating oxidative stress and improving water status of plants. Few studies have considered multiple aspects of plant responses to key components of global climate change, including higher temperature, elevated carbon dioxide (ECO₂), and drought. Hence, their individual and combinatorial effects on plants need to be investigated in the context of understanding climate change impact on plant growth and development. We investigated the interactive effects of temperature, CO₂, watering regime, and genotype on Arabidopsis thaliana (WT and ABA-insensitive mutant, abi1-1). Plants were grown in controlled-environment growth chambers under two temperature regimes (22/18 °C and 28/24 °C, 16 h light/8 h dark), two CO₂ concentrations (400 and 700 μmol mol^-1), and two watering regimes (well-watered and water-stressed) for 18 days. Plant growth, anatomical, physiological, molecular, and hormonal responses were determined. Our study provided valuable information about plant responses to the interactive effects of multiple environmental factors. We showed that drought and ECO₂ had larger effects on plants than higher temperatures. ECO₂ alleviated the detrimental effects of temperature and drought by mitigating oxidative stress and plant water status, and this positive effect was consistent across multiple response levels. The WT plants performed better than the abi1-1 plants; the former had higher rosette diameter, total dry mass, leaf and soil water potential, leaf moisture, proline, ethylene, trans-zeatin, isopentyladenine, and cis-zeatin riboside than the latter. The water-stressed plants of both genotypes accumulated more abscisic acid (ABA) than the well-watered plants; however, higher temperatures decreased the ability of WT plants to produce ABA in response to drought. We conclude that drought strongly, while higher temperature to a lesser extent, affects Arabidopsis seedlings, and ECO₂ reduces the adverse effects of these stressors more efficiently in the WT plants than in the abi1-1 plants. Findings from this study can be extrapolated to other plant species that share similar characteristics and/or family with Arabidopsis.

Nitric oxide molecular targets: reprogramming plant development upon stress

Plants are sessile organisms that need to complete their life cycle by the integration of different abiotic and biotic environmental signals, tailoring developmental cues and defense concomitantly. Commonly, stress responses are detrimental to plant growth and, despite the fact that intensive efforts have been made to understand both plant development and defense separately, most of the molecular basis of this trade-off remains elusive. To cope with such a diverse range of processes, plants have developed several strategies including the precise balance of key plant growth and stress regulators [i.e. phytohormones, reactive nitrogen species (RNS), and reactive oxygen species (ROS)]. Among RNS, nitric oxide (NO) is a ubiquitous gasotransmitter involved in redox homeostasis that regulates specific checkpoints to control the switch between development and stress, mainly by post-translational protein modifications comprising S-nitrosation of cysteine residues and metals, and nitration of tyrosine residues. In this review, we have sought to compile those known NO molecular targets able to balance the crossroads between plant development and stress, with special emphasis on the metabolism, perception, and signaling of the phytohormones abscisic acid and salicylic acid during abiotic and biotic stress responses.

Drought-induced mortality in Scots pine: opening the metabolic black box

Forests are sensitive to droughts, which increase the mortality rate of tree species. Various processes have been proposed to underlie drought-induced tree mortality, including hydraulic failure, carbon starvation and increased susceptibility to natural enemies. To give insights into these processes, we assessed the metabolic effects of a mortality-inducing drought on seedlings of Pinus sylvestris L. (Scots Pine), a widespread and important Eurasian species. We found divergence over time in the foliar metabolic composition of droughted vs well-watered seedlings, with the former showing increased abundance of aromatic amino acids and decreases in secondary metabolism associated with defence. We observed no significant differences amongst provenances in these effects: seedlings from drought-prone areas showed the same foliar metabolic changes under drought as seedlings from moist environments, although morphological effects of drought varied by provenance. Overall, our results demonstrate how severe drought prior to death may target particular primary and secondary metabolic pathways, weakening defences against natural enemies and contributing to the risk of drought-induced mortality in P. sylvestris.

Norway Spruce (Picea abies L.) Provenances Use Different Physiological Strategies to Cope with Water Deficit

Research Highlights: In this study, we found different regulatory mechanisms in two contrasting provenances of Norway spruce responding to progressive drought stress. Background and Objectives: In the context of climate and environmental changes, the following question of high importance in scientific studies is: How will Norway spruce, which forms a dominant component in European mountain and boreal forests, be able to cope with the increasing frequencies and intensities of drought periods? The aim of the study was to investigate the physiological responses of eight-month-old seedlings, as a most vulnerable phase of forest tree life, for different spruce provenances, to find out variable strategies in relation to controlled drought stress. Materials and Methods: We performed an experiment under controlled conditions with spruce seedlings from a climatologically warmer stand of 410 m above sea level (a.s.l.), and a moderately cool stand of 931 m a.s.l. The soil water content, leaf predawn water potential, the osmotic potential of needles, the relative water content of needles, and the photosynthetic performance and the contents of primary metabolites (proline and abscisic acid) were investigated as indicators of the spruce seedlings’ responses to water stress. The status of the indicators was analyzed at different temporal intervals, using repeated measures analysis of variance (ANOVA), reflecting the seedlings´ water conditions (early drought, drought, or recovery). On a daily basis, the differences among the indicators were tested with one-way ANOVA. A principal component analysis (PCA) was used to identify the provenance-specific responses of tree drought indicators in a multi-dimensional data set. Results: The responses to drought stress differed between the provenances. Whereas seedlings of ´warm´ provenance from a low altitude performed a conservation strategy, with high amounts of accumulated abscisic acid and closing the stomata faster, the reactions of ´cool´ provenance seedlings from a higher altitude were not so sensitive and the plants’ water supply and photosynthetic performance remained significantly higher. These findings indicate that a higher drought resistance in ´cool´ provenance could be related to greater amounts of proline amino-acids, which are accumulated from the beginning of the drought simulation. Furthermore, proline accumulation resulting in increased stress tolerance is controlled through another mechanism than osmotic adjustment. Conclusions: The observed variations in the regulatory mechanisms used to develop adaptive strategies in different provenances are an important factor for seedling survival under a changing climate.

Phytohormones Regulate Accumulation of Osmolytes Under Abiotic Stress

Plants face a variety of abiotic stresses, which generate reactive oxygen species (ROS), and ultimately obstruct normal growth and development of plants. To prevent cellular damage caused by oxidative stress, plants accumulate certain compatible solutes known as osmolytes to safeguard the cellular machinery. The most common osmolytes that play crucial role in osmoregulation are proline, glycine-betaine, polyamines, and sugars. These compounds stabilize the osmotic differences between surroundings of cell and the cytosol. Besides, they also protect the plant cells from oxidative stress by inhibiting the production of harmful ROS like hydroxyl ions, superoxide ions, hydrogen peroxide, and other free radicals. The accumulation of osmolytes is further modulated by phytohormones like abscisic acid, brassinosteroids, cytokinins, ethylene, jasmonates, and salicylic acid. It is thus important to understand the mechanisms regulating the phytohormone-mediated accumulation of osmolytes in plants during abiotic stresses. In this review, we have discussed the underlying mechanisms of phytohormone-regulated osmolyte accumulation along with their various functions in plants under stress conditions.