Control of Plant Water Use by ABA Induction of Senescence and Dormancy: An Overlooked Lesson from Evolution

Abscisic acid and ethylene coordinating fruit ripening under abiotic stress

Fleshy fruit metabolism is intricately influenced by environmental changes, yet the hormonal regulations underlying these responses remain poorly elucidated. ABA and ethylene, pivotal in stress responses across plant vegetative tissues, play crucial roles in triggering fleshy fruit ripening. Their actions are intricately governed by complex mechanisms, influencing key aspects such as nutraceutical compound accumulation, sugar content, and softening parameters. Both hormones are essential orchestrators of significant alterations in fruit development in response to stressors like drought, salt, and temperature fluctuations. These alterations encompass colour development, sugar accumulation, injury mitigation, and changes in cell-wall degradation and ripening progression. This review provides a comprehensive overview of recent research progress on the roles of ABA and ethylene in responding to drought, salt, and temperature stress, as well as the molecular mechanisms controlling ripening in environmental cues. Additionally, we propose further studies aimed at genetic manipulation of ABA and ethylene signalling, offering potential strategies to enhance fleshy fruit resilience in the face of future climate change scenarios.

Overexpression of SlWRKY6 enhances drought tolerance by strengthening antioxidant defense and stomatal closure via ABA signaling in Solanum lycopersicum L

Drought is a major handicap for plant growth and development. WRKY proteins comprise one of the largest families of plant transcription factors, playing important roles in plant growth and stress tolerance. In tomato (Solanum lycopersicum L.), different WRKY transcription factors differentially (positively or negatively) regulate drought tolerance, however, the role of SlWRKY6 in drought response and the associated molecular mechanisms of stress tolerance remain unclear. Here we report that SlWRKY6, a member of the WRKYII-b group, is involved in the functional aspects of drought resistance in tomato. Transcriptional activation assays show that SlWRKY6 is transcriptionally active in yeast cells, while the subcellular localization assay indicates that SlWRKY6 is localized in the nucleus. Overexpression of SlWRKY6 in tomato plants resulted in stronger antioxidant capacity and drought resistance as manifested by increased photosynthetic capacity and decreased reactive oxygen species accumulation, malondialdehyde content and relative electrolyte leakage in transgenic tomato plants compared with wild-type under drought stress. Moreover, increased abscisic acid (ABA) content and transcript abundance of ABA synthesis and signaling genes (NCED1, NCED4, PYL4, AREB1 and SnRK2.6) in the transgenic tomato plants indicated potential involvement of the ABA pathway in SlWRKY6-induced drought resistance in tomato plants. Inspection of 2-kb sequences upstream of the predicted binding sites in the promoter of SlNCED1/4 identified two copies of the core W-box (TTGACC/T) sequence in the promoter of SlNCED1/4, which correlates well with the expression of these genes in response to drought, further suggesting the involvement of ABA-dependent pathway in SlWRKY6-induced drought resistance. The study unveils a critical role of SlWRKY6, which can be useful to further reveal the drought tolerance mechanism and breeding of drought-resistant tomato varieties for sustainable vegetable production in the era of climate change.

Genome-wide identification, abiotic stress, and expression analysis of PYL family in Tartary buckwheat (Fagopyrum tataricum (L.) Gaertn.) during grain development

BACKGROUND

Abscisic acid (ABA) is a plant hormone that plays an important role in plant resistance to drought, salinity, cold, and pathogens. It is also important for regulating plant growth and development. Pyrabactin resistance/pyr1-like/regulatory components of the ABA receptor (PYL/RCAR) are ABA receptor proteins in plants and the core of ABA signal transduction pathways in plant regulatory factors. At present, there are no reports on the PYL family of Tartary buckwheat.

RESULTS

In this study, 19 paralogous form PYL genes in buckwheat were identified at the whole-genome level and named FtPYL1-FtPYL19 according to their positions on chromosomes. We further analyzed the gene structure, conserved motifs, cis-acting elements, gene duplication, phylogenetic relationships, and expression patterns under different stress treatments and during grain development of the 19 paralogous form PYL genes in Tartary buckwheat. The FtPYL gene exhibits a single exonic gene structure for about 68.4% of the duplicated forms from the total paralogous forms. The remaining subfamilies, such as I and II, contain three exons and two exons (e.g., FtPYL19), respectively. Nineteen FtPYL genes were evenly distributed across the eight chromosomes, with at least one FtPYL gene on each chromosome. In the FtPYL gene family, there was one tandem repeat event and five gene duplication events. We investigated the gene expression levels of FtPYL gene under four abiotic stresses and different stages of grain development. Under drought stress (PEG6000), the relative expression levels of FtPYL14 and FtPYL15 increased by fourfold. Under high temperature stress (38℃), the relative expression level of FtPYL16 dropped to 0.12, and that of FtPYL17 fell to 0.22. At different stages of grain development, the gene expression level of FtPY15 is extremely high at 19 D. The relative expression level of FtPYL7 in roots and stems reaches up to approximately 450, and the relative expression level of FtPYL10 in 13 D also reaches up to 248. In this study, the PYL gene family of Tartary buckwheat was identified and analyzed based on the whole genome, and 19 paralogous form FtPYL genes of Tartary buckwheat were bioinformatically analyzed. The expression patterns of 19 paralogous form FtPYL genes in Tartary buckwheat cultivars under different stress treatments and during grain development were analyzed. It was found that the FtPYL gene played an important role in grain development.

Bulbil initiation: a comprehensive review on resources, development, and utilisation, with emphasis on molecular mechanisms, advanced technologies, and future prospects

Bulbil is an important asexual reproductive structure of bulbil plants. It mainly grows in leaf axils, leaf forks, tubers and the upper and near ground ends of flower stems of plants. They play a significant role in the reproduction of numerous herbaceous plant species by serving as agents of plant propagation, energy reserves, and survival mechanisms in adverse environmental conditions. Despite extensive research on bulbil-plants regarding their resources, development mechanisms, and utilisation, a comprehensive review of bulbil is lacking, hindering progress in exploiting bulbil resources. This paper provides a systematic overview of bulbil research, including bulbil-plant resources, identification of development stages and maturity of bulbils, cellular and molecular mechanisms of bulbil development, factors influencing bulbil development, gene research related to bulbil development, multi-bulbil phenomenon and its significance, medicinal value of bulbils, breeding value of bulbils, and the application of plant tissue culture technology in bulbil production. The application value of the Temporary Immersion Bioreactor System (TIBS) and Terahertz (THz) in bulbil breeding is also discussed, offering a comprehensive blueprint for further bulbil resource development. Additionally, additive, seven areas that require attention are proposed: (1) Utilization of modern network technologies, such as plant recognition apps or websites, to collect and identify bulbous plant resources efficiently and extensively; (2) Further research on cell and tissue structures that influence bulb cell development; (3) Investigation of the network regulatory relationship between genes, proteins, metabolites, and epigenetics in bulbil development; (4) Exploration of the potential utilization value of multiple sprouts, including medicinal, ecological, and horticultural applications; (5) Innovation and optimization of the plant tissue culture system for bulbils; (6) Comprehensive application research of TIBS for large-scale expansion of bulbil production; (7) To find out the common share genetics between bulbils and flowers.

How plants sense and respond to osmotic stress

Drought is one of the most serious abiotic stresses to land plants. Plants sense and respond to drought stress to survive under water deficiency. Scientists have studied how plants sense drought stress, or osmotic stress caused by drought, ever since Charles Darwin, and gradually obtained clues about osmotic stress sensing and signaling in plants. Osmotic stress is a physical stimulus that triggers many physiological changes at the cellular level, including changes in turgor, cell wall stiffness and integrity, membrane tension, and cell fluid volume, and plants may sense some of these stimuli and trigger downstream responses. In this review, we emphasized water potential and movements in organisms, compared putative signal inputs in cell wall-containing and cell wall-free organisms, prospected how plants sense changes in turgor, membrane tension, and cell fluid volume under osmotic stress according to advances in plants, animals, yeasts, and bacteria, summarized multilevel biochemical and physiological signal outputs, such as plasma membrane nanodomain formation, membrane water permeability, root hydrotropism, root halotropism, Casparian strip and suberin lamellae, and finally proposed a hypothesis that osmotic stress responses are likely to be a cocktail of signaling mediated by multiple osmosensors. We also discussed the core scientific questions, provided perspective about the future directions in this field, and highlighted the importance of robust and smart root systems and efficient source-sink allocations for generating future high-yield stress-resistant crops and plants.

OsNAC103, a NAC Transcription Factor, Positively Regulates Leaf Senescence and Plant Architecture in Rice

Leaf senescence, the last stage of leaf development, is essential for crop yield by promoting nutrition relocation from senescence leaves to new leaves and seeds. NAC (NAM/ATAF1/ATAF2/CUC2) proteins, one of the plant-specific transcription factors, widely distribute in plants and play important roles in plant growth and development. Here, we identified a new NAC member OsNAC103 and found that it plays critical roles in leaf senescence and plant architecture in rice. OsNAC103 mRNA levels were dramatically induced by leaf senescence as well as different phytohormones such as ABA, MeJA and ACC and abiotic stresses including dark, drought and high salinity. OsNAC103 acts as a transcription factor with nuclear localization signals at the N terminal and a transcriptional activation signal at the C terminal. Overexpression of OsNAC103 promoted leaf senescence while osnac103 mutants delayed leaf senescence under natural condition and dark-induced condition, meanwhile, senescence-associated genes (SAGs) were up-regulated in OsNAC103 overexpression (OsNAC103-OE) lines, indicating that OsNAC103 positively regulates leaf senescence in rice. Moreover, OsNAC103-OE lines exhibited loose plant architecture with larger tiller angles while tiller angles of osnac103 mutants decreased during the vegetative and reproductive growth stages due to the response of shoot gravitropism, suggesting that OsNAC103 can regulate the plant architecture in rice. Taken together, our results reveal that OsNAC103 plays crucial roles in the regulation of leaf senescence and plant architecture in rice.

Suppression of SlDREB3 increases leaf ABA responses and promotes drought tolerance in transgenic tomato plants

Drought susceptibility is a major yield limiting factor in agricultural crops especially in hybrids/varieties that have been bred for high yields. We show that manipulation of the SlDREB3 gene in tomato alters ABA responses and thereby sensitivity of stomatal closure to ABA. SlDREB3 suppression lines show ABA hypersensitivity and rapid stomatal closure in response to ABA while over-expression lines show reduced sensitivity to ABA and open stomata even at high ABA levels with rapid water loss after 10 days of water stress. This is accompanied with high ROS levels and increased membrane damage due to senescence of leaves and drastically reduced survival in drought. The relative water content (RWC) of OEx lines is much reduced even when grown under well-watered conditions. In contrast, suppression lines show greater tolerance to water stress and almost complete survival to 10-day water stress. They show much reduced ROS levels, reduced membrane damage, higher RWC and reduced leaf water loss. These changes are associated with higher expression of ABA signalling pathway genes in suppression lines while these are highly reduced in OEx lines. The studies suggest that control of ABA signalling by SlDREB3 can help in withstanding severe drought.

CURLY LEAF modulates apoplast liquid water status in Arabidopsis leaves

The apoplast of plant leaves, the intercellular space between mesophyll cells, is normally largely filled with air with a minimal amount of liquid water in it, which is essential for key physiological processes such as gas exchange to occur. Phytopathogens exploit virulence factors to induce a water-rich environment, or "water-soaked" area, in the apoplast of the infected leaf tissue to promote disease. We propose that plants evolved a "water soaking" pathway, which normally keeps a nonflooded leaf apoplast for plant growth but is disturbed by microbial pathogens to facilitate infection. Investigation of the "water soaking" pathway and leaf water control mechanisms is a fundamental, yet previously overlooked, aspect of plant physiology. To identify key components in the "water soaking" pathway, we performed a genetic screen to isolate Arabidopsis (Arabidopsis thaliana) severe water soaking (sws) mutants that show liquid water overaccumulation in the leaf under high air humidity, a condition required for visible water soaking. Here, we report the sws1 mutant, which displays rapid water soaking upon high humidity treatment due to a loss-of-function mutation in CURLY LEAF (CLF), encoding a histone methyltransferase in the POLYCOMB REPRESSIVE COMPLEX 2 (PRC2). We found that the sws1 (clf) mutant exhibits enhanced abscisic acid (ABA) levels and stomatal closure, which are indispensable for its water soaking phenotype and mediated by CLF's epigenetic regulation of a group of ABA-associated NAM, ATAF, and CUC (NAC) transcription factor genes, NAC019/055/072. The clf mutant showed weakened immunity, which likely also contributes to the water soaking phenotype. In addition, the clf plant supports a substantially higher level of Pseudomonas syringae pathogen-induced water soaking and bacterial multiplication, in an ABA pathway and NAC019/055/072-dependent manner. Collectively, our study sheds light on an important question in plant biology and demonstrates CLF as a key modulator of leaf liquid water status via epigenetic regulation of the ABA pathway and stomatal movement.

Comparative transcriptome profiling reveals the multiple levels of crosstalk in phytohormone networks in Brassica napus

Plant hormones are the intrinsic factors that control plant development. The integration of different phytohormone pathways in a complex network of synergistic, antagonistic and additive interactions has been elucidated in model plants. However, the systemic level of transcriptional responses to hormone crosstalk in Brassica napus is largely unknown. Here, we present an in-depth temporal-resolution study of the transcriptomes of the seven hormones in B. napus seedlings. Differentially expressed gene analysis revealed few common target genes that co-regulated (up- and down-regulated) by seven hormones; instead, different hormones appear to regulate distinct members of protein families. We then constructed the regulatory networks between the seven hormones side by side, which allowed us to identify key genes and transcription factors that regulate the hormone crosstalk in B. napus. Using this dataset, we uncovered a novel crosstalk between gibberellin and cytokinin in which cytokinin homeostasis was mediated by RGA-related CKXs expression. Moreover, the modulation of gibberellin metabolism by the identified key transcription factors was confirmed in B. napus. Furthermore, all data were available online from http://yanglab.hzau.edu.cn/BnTIR/hormone. Our study reveals an integrated hormone crosstalk network in Brassica napus, which also provides a versatile resource for future hormone studies in plant species.

The Abscisic Acid-Responsive Element Binding Factors (ABFs)-MAPKKK18 module regulates ABA- induced leaf senescence in Arabidopsis

The Mitogen-Activated Protein Kinase Kinase Kinase 18 (MAPKKK18) has been reported to play a role in abiotic stress priming in long-term abscisic acid (ABA) response including drought tolerance and leaf senescence. However, the upstream transcriptional regulators of MAPKKK18 remain to be determined. Here, we report ABA-Responsive Element (ABRE) Binding Factors (ABFs) as upstream transcription factors (TFs) of MAPKKK18 expression. Mutants of abf2, abf3, abf4 and abf2abf3abf4 dramatically reduced the transcription of MAPKKK18. Our electrophoresis mobility shift assay (EMSA) and dual-luciferase reporter assay demonstrated that ABF2, ABF3, and ABF4 bound to ABRE cis-elements within the promoter of MAPKKK18 to transactivate its expression. Furthermore, enrichments of the promoter region of MAPKKK18 by ABF2, ABF3, and ABF4 were confirmed by in vivo chromatin immunoprecipitation coupled with qPCR (ChIP-qPCR). Additionally, we found that mutants of mapkkk18 exhibited obvious delayed leaf senescence. Moreover, a genetic study showed that overexpression of ABF2, ABF3, and ABF4 in the background of mapkkk18 mostly phenocopied the stay-green phenotype of mapkkk18 and, expression levels of five target genes of ABFs, that is, NYE1, NYE2, NYC1, PAO, and SAG29 were attenuated as a result of MAPKKK18 mutation. These findings demonstrate that ABF2, ABF3, and ABF4 act as transcription regulators of MAPKKK18, and also suggest that, at least in part, ABA acts in priming leaf senescence via ABF-induced expression of MAPKKK18.

Plant hormone regulation of abiotic stress responses

Plant hormones are signalling compounds that regulate crucial aspects of growth, development and environmental stress responses. Abiotic stresses, such as drought, salinity, heat, cold and flooding, have profound effects on plant growth and survival. Adaptation and tolerance to such stresses require sophisticated sensing, signalling and stress response mechanisms. In this Review, we discuss recent advances in understanding how diverse plant hormones control abiotic stress responses in plants and highlight points of hormonal crosstalk during abiotic stress signalling. Control mechanisms and stress responses mediated by plant hormones including abscisic acid, auxin, brassinosteroids, cytokinins, ethylene and gibberellins are discussed. We discuss new insights into osmotic stress sensing and signalling mechanisms, hormonal control of gene regulation and plant development during stress, hormone-regulated submergence tolerance and stomatal movements. We further explore how innovative imaging approaches are providing insights into single-cell and tissue hormone dynamics. Understanding stress tolerance mechanisms opens new opportunities for agricultural applications.

Role of abscisic acid in modulating drought acclimation, agronomic characteristics and β-N-oxalyl-L-α,β-diaminopropionic acid (β-ODAP) accumulation in grass pea (Lathyrus sativus L.)

BACKGROUND

β-N-oxalyl-l-α,β-diaminopropionic acid (β-ODAP) is a physiological indicator in response to drying soil. However, how abscisic acid (ABA) modulates β-ODAP accumulation and its related agronomic characteristics in drought stressed grass pea (Lathyrus sativus L.) continue to be unclear. The present study aimed to evaluate the effects of ABA addition on drought tolerance, agronomic characteristics and β-ODAP content in grass pea under drought stress.

RESULTS

Exogenous ABA significantly promoted ABA levels by 19.3% and 18.3% under moderate and severe drought stress, respectively, compared to CK (without ABA, used as control check treatment). ABA addition activated earlier trigger of non-hydraulic root-sourced signal at 69.1% field capacity (FC) (65.5% FC in CK) and accordingly prolonged its operation period to 45.6% FC (49.0% FC in CK). This phenomenon was mechanically associated with the physiological mediation of ABA, where its addition significantly promoted the activities of leaf superoxide dismutase, catalase and peroxidase enzymes and the biosynthesis of leaf proline, simultaneously lowering the accumulation of malondialdehyde and hydrogen peroxide under moderate and severe stresses. Interestingly, ABA application significantly increased seed β-ODAP content by 21.7% and 21.3% under moderate and severe drought stress, but did not change leaf β-ODAP content. Furthermore, ABA application produced similar shoot biomass and grain yield as control groups.

CONCLUSION

Exogenous ABA improved the drought adaptability of grass pea and promoted the synthesis of β-ODAP in seeds but not in leaves. Our findings provide novel insights into the agronomic role of ABA in relation to β-ODAP enrichment in grass pea subjected to drought stress. © 2021 Society of Chemical Industry.

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.

Understanding a Mechanistic Basis of ABA Involvement in Plant Adaptation to Soil Flooding: The Current Standing

Soil flooding severely impairs agricultural crop production. Plants can cope with flooding conditions by embracing an orchestrated set of morphological adaptations and physiological adjustments that are regulated by the elaborated hormonal signaling network. The most prominent of these hormones is ethylene, which has been firmly established as a critical signal in flooding tolerance. ABA (abscisic acid) is also known as a “stress hormone” that modulates various responses to abiotic stresses; however, its role in flooding tolerance remains much less established. Here, we discuss the progress made in the elucidation of morphological adaptations regulated by ABA and its crosstalk with other phytohormones under flooding conditions in model plants and agriculturally important crops.

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.

Current status of microRNA-mediated regulation of drought stress responses in cereals

Drought is one of the most important abiotic stress factors impeding crop productivity. With the uncovering of their role as potential regulators of gene expression, microRNAs (miRNAs) have been recognized as new targets for developing stress resistance. MicroRNAs are small noncoding RNAs whose abundance is significantly altered under stress conditions. Interestingly, plant miRNAs predominantly targets transcription factors (TFs), and some of which are also the most critical drought-responsive genes that in turn could regulate the expression of numerous loci with drought-adaptive potential. The phytohormone ABA plays important roles in regulating stomatal conductance and in initiating an adaptive response to drought stress. miRNAs are implicated in regulating ABA-(abscisic acid) and non-ABA-mediated drought resistance pathways. For instance, miR159-MYB module and miR169-NFYA module participates in an ABA-dependent pathway, whereas several other ABA-independent miRNA-target modules (miR156-SPL; miR393-TIR1; miR160-ARF10, ARF16, ARF17; miR167-ARF6 and ARF8; miR390/TAS3siRNA-ARF2, ARF3, ARF4) collectively regulate drought responses in plants. Overall, miRNA-mediated drought response manifests diverse molecular, biochemical and physiological processes. Because of their immense role in controlling gene expression, miRNA manipulation has significant potential to augment plant tolerance to drought stress. This review compiles the current understanding of drought-responsive miRNAs in major cereals. Also, potential miRNA manipulation strategies currently in use along with the challenges and future perspectives are discussed.

Overlapping and stress-specific transcriptomic and hormonal responses to flooding and drought in soybean

Flooding and drought are serious constraints that reduce crop productivity worldwide. Previous studies identified genes conferring tolerance to both water extremes in various plants. However, overlapping responses to flooding and drought at the genome-scale remain obscure. Here, we defined overlapping and stress-specific transcriptomic and hormonal responses to submergence, drought and recovery from these stresses in soybean (Glycine max). We performed comparative RNA-sequencing and hormone profiling, identifying genes, hormones and biological processes that are differentially regulated in an overlapping or stress-specific manner. Overlapping responses included positive regulation of trehalose and sucrose metabolism and negative regulation of cellulose, tubulin, photosystem II and I, and chlorophyll biosynthesis, facilitating the economization of energy reserves under both submergence and drought. Additional energy-consuming pathways were restricted in a stress-specific manner. Downregulation of distinct pathways for energy saving under each stress suggests energy-consuming processes that are relatively unnecessary for each stress adaptation are turned down. Our newly developed transcriptomic-response analysis revealed that abscisic acid and ethylene responses were activated in common under both stresses, whereas stimulated auxin response was submergence-specific. The energy-saving strategy is the key overlapping mechanism that underpins adaptation to both submergence and drought in soybean. Abscisic acid and ethylene are candidate hormones that coordinate transcriptomic energy-saving processes under both stresses. Auxin may be a signaling component that distinguishes submergence-specific regulation of the stress response.

Genome-wide analysis of long noncoding RNAs affecting floral bud dormancy in pears in response to cold stress

The versatile role of long noncoding RNAs (lncRNAs) in plant growth and development has been established, but a systematic identification and analysis of lncRNAs in the pear has not been reported. Bud dormancy is a crucial and complicated protective mechanism for plants in winter. The roles of lncRNAs in the dormancy process remain largely unclear. In this study, we induced pear floral buds to enter into different dormant statuses by simulating four different chilling accumulation conditions. Then, a time series of RNA-seq analysis was performed and we identified 7594 lncRNAs in Pyrus pyrifolia (Burm. F.) Nakai that have not been identified. The sequence and expression of the lncRNAs were confirmed by PCR analysis. In total, 6253 lncRNAs were predicted to target protein-coding genes including 692 cis-regulated pairs (596 lncRNAs) and 13,158 trans-regulated pairs (6181 lncRNAs). Gene Ontology analysis revealed that most of lncRNAs' target genes were involved in catalytic activity, metabolic processes and cellular processes. In the trend analysis, 124 long-term cold response lncRNAs and 80 short-term cold response lncRNAs were predicted. Regarding the lncRNA-miRNA regulatory networks, 59 lncRNAs were identified as potential precursors for miRNA members of 20 families, 586 lncRNAs were targets of 261 pear miRNAs and 53 lncRNAs were endogenous target mimics for 26 miRNAs. In addition, three cold response lncRNAs, two miRNAs and their target genes were selected for expression confirmed. The trend of their expression was consistent with the predicted relationships among them and suggested possible roles of lncRNAs in ABA metabolic pathway. Our findings not only suggest the potential roles of lncRNAs in regulating the dormancy of pear floral buds but also provide new insights into the lncRNA-miRNA-mRNA regulatory network in plants.

Chemical activation of Arabidopsis SnRK2.6 by pladienolide B

Abscisic acid (ABA) is an important phytohormone mediating osmotic stress responses. SUCROSE NONFERMENTING 1 (SNF1)-RELATED PROTEIN KINASE 2.6 (SnRK2.6, also named OPEN STOMATA1 and SNF1-RELATED KINASE 2E) is central in the ABA signaling pathway; therefore, manipulating its activity may be useful to confer stress tolerance in plants. Pladienolide B (PB) is an mRNA splicing inhibitor and enhances ABA responses. Here, we analyzed the effect of PB on Arabidopsis SnRK2.6. PB enhanced the activity of recombinant SnRK2.6 in vitro through direct physical interaction as predicted by molecular docking simulations followed by mutation experiments and isothermal titration calorimetry. Structural modeling predicted probable interaction sites between PB and SnRK2.6, and experiments with mutated SnRK2.6 revealed that Leu-46 was the most essential amino acid residue for SnRK2.6 activation by PB. This study demonstrates the feasibility of SnRK2.6 chemical manipulation and paves the way for the modification of plant osmotic stress responses.

Abscisic Acid-Induced Stomatal Closure: An Important Component of Plant Defense Against Abiotic and Biotic Stress

Abscisic acid (ABA) is a stress hormone that accumulates under different abiotic and biotic stresses. A typical effect of ABA on leaves is to reduce transpirational water loss by closing stomata and parallelly defend against microbes by restricting their entry through stomatal pores. ABA can also promote the accumulation of polyamines, sphingolipids, and even proline. Stomatal closure by compounds other than ABA also helps plant defense against both abiotic and biotic stress factors. Further, ABA can interact with other hormones, such as methyl jasmonate (MJ) and salicylic acid (SA). Such cross-talk can be an additional factor in plant adaptations against environmental stresses and microbial pathogens. The present review highlights the recent progress in understanding ABA's multifaceted role under stress conditions, particularly stomatal closure. We point out the importance of reactive oxygen species (ROS), reactive carbonyl species (RCS), nitric oxide (NO), and Ca²⁺ in guard cells as key signaling components during the ABA-mediated short-term plant defense reactions. The rise in ROS, RCS, NO, and intracellular Ca²⁺ triggered by ABA can promote additional events involved in long-term adaptive measures, including gene expression, accumulation of compatible solutes to protect the cell, hypersensitive response (HR), and programmed cell death (PCD). Several pathogens can counteract and try to reopen stomata. Similarly, pathogens attempt to trigger PCD of host tissue to their benefit. Yet, ABA-induced effects independent of stomatal closure can delay the pathogen spread and infection within leaves. Stomatal closure and other ABA influences can be among the early steps of defense and a crucial component of plants' innate immunity response. Stomatal guard cells are quite sensitive to environmental stress and are considered good model systems for signal transduction studies. Further research on the ABA-induced stomatal closure mechanism can help us design strategies for plant/crop adaptations to stress.

Gene expression changes occurring at bolting time are associated with leaf senescence in Arabidopsis

In plants, the vegetative to reproductive phase transition (termed bolting in Arabidopsis) generally precedes age-dependent leaf senescence (LS). Many studies describe a temporal link between bolting time and LS, as plants that bolt early, senesce early, and plants that bolt late, senesce late. The molecular mechanisms underlying this relationship are unknown and are potentially agriculturally important, as they may allow for the development of crops that can overcome early LS caused by stress-related early-phase transition. We hypothesized that leaf gene expression changes occurring in synchrony with bolting were regulating LS. ARABIDOPSIS TRITHORAX (ATX) enzymes are general methyltransferases that regulate the adult vegetative to reproductive phase transition. We generated an atx1, atx3, and atx4 (atx1,3,4) triple T-DNA insertion mutant that displays both early bolting and early LS. This mutant was used in an RNA-seq time-series experiment to identify gene expression changes in rosette leaves that are likely associated with bolting. By comparing the early bolting mutant to vegetative WT plants of the same age, we were able to generate a list of differentially expressed genes (DEGs) that change expression with bolting as the plants age. We trimmed the list by intersection with publicly available WT datasets, which removed genes from our DEG list that were atx1,3,4 specific. The resulting 398 bolting-associated genes (BAGs) are differentially expressed in a mature rosette leaf at bolting. The BAG list contains many well-characterized LS regulators (ORE1, WRKY45, NAP, WRKY28), and GO analysis revealed enrichment for LS and LS-related processes. These bolting-associated LS regulators may contribute to the temporal coupling of bolting time to LS.

Wheat Transcription Factor TaSNAC11-4B Positively Regulates Leaf Senescence through Promoting ROS Production in Transgenic Arabidopsis

Senescence is the final stage of leaf development which is accompanied by highly coordinated and complicated reprogramming of gene expression. Genetic manipulation of leaf senescence in major crops including wheat has been shown to be able to increase stress tolerance and grain yield. NAC(No apical meristem (NAM), ATAF1/2, and cup-shaped cotyledon (CUC)) transcription factors (TFs) play important roles in regulating gene expression changes during leaf senescence and in response to abiotic stresses. Here, we report the characterization of TaSNAC11-4B (Uniprot: A0A1D5XI64), a wheat NAC family member that acts as a functional homolog of AtNAP, a key regulator of leaf senescence in Arabidopsis. The expression of TaSNAC11-4B was up-regulated with the progression of leaf senescence, in response to abscisic acid (ABA) and drought treatments in wheat. Ectopic expression of TaSNAC11-4B in Arabidopsis promoted ROS accumulation and significantly accelerated age-dependent as well as drought- and ABA-induced leaf senescence. Results from transcriptional activity assays indicated that the TaSNAC11-4B protein displayed transcriptional activation activities that are dependent on its C terminus. Furthermore, qRT-PCR and dual-Luciferase assay results suggested that TaSNAC11-4B could positively regulate the expression of AtrbohD and AtrbohF, which encode catalytic subunits of the ROS-producing NADPH oxidase. Further analysis of TaSNAC11-4B in wheat senescence and the potential application of this gene in manipulating leaf senescence with the purpose of yield increase and stress tolerance is discussed.

Genome-wide identification and characterization of ABA receptor PYL gene family in rice

BACKGROUND

Abscisic acid (ABA), a key phytohormone that controls plant growth and stress responses, is sensed by the pyrabactin resistance 1(PYR1)/PYR1-like (PYL)/regulatory components of the ABA receptor (RCAR) family of proteins. Comprehensive information on evolution and function of PYL gene family in rice (Oryza sativa) needs further investigation. This study made detailed analysis on evolutionary relationship between PYL family members, collinearity, synteny, gene structure, protein motifs, cis-regulatory elements (CREs), SNP variations, miRNAs targeting PYLs and expression profiles in different tissues and stress responses.

RESULTS

Based on sequence homology with Arabidopsis PYL proteins, we identified a total of 13 PYLs in rice (BOP clade) and maize (PACCMAD clade), while other members of BOP (wheat - each diploid genome, barley and Brachypodium) and PACCMAD (sorghum and foxtail millet) have 8-9 PYLs. The phylogenetic analysis divided PYLs into three subfamilies that are structurally and functionally conserved across species. Gene structure and motif analysis of OsPYLs revealed that members of each subfamily have similar gene and motif structure. Segmental duplication appears be the driving force for the expansion of PYLs, and the majority of the PYLs underwent evolution under purifying selection in rice. 32 unique potential miRNAs that might target PYLs were identified in rice. Thus, the predicted regulation of PYLs through miRNAs in rice is more elaborate as compared with B. napus. Further, the miRNAs identified to in this study were also regulated by stresses, which adds additional layer of regulation of PYLs. The frequency of SAPs identified was higher in indica cultivars and were predominantly located in START domain that participate in ABA binding. The promoters of most of the OsPYLs have cis-regulatory elements involved in imparting abiotic stress responsive expression. In silico and q-RT-PCR expression analyses of PYL genes revealed multifaceted role of ABARs in shaping plant development as well as abiotic stress responses.

CONCLUSION

The predicted miRNA mediated regulation of OsPYLs and stress regulated expression of all OsPYLs, at least, under one stress, lays foundation for further validation and fine tuning ABA receptors for stress tolerance without yield penalty in rice.

Drought resistance of Camellia oleifera under drought stress: Changes in physiology and growth characteristics

To investigate the growth, physiological changes and mechanism of drought resistance of Camellia oleifera GWu-2 under drought stress conditions, changes in the main growth and physiological indices of GWu-2 under different water gradients were studied. Factor analysis was used to study the differences between indicators under different water gradients, and correlation analysis was implemented to analyze the relationship between different factors. We observed that the growth state, enzyme secretion, stomatal morphology and leaf osmotic adjustment substances were significantly affected by drought stress. In particular, increases in leaf abscisic acid (ABA), indole acetic acid (IAA) and methyl jasmonate (MeJA) contents under drought stress were negatively correlated with the stomatal opening degree, and the ratio of ZR/GA3 was significantly correlated with the growth and physiological indicators of GWu-2, indicating that different hormones respond differently to drought stress and have different functions in the growth regulation and drought resistance of GWu-2. We concluded that the drought resistance mechanism of GWu-2 was controlled by maintaining root growth to obtain the necessary water, increasing the contents of osmotic substances of leaves to maintain water holding capacity, reducing the transpiration of water by increasing leaf ABA, IAA and MeJA content to close stomata and reducing the damage caused by drought by increasing the activity of superoxide dismutase (SOD).

Comparative functional analyses of DWARF14 and KARRIKIN INSENSITIVE 2 in drought adaptation of Arabidopsis thaliana

Functional analyses of various strigolactone-deficient mutants have demonstrated that strigolactones enhance drought resistance; however, the mechanistic involvement of the strigolactone receptor DWARF14 (D14) in this trait remains elusive. In this study, loss-of-function analysis of the D14 gene in Arabidopsis thaliana revealed that d14 mutant plants were more drought-susceptible than wild-type plants, which was associated with their larger stomatal aperture, slower abscisic acid (ABA)-mediated stomatal closure, lower anthocyanin content and delayed senescence under drought stress. Transcriptome analysis revealed a consistent alteration in the expression levels of many genes related to the observed physiological and biochemical changes in d14 plants when compared with the wild type under normal and dehydration conditions. A comparative drought resistance assay confirmed that D14 plays a less critical role in Arabidopsis drought resistance than its paralog karrikin receptor KARRIKIN INSENSITIVE 2 (KAI2). In-depth comparative analyses of the single mutants d14 and kai2 and the double mutant d14 kai2, in relation to various drought resistance-associated mechanisms, revealed that D14 and KAI2 exhibited a similar effect on stomatal closure. On the other hand, D14 had a lesser role in the maintenance of cell membrane integrity, leaf cuticle structure and ABA-induced leaf senescence, but a greater role in drought-induced anthocyanin biosynthesis, than KAI2. Interestingly, a possible additive relationship between D14 and KAI2 could be observed in regulating cell membrane integrity and leaf cuticle development. In addition, our findings also suggest the existence of a complex interaction between the D14 and ABA signaling pathways in the adaptation of Arabidopsis to drought.

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.

Signal Transduction in Leaf Senescence: Progress and Perspective

Leaf senescence is a degenerative process that is genetically controlled and involves nutrient remobilization prior to the death of leaf tissues. Age is a key developmental determinant of the process along with other senescence inducing factors. At the cellular level, different hormones, signaling molecules, and transcription factors contribute to the regulation of senescence. This review summarizes the recent progress in understanding the complexity of the senescence process with primary focuses on perception and transduction of senescence signals as well as downstream regulatory events. Future directions in this field and potential applications of related techniques in crop improvement will be discussed.

Genome-wide identification and characterization of ABA receptor PYL/RCAR gene family reveals evolution and roles in drought stress in Nicotiana tabacum

BACKGROUND

Abscisic acid (ABA) is an important phytohormone for plant growth, development and responding to stresses such as drought, salinity, and pathogen infection. Pyrabactin Resistance 1 (PYR1)/PYR1-Like (PYL)/Regulatory Component of ABA Receptor (RCAR) (hereafter referred to as PYLs) has been identified as the ABA receptors. The PYL family members have been well studied in many plants. However, the members of PYL family have not been systematically identified at genome level in cultivated tobacco (Nicotiana tabacum) and its two ancestors. In this study, the phylogenic relationships, chromosomal distribution, gene structures, conserved motifs/regions, and expression profiles of NtPYLs were analyzed.

RESULTS

We identified 29, 11, 16 PYLs in the genomes of allotetraploid N. tabacum, and its two diploid ancestors N. tomentosiformis and N. sylvestris, respectively. The phylogenetic analysis revealed that NtPYLs can be divided into three subfamilies, and each NtPYL has one counterpart in N. sylvestris or N. tomentosiformis. Based on microarray analysis of NtPYL transcripts, four NtPYLs (from subfamily II, III), and five NtPYLs (from subfamily I) are highlighted as potential candidates for further functional characterization in N. tabacum seed development, response to ABA, and germination, and resistance to abiotic stresses, respectively. Interestingly, the expression profiles of members in the same NtPYL subfamily showed somehow similar patterns in tissues at different developmental stages and in leaves of seedlings under drought stress, suggesting particular NtPYLs might have multiple functions in both plant development and drought stress response.

CONCLUSIONS

NtPYLs are highlighted for important functions in seed development, germination and response to ABA, and particular in drought tolerance. This work will not only shed light on the PYL family in tobacco, but also provides some valuable information for functional characterization of ABA receptors in N. tabacum.

Genome-wide identification and comparative analysis of drought-related microRNAs in two maize inbred lines with contrasting drought tolerance by deep sequencing

Drought has become one of the most serious abiotic stresses influencing crop production worldwide. Understanding the molecular regulatory networks underlying drought adaption and tolerance in crops is of great importance for future breeding. microRNAs (miRNAs), as important components of post-transcriptional regulation, play crucial roles in drought response and adaptation in plants. Here, we report a miRNome analysis of two maize inbred lines with contrasting levels of drought tolerance under soil drought in the field. Differential expression analysis showed 11 and 34 miRNAs were uniquely responded to drought in H082183 (drought tolerant) and Lv28 (drought sensitive), respectively, in leaves. In roots, 19 and 23 miRNAs uniquely responded to drought in H082183 and Lv28, respectively. Expression analysis of these drought-responsive miRNA-mRNA modules revealed miR164-MYB, miR164-NAC, miR159-MYB, miR156-SPL and miR160-ARF showed a negative regulatory relationship. Further analysis showed that the miR164-MYB and miR164-NAC modules in the tolerant line modulated the stress response in an ABA (abscisic acid)-dependent manner, while the miR156-SPL and miR160-ARF modules in the sensitive line participated in the inhibition of metabolism in drought-exposed leaves. Together, our results provide new insight into not only drought-tolerance-related miRNA regulation networks in maize but also key miRNAs for further characterization and improvement of maize drought tolerance.

The adaptation strategies of Herpetospermum pedunculosum (Ser.) Baill at altitude gradient of the Tibetan plateau by physiological and metabolomic methods

BACKGROUND

Herpetospermum pedunculosum (Ser.) Baill is annual scandent herbs. They are used in the treatment of piles, inflammation of the stomach and the intestines. It can survive the extreme environment of the Tibetan Plateau (TP). However, the underlying mechanisms of this adaptation to H. pedunculosum from TP remain unclear. Here, we combined physiological and metabolomics methods to analyze H. pedunculosum response to altitude gradient differences.

RESULTS

At high altitude, increases in the activities of Ascorbate peroxidase (APX), Glutathione reductase (GR), Dehydroascorbate reductase (DHAR), Monodehydroascorbate reductase (MDHAR), Superoxide dismutase (SOD) have been observed in leaves. Total Glutathion content, total Ascorbate content and the ASA (ascorbic acid)/docosahexaenoic acid (DHA) ration were highly elevated from low altitude to high altitude. In addition, high altitude induces decrease of the Anthocyanidin content (ANTH) and increase of abscisic acid content (ABA). The GC-MS analyses identified of 50 metabolites from leaves of H. pedunculosum. In addition, a metabolic network was constructed based on metabolomic datasets using a weighted correlation network analysis (WGCNA) approach. The network analysis uncovered 4 distinguished metabolic modules highly associated with I, II, III and IV respectively. Furthermore, the analysis successfully classified 50 samples into seven groups: carbohydrates, amino acids, organic acids, lipid components, polyamine, secondary metabolism and others.

CONCLUSIONS

In the present study, the content of parts of amino acid components increased in samples collected at higher altitudes, and most of metabolites, including carbohydrates and organic acids were assigned to the carbon metabolic pathway comprising reductive pentose phosphate pathway, glycolysis and TCA cycle, indicating the direct relationship between adaptability and the carbon metabolic pathway and amino acids in H. pedunculosum response to high altitude. The results of this study laid the foundation of the molecular mechanism on H. pedunculosum from high altitude.

Tomato fruit ripening factor NOR controls leaf senescence

NAC transcription factors (TFs) are important regulators of expressional reprogramming during plant development, stress responses, and leaf senescence. NAC TFs also play important roles in fruit ripening. In tomato (Solanum lycopersicum), one of the best characterized NACs involved in fruit ripening is NON-RIPENING (NOR), and the non-ripening (nor) mutation has been widely used to extend fruit shelf life in elite varieties. Here, we show that NOR additionally controls leaf senescence. Expression of NOR increases with leaf age, and developmental as well as dark-induced senescence are delayed in the nor mutant, while overexpression of NOR promotes leaf senescence. Genes associated with chlorophyll degradation as well as senescence-associated genes (SAGs) show reduced and elevated expression, respectively, in nor mutants and NOR overexpressors. Overexpression of NOR also stimulates leaf senescence in Arabidopsis thaliana. In tomato, NOR supports senescence by directly and positively regulating the expression of several senescence-associated genes including, besides others, SlSAG15 and SlSAG113, SlSGR1, and SlYLS4. Finally, we find that another senescence control NAC TF, namely SlNAP2, acts upstream of NOR to regulate its expression. Our data support a model whereby NAC TFs have often been recruited by higher plants for both the control of leaf senescence and fruit ripening.

Micromanagement of Developmental and Stress-Induced Senescence: The Emerging Role of MicroRNAs

MicroRNAs are short (19⁻24-nucleotide-long), non-coding RNA molecules. They downregulate gene expression by triggering the cleavage or translational inhibition of complementary mRNAs. Senescence is a stage of development following growth completion and is dependent on the expression of specific genes. MicroRNAs control the gene expression responsible for plant competence to answer senescence signals. Therefore, they coordinate the juvenile-to-adult phase transition of the whole plant, the growth and senescence phase of each leaf, age-related cellular structure changes during vessel formation, and remobilization of resources occurring during senescence. MicroRNAs are also engaged in the ripening and postharvest senescence of agronomically important fruits. Moreover, the hormonal regulation of senescence requires microRNA contribution. Environmental cues, such as darkness or drought, induce senescence-like processes in which microRNAs also play regulatory roles. In this review, we discuss recent findings concerning the role of microRNAs in the senescence of various plant species.

It's Hard to Avoid Avoidance: Uncoupling the Evolutionary Connection between Plant Growth, Productivity and Stress "Tolerance"

In the last 100 years, agricultural developments have favoured selection for highly productive crops, a fact that has been commonly associated with loss of key traits for environmental stress tolerance. We argue here that this is not exactly the case. We reason that high yield under near optimal environments came along with hypersensitization of plant stress perception and consequently early activation of stress avoidance mechanisms, such as slow growth, which were originally needed for survival over long evolutionary time periods. Therefore, mechanisms employed by plants to cope with a stressful environment during evolution were overwhelmingly geared to avoid detrimental effects so as to ensure survival and that plant stress "tolerance" is fundamentally and evolutionarily based on "avoidance" of injury and death which may be referred to as evolutionary avoidance (EVOL-Avoidance). As a consequence, slow growth results from being exposed to stress because genes and genetic programs to adjust growth rates to external circumstances have evolved as a survival but not productivity strategy that has allowed extant plants to avoid extinction. To improve productivity under moderate stressful conditions, the evolution-oriented plant stress response circuits must be changed from a survival mode to a continued productivity mode or to avoid the evolutionary avoidance response, as it were. This may be referred to as Agricultural (AGRI-Avoidance). Clearly, highly productive crops have kept the slow, reduced growth response to stress that they evolved to ensure survival. Breeding programs and genetic engineering have not succeeded to genetically remove these responses because they are polygenic and redundantly programmed. From the beginning of modern plant breeding, we have not fully appreciated that our crop plants react overly-cautiously to stress conditions. They over-reduce growth to be able to survive stresses for a period of time much longer than a cropping season. If we are able to remove this polygenic redundant survival safety net we may improve yield in moderately stressful environments, yet we will face the requirement to replace it with either an emergency slow or no growth (dormancy) response to extreme stress or use resource management to rescue crops under extreme stress (or both).