Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Can we improve the chilling tolerance of maize photosynthesis through breeding?

Chilling tolerance is necessary for crops to thrive in temperate regions where cold snaps and lower baseline temperatures place limits on life processes; this is particularly true for crops of tropical origin such as maize. Photosynthesis is often adversely affected by chilling stress, yet the maintenance of photosynthesis is essential for healthy growth and development, and most crucially for yield. In this review, we describe the physiological basis for enhancing chilling tolerance of photosynthesis in maize by examining nine key responses to chilling stress. We synthesize current knowledge of genetic variation for photosynthetic chilling tolerance in maize with respect to each of these traits and summarize the extent to which genetic mapping and candidate genes have been used to understand the genomic regions underpinning chilling tolerance. Finally, we provide perspectives on the future of breeding for photosynthetic chilling tolerance in maize. We advocate for holistic and high-throughput approaches to screen for chilling tolerance of photosynthesis in research and breeding programmes in order to develop resilient crops for the future.

Physiological mechanisms of ABA-induced salinity tolerance in leaves and roots of rice

Abscisic acid (ABA) plays a crucial role in response to abiotic stress as important small molecules in regulating metabolism. This study aimed to evaluate the ability of foliar spraying ABA to regulate growth quality at rice seedling stage under salt stress. Results demonstrated that salt stress strongly reduced all the growth parameters of two rice seedlings ('Chaoyouqianhao' and 'Huanghuazhan'), caused prominent decrease in the levels of photosynthetic pigments (mainly in Huanghuazhan), photosynthesis and fluorescence parameters. Salinity treatment increased the concentration of malondialdehyde (MDA) and hydrogen peroxide (H2O2) in roots, whereas significant decreased H2O2 was found in leaves of Huanghuazhan. Additionally, salinity triggered high Na+ content particularly in leaves and enhanced catalase (CAT) activities, ascorbate peroxidase (APX) and peroxidase (POD) activities of the two rice seedlings. Nevertheless, salinity-induced increased root ascorbic acid (AsA) and glutathione (GSH) levels while decreased in leaves, which depended on treatment time. Conversely, ABA application partially or completely mitigated salinity toxicity on the seedlings. ABA could reverse most of the changed physiological parameters triggered by salt stress. Specially, ABA treatment improved antioxidant enzyme levels and significantly reduced the Na+ content of two varieties as well as increased the K+, Mg2+ and Ca2+ content in leaves and roots. ABA treatment increased the hormone contents of 1-aminocclopropane carboxylic acid (ACC), trans-zeatin (TZ), N6-isopentyladenosine (IPA), Indole-3-acetic acid (IAA), and ABA in leaves of two rice varieties under salt stress. It is suggested that ABA was beneficial to protect membrane lipid peroxidation, the modulation of antioxidant defense systems and endogenous hormonal balance with imposition to salt stress.

Thermo-Priming Mediated Cellular Networks for Abiotic Stress Management in Plants

Plants can adapt to different environmental conditions and can survive even under very harsh conditions. They have developed elaborate networks of receptors and signaling components, which modulate their biochemistry and physiology by regulating the genetic information. Plants also have the abilities to transmit information between their different parts to ensure a holistic response to any adverse environmental challenge. One such phenomenon that has received greater attention in recent years is called stress priming. Any milder exposure to stress is used by plants to prime themselves by modifying various cellular and molecular parameters. These changes seem to stay as memory and prepare the plants to better tolerate subsequent exposure to severe stress. In this review, we have discussed the various ways in which plants can be primed and illustrate the biochemical and molecular changes, including chromatin modification leading to stress memory, with major focus on thermo-priming. Alteration in various hormones and their subsequent role during and after priming under various stress conditions imposed by changing climate conditions are also discussed.

Heavy metal induced regulation of plant biology: Recent insights

The presence of different forms of heavy metals in the earth crust is very primitive and probably associated with the origin of plant life. However, since the beginning of human civilisation, heavy metal use and its contamination to all living systems on earth have significantly increased due to human anthropogenic activities. Heavy metals are nonbiodegradable, which directly or indirectly impact photosynthesis, antioxidant system, mineral nutrition status, phytohormones and amino acid-derived molecules. Due to the toxic behaviour of some heavy metals, the endogenous status of chemical messengers like phytohormones may get significantly influenced, leading to harmful impacts on plant growth, development and overall yield of the plants. It has been noticed that exogenous application of phytohormones, that is, abscisic acid, salicylic acid, auxins, brassinosteroids, cytokinins, ethylene and gibberellins can positively regulate the heavy metal toxicity in plants through the regulation of the ascorbate-glutathione cycle, nitrogen metabolism, proline metabolisms, transpiration rate, and cell division. Furthermore, it may also restrict the entry of heavy metals into the plant cells, which aids in the recovery of plant growth and productivity. Besides these, some defence molecules also assist the plant in dealing with heavy metal toxicity. Therefore, the present review aims to bridge the knowledge gap in this context and present outstanding discoveries related to plant life supportive processes during stressful conditions including phytohormones and heavy metal crosstalk along with suggestions for future research in this field.

Effect of ACGT motif in spatiotemporal regulation of AtAVT6D, which improves tolerance to osmotic stress and nitrogen-starvation

Plasma membrane-localized AtAVT6D importing aspartic acid can be targeted to develop plants with enhanced osmotic and nitrogen-starvation tolerance. AtAVT6D promoter can be exploited as a stress-inducible promoter for genetic improvements to raise stress-resilient crops. The AtAVT6 family of amino acid transporters in Arabidopsis thaliana has been predicted to export amino acids like aspartate and glutamate. However, the functional characterization of these amino acid transporters in plants remains unexplored. The present study investigates the expression patterns of AtAVT6 genes in different tissues and under various abiotic stress conditions using quantitative Real-time PCR. The expression analysis demonstrated that the member AtAVT6D was significantly induced in response to phytohormone ABA and stresses like osmotic and drought. The tissue-specific expression analysis showed that AtAVT6D was strongly expressed in the siliques. Taking together these results, we can speculate that AtAVT6D might play a vital role in silique development and abiotic stress tolerance. Further, subcellular localization study showed AtAVT6D was localized to the plasma membrane. The heterologous expression of AtAVT6D in yeast cells conferred significant tolerance to nitrogen-deficient and osmotic stress conditions. The Xenopus oocyte studies revealed that AtAVT6D is involved in the uptake of Aspartic acid. While overexpression of AtAVT6D resulted in smaller siliques in Arabidopsis thaliana. Additionally, transient expression studies were performed with the full-length AtAVT6D promoter and its deletion constructs to study the effect of ACGT-N₂₄-ACGT motifs on the reporter gene expression in response to abiotic stresses and ABA treatment. The fluorometric GUS analyses revealed that the promoter deletion construct-2 (Pro.C2) possessing a single copy of ACGT-N₂₄-ACGT motif directed the strongest GUS expression under all the abiotic conditions tested. These results suggest that Pro.C2 can be used as a stress-inducible promoter to drive a significant transgene expression.

RING-box proteins regulate leaf senescence and stomatal closure via repression of ABA transporter gene ABCG40

Plant hormone abscisic acid (ABA) plays an indispensable role in the control of leaf senescence, during which ABA signaling depends on its biosynthesis. Nevertheless, the role of ABA transport in leaf senescence remains unknown. Here, we identified two novel RING-box protein-encoding genes UBIQUITIN LIGASE of SENESCENCE 1 and 2 (ULS1 and ULS2) involved in leaf senescence. Lack of ULS1 and ULS2 accelerates leaf senescence, which is specifically promoted by ABA treatment. Furthermore, the expression of senescence-related genes is significantly affected in mature leaves of uls1/uls2 double mutant (versus wild type (WT)) in an ABA-dependent manner, and the ABA content is substantially increased. ULS1 and ULS2 are mainly expressed in the guard cells and aging leaves, and the expression is induced by ABA. Further RNA-seq and quantitative proteomics of ubiquitination reveal that ABA transporter ABCG40 is highly expressed in uls1/uls2 mutant versus WT, though it is not the direct target of ULS1/2. Finally, we show that the acceleration of leaf senescence, the increase of leaf ABA content, and the promotion of stomatal closure in uls1/usl2 mutant are suppressed by abcg40 loss-of-function mutation. These results indicate that ULS1 and ULS2 function in feedback inhibition of ABCG40-dependent ABA transport during ABA-induced leaf senescence and stomatal closure.

Auxin response factors in plant adaptation to drought and salinity stress

Salinity and drought stresses affect plant growth worldwide and limit crop production. Auxin is crucial in regulating plants' salinity and drought stress adaptative response. As a chemical messenger, auxin influences gene expression through a family of functionally distinct transcription factors, the DNA-binding AUXIN RESPONSE FACTORS (ARFs). Various studies have revealed the important roles of ARFs in regulating drought and salinity stress responses in plants. Different ARFs regulate soluble sugar content, promote root development, and maintain chlorophyll content under drought and saline stress conditions to help plants adapt to these stresses. The functional characterization of ARFs pertaining to the regulation of drought and salinity stress responses is still in its infancy. Interestingly, the small RNA-mediated post-transcriptional regulation of ARF expression has been shown to influence plant responses to both stresses. The current knowledge on the diverse roles of ARFs in conferring specificity to auxin-mediated drought and salinity stress responses has not been reviewed to date. In this review, we summarize the recent research concerning the role of ARFs in response to drought and salinity stresses: gene expression patterns, functional characterization, and post-transcriptional regulation under drought and salinity stresses. We have also reviewed the modulation of ARF expression by other molecular regulators in the context of drought and salt stress signaling.

Genome-Wide Identification and Expressional Profiling of the Metal Tolerance Protein Gene Family in Brassica napus

The Cation Diffusion Facilitator (CDF) family, also named Metal Tolerance Protein (MTP), is one of the gene families involved in heavy metal transport in plants. However, a comprehensive study of MTPs in Brassica napus has not been reported yet. In the present study, we identified 33 BnMTP genes from the rapeseed genome using bioinformatic analyses. Subsequently, we analyzed the phylogenetic relationship, gene structure, chromosome distribution, conserved domains, and motifs of the BnMTP gene family. The 33 BnMTPs were phylogenetically divided into three major clusters (Zn-CDFs, Fe/Zn-CDFs, and Mn-CDFs) and seven groups (group 1, 5, 6, 7, 8, 9, and 12). The structural characteristics of the BnMTP members were similar in the same group, but different among groups. Evolutionary analysis indicated that the BnMTP gene family mainly expanded through whole-genome duplication (WGD) and segmental duplication events. Moreover, the prediction of cis-acting elements and microRNA target sites suggested that BnMTPs might be involved in plant growth, development, and stress responses. In addition, we found the expression of 24 BnMTPs in rapeseed leaves or roots could respond to heavy metal ion treatments. These results provided an important basis for clarifying the biological functions of BnMTPs, especially in heavy metal detoxification, and will be helpful in the phytoremediation of heavy metal pollution in soil.

HvbZIP21, a Novel Transcription Factor From Wild Barley Confers Drought Tolerance by Modulating ROS Scavenging

Drought stress is a common environmental stress, which adversely affects the yield and quality of crops. Due to its excellent drought tolerance, wild barley from the Middle East region is considered a valuable source for barley improvement. Here, we compared the growth rate, stomatal regulation and capacity to metabolize reactive oxygen species (ROS) of two barley cultivars and one wild barley accession. The results indicated the wild barley EC_S1 showed a more significant decline in stomatal aperture and less ROS production. Transcriptomic analysis revealed that EC_S1 has slower transcriptional regulation (5,050 DEGs) in the early stage of drought stress (14 days) than Baudin (7,022 DEGs) and Tadmor (6,090 DEGs). In addition, 30 hub genes, including nine known drought-related genes were identified by WGCNA analysis. Then, we cloned a novel bZIP transcription factor, HvbZIP21, from EC_S1. HvbZIP21 was subcellularly targeted to the nucleus. Overexpression of HvbZIP21 in Arabidopsis enhanced drought tolerance due to increasing activities of superoxide dismutase, peroxidase, and catalase activities as well as glutathione content. Silencing of HvbZIP21 in EC_S1 suppressed drought tolerance in BSMV:HvbZIP21-inoculated plants. Taken together, our findings suggest that HvbZIP21 play a critical role in drought tolerance by manipulating ROS scavenging.

PYR/PYL/RCAR Receptors Play a Vital Role in the Abscisic-Acid-Dependent Responses of Plants to External or Internal Stimuli

Abscisic acid (ABA) is a phytohormone that plays a key role in regulating several developmental processes as well as in response to stressful conditions such as drought. Activation of the ABA signaling cascade allows the induction of an appropriate physiological response. The basic components of the ABA signaling pathway have been recognized and characterized in recent years. Pyrabactin resistance, pyrabactin resistance-like, and the regulatory component of ABA receptors (PYR/PYL/RCAR) are the major components responsible for the regulation of the ABA signaling pathway. Here, we review recent findings concerning the PYR/PYL/RCAR receptor structure, function, and interaction with other components of the ABA signaling pathway as well as the termination mechanism of ABA signals in plant cells. Since ABA is one of the basic elements related to abiotic stress, which is increasingly common in the era of climate changes, understanding the perception and transduction of the signal related to this phytohormone is of paramount importance in further increasing crop tolerance to various stress factors.

RING Zinc Finger Proteins in Plant Abiotic Stress Tolerance

RING zinc finger proteins have a conserved RING domain, mainly function as E3 ubiquitin ligases, and play important roles in plant growth, development, and the responses to abiotic stresses such as drought, salt, temperature, reactive oxygen species, and harmful metals. RING zinc finger proteins act in abiotic stress responses mainly by modifying and degrading stress-related proteins. Here, we review the latest progress in research on RING zinc finger proteins, including their structural characteristics, classification, subcellular localization, and physiological functions, with an emphasis on abiotic stress tolerance. Under abiotic stress, RING zinc finger proteins on the plasma membrane may function as sensors or abscisic acid (ABA) receptors in abiotic stress signaling. Some RING zinc finger proteins accumulate in the nucleus may act like transcription factors to regulate the expression of downstream abiotic stress marker genes through direct or indirect ways. Most RING zinc finger proteins usually accumulate in the cytoplasm or nucleus and act as E3 ubiquitin ligases in the abiotic stress response through ABA, mitogen-activated protein kinase (MAPK), and ethylene signaling pathways. We also highlight areas where further research on RING zinc finger proteins in plants is needed.

Signal Transduction in Cereal Plants Struggling with Environmental Stresses: From Perception to Response

Cereal plants under abiotic or biotic stressors to survive unfavourable conditions and continue growth and development, rapidly and precisely identify external stimuli and activate complex molecular, biochemical, and physiological responses. To elicit a response to the stress factors, interactions between reactive oxygen and nitrogen species, calcium ions, mitogen-activated protein kinases, calcium-dependent protein kinases, calcineurin B-like interacting protein kinase, phytohormones and transcription factors occur. The integration of all these elements enables the change of gene expression, and the release of the antioxidant defence and protein repair systems. There are still numerous gaps in knowledge on these subjects in the literature caused by the multitude of signalling cascade components, simultaneous activation of multiple pathways and the intersection of their individual elements in response to both single and multiple stresses. Here, signal transduction pathways in cereal plants under drought, salinity, heavy metal stress, pathogen, and pest attack, as well as the crosstalk between the reactions during double stress responses are discussed. This article is a summary of the latest discoveries on signal transduction pathways and it integrates the available information to better outline the whole research problem for future research challenges as well as for the creative breeding of stress-tolerant cultivars of cereals.

Drought stress increases the expression of barley defence genes with negative consequences for infesting cereal aphids

Crops are exposed to myriad abiotic and biotic stressors with negative consequences. Two stressors that are expected to increase under climate change are drought and infestation with herbivorous insects, including important aphid species. Expanding our understanding of the impact drought has on the plant-aphid relationship will become increasingly important under future climate scenarios. Here we use a previously characterized plant-aphid system comprising a susceptible variety of barley, a wild relative of barley with partial aphid resistance, and the bird cherry-oat aphid to examine the drought-plant-aphid relationship. We show that drought has a negative effect on plant physiology and aphid fitness, and provide evidence to suggest that plant resistance influences aphid responses to drought stress. Furthermore, we show that the expression of thionin genes, plant defensive compounds that contribute to aphid resistance, increase in susceptible plants exposed to drought stress but remain at constant levels in the partially resistant plant, suggesting that they play an important role in determining the success of aphid populations. This study highlights the role of plant defensive processes in mediating the interactions between the environment, plants, and herbivorous insects.

Transcriptome Analyses of Near Isogenic Lines Reveal Putative Drought Tolerance Controlling Genes in Wheat

Drought stress, especially at the grain-filling stage, is a major constraint for wheat production. Drought tolerance is a complex trait controlled by a large array of genes and pathways. This study conducted gene expression profiling on two pairs of near-isogenic lines (NILs) for an important qDSI.4B.1 QTL conferring drought tolerance on the short arm of chromosome 4B in wheat. Analysis showed 1,614 genome-wide differentially expressed genes (DEGs) between the tolerant and susceptible isolines in both NIL pairs. Six common DEGs were found between NIL1 and NIL2 at both 7 and 14 days after stress induction, with two of them having single nucleotide polymorphism (SNP) variants. These six genes that were confirmed by quantitative real-time PCR (qRT-PCR) expression analysis are considered candidate genes for drought tolerance mediated by qDSI.4B.1 QTL with their main contributions to gene regulation, cell elongation, protein quality control, secondary metabolism, and hormone signaling. These six candidate genes and the highest number of DEGs and variants (SNPs/indels) were located between 49 and 137 Mbp of 4BS, making this interval the most probable location for the qDSI.4B.1 locus. Additionally, 765 and 84 DEGs were detected as responsive genes to drought stress in tolerant and susceptible isolines, respectively. According to gene ontology (GO), protein phosphorylation, oxidation reduction, and regulation of transcription were top biological processes involved in the drought response and tolerance. These results provide insights into stress responses regulated by the 4BS locus and have identified candidate genes and genetic markers that can be used for fine mapping of the qDSI.4B.1 locus and, ultimately, in wheat breeding programs for drought tolerance.

Analysis of metabolic dynamics during drought stress in Arabidopsis plants

Drought is a major cause of agricultural losses worldwide. Climate change will intensify drought episodes threatening agricultural sustainability. Gaining insights into drought response mechanisms is vital for crop adaptation to climate emergency. To date, only few studies report comprehensive analyses of plant metabolic adaptation to drought. Here, we present a multifactorial metabolomic study of early-mid drought stages in the model plant Arabidopsis thaliana. We sampled root and shoot tissues of plants subjected to water withholding over a six-day time course, including brassinosteroids receptor mutants previously reported to show drought tolerance phenotypes. Furthermore, we sequenced the root transcriptome at basal and after 5 days drought, allowing direct correlation between metabolic and transcriptomic changes and the multi-omics integration. Significant abiotic stress signatures were already activated at basal conditions in a vascular-specific receptor overexpression (BRL3ox). These were also rapidly mobilized under drought, revealing a systemic adaptation strategy driven from inner tissues of the plant. Overall, this dataset provides a significant asset to study drought metabolic adaptation and allows its analysis from multiple perspectives.

Abscisic acid-catabolizing bacteria: A useful tool for enhancing phytoremediation

Bacteria-facilitated phytoextraction has been gaining recognition for the phytoremediation of heavy metal (HM)-contaminated soils. Nevertheless, it remains unclear whether catabolizing abscisic acid (ABA) in hyperaccumulating plants via rhizobacteria could facilitate HM phytoextraction. In this study, inoculation with the ABA-catabolizing bacterium, Rhodococcus qingshengii, increased HM (Cd, Zn, Pb, and Cu) concentrations in the shoots of hyperaccumulators Vetiveria zizanioides, Brassica juncea, Lolium perenne L., Solanum nigrum L., and Sedum alfredii Hance grown in mildly and severely contaminated soils by 28.8%-331.3%, 8.5%-393.4%, 21.2%-222.5%, 14.7%-115.5%, and 28.3%-174.2%, respectively, compared with non-inoculated plants. The fresh biomass of these hyperaccumulators was elevated by 16.5%-94.4%, compared to that of the bacteria-free control. Phytoremediation potential indices, including bioconcentration and translocation factors, also revealed that the bacteria markedly boosted the phytoextraction efficacy from soil. Furthermore, principal component analysis (PCA) suggested that the effects of bacteria on the concentrations of Cd and Zn in hyperaccumulators were significantly correlated with ABA metabolism, but not with Pb and Cu. Combined with the synergistic effects on plant biomass, the bacteria also improved the phytoextraction of Pb and Cu in hyperaccumulators. Overall, the application of microorganism-assisted remediation based on ABA-catabolizing bacteria might be an alternative strategy for enhancing phytoremediation efficiency in HM-contaminated soils.

Microbe-Mediated Thermotolerance in Plants and Pertinent Mechanisms- A Meta-Analysis and Review

Microbial symbionts can mediate plant stress responses by enhancing thermal tolerance, but less attention has been paid to measuring these effects across plant-microbe studies. We performed a meta-analysis of published studies as well as discussed with relevant literature to determine how the symbionts influence plant responses under non-stressed versus thermal-stressed conditions. As compared to non-inoculated plants, inoculated plants had significantly higher biomass and photosynthesis under heat stress conditions. A significantly decreased accumulation of malondialdehyde (MDA) and hydrogen peroxide (H2O2) indicated a lower oxidation level in the colonized plants, which was also correlated with the higher activity of catalase, peroxidase, glutathione reductase enzymes due to microbial colonization under heat stress. However, the activity of superoxide dismutase, ascorbate oxidase, ascorbate peroxidase, and proline were variable. Our meta-analysis revealed that microbial colonization influenced plant growth and physiology, but their effects were more noticeable when their host plants were exposed to high-temperature stress than when they grew under ambient temperature conditions. We discussed the mechanisms of microbial conferred plant thermotolerance, including at the molecular level based on the available literature. Further, we highlighted and proposed future directions toward exploring the effects of symbionts on the heat tolerances of plants for their implications in sustainable agricultural production.

The network centered on ICEs play roles in plant cold tolerance, growth and development

ICEs are key transcription factors in response to cold in plant, they also balance plant growth and stress tolerance. Thus, we systematize the information about ICEs published to date. Low temperature is an important factor affecting plant growth and development. Exposing to cold condition results in a suit of effects on plants including reduction of plant growth and reproduction, and decrease in crop yield and quality. Plants have evolved a series of strategies to deal with cold stress such as reprogramming of the expression of genes and transcription factors. ICEs (Inducer of CBF Expression), as transcription factors regulating CBFs (C-repeat binding factor), play key roles in balancing plant growth and stress tolerance. Studies on ICEs focused on the function of ICEs on cold tolerance, growth and development; post-translational modifications of ICEs and crosstalk between the ICEs and phytohormones. In this review, we focus on systematizing the information published to date. We summarized the main advances of the functions of ICEs on the cold tolerance, growth and development. And we also elaborated the regulation of ICEs protein stability including phosphorylation, ubiquitination and SUMOylation of ICE. Finally, we described the function of ICEs in the crosstalk among different phytohormone signaling pathway and cold stress. This review provides perspectives for ongoing research about cold tolerance, growth and development in plant.

Cellulose synthase-like protein OsCSLD4 plays an important role in the response of rice to salt stress by mediating abscisic acid biosynthesis to regulate osmotic stress tolerance

Cell wall polysaccharide biosynthesis enzymes play important roles in plant growth, development and stress responses. The functions of cell wall polysaccharide synthesis enzymes in plant growth and development have been well studied. In contrast, their roles in plant responses to environmental stress are poorly understood. Previous studies have demonstrated that the rice cell wall cellulose synthase-like D4 protein (OsCSLD4) is involved in cell wall polysaccharide synthesis and is important for rice growth and development. This study demonstrated that the OsCSLD4 function-disrupted mutant nd1 was sensitive to salt stress, but insensitive to abscisic acid (ABA). The expression of some ABA synthesis and response genes was repressed in nd1 under both normal and salt stress conditions. Exogenous ABA can restore nd1-impaired salt stress tolerance. Moreover, overexpression of OsCSLD4 can enhance rice ABA synthesis gene expression, increase ABA content and improve rice salt tolerance, thus implying that OsCSLD4-regulated rice salt stress tolerance is mediated by ABA synthesis. Additionally, nd1 decreased rice tolerance to osmotic stress, but not ion toxic tolerance. The results from the transcriptome analysis showed that more osmotic stress-responsive genes were impaired in nd1 than salt stress-responsive genes, thus indicating that OsCSLD4 is involved in rice salt stress response through an ABA-induced osmotic response pathway. Intriguingly, the disruption of OsCSLD4 function decreased grain width and weight, while overexpression of OsCSLD4 increased grain width and weight. Taken together, this study demonstrates a novel plant salt stress adaptation mechanism by which crops can coordinate salt stress tolerance and yield.

Synergistic interaction between ABA and IAA due to moderate soil drying promotes grain filling of inferior spikelets in rice

Poor grain filling of inferior spikelets is becoming a severe problem in some super rice varieties with large panicles. Moderate soil drying (MD) after pollination has been proven to be a practical strategy to promote grain filling. However, the molecular mechanisms underlying this phenomenon remain largely unexplored. Here, transcriptomic analysis of the most active grain filling stage revealed that both starch metabolism and phytohormone signaling were significantly promoted by MD treatment, accompanied by increased enzyme activities of starch synthesis and elevated abscisic acid (ABA) and indole-3-acetic acid (IAA) content in the inferior spikelet. Moreover, the IAA biosynthesis genes OsYUC11 and OsTAR2 were upregulated, while OsIAA29 and OsIAA24, which encode two repressors of auxin signaling, were downregulated by MD, implying a regulation of both IAA biosynthesis and auxin signal transduction in the inferior spikelet by MD. A notable improvement in grain filling of the inferior spikelet was found in the aba8ox2 mutant, which is mutated in an ABA catabolism gene. In contrast, overexpression of OsABA8ox2 significantly reduced grain filling. Interestingly, not only the IAA content, but also the expression of IAA biosynthesis and auxin-responsive genes displayed a similar trend to that in the inferior spikelet under MD. In addition, several OsTPP genes were downregulated in the inferior spikelets of both MD/ABA-treated wild-type plants and the aba8ox2 mutant, resulting in lower trehalose content and higher levels of -6-phosphate (T6P), thereby increasing the expression of OsTAR2, a target of T6P. Taken together, our results suggest that the synergistic interaction of ABA-mediated accumulation of IAA promotes grain filling of inferior spikelets under MD.

The miR159-MYB33-ABI5 module regulates seed germination in Arabidopsis

Drought stress restricts crop productivity and exacerbates food shortages. The plant hormone, abscisic acid (ABA), has been shown to be a pivotal player in the regulation of drought tolerance and seed germination in plants. ABA accumulates under abiotic stresses to promote miR159 expression. miR159 is an ancient and conserved plant miRNA that plays diverse roles in plant development, seed germination, and drought response in Arabidopsis. Our previous studies demonstrated that miR159 regulates the vegetative phase change by repressing the ABI5 activation and thereafter preventing hyperactivation of miR156. However, whether the miR159-MYB33-ABI5 module plays a role in seed germination and drought response, and if so, how they interact genetically, remain largely unexplored. Here, we show that loss-of-function of miR159 (mir159ab) confers enhanced drought tolerance and hypersensitivity of seed germination to ABA. Genetic analyses demonstrated that loss-of-function mutation in the ABI5 gene suppresses the hypersensitivity of mir159ab to ABA, and the insensitivity of myb33 seeds to ABA treatment is ABI5 dependent. ABI5 functions downstream of MYB33 and miR159 in response to ABA. Therefore, our results uncover a new role for the miR159-MYB33-ABI5 module in the regulation of drought response and seed germination in plants.

The Effects of Exogenous Salicylic Acid on Endogenous Phytohormone Status in Hordeum vulgare L. under Salt Stress

Acclimation to salt stress in plants is regulated by complex signaling pathways involving endogenous phytohormones. The signaling role of salicylic acid (SA) in regulating crosstalk between endogenous plant growth regulators' levels was investigated in barley (Hordeum vulgare L. 'Ince'; 2n = 14) leaves and roots under salt stress. Salinity (150 and 300 mM NaCl) markedly reduced leaf relative water content (RWC), growth parameters, and leaf water potential (LWP), but increased proline levels in both vegetative organs. Exogenous SA treatment did not significantly affect salt-induced negative effects on RWC, LWP, and growth parameters but increased the leaf proline content of plants under 150 mM salt stress by 23.1%, suggesting that SA enhances the accumulation of proline, which acts as a compatible solute that helps preserve the leaf's water status under salt stress. Changes in endogenous phytohormone levels were also investigated to identify agents that may be involved in responses to increased salinity and exogenous SA. Salt stress strongly affected endogenous cytokinin (CK) levels in both vegetative organs, increasing the concentrations of CK free bases, ribosides, and nucleotides. Indole-3-acetic acid (IAA, auxin) levels were largely unaffected by salinity alone, especially in barley leaves, but SA strongly increased IAA levels in leaves at high salt concentration and suppressed salinity-induced reductions in IAA levels in roots. Salt stress also significantly increased abscisic acid (ABA) and ethylene levels; the magnitude of this increase was reduced by treatment with exogenous SA. Both salinity and SA treatment reduced jasmonic acid (JA) levels at 300 mM NaCl but had little effect at 150 mM NaCl, especially in leaves. These results indicate that under high salinity, SA has antagonistic effects on levels of ABA, JA, ethylene, and most CKs, as well as basic morphological and physiological parameters, but has a synergistic effect on IAA, which was well exhibited by principal component analysis (PCA).

Interplay between Ca2+/Calmodulin-Mediated Signaling and AtSR1/CAMTA3 during Increased Temperature Resulting in Compromised Immune Response in Plants

Changing temperatures are known to affect plant–microbe interactions; however, the molecular mechanism involved in plant disease resistance is not well understood. Here, we report the effects of a moderate change in temperature on plant immune response through Ca²⁺/calmodulin-mediated signaling. At 30 °C, Pst DC3000 triggered significantly weak and relatively slow Ca²⁺ influx in plant cells, as compared to that at 18 °C. Increased temperature contributed to an enhanced disease susceptibility in plants; the enhanced disease susceptibility is the result of the compromised stomatal closure induced by pathogens at high temperature. A Ca²⁺ receptor, AtSR1, contributes to the decreased plant immunity at high temperatures and the calmodulin-binding domain (CaMBD) is required for its function. Furthermore, both salicylic acid biosynthesis (ICS) and salicylic acid receptor (NPR1) are involved in this process. In addition to stomatal control, AtSR1 is involved in high temperature-compromised apoplastic immune response through the salicylic acid signaling pathway. The qRT-PCR data revealed that AtSR1 contributed to increased temperatures-mediated susceptible immune response by regulating SA-related genes in atsr1, such as PR1, ICS1, NPR1, as well as EDS1. Our results indicate that Ca²⁺ signaling has broad effects on the molecular interplay between changing temperatures as well as plant defense during plant–pathogen interactions.

Spray-induced gene silencing targeting a glutathione S-transferase gene improves resilience to drought in grapevine

Along with the ongoing climate change, drought events are predicted to become more severe. In this context, the spray-induced gene silencing (SIGS) technique could represent a useful strategy to improve crop stress resilience. A previous study demonstrated that the Arabidopsis mutants for a glutathione S-transferase (GST) gene had increased abscisic acid (ABA) levels and a more activated antioxidant system, both features that improved drought resilience. Here, we used SIGS to target a putative grape GST gene (VvGST40). Then, ecophysiological, biochemical and molecular responses of 'Chardonnay' cuttings were analysed during a drought and recovery time-course. Gas exchange, ABA and t-resveratrol concentration as well as expression of stress-related genes were monitored in not treated controls, dsRNA-VvGST40- and dsRNA-GFP- (negative control of the technique) treated plants, either submitted or not to drought. VvGST40-treated plants revealed increased resilience to severe drought as attested by the ecophysiological data. Analysis of target metabolites and antioxidant- and ABA-related transcripts confirmed that VvGST40-treated plants were in a priming status compared with controls. SIGS targeting an endogenous gene was successfully applied in grapevine, confirming the ability of this technique to be exploited not only for plant protection issues but also for functional genomic studies.

Interaction between fructan metabolism and plant growth regulators

The relationship of fructan to plant growth regulators is clearly more complicated than it looks and is likely related to differences between fructan molecules in size and structure as well as localization. Fructans are a complex group of carbohydrates composed mainly of fructose units linked to a sucrose molecule. Fructans are present in plants as heterogeneous mixtures with diverse molecular structures and mass, different polymerization degrees, and linkage types between fructosyl residues. Like sucrose, they are frequently stored in leaves and other organs, acting as carbohydrate reserves. Fructans are synthesized in the cell vacuole by fructosyltransferase enzymes and catabolized by fructan exohydrolase enzymes. Several publications have shown that fructan metabolism varies with the stage of plant development and in response to the environment. Recent studies have shown a correlation between plant growth regulators (PGR), fructan metabolism, and tolerance to drought and cold. PGR are compounds that profoundly influence the growth and differentiation of plant cells, tissues, and organs. They play a fundamental role in regulating plant responses to developmental and environmental signals. In this review, we summarize the most up-to-date knowledge on the metabolism of fructans and their crosstalk with PGR signaling pathways. We identify areas that require more research to complete our understanding of the role of fructans in plants.

Physiological and Multi-Omics Approaches for Explaining Drought Stress Tolerance and Supporting Sustainable Production of Rice

Drought differs from other natural disasters in several respects, largely because of the complexity of a crop's response to it and also because we have the least understanding of a crop's inductive mechanism for addressing drought tolerance among all abiotic stressors. Overall, the growth and productivity of crops at a global level is now thought to be an issue that is more severe and arises more frequently due to climatic change-induced drought stress. Among the major crops, rice is a frontline staple cereal crop of the developing world and is critical to sustaining populations on a daily basis. Worldwide, studies have reported a reduction in rice productivity over the years as a consequence of drought. Plants are evolutionarily primed to withstand a substantial number of environmental cues by undergoing a wide range of changes at the molecular level, involving gene, protein and metabolite interactions to protect the growing plant. Currently, an in-depth, precise and systemic understanding of fundamental biological and cellular mechanisms activated by crop plants during stress is accomplished by an umbrella of -omics technologies, such as transcriptomics, metabolomics and proteomics. This combination of multi-omics approaches provides a comprehensive understanding of cellular dynamics during drought or other stress conditions in comparison to a single -omics approach. Thus a greater need to utilize information (big-omics data) from various molecular pathways to develop drought-resilient crop varieties for cultivation in ever-changing climatic conditions. This review article is focused on assembling current peer-reviewed published knowledge on the use of multi-omics approaches toward expediting the development of drought-tolerant rice plants for sustainable rice production and realizing global food security.

Abscisic acid insensitive 4 interacts with ICE1 and JAZ proteins to regulate ABA signaling-mediated cold tolerance in apple

Abscisic acid is involved in the regulation of cold stress response, but its molecular mechanism remains to be elucidated. In this study, we demonstrated that the APETALA2/ethylene responsive factor (AP2/ERF) family protein MdABI4 positively regulates abscisic acid-mediated cold tolerance in apple. We found that MdABI4 interacts with MdICE1, a key regulatory protein involved in the cold stress response, and enhances the transcriptional regulatory function of MdICE1 on its downstream target gene MdCBF1, thus improving abscisic acid-mediated cold tolerance. The jasmonate-ZIM domain (JAZ) proteins MdJAZ1 and MdJAZ2 negatively modulate MdABI4-improved cold tolerance in apple by interacting with the MdABI4 protein. Further investigation showed that MdJAZ1 and MdJAZ2 interfere with the interaction between the MdABI4 and MdICE1 proteins. Together, our data revealed that MdABI4 integrates jasmonic acid and abscisic acid signals to precisely modulate cold tolerance in apple through the JAZ-ABI4-ICE1-CBF regulatory cascade. These findings provide insights into the crosstalk between jasmonic acid and abscisic acid signals in response to cold stress.

The Loss of Function of the NODULE INCEPTION-Like PROTEIN 7 Enhances Salt Stress Tolerance in Arabidopsis Seedlings

Plants acquire nitrogen, an essential macronutrient, from the soil as nitrate. Since nitrogen availability is a major determinant of crop productivity, the soil is amended with nitrogenous fertilizers. Extensive use of irrigation can lead to the accumulation of salt in the soil, which compromises crop productivity. Our characterization of NODULE INCEPTION (NIN)-like PROTEIN 7 (NLP7), a transcription factor regulating the primary response to nitrate, revealed an intersection of salt stress and nitrate metabolism. The growth of loss-of-function mutant nlp7 was tolerant to high salinity that normally reduces the fresh weight and chlorophyll and protein content of wild type (Col-0). On a medium with high salinity, the nlp7 experienced less stress, accumulating less proline, producing less nitric oxide (NO) and reactive oxygen species (ROS), and expressing lower transcript levels of marker genes, such as RD29A and COR47, than Col-0. Nevertheless, more sodium ions were translocated to and accumulated in the shoots of nlp7 than that of Col-0. Since nlp7 also expressed less nitrate reductase (NR) activity, nitrate accumulated to abnormally high levels with or without salinity. We attributed the enhanced salt tolerance of nlp7 to the balanced accumulation of nitrate anions and sodium cations. Our results suggest that nitrate metabolism and signaling might be targeted to improve salt tolerance.

Phenotyping and Quantitative Trait Locus Analysis for the Limited Transpiration Trait in an Upper-Mid South Soybean Recombinant Inbred Line Population ("Jackson" × "KS4895"): High Throughput Aquaporin Inhibitor Screening

Soybean is most often grown under rainfed conditions and negatively impacted by drought stress in the upper mid-south of the United States. Therefore, identification of drought-tolerance traits and their corresponding genetic components are required to minimize drought impacts on productivity. Limited transpiration (TRlim) under high vapor pressure deficit (VPD) is one trait that can help conserve soybean water-use during late-season drought. The main research objective was to evaluate a recombinant inbred line (RIL) population, from crossing two mid-south soybean lines ("Jackson" × "KS4895"), using a high-throughput technique with an aquaporin inhibitor, AgNO3, for the TRlim trait. A secondary objective was to undertake a genetic marker/quantitative trait locus (QTL) genetic analysis using the AgNO3 phenotyping results. A set of 122 soybean genotypes (120-RILs and parents) were grown in controlled environments (32/25-d/n °C). The transpiration rate (TR) responses of derooted soybean shoots before and after application of AgNO3 were measured under 37°C and >3.0 kPa VPD. Then, the decrease in transpiration rate (DTR) for each genotype was determined. Based on DTR rate, a diverse group (slow, moderate, and high wilting) of 26 RILs were selected and tested for the whole plant TRs under varying levels of VPD (0.0-4.0 kPa) at 32 and 37°C. The phenotyping results showed that 88% of slow, 50% of moderate, and 11% of high wilting genotypes expressed the TRlim trait at 32°C and 43, 10, and 0% at 37°C, respectively. Genetic mapping with the phenotypic data we collected revealed three QTL across two chromosomes, two associated with TRlim traits and one associated with leaf temperature. Analysis of Gene Ontologies of genes within QTL regions identified several intriguing candidate genes, including one gene that when overexpressed had previously been shown to confer enhanced tolerance to abiotic stress. Collectively these results will inform and guide ongoing efforts to understand how to deploy genetic tolerance for drought stress.

Phytohormones Trigger Drought Tolerance in Crop Plants: Outlook and Future Perspectives

In the past and present, human activities have been involved in triggering global warming, causing drought stresses that affect animals and plants. Plants are more defenseless against drought stress; and therefore, plant development and productive output are decreased. To decrease the effect of drought stress on plants, it is crucial to establish a plant feedback mechanism of resistance to drought. The drought reflex mechanisms include the physical stature physiology and biochemical, cellular, and molecular-based processes. Briefly, improving the root system, leaf structure, osmotic-balance, comparative water contents and stomatal adjustment are considered as most prominent features against drought resistance in crop plants. In addition, the signal transduction pathway and reactive clearance of oxygen are crucial mechanisms for coping with drought stress via calcium and phytohormones such as abscisic acid, salicylic acid, jasmonic acid, auxin, gibberellin, ethylene, brassinosteroids and peptide molecules. Furthermore, microorganisms, such as fungal and bacterial organisms, play a vital role in increasing resistance against drought stress in plants. The number of characteristic loci, transgenic methods and the application of exogenous substances [nitric oxide, (C28H48O6) 24-epibrassinolide, proline, and glycine betaine] are also equally important for enhancing the drought resistance of plants. In a nutshell, the current review will mainly focus on the role of phytohormones and related mechanisms involved in drought tolerance in various crop plants.

Plant Flavonoids in Mediterranean Species: A Focus on Flavonols as Protective Metabolites under Climate Stress

Flavonoids are specialized metabolites largely widespread in plants where they play numerous roles including defense and signaling under stress conditions. These compounds encompass several chemical subgroups such as flavonols which are one the most represented classes. The most studied flavonols are kaempferol, quercetin and myricetin to which research attributes antioxidative properties and a potential role in UV-defense through UV-screening mechanisms making them critical for plant adaptation to climate change. Despite the great interest in flavonol functions in the last decades, some functional aspects remain under debate. This review summarizes the importance of flavonoids in plant defense against climate stressors and as signal molecules with a focus on flavonols in Mediterranean plant species. The review emphasizes the relationship between flavonol location (at the organ, tissue and cellular scales) and their function as defense metabolites against climate-related stresses. It also provides evidence that biosynthesis of flavonols, or flavonoids as a whole, could be a crucial process allowing plants to adapt to climate change, especially in the Mediterranean area which is considered as one of the most sensitive regions to climate change over the globe.

Discovery of potential protein biomarkers associated with sugarcane white leaf disease susceptibility using a comparative proteomic approach

Sugarcane white leaf disease (SCWLD) is caused by phytoplasma, a serious sugarcane phytoplasma pathogen, which causes significant decreases in crop yield and sugar quality. The identification of proteins involved in the defense mechanism against SCWLD phytoplasma may help towards the development of varieties resistant to SCWLD. We investigated the proteomes of four sugarcane varieties with different levels of susceptibility to SCWLD phytoplasma infection, namely K88-92 and K95-84 (high), KK3 (moderate), and UT1 (low) by quantitative label-free nano-liquid chromatography-tandem mass spectrometry (nano LC-MS/MS). A total of 248 proteins were identified and compared among the four sugarcane varieties. Two potential candidate protein biomarkers for reduced susceptibility to SCWLD phytoplasma were identified as proteins detected only in UT1. The functions of these proteins are associated with protein folding, metal ion binding, and oxidoreductase. The candidate biomarkers could be useful for further study of the sugarcane defense mechanism against SCWLD phytoplasma, and in molecular and conventional breeding strategies for variety improvement.

Inferring the genetic responses to acute drought stress across an ecological gradient

BACKGROUND

How do xerophytic species thrive in environments that experience extreme annual drought? Although critical to the survival of many species, the genetic responses to drought stress in many non-model organisms has yet to be explored. We investigated this question in Mentzelia section Bartonia (Loasaceae), which occurs throughout western North America, including arid lands. To better understand the genetic responses to drought stress among species that occur in different habitats, the gene expression levels of three species from Mentzelia were compared across a precipitation gradient. Two de novo reference transcriptomes were generated and annotated. Leaf and root tissues were collected from control and drought shocked plants and compared to one another for differential expression. A target-gene approach was also implemented to better understand how drought-related genes from model and crop species function in non-model systems.

RESULTS

When comparing the drought-shock treatment plants to their respective control plants, we identified 165 differentially expressed clusters across all three species. Differentially expressed genes including those associated with water movement, photosynthesis, and delayed senescence. The transcriptome profiling approach was coupled with a target genes approach that measured expression of 90 genes associated with drought tolerance in model organisms. Comparing differentially expressed genes with a ≥ 2 log-fold value between species and tissue types showed significant differences in drought response. In pairwise comparisons, species that occurred in drier environments differentially expressed greater genes in leaves when drought shocked than those from wetter environments, but expression in the roots mostly produced opposite results.

CONCLUSIONS

Arid-adapted species mount greater genetic responses compared to the mesophytic species, which has likely evolved in response to consistent annual drought exposure across generations. Drought responses also depended on organ type. Xerophytes, for example, mounted a larger response in leaves to downregulate photosynthesis and senescence, while mobilizing carbon and regulating water in the roots. The complexity of drought responses in Mentzelia suggest that whole organism responses need to be considered when studying drought and, in particular, the physiological mechanisms in which plants regulate water, carbon, cell death, metabolism, and secondary metabolites.

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.

Root exudates impact plant performance under abiotic stress

Plant root exudates serve pivotal roles in supporting plant development and interactions with the physicochemical and biological factors in the rhizosphere. Under stress conditions, root exudation is involved in enhancing plant resource-use efficiency and facilitating the crosstalk between plant and soil microbes to ameliorate stress. Although there are a large number of root exudates that remain to be characterized, recent technological advancements have allowed for the function of many exudate compounds to be elucidated. In this review, we discuss current knowledge about the key root exudates that modulate plant resource-use efficiency under various abiotic stresses including drought, aluminum toxicity, phosphorus, nitrogen, and iron deficiency. The role that key root exudates play in shaping microbial communities in the rhizosphere under stress conditions is also an important consideration addressed in this review.

Differential regulations of abscisic acid-induced desiccation tolerance and vegetative dormancy by group B3 Raf kinases in liverworts

Phytohormone abscisic acid (ABA) plays a key role in stomata closure, osmostress acclimation, and vegetative and embryonic dormancy. Group B3 Raf protein kinases (B3-Rafs) serve as positive regulators of ABA and osmostress signaling in the moss Physcomitrium patens and the angiosperm Arabidopsis thaliana. While P. patens has a single B3-Raf called ARK, specific members of B3-Rafs among six paralogs regulate ABA and osmostress signaling in A. thaliana, indicating functional diversification of B3-Rafs in angiosperms. However, we found that the liverwort Marchantia polymorpha, belonging to another class of bryophytes, has three paralogs of B3-Rafs, MpARK1, MpARK2, and MpARK3, with structural variations in the regulatory domains of the polypeptides. By reporter assays of the P. patens ark line and analysis of genome-editing lines of M. polymorpha, we found that these B3-Rafs are functionally redundant in ABA response, with respect to inhibition of growth, tolerance to desiccation and expression of stress-associated transcripts, the majority of which are under the control of the PYR/PYL/RCAR-like receptor MpPYL1. Interestingly, gemmae in gemma cups were germinating only in mutant lines associated with MpARK1, indicating that dormancy in the gametophyte is controlled by a specific B3-Raf paralog. These results indicated not only conservation of the role of B3-Rafs in ABA and osmostress response in liverworts but also functional diversification of B3-Rafs, which is likely to have occurred in the early stages of land plant evolution.

The BBX gene CmBBX22 negatively regulates drought stress tolerance in chrysanthemum

BBX transcription factors play vital roles in plant growth, development, and stress responses. Although BBX proteins have been studied in great detail in the model plant Arabidopsis, their roles in crop plants such as chrysanthemum are still largely uninvestigated. Here, we cloned CmBBX22 and further determined the function of CmBBX22 in response to drought treatment. Subcellular localization and transactivation assay analyses revealed that CmBBX22 was localized in the nucleus and possessed transactivation activity. Overexpression of CmBBX22 in chrysanthemum was found to reduce plant drought tolerance, whereas expression of the chimeric repressor CmBBX22-SRDX was found to promote a higher drought tolerance than that shown by wild-type plants, indicating that CmBBX22 negatively regulates drought tolerance in chrysanthemum. Transcriptome analysis and physiological measurements indicated the potential involvement of the CmBBX22-mediated ABA response, stomatal conductance, and antioxidant responses in the negative regulation of drought tolerance in chrysanthemum. Based on the findings of this study, we were thus able to establish the mechanisms whereby the transcriptional activator CmBBX22 negatively regulates drought tolerance in chrysanthemum via the regulation of the abscisic acid response, stomatal conductance, and antioxidant responses.

Climate change regulated abiotic stress mechanisms in plants: a comprehensive review

Global climate change is identified as a major threat to survival of natural ecosystems. Climate change is a dynamic, multifaceted system of alterations in environmental conditions that affect abiotic and biotic components of the world. It results in alteration in environmental conditions such as heat waves, intensity of rainfall, CO₂ concentration and temperature that lead to rise in new pests, weeds and pathogens. Climate change is one of the major constraints limiting plant growth and development worldwide. It impairs growth, disturbs photosynthesis, and reduces physiological responses in plants. The variations in global climate have gained the attention of researchers worldwide, as these changes negatively affect the agriculture by reducing crop productivity and food security. With this background, this review focuses on the effects of elevated atmospheric CO₂ concentration, temperature, drought and salinity on the morphology, physiology and biochemistry of plants. Furthermore, this paper outlines an overview on the reactive oxygen species (ROS) production and their impact on the biochemical and molecular status of plants with increased climatic variations. Also additionally, different tolerance strategies adopted by plants to combat environmental adversities have been discussed.

Liquid Chromatography-Tandem Mass Spectrometry-Based Profiling of Plant Hormones

Phytohormones plays crucial physiological functions in plants, where they are involved in plant development, reproduction, defense, and many other functions. Phytohormones production has been found to be regulated in response to abiotic and biotic factors affecting the plant metabolism, and therefore, biosynthesis of primary and secondary metabolites. Thus, the detection and quantification of phytohormones in different plant tissues are essential to be determined unraveling the various plant metabolic pathways and behavior. Yet phytohormones analysis is always problematic, since they are found in extremely low concentrations and have a wide range of chemical and physicochemical properties. As a result, the ideal method should start with an appropriate extraction procedure followed by quantification by highly sensitive instrumental techniques providing precise and robust results. The current chapter presents an improved extraction method based on liquid-liquid extraction from a 50-mg aliquot of plant tissue for analysis of the major classes of phytohormones. Then, mass spectrometry (MS) analysis is conducted using quadrupole/linear ion trap (QLIT) mass analyzer equipped with electrospray ionization (ESI) source after a liquid chromatographic separation step. The developed method demonstrates an appropriate feasibility addressing biological questions related to phytohormones production and regulation.

Assessing Postgermination Development in Arabidopsis thaliana Under Abiotic Stress

The abscisic acid (ABA) phytohormone is well known to regulate responses to abiotic stress, particularly tolerance to osmotic stress. Screening for phenotypes at the early plant development stages is fundamental to identify new regulators of the ABA pathway, which in turn is extremely relevant for agriculture in a global climate change context. Typically, under experimental conditions, seeds are germinated in hormone-containing plates, and postgermination development is then assessed through scoring of the appearance of green or expanded cotyledons. However, postgermination phenotypes may be either masked or exacerbated by prior defects in seed germination. To circumvent this, we propose a transfer assay to screen specifically and quickly for postgermination phenotypes affected by exogenous ABA. The assay can be applied to different forms of abiotic stress, and we provide tips to score for postgermination phenotypes in genotypes exhibiting differential development.

Genetic Modification of Bergera koenigii for Expression of the Bacterial Pesticidal Protein Cry1Ba1

The curry leaf tree, Bergera koenigii, is highly attractive to the Asian citrus psyllid, Diaphorina citri, which vectors the bacterial causative agent of citrus greening or huanglongbing disease. This disease has decimated citrus production in Florida and in other citrus-producing countries. As D. citri exhibits high affinity for feeding on young leaves of B. koenigii, transgenic B. koenigii expressing bacteria-derived pesticidal proteins such as Cry1Ba1 have potential for D. citri management when planted in or adjacent to citrus groves. Importantly, the plant pathogenic bacterium that causes citrus greening does not replicate in B. koenigii. Transgenic plants of B. koenigii were produced by insertion of the gene encoding the active core of the pesticidal protein Cry1Ba1 derived from Bacillus thuringiensis. The transformation success rate was low relative to that of other citrus, at 0.89%. T-DNA integration into the genome and cry1ba1 transcription in transgenic plants were confirmed. Transgenic plants expressing Cry1Ba1 differed from wild-type plants, differed in photosynthesis parameters and hormone levels in some instances, and a marked delay in wilting of detached leaves. The gut epithelium of D. citri fed on transgenic plants was severely damaged, consistent with Cry1Ba1-mediated pore formation, confirming expression of the pesticidal protein by transgenic B. koenigii. These results demonstrate that transgenic B. koenigii expressing bacteria-derived pesticidal proteins can be produced for potential use as trap plants for suppression of D. citri populations toward protection of citrus groves from citrus greening.

RNA-Binding Proteins: The Key Modulator in Stress Granule Formation and Abiotic Stress Response

To cope with abiotic environmental stress, plants rapidly change their gene expression transcriptionally and post-transcriptionally, the latter by translational suppression of selected proteins and the assembly of cytoplasmic stress granules (SGs) that sequester mRNA transcripts. RNA-binding proteins (RBPs) are the major players in these post-transcriptional processes, which control RNA processing in the nucleus, their export from the nucleus, and overall RNA metabolism in the cytoplasm. Because of their diverse modular domain structures, various RBP types dynamically co-assemble with their targeted RNAs and interacting proteins to form SGs, a process that finely regulates stress-responsive gene expression. This review summarizes recent findings on the involvement of RBPs in adapting plants to various abiotic stresses via modulation of specific gene expression events and SG formation. The relationship of these processes with the stress hormone abscisic acid (ABA) is discussed.

Plant growth-promoting properties of bacterial endophytes isolated from roots of Thymus vulgaris L. and investigate their role as biofertilizers to enhance the essential oil contents

The main objective of the current study was to improve the essential oil contents of Thymus vulgaris L. using bio-inoculation with bacterial endophytes. Therefore, out of fourteen endophytic bacterial isolates obtained from roots of T. vulgaris, five isolates were selected based on the highest nitrogen-fixation and phosphate solubilization activity and identified as: Bacillus haynesii T9r, Citrobacter farmeri T10r, Bacillus licheniformis T11r, Bacillus velezensis T12r, and Bacillus velezensis T13r. These five strains have been recorded as ammonia, hydrogen cyanide (HCN), siderophores, and indole-3-acetic acid (IAA) producers. These strains have the efficacy to fix-nitrogen by reduction of acetylene with values of 82.133±1.4-346.6±1.4 n-mole-C₂H₄/ml/24 h. The IAA, gibberellic acid, abscisic acid, benzyl, kinten, and ziaten production were confirmed using HPLC. Two strains of T11r and T13r showed the highest plant growth-promoting properties and were selected for bio-inoculation of T. vulgaris individually or in a consortium with different mineral fertilization doses (0, 50, 75, and 100%) under field conditions. The highest growth performance was attained with the endophytic consortium (T11r+T13r) in the presence of 100% mineral fertilization. The GC-MS analysis of thyme oil contents showed the presence of 23 various compounds with varying percentages and the thymol fraction represented the highest percentages (39.1%) in the presence of the bacterial consortium.

To Fight or to Grow: The Balancing Role of Ethylene in Plant Abiotic Stress Responses

Plants often live in adverse environmental conditions and are exposed to various stresses, such as heat, cold, heavy metals, salt, radiation, poor lighting, nutrient deficiency, drought, or flooding. To adapt to unfavorable environments, plants have evolved specialized molecular mechanisms that serve to balance the trade-off between abiotic stress responses and growth. These mechanisms enable plants to continue to develop and reproduce even under adverse conditions. Ethylene, as a key growth regulator, is leveraged by plants to mitigate the negative effects of some of these stresses on plant development and growth. By cooperating with other hormones, such as jasmonic acid (JA), abscisic acid (ABA), brassinosteroids (BR), auxin, gibberellic acid (GA), salicylic acid (SA), and cytokinin (CK), ethylene triggers defense and survival mechanisms thereby coordinating plant growth and development in response to abiotic stresses. This review describes the crosstalk between ethylene and other plant hormones in tipping the balance between plant growth and abiotic stress responses.

Structural and Functional Insights into the Role of Guard Cell Ion Channels in Abiotic Stress-Induced Stomatal Closure

A stomatal pore is formed by a pair of specialized guard cells and serves as a major gateway for water transpiration and atmospheric CO₂ influx for photosynthesis in plants. These pores must be tightly controlled, as inadequate CO₂ intake and excessive water loss are devastating for plants. When the plants are exposed to extreme weather conditions such as high CO₂ levels, O₃, low air humidity, and drought, the turgor pressure of the guard cells exhibits an appropriate response against these stresses, which leads to stomatal closure. This phenomenon involves a complex network of ion channels and their regulation. It is well-established that the turgor pressure of guard cells is regulated by ions transportation across the membrane, such as anions and potassium ions. In this review, the guard cell ion channels are discussed, highlighting the structure and functions of key ion channels; the SLAC1 anion channel and KAT1 potassium channel, and their regulatory components, emphasizing their significance in guard cell response to various stimuli.

Molecular and Physiological Perspectives of Abscisic Acid Mediated Drought Adjustment Strategies

Drought is the most prevalent unfavorable condition that impairs plant growth and development by altering morphological, physiological, and biochemical functions, thereby impeding plant biomass production. To survive the adverse effects, water limiting condition triggers a sophisticated adjustment mechanism orchestrated mainly by hormones that directly protect plants via the stimulation of several signaling cascades. Predominantly, water deficit signals cause the increase in the level of endogenous ABA, which elicits signaling pathways involving transcription factors that enhance resistance mechanisms to combat drought-stimulated damage in plants. These responses mainly include stomatal closure, seed dormancy, cuticular wax deposition, leaf senescence, and alteration of the shoot and root growth. Unraveling how plants adjust to drought could provide valuable information, and a comprehensive understanding of the resistance mechanisms will help researchers design ways to improve crop performance under water limiting conditions. This review deals with the past and recent updates of ABA-mediated molecular mechanisms that plants can implement to cope with the challenges of drought stress.

Sorghum in dryland: morphological, physiological, and molecular responses of sorghum under drought stress

Droughts negatively affect sorghum's productivity and nutritional quality. Across its diversity centers, however, there exist resilient genotypes that function differently under drought stress at various levels, including molecular and physiological. Sorghum is an economically important and a staple food crop for over half a billion people in developing countries, mostly in arid and semi-arid regions where drought stress is a major limiting factor. Although sorghum is generally considered tolerant, drought stress still significantly hampers its productivity and nutritional quality across its major cultivation areas. Hence, understanding both the effects of the stress and plant response is indispensable for improving drought tolerance of the crop. This review aimed at enhancing our understanding and provide more insights on drought tolerance in sorghum as a contribution to the development of climate resilient sorghum cultivars. We summarized findings on the effects of drought on the growth and development of sorghum including osmotic potential that impedes germination process and embryonic structures, photosynthetic rates, and imbalance in source-sink relations that in turn affect seed filling often manifested in the form of substantial reduction in grain yield and quality. Mechanisms of sorghum response to drought-stress involving morphological, physiological, and molecular alterations are presented. We highlighted the current understanding about the genetic basis of drought tolerance in sorghum, which is important for maximizing utilization of its germplasm for development of improved cultivars. Furthermore, we discussed interactions of drought with other abiotic stresses and biotic factors, which may increase the vulnerability of the crop or enhance its tolerance to drought stress. Based on the research reviewed in this article, it appears possible to develop locally adapted cultivars of sorghum that are drought tolerant and nutrient rich using modern plant breeding techniques.