The nuclear interactor PYL8/RCAR3 of Fagus sylvatica FsPP2C1 is a positive regulator of abscisic acid signaling in seeds and stress

Reversible S-nitrosylation of bZIP67 by peroxiredoxin IIE activity and nitro-fatty acids regulates the plant lipid profile

Nitric oxide (NO) is a gasotransmitter required in a broad range of mechanisms controlling plant development and stress conditions. However, little is known about the specific role of this signaling molecule during lipid storage in the seeds. Here, we show that NO is accumulated in developing embryos and regulates the fatty acid profile through the stabilization of the basic/leucine zipper transcription factor bZIP67. NO and nitro-linolenic acid target and accumulate bZIP67 to induce the downstream expression of FAD3 desaturase, which is misregulated in a non-nitrosylable version of the protein. Moreover, the post-translational modification of bZIP67 is reversible by the trans-denitrosylation activity of peroxiredoxin IIE and defines a feedback mechanism for bZIP67 redox regulation. These findings provide a molecular framework to control the seed fatty acid profile caused by NO, and evidence of the in vivo functionality of nitro-fatty acids during plant developmental signaling.

Genome-wide identification of core components of ABA signaling and transcriptome analysis reveals gene circuits involved in castor bean (Ricinus communis L.) response to drought

Castor bean (Ricinus communis L.) can withstand long periods of water deficit and high temperatures, and therefore has been recognized as a drought-resistant plant species, allowing the study of gene networks involved in drought response and tolerance. The identification of genes networks related to drought response in this plant may yield important information in the characterization of molecular mechanisms correlating changes in the gene expression with the physiological adaptation processes. In this context, gene families related to abscisic acid (ABA) signaling play a crucial role in developmental and environmental adaptation processes of plants to drought stress. However, the families that function as the core components of ABA signaling, as well as genes networks related to drought response, are not well understood in castor bean. In this study 7 RcPYL, 63 RcPP2C, and 6 RcSnRK2 genes were identified in castor bean genome, which was further supported by chromosomal distribution, gene structure, evolutionary relationships, and conserved motif analyses. The castor bean general expression profile was investigated by RNAseq in root and leaf tissues in response to drought stress. These analyses allowed the identification of genes differentially expressed, including genes from the ABA signaling core, genes related to photosynthesis, cell wall, energy transduction, antioxidant response, and transcription factors. These analyses provide new insights into the core components of ABA signaling in castor bean, allow the identification of several molecular responses associated with the high physiological adaptation of castor bean to drought stress, and contribute to the identification of candidate genes for genetic improvement.

Genome-wide identification and characterization of ABA receptor pyrabactin resistance 1-like protein (PYL) family in oat

Abscisic acid (ABA) is a phytohormone that plays an important role in plant growth and development. Meanwhile, ABA also plays a key role in the plant response to abiotic stressors such as drought and high salinity. The pyrabactin resistance 1-like (PYR/PYL) protein family of ABA receptors is involved in the initial step of ABA signal transduction. However, no systematic studies of the PYL family in "Avena sativa, a genus Avena in the grass family Poaceae," have been conducted to date. Thus, in this study, we performed a genome-wide screening to identify PYL genes in oat and characterized their responses to drought stress. A total of 12 AsPYL genes distributed on nine chromosomes were identified. The phylogenetic analysis divided these AsPYLs into three subfamilies, based on structural and functional similarities. Gene and motif structure analysis of AsPYLs revealed that members of each subfamily share similar gene and motif structure. Segmental duplication appears to be the driving force for the expansion of PYLs, Furthermore, stress-responsive AsPYLs were detected through RNA-seq analysis. The qRT-PCR analysis of 10 AsPYL genes under drought, salt, and ABA stress revealed that AsPYL genes play an important role in stress response. These data provide a reference for further studies on the oat PYL gene family and its function.

PYL Family Genes from Liriodendron chinense Positively Respond to Multiple Stresses

The phytohormone abscisic acid (ABA) plays important roles in response to abiotic and biotic stresses in plants. Pyrabactin resistance 1-like (PYR/PYL) proteins are well-known as ABA receptors, which are responsible for ABA signal transduction. Nevertheless, the characteristics of PYL genes from Liriodendron chinense, an endangered timber tree, remain unclear in coping with various stresses. In this study, five PYLs were identified from the genome of Liriodendron chinense by sequence alignment and conserved motif analysis, which revealed that these LcPYLs contain a conserved gate and latch motif for ABA binding. The LcPYL promoters possess a series of cis-acting elements involved in response to various hormone and abiotic stresses. Moreover, the transcriptome data of Liriodendron hybrid leaves reveal that LcPYL genes specifically transcript under different abiotic stresses; Lchi11622 transcription was induced by drought and cold treatment, and Lchi01385 and Lchi16997 transcription was upregulated under cold and hot stress, respectively. Meanwhile, the LcPYLs with high expression levels shown in the transcriptomes were also found to be upregulated in whole plants treated with the same stresses tested by qPCR. Moreover, under biotic stress caused by scale insect and whitefly, Liriodendron hybrid leaves exhibited a distinct phenotype including disease spots that are dark green in the middle and yellow on the margin; the qPCR results showed that the relative expression levels of Lchi13641 and Lchi11622 in infected leaves were upregulated by 1.76 and 3.75 folds relative to normal leaves, respectively. The subcellular localizations of these stress-responsive LcPYLs were also identified in protoplasts of Liriodendron hybrid. These results provide a foundation to elucidate the function of PYLs from this elite tree species and assist in understanding the molecular mechanism of Liriodendron hybrid in dealing with abiotic and biotic stresses. In future research, the detailed biological function of LcPYLs and the genetic redundancy between LcPYLs can be explored by gene overexpression and knockout based on this study.

Dry side of the core: a meta-analysis addressing the original nature of the ABA signalosome at the onset of seed imbibition

The timing of seedling emergence is a major agricultural and ecological fitness trait, and seed germination is controlled by a complex molecular network including phytohormone signalling. One such phytohormone, abscisic acid (ABA), controls a large array of stress and developmental processes, and researchers have long known it plays a crucial role in repressing germination. Although the main molecular components of the ABA signalling pathway have now been identified, the molecular mechanisms through which ABA elicits specific responses in distinct organs is still enigmatic. To address the fundamental characteristics of ABA signalling during germination, we performed a meta-analysis focusing on the Arabidopsis dry seed proteome as a reflexion basis. We combined cutting-edge proteome studies, comparative functional analyses, and protein interaction information with genetic and physiological data to redefine the singular composition and operation of the ABA core signalosome from the onset of seed imbibition. In addition, we performed a literature survey to integrate peripheral regulators present in seeds that directly regulate core component function. Although this may only be the tip of the iceberg, this extended model of ABA signalling in seeds already depicts a highly flexible system able to integrate a multitude of information to fine-tune the progression of germination.

Wheat ABA Receptor TaPYL5 Constitutes a Signaling Module with Its Downstream Partners TaPP2C53/TaSnRK2.1/TaABI1 to Modulate Plant Drought Response

Abscisic acid receptors (ABR) play crucial roles in transducing the ABA signaling initiated by osmotic stresses, which has a significant impact on plant acclimation to drought by modulating stress-related defensive physiological processes. We characterized TaPYL5, a member of the ABR family in wheat (Triticum aestivum), as a mediator of drought stress adaptation in plants. The signals derived from the fusion of TaPYL5-GFP suggest that the TaPYL5 protein was directed to various subcellular locations, namely stomata, plasma membrane, and nucleus. Drought stress significantly upregulated the TaPYL5 transcripts in roots and leaves. The biological roles of ABA and drought responsive cis-elements, specifically ABRE and recognition sites MYB, in mediating gene transcription under drought conditions were confirmed by histochemical GUS staining analysis for plants harbouring a truncated TaPYL5 promoter. Yeast two-hybrid and BiFC assays indicated that TaPYL5 interacted with TaPP2C53, a clade A member of phosphatase (PP2C), and the latter with TaSnRK2.1, a kinase member of the SnRK2 family, implying the formation of an ABA core signaling module TaPYL5/TaPP2C53/TaSnRK2.1. TaABI1, an ABA responsive transcription factor, proved to be a component of the ABA signaling pathway, as evidenced by its interaction with TaSnRK2.1. Transgene analysis of TaPYL5 and its module partners, as well as TaABI1, revealed that they have an effect on plant drought responses. TaPYL5 and TaSnRK2.1 positively regulated plant drought acclimation, whereas TaPP2C53 and TaABI1 negatively regulated it. This coincided with the osmotic stress-related physiology shown in their transgenic lines, such as stomata movement, osmolytes biosynthesis, and antioxidant enzyme function. TaPYL5 significantly altered the transcription of numerous genes involved in biological processes related to drought defense. Our findings suggest that TaPYL5 is one of the most important regulators in plant drought tolerance and a valuable target for engineering drought-tolerant cultivars in wheat.

Transcriptome and metabolome profiling provide insights into hormone-mediated enhanced growth in autotetraploid seedlings of banana (Musa spp.)

Introduction

Reconstructive breeding based on autotetraploids to generate triploid varieties is a promising breeding strategy in banana (Musa spp.). Therefore understanding the molecular mechanisms underlying the phenotypic differences between the original diploid and its autopolyploid derivatives is of significant importance in such breeding programs of banana.

Methods

In this study, a number of non-chimeric autotetraploid plants, confirmed by flow cytometry and chromosome counting were obtained using colchicine treatment of ‘Pisang Berlin' (AA Group), a diploid banana cultivar highly resistant to Fusarium wilt Tropical Race 4 (Foc TR4) and widely cultivated in Asia.

Results and discussion

The autotetraploids showed significant increase in plant height, pseudostem diameter, root length, leaf thickness, leaf area, and leaf chlorophyll content. Transcriptomic analysis indicated that differentially expressed genes were mainly enriched in plant hormone signal transduction, mitogen-activated protein kinase (MAPK) signaling pathway, and carbon fixation in photosynthetic organelles. The genes related to the metabolism, transport or signaling of auxin, abscisic acid (ABA), cytokinin (CTK) and gibberellin (GA), as well as the genes encoding essential enzymes in photosynthetic CO2 fixation were differentially expressed in leaves of autotetraploids and most of them were up-regulated. Metabolomic analysis revealed that the differentially accumulated metabolites were mainly involved in plant hormone signal transduction, porphyrin and chlorophyll metabolism, indole alkaloid biosynthesis, and carbon fixation in photosynthetic organelles. The results therefore, demonstrate that the hormones IAA, ABA, and photosynthetic regulation may play a vital role in the observed enhancement in the autotetraploids. These could be used as molecular and biochemical markers to facilitate the generation of triploid progenies as suitable new varieties for cultivation.

Tartary buckwheat FtMYB30 transcription factor improves the salt/drought tolerance of transgenic Arabidopsis in an ABA-dependent manner

Drought and high salinity affect plant growth, development, yield, and quality. MYB transcription factors (TFs) in plants play an indispensable regulatory role in resisting adverse stress. In this study, screening and functional validation of the TF FtMYB30, which can respond extensively to abiotic stress and abscisic acid (ABA), was achieved in Tartary buckwheat. FtMYB30, one of the SG22 (sub-group 22) family of R2R3-MYB TFs, localized in the nucleus and had transcriptional activation activity. Under drought and salt stress, FtMYB30 overexpression reduced the oxidative damage in transgenic plants by increasing the activity of proline content and antioxidant enzymes and significantly upregulate the expression of RD29A, RD29B, and Cu/ZnSOD, thereby enhancing drought/salt tolerance in transgenic Arabidopsis. Additionally, overexpression of FtMYB30 can reduce the sensitivity of transgenic plants to ABA. Moreover, AtRCAR1/2/3 and AtMPK6 directly interact with the FtMYB30 TF, possibly through the crosstalk between MAPKs (mitogen-activated protein kinases) and the ABA signaling pathway. Taken together, these results suggest that FtMYB30, as a positive regulator, mediates plant tolerance to salt and drought through an ABA-dependent signaling pathway.

Overexpression VaPYL9 improves cold tolerance in tomato by regulating key genes in hormone signaling and antioxidant enzyme

BACKGROUND

Abscisic acid (ABA) has been reported in controlling plant growth and development, and particularly dominates a role in resistance to abiotic stress. The Pyrabactin Resistance1/PYR1-Like /Regulatory Components of ABA receptors (PYR1/PYL/RCAR) gene family, of which the PYL9 is a positive regulator related to stress response in ABA signaling transduction. Although the family has been identified in grape, detailed VaPYL9 function in cold stress remains unknown.

RESULTS

In order to explore the cold tolerance mechanism in grape, VaPYL9 was cloned from Vitis amurensis. The subcellular localization showed that VaPYL9 was mainly expressed in the plasma membrane. Yeast two-hybrid (Y2H) showed VaPCMT might be a potential interaction protein of VaPYL9. Through the overexpression of VaPYL9 in tomatoes, results indicated transgenic plants had higher antioxidant enzyme activities and proline content, lower malondialdehyde (MDA) and H₂O₂ content, and improving the ability to scavenge reactive oxygen species than wild-type (WT). Additionally, ABA content and the ratio of ABA/IAA kept a higher level than WT. Quantitative real-time PCR (qRT-PCR) showed that VaPYL9, SlNCED3, SlABI5, and antioxidant enzyme genes (POD, SOD, CAT) were up-regulated in transgenic tomatoes. Transcriptome sequencing (RNA-seq) found that VaPYL9 overexpression caused the upregulation of key genes PYR/PYL, PYL4, MAPK17/18, and WRKY in transgenic tomatoes under cold stress.

CONCLUSION

Overexpression VaPYL9 enhances cold resistance of transgenic tomatoes mediated by improving antioxidant enzymes activity, reducing membrane damages, and regulating key genes in plant hormones signaling and antioxidant enzymes.

Genome-wide identification and comparative analysis of the PYL gene family in eight Rosaceae species and expression analysis of seeds germination in pear

Abscisic acid (ABA) is a phytohormone that plays important roles in the regulation of plant growth, seed germination, and stress responses. The pyrabactin resistance 1-like (PYR/PYL) protein, an ABA receptor, was involved in the initial step in ABA signal transduction. However, the evolutionary history and characteristics of PYL genes expression remain unclear in Chinese white pear (Pyrus bretschneideri) or other Rosaceae species. In this study, 67 PYL genes were identified in eight Rosaceae species, and have been classified into three subgroups based on specific motifs and phylogenetic analysis. Intriguingly, we observed that whole-genome duplication (WGD) and dispersed duplication (DSD) have a major contribution to PYL family expansion. Purifying selection was the major force in PYL genes evolution. Expression analysis finds that PYL genes may function in multiple pear tissues. qRT-PCR validation of 11 PbrPYL genes indicates their roles in seed germination and abiotic stress responses. Our study provides a basis for further elucidation of the function of PYL genes and analysis of their expansion, evolution and expression patterns, which helps to understand the molecular mechanism of pear response to seed germination and seedling abiotic stress.

DNA and Histone Methylation Regulates Different Types of Fruit Ripening by Transcriptome and Proteome Analyses

Methylation affects different aspects of genetic material stability, gene expression regulation, and histone modification. The previous reports depicted that DNA and histone methylation regulates plant growth and development. In this study, we evaluated the effects of DNA and histone methylation on 'Hongjia' strawberry and 'Lichun' tomato. We investigated the transient transformation system for arginine methyltransferase (FvPRMT1.5) overexpression and interference and assessed the phenotypic appearance and mRNA and protein expression levels. Results depicted that changes in methylation levels caused inhibition of carotenoids and anthocyanins. Furthermore, the profiling of aroma components was altered in response to 5-azacytidine. DNA hypomethylation induced the expression levels of genes involved in photosynthesis, flavonoid biosynthesis, and hormone signal transduction pathways, while the expression levels of related proteins showed a downward trend. Overall, we proposed a model that reveals the possible regulatory effects of DNA and histone methylation during fruit ripening.

Mechanisms of Abscisic Acid-Mediated Drought Stress Responses in Plants

Drought is one of the major constraints to rain-fed agricultural production, especially under climate change conditions. Plants evolved an array of adaptive strategies that perceive stress stimuli and respond to these stress signals through specific mechanisms. Abscisic acid (ABA) is a premier signal for plants to respond to drought and plays a critical role in plant growth and development. ABA triggers a variety of physiological processes such as stomatal closure, root system modulation, organizing soil microbial communities, activation of transcriptional and post-transcriptional gene expression, and metabolic alterations. Thus, understanding the mechanisms of ABA-mediated drought responses in plants is critical for ensuring crop yield and global food security. In this review, we highlighted how plants adjust ABA perception, transcriptional levels of ABA- and drought-related genes, and regulation of metabolic pathways to alter drought stress responses at both cellular and the whole plant level. Understanding the synergetic role of drought and ABA will strengthen our knowledge to develop stress-resilient crops through integrated advanced biotechnology approaches. This review will elaborate on ABA-mediated drought responses at genetic, biochemical, and molecular levels in plants, which is critical for advancement in stress biology research.

Genome-wide identification of ABA receptor PYL/RCAR gene family and their response to cold stress in Medicago sativa L

Abscisic acid (ABA) is an important phytohormone involved in plant growth, plant development, and the protection of plants against abiotic stresses. PYL/RCAR (pyrabactin resistance/pyr1-like/regulatory components of ABA receptor) is the receptor protein of ABA and the core component of the ABA signal transduction network. The PYL gene family has been identified and analyzed in many species, however, there is no report about the research on the whole genome-wide identification of the alfalfa (Medicago sativa L.) PYL gene family. Therefore, to explore the function of alfalfa PYL genes, 39 MsPYL genes were identified by analyzing the recently published genome of alfalfa. Using bioinformatics methods, we systematically analyzed the chromosome location, protein physicochemical properties, evolutionary relationship, conserved motifs, and response to low-temperature stress of the MsPYL family of alfalfa. The results showed that 39 alfalfa MsPYL genes were distributed on 24 chromosomes, and the analysis of gene duplication events showed that fragment duplication was predominant duplication in alfalfa MsPYL family gene expansion. The phylogenetic tree of MsPYL protein of alfalfa and the phylogenetic tree of PYL genes of 3 species show that the MsPYL gene family can be divided into 3 subfamilies, and the structures of the same subfamilies are relatively similar. The 39 MsPYL gene family members of alfalfa contain 10 Motifs. Motif1, Motif2, Motif3, and Motif5 are the conserved motifs shared by these genes; cis-regulatory elements in promoter regions indicate that regulatory elements related to transcription, cell cycle, development, hormone, and stress response are abundantly present in the MsPYL promoter sequences; Real-time fluorescence quantitative PCR analysis showed that the expression of MsPYL genes can be induced by low-temperature treatment. This study provides a reference for further exploring the structural and functional characterization of the alfalfa PYL gene family.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01066-3.

New Abscisic Acid Derivatives Revealed Adequate Regulation of Stomatal, Transcriptional, and Developmental Responses to Conquer Drought

The phytohormone abscisic acid (ABA) plays an important role in plant stress response, mainly against desiccation. Hence, ABA receptor agonists may function as agents to enhance drought tolerance in crops. ABA exhibits diverse functions that impact plant development and are regulated by various ABA receptor subfamilies. Indeed, we previously reported that 3'-alkyl ABAs exhibit diverse receptor specificities and that 3'-butyl ABA induced a drought stress response without eliciting growth inhibitory effects in Arabidopsis seedlings. Thus, to further investigate plant responses induced by 3'-butyl ABA, as well as the receptors that control the opposing stress and growth responses, we designed new 3'-alkyl ABA derivatives. In addition to the 3'-alkyl chain, a cyclopropyl group was attached to position 3 of ABA to occupy the C6 cleft in the ABA-binding pocket of the receptors, which served to increase the binding affinity and specificity to a certain receptor set. Additionally, the inhibitory activity of pyrabactin resistance 1 (PYR1) and PYR1-like (PYL1) proteins against type 2C protein phosphatase increased following incorporation of the 3-cyclopropyl group in all tested 3'-alkyl ABAs. Interestingly, 3'-butyl ABA induced the highest tolerance against drought stress, compared with 3-cyclopropyl derivatives. To investigate the molecular mechanism underlying the effects elicited by different chemical treatments, those of ABA derivatives on stomatal closure, growth, and gene expression were studied. Evaluation of the receptors activated by ABA derivatives and the plant responses revealed the induction of PYR1, PYL1, PYL2, and PYL5, mediated stomatal closure, and regulated transcription, consequently leading to drought tolerance in plants.

Redox feedback regulation of ANAC089 signaling alters seed germination and stress response

The interplay between the phytohormone abscisic acid (ABA) and the gasotransmitter nitric oxide (NO) regulates seed germination and post-germinative seedling growth. We show that GAP1 (germination in ABA and cPTIO 1) encodes the transcription factor ANAC089 with a critical membrane-bound domain and extranuclear localization. ANAC089 mutants lacking the membrane-tethered domain display insensitivity to ABA, salt, and osmotic and cold stresses, revealing a repressor function. Whole-genome transcriptional profiling and DNA-binding specificity reveals that ANAC089 regulates ABA- and redox-related genes. ANAC089 truncated mutants exhibit higher NO and lower ROS and ABA endogenous levels, alongside an altered thiol and disulfide homeostasis. Consistently, translocation of ANAC089 to the nucleus is directed by changes in cellular redox status after treatments with NO scavengers and redox-related compounds. Our results reveal ANAC089 to be a master regulator modulating redox homeostasis and NO levels, able to repress ABA synthesis and signaling during Arabidopsis seed germination and abiotic stress.

The abscisic acid receptor OsPYL6 confers drought tolerance to indica rice through dehydration avoidance and tolerance mechanisms

Abscisic acid (ABA) is a key regulator of plant development and stress tolerance. Here we report functional validation of the ABA receptor OsPYL6 by constitutive and stress-inducible overexpression and RNAi silencing, in an indica rice cultivar 'Pusa Sugandh 2'. Overexpression of OsPYL6 conferred ABA hypersensitivity during germination and promoted total root length. Overexpression and RNAi silencing of OsPYL6 resulted in enhanced accumulation of ABA in seedlings under non-stress conditions, at least, in part through up-regulation of different 9-cis epoxycarotenoid dioxygenase (NCED )genes. This suggests that PYL6 expression is crucial for ABA homeostasis. Analysis of drought tolerance of OsPYL6 transgenic and wild type plants showed that OsPYL6 overexpression enhanced the expression of stress-responsive genes and dehydration tolerance. Transgenic rice plants overexpressing OsPYL6 with AtRD29A (Arabidopsis thaliana Responsive to Dehydration 29A) promoter also exhibited about 25% less whole plant transpiration, compared with wild type plants under drought, confirming its role in activation of dehydration avoidance mechanisms. However, overexpression of PYL6 reduced grain yield under non-stress conditions due to reduction in height, biomass, panicle branching and spikelet fertility. RNAi silencing of OsPYL6 also reduced grain yield under drought. These results showed that rice OsPYL6 is a key regulator of plant development and drought tolerance, and fine-tuning of its expression is critical for improving yield and stress tolerance.

Transcriptional Regulation of Protein Phosphatase 2C Genes to Modulate Abscisic Acid Signaling

The plant hormone abscisic acid (ABA) triggers cellular tolerance responses to osmotic stress caused by drought and salinity. ABA controls the turgor pressure of guard cells in the plant epidermis, leading to stomatal closure to minimize water loss. However, stomatal apertures open to uptake CO₂ for photosynthesis even under stress conditions. ABA modulates its signaling pathway via negative feedback regulation to maintain plant homeostasis. In the nuclei of guard cells, the clade A type 2C protein phosphatases (PP2Cs) counteract SnRK2 kinases by physical interaction, and thereby inhibit activation of the transcription factors that mediate ABA-responsive gene expression. Under osmotic stress conditions, PP2Cs bind to soluble ABA receptors to capture ABA and release active SnRK2s. Thus, PP2Cs function as a switch at the center of the ABA signaling network. ABA induces the expression of genes encoding repressors or activators of PP2C gene transcription. These regulators mediate the conversion of PP2C chromatins from a repressive to an active state for gene transcription. The stress-induced chromatin remodeling states of ABA-responsive genes could be memorized and transmitted to plant progeny; i.e., transgenerational epigenetic inheritance. This review focuses on the mechanism by which PP2C gene transcription modulates ABA signaling.

Genome-Wide Identification of the ABA Receptors Genes and Their Response to Abiotic Stress in Apple

The pyrabactin resistance (PYR)/PYR1-like (PYL)/regulatory components of ABA receptor (RCAR) (known as PYLs for short) have been identified and characterized as the ABA receptors in some plants. However, little is known about the details regarding PYL family genes in the apple (Malusdomestica). In this study, we identified 13 apple PYLs, termed MdPYL1-13, which could be classified into four groups according to structural features of the amino acid sequence. The gene structures and conserved motifs analysis found that the majority of MdPYLs had a similar number of exons and similar conserved motif profile in the same group. In addition, 11 gene pairs were identified to exhibit synteny by synteny analysis between the apple and Arabidopsis. Furthermore, we investigated MdPYLs transcript level in various organs of the red-fleshed apple (Malussieversii f. Neidzwetzkyana (Dieck) Langenf) ‘Xinjiang No.1’. The results suggested all MdPYLs within group I were expressed at relatively higher levels in all of the organs tested. However, the genes of group IV had little or no variation. Additionally, we found various hormone and stress-related cis-elements in the promoters of MdPYLs by analyzing cis-elements. Therefore, the expression levels of all MdPYLs were further detected under ABA, PEG, salt, and cold stresses in ‘Xinjiang No.1’ seedlings. We found that all MdPYLs except for MdPYL11 were upregulated by ABA treatment, 10 genes were upregulated by PEG treatment, 12 genes were upregulated by NaCl treatment, and six genes were upregulated by cold treatment (4 °C) while seven genes were downregulated. Thus, these MdPYLs might be involved in the defense against abiotic stresses. In addition, the interaction between 13 MdPYLs and two 2C protein phosphatases in the apple (MdPP2C65 and MdPP2C72) was investigated in yeast two-hybrid assays. These results suggested that MdPYLs may bind to MdPP2C65 and MdPP2C72 in different manners and with different intensity. Our studies provide useful information for further investigating and researching the regulatory mechanisms of PYL family genes in response to abiotic stresses in the apple.

Seed comparative genomics in three coffee species identify desiccation tolerance mechanisms in intermediate seeds

In contrast to desiccation-tolerant 'orthodox' seeds, so-called 'intermediate' seeds cannot survive complete drying and are short-lived. All species of the genus Coffea produce intermediate seeds, but they show a considerable variability in seed desiccation tolerance (DT), which may help to decipher the molecular basis of seed DT in plants. We performed a comparative transcriptome analysis of developing seeds in three coffee species with contrasting desiccation tolerance. Seeds of all species shared a major transcriptional switch during late maturation that governs a general slow-down of metabolism. However, numerous key stress-related genes, including those coding for the late embryogenesis abundant protein EM6 and the osmosensitive calcium channel ERD4, were up-regulated during DT acquisition in the two species with high seed DT, C. arabica and C. eugenioides. By contrast, we detected up-regulation of numerous genes involved in the metabolism, transport, and perception of auxin in C. canephora seeds with low DT. Moreover, species with high DT showed a stronger down-regulation of the mitochondrial machinery dedicated to the tricarboxylic acid cycle and oxidative phosphorylation. Accordingly, respiration measurements during seed dehydration demonstrated that intermediate seeds with the highest DT are better prepared to cease respiration and avoid oxidative stresses.

Abscisic acid dynamics, signaling, and functions in plants

Abscisic acid (ABA) is an important phytohormone regulating plant growth, development, and stress responses. It has an essential role in multiple physiological processes of plants, such as stomatal closure, cuticular wax accumulation, leaf senescence, bud dormancy, seed germination, osmotic regulation, and growth inhibition among many others. Abscisic acid controls downstream responses to abiotic and biotic environmental changes through both transcriptional and posttranscriptional mechanisms. During the past 20 years, ABA biosynthesis and many of its signaling pathways have been well characterized. Here we review the dynamics of ABA metabolic pools and signaling that affects many of its physiological functions.

Analysis of differential gene expression in cold-tolerant vs. cold-sensitive varieties of snap bean (Phaseolus vulgaris L.) in response to low temperature stress

BACKGROUND

Snap bean, Phaseolus vulgaris L., as a warm-season vegetable, low temperature stress seriously affect the yield and quality. At present, little is known about the genes and molecular regulation mechanism in cold response in snap bean exposed to low temperature.

OBJECTIVES

Our objectives were to identify the low temperature response genes in snap bean and to examine differences in the gene response between cold-tolerant and cold-sensitive genotypes.

METHODS

We used two highly inbred snap bean lines in this study, the cold-tolerant line '120', and the cold-sensitive line '093'. The plants were grown to the three leaf and one heart stage and exposed to 4 °C low temperature. We used RNA sequencing (RNA-seq) to analyze the differences of gene expression.

RESULTS

988 and 874 cold-responsive genes were identified in 'T120 vs CK120' and 'T093 vs CK093' ('T' stands for low temperature treatment, and 'CK' stands for control at room temperature), respectively. Of these, 555 and 442 genes were unique to cold-stressed lines '120' and '093', respectively compared to the control. Our analysis of these differentially expressed genes indicates that Ca²⁺, ROS, and hormones act as signaling molecules that play important roles in low temperature response in P. vulgaris. Altering the expression of genes in these signaling pathways activates expression of downstream response genes which can interact with other signaling regulatory networks. This may maintained the balance of ROS and hormones, making line '120' more cold-tolerant than line '093'.

CONCLUSION

Our results provide a preliminarily understanding of the molecular basis of low temperature response in snap bean, and also establish a foundation for the future genetic improvement of cold sensitivity in snap bean by incorporating genes for cold tolerance.

Over-expression of an R2R3 MYB Gene, GhMYB73, increases tolerance to salt stress in transgenic Arabidopsis

MYB family genes act as important regulators modulating the response to abiotic stress in plants. However, much less is known about MYB proteins in cotton. Here, we found that a cotton MYB gene, GhMYB73, was induced by NaCl and abscisic acid (ABA). Silencing GhMYB73 expression in cotton increased sensitivity to salt stress. The cotyledon greening rate of Arabidopsis thaliana over-expressing GhMYB73 under NaCl or mannitol treatment was significantly enhanced during the seedling germination stage. What's more, several osmotic stress-induced genes, such as AtNHX1, AtSOS3 and AtP5CS1, were more highly induced in the over-expression lines than in wild type under salt treatment, supporting the hypothesis that GhMYB73 contributes to salinity tolerance by improving osmotic stress resistance. Arabidopsis lines over-expressing GhMYB73 had superior germination and cotyledon greening under ABA treatment, and some abiotic stress-induced genes involved in ABA pathways (AtPYL8, AtABF3, AtRD29B and AtABI5), had increased transcription levels under salt-stress conditions in these lines. Furthermore, we found that GhMYB73 physically interacts with GhPYL8 and AtPYL8, suggesting that GhMYB73 regulates ABA signaling during salinity stress response. Taken together, over-expression of GhMYB73 significantly increases tolerance to salt and ABA stress, indicating that it can potentially be used in transgenic technology approaches to improve cotton salt tolerance.

The wheat TabZIP2 transcription factor is activated by the nutrient starvation-responsive SnRK3/CIPK protein kinase

The understanding of roles of bZIP factors in biological processes during plant development and under abiotic stresses requires the detailed mechanistic knowledge of behaviour of TFs. Basic leucine zipper (bZIP) transcription factors (TFs) play key roles in the regulation of grain development and plant responses to abiotic stresses. We investigated the role and molecular mechanisms of function of the TabZIP2 gene isolated from drought-stressed wheat plants. Molecular characterisation of TabZIP2 and derived protein included analyses of gene expression and its target promoter, and the influence of interacting partners on the target promoter activation. Two interacting partners of TabZIP2, the 14-3-3 protein, TaWIN1 and the bZIP transcription factor TaABI5L, were identified in a Y2H screen. We established that under elevated ABA levels the activity of TabZIP2 was negatively regulated by the TaWIN1 protein and positively regulated by the SnRK3/CIPK protein kinase WPK4, reported previously to be responsive to nutrient starvation. The physical interaction between the TaWIN1 and the WPK4 was detected. We also compared the influence of homo- and hetero-dimerisation of TabZIP2 and TaABI5L on DNA binding. TabZIP2 gene functional analyses were performed using drought-inducible overexpression of TabZIP2 in transgenic wheat. Transgenic plants grown under moderate drought during flowering, were smaller than control plants, and had fewer spikes and seeds per plant. However, a single seed weight was increased compared to single seed weights of control plants in three of four evaluated transgenic lines. The observed phenotypes of transgenic plants and the regulation of TabZIP2 activity by nutrient starvation-responsive WPK4, suggest that the TabZIP2 could be the part of a signalling pathway, which controls the rearrangement of carbohydrate and nutrient flows in plant organs in response to drought.

Identification of Blueberry miRNAs and Their Targets Based on High-Throughput Sequencing and Degradome Analyses

miRNAs are important regulators of plant gene expression. To better characterize their functions, we applied high-throughput sequencing and degradome analyses to investigate three blueberry (Vaccinium ashei) tissues. A total of 127 known and 101 novel miRNAs were identified. Moreover, 141 targets for 42 known and 19 novel miRNAs were experimentally validated by degradome sequencing. A functional analysis of these miRNA targets revealed they were associated with diverse biological activities and several pathways, e.g., anthocyanin biosynthesis and cytokinin signal transduction. The data presented herein expand our understanding of the regulation of blueberry miRNAs during floral and fruit development stages. They may also provide new insights into the roles of miRNAs during anthocyanin biosynthesis in blueberry fruits.

Interaction network of core ABA signaling components in maize

We defined a comprehensive core ABA signaling network in monocot maize, including the gene expression, subcellular localization and interaction network of ZmPYLs, ZmPP2Cs, ZmSnRK2s and the putative substrates. The phytohormone abscisic acid (ABA) plays an important role in plant developmental processes and abiotic stress responses. In Arabidopsis, ABA is sensed by the PYL ABA receptors, which leads to binding of the PP2C protein phosphatase and activation of the SnRK2 protein kinases. These components functioning diversely and redundantly in ABA signaling are little known in maize. Using Arabidopsis pyl112458 and snrk2.2/3/6 mutants, we identified several ABA-responsive ZmPYLs and ZmSnRK2s, and also ZmPP2Cs. We showed the gene expression, subcellular localization and interaction network of ZmPYLs, ZmPP2Cs, and ZmSnRK2s, and the isolation of putative ZmSnRK2 substrates by mass spectrometry in monocot maize. We found that the ABA dependency of PYL-PP2C interactions is contingent on the identity of the PP2Cs. Among 238 candidate substrates for ABA-activated protein kinases, 69 are putative ZmSnRK2 substrates. Besides homologs of previously reported putative AtSnRK2 substrates, 23 phosphoproteins have not been discovered in the dicot Arabidopsis. Thus, we have defined a comprehensive core ABA signaling network in monocot maize and shed new light on ABA signaling.

Regulation of Seed Germination and Abiotic Stresses by Gibberellins and Abscisic Acid

Overall growth and development of a plant is regulated by complex interactions among various hormones, which is critical at different developmental stages. Some of the key aspects of plant growth include seed development, germination and plant survival under unfavorable conditions. Two of the key phytohormones regulating the associated physiological processes are gibberellins (GA) and abscisic acid (ABA). GAs participate in numerous developmental processes, including, seed development and seed germination, seedling growth, root proliferation, determination of leaf size and shape, flower induction and development, pollination and fruit expansion. Despite the association with abiotic stresses, ABA is essential for normal plant growth and development. It plays a critical role in different abiotic stresses by regulating various downstream ABA-dependent stress responses. Plants maintain a balance between GA and ABA levels constantly throughout the developmental processes at different tissues and organs, including under unfavorable environmental or physiological conditions. Here, we will review the literature on how GA and ABA control different stages of plant development, with focus on seed germination and selected abiotic stresses. The possible crosstalk of ABA and GA in specific events of the above processes will also be discussed, with emphasis on downstream stress signaling components, kinases and transcription factors (TFs). The importance of several key ABA and GA signaling intermediates will be illustrated. The knowledge gained from such studies will also help to establish a solid foundation to develop future crop improvement strategies.

Screening and identification of key genes regulating fall dormancy in alfalfa leaves

Fall dormancy (FD) determines the adaptation of an alfalfa variety and affects alfalfa production and quality. However, the molecular mechanism underlying FD remains poorly understood. Here, 44 genes regulating FD were identified by comparison of the transcriptomes from leaves of Maverick (fall-dormant alfalfa) and CUF101(non-fall-dormant), during FD and non-FD and were classified them depending on their function. The transcription of IAA-amino acid hydrolase ILR1-like 1, abscisic acid receptor PYL8, and monogalactosyldiacylglycerol synthase-3 in Maverick leaves was regulated by daylength and temperature, and the transcription of the abscisic acid receptor PYL8 was mainly affected by daylength. The changes in the expression of these genes and the abundance of their messenger RNA (mRNA) in Maverick leaves differed from those in CUF101 leaves, as evidenced by the correlation analysis of their mRNA abundance profiles obtained from April to October. The present findings suggested that these genes are involved in regulating FD in alfalfa.

Genome-wide identification of ABA receptor PYL family and expression analysis of PYLs in response to ABA and osmotic stress in Gossypium

Abscisic acid (ABA) receptor pyrabactin resistance1/PYR1-like/regulatory components of ABA receptor (PYR1/PYL/RCAR) (named PYLs for simplicity) are core regulators of ABA signaling, and have been well studied in Arabidopsis and rice. However, knowledge is limited about the PYL family regarding genome organization, gene structure, phylogenesis, gene expression and protein interaction with downstream targets in Gossypium. A comprehensive analysis of the Gossypium PYL family was carried out, and 21, 20, 40 and 39 PYL genes were identified in the genomes from the diploid progenitor G. arboretum, G. raimondii and the tetraploid G. hirsutum and G. barbadense, respectively. Characterization of the physical properties, chromosomal locations, structures and phylogeny of these family members revealed that Gossypium PYLs were quite conservative among the surveyed cotton species. Segmental duplication might be the main force promoting the expansion of PYLs, and the majority of the PYLs underwent evolution under purifying selection in Gossypium. Additionally, the expression profiles of GhPYL genes were specific in tissues. Transcriptions of many GhPYL genes were inhibited by ABA treatments and induced by osmotic stress. A number of GhPYLs can interact with GhABI1A or GhABID in the presence and/or absence of ABA by the yeast-two hybrid method in cotton.

The spatio-temporal specificity of PYR1/PYL/RCAR ABA receptors in response to developmental and environmental cues

From the different functions ABA exerted between the aboveground and belowground, seed and vegetative tissues, primary root and lateral root, stimulating stomatal closure and inhibiting stomatal opening, between young and senescence leaves in stomatal movement, among different cells in plasma membrane water permeability, we addressed the organ-, tissue-, cell-, physiological processes-, and development stage specificities of PYR1/PYL/RCAR ABA receptors. This specificity may reflect the spatio-temporal properties of water potentials as well as the endogenous ABA levels in detail context, which plus the various affinities among this receptor families, resulted in the specificity of the transcripts as well as genes functions. PYR1/PYL/RCAR ABA receptors may integrate the message of ABA resource (local signaling or long distance signaling) and concentration, thus fine-tuning ABA response to environmental- and developmental cues. It also evolutionally affording land plants sophisticated mechanism to survival adverse environments.

The core regulatory network of the abscisic acid pathway in banana: genome-wide identification and expression analyses during development, ripening, and abiotic stress

BACKGROUND

Abscisic acid (ABA) signaling plays a crucial role in developmental and environmental adaptation processes of plants. However, the PYL-PP2C-SnRK2 families that function as the core components of ABA signaling are not well understood in banana.

RESULTS

In the present study, 24 PYL, 87 PP2C, and 11 SnRK2 genes were identified from banana, which was further supported by evolutionary relationships, conserved motif and gene structure analyses. The comprehensive transcriptomic analyses showed that banana PYL-PP2C-SnRK2 genes are involved in tissue development, fruit development and ripening, and response to abiotic stress in two cultivated varieties. Moreover, comparative expression analyses of PYL-PP2C-SnRK2 genes between BaXi Jiao (BX) and Fen Jiao (FJ) revealed that PYL-PP2C-SnRK2-mediated ABA signaling might positively regulate banana fruit ripening and tolerance to cold, salt, and osmotic stresses. Finally, interaction networks and co-expression assays demonstrated that the core components of ABA signaling were more active in FJ than in BX in response to abiotic stress, further supporting the crucial role of the genes in tolerance to abiotic stress in banana.

CONCLUSIONS

This study provides new insights into the complicated transcriptional control of PYL-PP2C-SnRK2 genes, improves the understanding of PYL-PP2C-SnRK2-mediated ABA signaling in the regulation of fruit development, ripening, and response to abiotic stress, and identifies some candidate genes for genetic improvement of banana.

Endosperm-specific OsPYL8 and OsPYL9 act as positive regulators of the ABA signaling pathway in rice seed germination

Pyrabactin resistance-like (PYL) proteins were identified as receptors of the plant hormone ABA. The PYL family consists of multiple members that are differently expressed in various tissues, exhibit distinct biochemical properties and have diverse biological functions. In the present study, we explored the expression patterns of the rice (Oryza sativa L.) OsPYL family genes and determined that OsPYL8 and OsPYL9 are specifically expressed in the endosperms. Sequence analysis and deletion experiments revealed that the OsPYL8 and OsPYL9 promoters contain multiple motifs involved in endosperm-specific expression. Transgenic rice plants overexpressing OsPYL8 or OsPYL9 showed hypersensitivity to ABA during seed germination, suggesting that both OsPYL8 and OsPYL9 act as positive regulators of the ABA signalling pathway in the seed. OsPYL8 and OsPYL9 interact with OsPP2C51 and OsPP2C68, whose expression is induced in the endosperms by ABA. Our results provided a foundation for future studies on OsPYL8- and OsPYL9-mediated ABA signalling in the rice endosperms.

Overexpression of Pyrabactin Resistance-Like Abscisic Acid Receptors Enhances Drought, Osmotic, and Cold Tolerance in Transgenic Poplars

Abscisic acid (ABA) has been known participate in a wider range of adaptive responses to diverse environmental abiotic stresses such as drought, osmosis, and low temperatures. ABA signaling is initiated by its receptors PYR/PYL/RCARs, a type of soluble proteins with a conserved START domain which can bind ABA and trigger the downstream pathway. Previously, we discovered that poplar (Populus trichocarpa) genome encodes 14 PYR/PYL/RCAR orthologs (PtPYRLs), and two of them, PtPYRL1 and PtPYRL5 have been functionally characterized to positively regulate drought tolerance. However, the physiological function of these ABA receptors in poplar remains uncharacterized. Here, we generated transgenic poplar plants overexpressing PtPYRL1 and PtPYRL5 and found that they exhibited more vigorous growth and produced greater biomass when exposed to drought stress. The improved drought tolerance was positively correlated with the key physiological responses dictated by the ABA signaling pathway, including increase in stomatal closure and decrease in leaf water loss. Further analyses revealed that overexpression lines showed improved capacity in scavenging reactive oxygen species and enhanced the activation of antioxidant enzymes under drought stress. Moreover, overexpression of PtPYRL1 or PtPYRL5 significantly increased the poplar resistance to osmotic and cold stresses. In summary, our results suggest that constitutive expression of PtPYRL1 and PtPYRL5 significantly enhances the resistance to drought, osmotic and cold stresses by positively regulating ABA signaling in poplar.

Abscisic-acid-dependent basic leucine zipper (bZIP) transcription factors in plant abiotic stress

One of the major causes of significant crop loss throughout the world is the myriad of environmental stresses including drought, salinity, cold, heavy metal toxicity, and ultraviolet-B (UV-B) rays. Plants as sessile organisms have evolved various effective mechanism which enable them to withstand this plethora of stresses. Most of such regulatory mechanisms usually follow the abscisic-acid (ABA)-dependent pathway. In this review, we have primarily focussed on the basic leucine zipper (bZIP) transcription factors (TFs) activated by the ABA-mediated signalosome. Upon perception of ABA by specialized receptors, the signal is transduced via various groups of Ser/Thr kinases, which phosphorylate the bZIP TFs. Following such post-translational modification of TFs, they are activated so that they bind to specific cis-acting sequences called abscisic-acid-responsive elements (ABREs) or GC-rich coupling elements (CE), thereby influencing the expression of their target downstream genes. Several in silico techniques have been adopted so far to predict the structural features, recognize the regulatory modification sites, undergo phylogenetic analyses, and facilitate genome-wide survey of TF under multiple stresses. Current investigations on the epigenetic regulation that controls greater accessibility of the inducible regions of DNA of the target gene to the bZIP TFs exclusively under stress situations, along with the evolved stress memory responses via genomic imprinting mechanism, have been highlighted. The potentiality of overexpression of bZIP TFs, either in a homologous or in a heterologous background, in generating transgenic plants tolerant to various abiotic stressors have also been addressed by various groups. The present review will provide a coherent documentation on the functional characterization and regulation of bZIP TFs under multiple environmental stresses, with the major goal of generating multiple-stress-tolerant plant cultivars in near future.

The Role of MAPK Modules and ABA during Abiotic Stress Signaling

To respond to abiotic stresses, plants have developed specific mechanisms that allow them to rapidly perceive and respond to environmental changes. The phytohormone abscisic acid (ABA) was shown to be a pivotal regulator of abiotic stress responses in plants, triggering major changes in plant physiology. The ABA core signaling pathway largely relies on the activation of SnRK2 kinases to mediate several rapid responses, including gene regulation, stomatal closure, and plant growth modulation. Mitogen-activated protein kinases (MAPKs) have also been implicated in ABA signaling, but an entire ABA-activated MAPK module was uncovered only recently. In this review, we discuss the evidence for a role of MAPK modules in the context of different plant ABA signaling pathways.

An ABA-increased interaction of the PYL6 ABA receptor with MYC2 Transcription Factor: A putative link of ABA and JA signaling

Abscisic acid (ABA) is a plant hormone that mediates abiotic stress tolerance and regulates growth and development. ABA binds to members of the PYL/RCAR ABA receptor family that initiate signal transduction inhibiting type 2C protein phosphatases. Although crosstalk between ABA and the hormone Jasmonic Acid (JA) has been shown, the molecular entities that mediate this interaction have yet to be fully elucidated. We report a link between ABA and JA signaling through a direct interaction of the ABA receptor PYL6 (RCAR9) with the basic helix-loop-helix transcription factor MYC2. PYL6 and MYC2 interact in yeast two hybrid assays and the interaction is enhanced in the presence of ABA. PYL6 and MYC2 interact in planta based on bimolecular fluorescence complementation and co-immunoprecipitation of the proteins. Furthermore, PYL6 was able to modify transcription driven by MYC2 using JAZ6 and JAZ8 DNA promoter elements in yeast one hybrid assays. Finally, pyl6 T-DNA mutant plants show an increased sensitivity to the addition of JA along with ABA in cotyledon expansion experiments. Overall, the present study identifies a direct mechanism for transcriptional modulation mediated by an ABA receptor different from the core ABA signaling pathway, and a putative mechanistic link connecting ABA and JA signaling pathways.

An ABA-increased interaction of the PYL6 ABA receptor with MYC2 Transcription Factor: A putative link of ABA and JA signaling

Abscisic acid (ABA) is a plant hormone that mediates abiotic stress tolerance and regulates growth and development. ABA binds to members of the PYL/RCAR ABA receptor family that initiate signal transduction inhibiting type 2C protein phosphatases. Although crosstalk between ABA and the hormone Jasmonic Acid (JA) has been shown, the molecular entities that mediate this interaction have yet to be fully elucidated. We report a link between ABA and JA signaling through a direct interaction of the ABA receptor PYL6 (RCAR9) with the basic helix-loop-helix transcription factor MYC2. PYL6 and MYC2 interact in yeast two hybrid assays and the interaction is enhanced in the presence of ABA. PYL6 and MYC2 interact in planta based on bimolecular fluorescence complementation and co-immunoprecipitation of the proteins. Furthermore, PYL6 was able to modify transcription driven by MYC2 using JAZ6 and JAZ8 DNA promoter elements in yeast one hybrid assays. Finally, pyl6 T-DNA mutant plants show an increased sensitivity to the addition of JA along with ABA in cotyledon expansion experiments. Overall, the present study identifies a direct mechanism for transcriptional modulation mediated by an ABA receptor different from the core ABA signaling pathway, and a putative mechanistic link connecting ABA and JA signaling pathways.

Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Abiotic stress is a primary threat to fulfill the demand of agricultural production to feed the world in coming decades. Plants reduce growth and development process during stress conditions, which ultimately affect the yield. In stress conditions, plants develop various stress mechanism to face the magnitude of stress challenges, although that is not enough to protect them. Therefore, many strategies have been used to produce abiotic stress tolerance crop plants, among them, abscisic acid (ABA) phytohormone engineering could be one of the methods of choice. ABA is an isoprenoid phytohormone, which regulates various physiological processes ranging from stomatal opening to protein storage and provides adaptation to many stresses like drought, salt, and cold stresses. ABA is also called an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to different environmental stress conditions. In this review, we will discuss the role of ABA in response to abiotic stress at the molecular level and ABA signaling. The review also deals with the effect of ABA in respect to gene expression.

Abscisic Acid and Abiotic Stress Tolerance in Crop Plants

Abiotic stress is a primary threat to fulfill the demand of agricultural production to feed the world in coming decades. Plants reduce growth and development process during stress conditions, which ultimately affect the yield. In stress conditions, plants develop various stress mechanism to face the magnitude of stress challenges, although that is not enough to protect them. Therefore, many strategies have been used to produce abiotic stress tolerance crop plants, among them, abscisic acid (ABA) phytohormone engineering could be one of the methods of choice. ABA is an isoprenoid phytohormone, which regulates various physiological processes ranging from stomatal opening to protein storage and provides adaptation to many stresses like drought, salt, and cold stresses. ABA is also called an important messenger that acts as the signaling mediator for regulating the adaptive response of plants to different environmental stress conditions. In this review, we will discuss the role of ABA in response to abiotic stress at the molecular level and ABA signaling. The review also deals with the effect of ABA in respect to gene expression.

Contrasting transcriptional responses of PYR1/PYL/RCAR ABA receptors to ABA or dehydration stress between maize seedling leaves and roots

BACKGROUND

The different actions of abscisic acid (ABA) in the aboveground and belowground parts of plants suggest the existence of a distinct perception mechanism between these organs. Although characterization of the soluble ABA receptors PYR1/PYL/RCAR as well as core signaling components has greatly advanced our understanding of ABA perception, signal transduction, and responses, the environment-dependent organ-specific sensitivity of plants to ABA is less well understood.

RESULTS

By performing real-time quantitative PCR assays, we comprehensively compared transcriptional differences of core ABA signaling components in response to ABA or osmotic/dehydration stress between maize (Zea mays L.) roots and leaves. Our results demonstrated up-regulation of the transcript levels of ZmPYLs homologous to dimeric-type Arabidopsis ABA receptors by ABA in maize primary roots, whereas those of ZmPYLs homologous to monomeric-type Arabidopsis ABA receptors were down-regulated. However, this trend was reversed in the leaves of plants treated with ABA via the root medium. Although the mRNA levels of ZmPYL1-3 increased significantly in roots subjected to polyethylene glycol (PEG)-induced osmotic stress, ZmPYL4-11 transcripts were either maintained at a stable level or increased only slightly. In detached leaves subjected to dehydration, the transcripts of ZmPYL1-3 together with ZmPYL5, ZmPYL6, ZmPYL10 and ZmPYL11 were decreased, whereas those of ZmPYL4, ZmPYL7 and ZmPYL8 were significantly increased. Our results also showed that all of the evaluated transcripts of PP2Cs and SnRK2 were quickly up-regulated in roots by ABA or osmotic stress; conversely they were either up-regulated or maintained at a constant level in leaves, depending on the isoforms within each family.

CONCLUSIONS

There is a distinct profile of PYR/PYL/RCAR ABA receptor gene expression between maize roots and leaves, suggesting that monomeric-type ABA receptors are mainly involved in the transmission of ABA signals in roots but that dimeric-type ABA receptors primarily carry out this function in leaves. Given that ZmPYL1 and ZmPYL4 exhibit similar transcript abundance under normal conditions, our findings may represent a novel mechanism for species-specific regulation of PYR/PYL/RCAR ABA receptor gene expression. A difference in the preference for core signaling components in the presence of exogenous ABA versus stress-induced endogenous ABA was observed in both leaves and roots. It appears that core ABA signaling components perform their osmotic/dehydration stress response functions in a stress intensity-, duration-, species-, organ-, and isoform-specific manner, leading to plasticity in response to adverse conditions and, thus, acclimation to life on land. These results deepen our understanding of the diverse biological effects of ABA between plant leaves and roots in response to abiotic stress at the stimulus-perception level.

Identification of Multiple Stress Responsive Genes by Sequencing a Normalized cDNA Library from Sea-Land Cotton (Gossypium barbadense L.)

Background

Plants often face multiple stresses including drought, extreme temperature, salinity, nutrition deficiency and biotic stresses during growth and development. All the stresses result in a series of physiological and metabolic reactions and then generate reversible inhibition of metabolism and growth and can cause seriously irreversible damage, even death. At each stage of cotton growth, environmental stress conditions pose devastating threats to plant growth and development, especially yield and quality. Due to the complex stress conditions and unclear molecular mechanisms of stress response, there is an urgent need to explore the mechanisms of cotton response against abiotic stresses.

Methodology and Principal Findings

A normalized cDNA library was constructed using Gossypium barbadense Hai-7124 treated with different stress conditions (heat, cold, salt, drought, potassium and phosphorus deficit and Verticillium dahliae infection). Random sequencing of this library generated 6,047 high-quality expressed sequence tags (ESTs). The ESTs were clustered and assembled into 3,135 uniESTs, composed of 2,497 contigs and 638 singletons. The blastx results demonstrated 2,746 unigenes showing significant similarity to known genes, 74 uniESTs displaying significant similarity to genes of predicted proteins, and 315 uniESTs remain uncharacterized. Functional classification unveiled the abundance of uniESTs in binding, catalytic activity, and structural molecule activity. Annotations of the uniESTs by the plant transcription factor database (PlantTFDB) and Plant Stress Protein Database (PSPDB) disclosed that transcription factors and stress-related genes were enriched in the current library. The expression of some transcription factors and specific stress-related genes were verified by RT-PCR under various stress conditions.

Conclusions/Significance

Annotation results showed that a huge number of genes respond to stress in our study, such as MYB-related, C2H2, FAR1, bHLH, bZIP, MADS, and mTERF. These results will improve our knowledge of stress tolerance in cotton. In addition, they are also helpful in discovering candidate genes related to stress tolerance. The publicly available ESTs from G. barbadense are a valuable genomic resource that will facilitate further molecular study and breeding of stress-tolerant cotton.

Transcriptome-wide functional characterization reveals novel relationships among differentially expressed transcripts in developing soybean embryos

BACKGROUND

Transcriptomics reveals the existence of transcripts of different coding potential and strand orientation. Alternative splicing (AS) can yield proteins with altered number and types of functional domains, suggesting the global occurrence of transcriptional and post-transcriptional events. Many biological processes, including seed maturation and desiccation, are regulated post-transcriptionally (e.g., by AS), leading to the production of more than one coding or noncoding sense transcript from a single locus.

RESULTS

We present an integrated computational framework to predict isoform-specific functions of plant transcripts. This framework includes a novel plant-specific weighted support vector machine classifier called CodeWise, which predicts the coding potential of transcripts with over 96 % accuracy, and several other tools enabling global sequence similarity, functional domain, and co-expression network analyses. First, this framework was applied to all detected transcripts (103,106), out of which 13 % was predicted by CodeWise to be noncoding RNAs in developing soybean embryos. Second, to investigate the role of AS during soybean embryo development, a population of 2,938 alternatively spliced and differentially expressed splice variants was analyzed and mined with respect to timing of expression. Conserved domain analyses revealed that AS resulted in global changes in the number, types, and extent of truncation of functional domains in protein variants. Isoform-specific co-expression network analysis using ArrayMining and clustering analyses revealed specific sub-networks and potential interactions among the components of selected signaling pathways related to seed maturation and the acquisition of desiccation tolerance. These signaling pathways involved abscisic acid- and FUSCA3-related transcripts, several of which were classified as noncoding and/or antisense transcripts and were co-expressed with corresponding coding transcripts. Noncoding and antisense transcripts likely play important regulatory roles in seed maturation- and desiccation-related signaling in soybean.

CONCLUSIONS

This work demonstrates how our integrated framework can be implemented to make experimentally testable predictions regarding the coding potential, co-expression, co-regulation, and function of transcripts and proteins related to a biological process of interest.

De novo assembly and characterization of germinating lettuce seed transcriptome using Illumina paired-end sequencing

At supraoptimal temperature, germination of lettuce (Lactuca sativa L.) seeds exhibits a typical germination thermoinhibition, which can be alleviated by sodium nitroprusside (SNP) in a nitric oxide-dependent manner. However, the molecular mechanism of seed germination thermoinhibition and its alleviation by SNP are poorly understood. In the present study, the lettuce seeds imbibed at optimal temperature in water or at supraoptimal temperature with or without 100 μM SNP for different periods of time were used as experimental materials, the total RNA was extracted and sequenced, we gained 147,271,347 raw reads using Illumina paired-end sequencing technique and assembled the transcriptome of germinating lettuce seeds. A total of 51,792 unigenes with a mean length of 849 nucleotides were obtained. Of these unigenes, a total of 29,542 unigenes were annotated by sequence similarity searching in four databases, NCBI non-redundant protein database, SwissProt protein database, euKaryotic Ortholog Groups database, and NCBI nucleotide database. Among the annotated unigenes, 22,276 unigenes were assigned to Gene Ontology database. When all the annotated unigenes were searched against the Kyoto Encyclopedia of Genes and Genomes Pathway database, a total of 8,810 unigenes were mapped to 5 main categories including 260 pathways. We first obtained a lot of unigenes encoding proteins involved in abscisic acid (ABA) signaling in lettuce, including 11 ABA receptors, 94 protein phosphatase 2Cs and 16 sucrose non-fermenting 1-related protein kinases. These results will help us to better understand the molecular mechanism of seed germination, thermoinhibition of seed germination and its alleviation by SNP.

S-nitrosylation triggers ABI5 degradation to promote seed germination and seedling growth

Plant survival depends on seed germination and progression through post-germinative developmental checkpoints. These processes are controlled by the stress phytohormone abscisic acid (ABA). ABA regulates the basic leucine zipper transcriptional factor ABI5, a central hub of growth repression, while the reactive nitrogen molecule nitric oxide (NO) counteracts ABA during seed germination. However, the molecular mechanisms by which seeds sense more favourable conditions and start germinating have remained elusive. Here we show that ABI5 promotes growth via NO, and that ABI5 accumulation is altered in genetic backgrounds with impaired NO homeostasis. S-nitrosylation of ABI5 at cysteine-153 facilitates its degradation through CULLIN4-based and KEEP ON GOING E3 ligases, and promotes seed germination. Conversely, mutation of ABI5 at cysteine-153 deregulates protein stability and inhibition of seed germination by NO depletion. These findings suggest an inverse molecular link between NO and ABA hormone signalling through distinct posttranslational modifications of ABI5 during early seedling development.

Nitric oxide counteracts the inhibitory effects of the plant hormone ABA during seed germination and seedling growth. Here, Albertos et al. show that nitric oxide can act antagonistically to ABA by inducing the S-nitrosylation and degradation of the ABI5 transcription factor involved in ABA signalling.

Complex Interplay of Hormonal Signals during Grape Berry Ripening

Grape and wine production and quality is extremely dependent on the fruit ripening process. Sensory and nutritional characteristics are important aspects for consumers and their development during fruit ripening involves complex hormonal control. In this review, we explored data already published on grape ripening and compared it with the hormonal regulation of ripening of other climacteric and non-climacteric fruits. The roles of abscisic acid, ethylene, and brassinosteroids as promoters of ripening are discussed, as well as the role of auxins, cytokinins, gibberellins, jasmonates, and polyamines as inhibitors of ripening. In particular, the recently described role of polyamine catabolism in grape ripening is discussed, together with its putative interaction with other hormones. Furthermore, other recent examples of cross-talk among the different hormones are presented, revealing a complex interplay of signals during grape development and ripening.

Abscisic acid receptor PYRABACTIN RESISTANCE-LIKE 8, PYL8, is involved in glucose response and dark-induced leaf senescence in Arabidopsis

Abscisic acid (ABA) receptors in plants are thought to be involved in various cellular processes mediated by signal transduction pathways. There are about 14 ABA receptors in Arabidopsis, but only a few have been studied. In this study, we investigated the effect of the disruption and overexpression of an ABA receptor gene, PYL8 (At5g53160) on plant responses to glucose (Glc) and dark-induced leaf senescence. Expression of PYL8 was strongly reduced by Glc treatment. Overexpression of PYL8 in Arabidopsis resulted in significantly reduced seed germination and cotyledon greening under high Glc conditions, while RNAi transgenic lines were more insensitive to Glc stress. Activities of two Glc-responsive genes, Arabidopsis thaliana Hexokinase 1 (AtHXK1) and ABA insensitive 5 (ABI5) were higher in PYL8-overexpressing plants than in the wild-type (WT) plants after Glc treatment, whereas the transcript levels of these genes in RNAi plants decreased. Furthermore, PYL8-overexpressing plants displayed increased yellowing, membrane ion leakage, and reduced chlorophyll content due to dark-induced senescence, and exhibited stronger expression of a group of senescence-inducible genes than did WT. The data show that PYL8 plays essential roles in responses to both Glc and dark-induced senescence in A. thaliana.

Comparative transcriptome analysis of the Asteraceae halophyte Karelinia caspica under salt stress

BACKGROUND

Much attention has been given to the potential of halophytes as sources of tolerance traits for introduction into cereals. However, a great deal remains unknown about the diverse mechanisms employed by halophytes to cope with salinity. To characterize salt tolerance mechanisms underlying Karelinia caspica, an Asteraceae halophyte, we performed Large-scale transcriptomic analysis using a high-throughput Illumina sequencing platform. Comparative gene expression analysis was performed to correlate the effects of salt stress and ABA regulation at the molecular level.

RESULTS

Total sequence reads generated by pyrosequencing were assembled into 287,185 non-redundant transcripts with an average length of 652 bp. Using the BLAST function in the Swiss-Prot, NCBI nr, GO, KEGG, and KOG databases, a total of 216,416 coding sequences associated with known proteins were annotated. Among these, 35,533 unigenes were classified into 69 gene ontology categories, and 18,378 unigenes were classified into 202 known pathways. Based on the fold changes observed when comparing the salt stress and control samples, 60,127 unigenes were differentially expressed, with 38,122 and 22,005 up- and down-regulated, respectively. Several of the differentially expressed genes are known to be involved in the signaling pathway of the plant hormone ABA, including ABA metabolism, transport, and sensing as well as the ABA signaling cascade.

CONCLUSIONS

Transcriptome profiling of K. caspica contribute to a comprehensive understanding of K. caspica at the molecular level. Moreover, the global survey of differentially expressed genes in this species under salt stress and analyses of the effects of salt stress and ABA regulation will contribute to the identification and characterization of genes and molecular mechanisms underlying salt stress responses in Asteraceae plants.

Intra-specific variations in expression of stress-related genes in beech progenies are stronger than drought-induced responses

Rapidly decreasing water availability as a consequence of climate change is likely to endanger the range of long-lived tree species. A pressing question is, therefore, whether adaptation to drought exists in important temperate tree species like European beech (Fagus sylvatica L.), a wide-spread, dominant forest tree in Central Europe. Here, five beech stands were selected along a precipitation gradient from moist to dry conditions. Neutral genetic markers revealed strong variation within and little differentiation between the populations. Natural regeneration from these stands was transferred to a common garden and used to investigate the expression of genes for abscisic acid (ABA)-related drought signaling [9-cis-epoxy-dioxygenase (NCED), protein phosphatase 2C (PP2C), early responsive to dehydration (ERD)] and stress protection [ascorbate peroxidase (APX), superoxide dismutase (SOD), aldehyde dehydrogenase (ALDH), glutamine amidotransferase (GAT)] that are involved in drought acclimation. We hypothesized that progenies from dry sites exhibit constitutively higher expression levels of ABA- and stress-related genes and are less drought responsive than progenies from moist sites. Transcript levels and stress responses (leaf area loss, membrane integrity) of well-irrigated and drought-stressed plants were measured during the early, mid- and late growing season. Principal component (PC) analysis ordered the beech progenies according to the mean annual precipitation at tree origin by the transcript levels of SOD, ALDH, GAT and ERD as major loadings along PC1. PC2 separated moist and drought treatments with PP2C levels as important loading. These results suggest that phosphatase-mediated signaling is flexibly acclimated to the current requirements, whereas stress compensatory measures exhibited genotypic variation, apparently underlying climate selection. In contrast to expectation, the drought responses were less pronounced than the progeny-related differences and the transcript levels were constitutively lower in beeches from dry than from moist sites. These results imply that beeches from dry origins may have evolved mechanisms to avoid oxidative stress.

Type 2C phosphatase 1 of Artemisia annua L. is a negative regulator of ABA signaling

The phytohormone abscisic acid (ABA) plays an important role in plant development and environmental stress response. Additionally, ABA also regulates secondary metabolism such as artemisinin in the medicinal plant Artemisia annua L. Although an earlier study showed that ABA receptor, AaPYL9, plays a positive role in ABA-induced artemisinin content improvement, many components in the ABA signaling pathway remain to be elucidated in Artemisia annua L. To get insight of the function of AaPYL9, we isolated and characterized an AaPYL9-interacting partner, AaPP2C1. The coding sequence of AaPP2C1 encodes a deduced protein of 464 amino acids, with all the features of plant type clade A PP2C. Transcriptional analysis showed that the expression level of AaPP2C1 is increased after ABA, salt, and drought treatments. Yeast two-hybrid and bimolecular fluorescence complementation assays (BiFC) showed that AaPYL9 interacted with AaPP2C1. The P89S, H116A substitution in AaPYL9 as well as G199D substitution or deletion of the third phosphorylation site-like motif in AaPP2C1 abolished this interaction. Furthermore, constitutive expression of AaPP2C1 conferred ABA insensitivity compared with the wild type. In summary, our data reveals that AaPP2C1 is an AaPYL9-interacting partner and involved in the negative modulation of the ABA signaling pathway in A. annua L.