Rheostatic Control of ABA Signaling through HOS15-Mediated OST1 Degradation

The lowdown on breakdown: Open questions in plant proteolysis

Proteolysis, including post-translational proteolytic processing as well as protein degradation and amino acid recycling, is an essential component of the growth and development of living organisms. In this article, experts in plant proteolysis pose and discuss compelling open questions in their areas of research. Topics covered include the role of proteolysis in the cell cycle, DNA damage response, mitochondrial function, the generation of N-terminal signals (degrons) that mark many proteins for degradation (N-terminal acetylation, the Arg/N-degron pathway, and the chloroplast N-degron pathway), developmental and metabolic signaling (photomorphogenesis, abscisic acid and strigolactone signaling, sugar metabolism, and postharvest regulation), plant responses to environmental signals (endoplasmic-reticulum-associated degradation, chloroplast-associated degradation, drought tolerance, and the growth-defense trade-off), and the functional diversification of peptidases. We hope these thought-provoking discussions help to stimulate further research.

N-glycosylation of SnRK2s affects NADPH maintenance in peroxisomes during prolonged ABA signalling

Unfavourable conditions, such as prolonged drought and high salinity, pose a threat to the survival and agricultural yield of plants. The phytohormone ABA plays a key role in the regulation of plant stress adaptation and is often maintained at high levels for extended periods. While much is known about ABA signal perception and activation in the early signalling stage, the molecular mechanism underlying desensitization of ABA signalling remains largely unknown. Here we demonstrate that in the endoplasmic reticulum (ER)-Golgi network, the key regulators of ABA signalling, SnRK2.2/2.3, undergo N-glycosylation, which promotes their redistribution from the nucleus to the peroxisomes in Arabidopsis roots and influences the transcriptional response in the nucleus during prolonged ABA signalling. On the peroxisomal membrane, SnRK2s can interact with glucose-6-phosphate (G6P)/phosphate translocator 1 (GPT1) to maintain NADPH homeostasis through increased activity of the peroxisomal oxidative pentose phosphate pathway (OPPP). The resulting maintenance of NADPH is essential for the modulation of hydrogen peroxide (H2O2) accumulation, thereby relieving ABA-induced root growth inhibition. The subcellular dynamics of SnRK2s, mediated by N-glycosylation suggest that ABA responses transition from transcriptional regulation in the nucleus to metabolic processes in the peroxisomes, aiding plants in adapting to long-term environmental stress.

Research Progress in the Regulation of the ABA Signaling Pathway by E3 Ubiquitin Ligases in Plants

E3 ubiquitin ligases (UBLs), as enzymes capable of specifically recognizing target proteins in the process of protein ubiquitination, play crucial roles in regulating responses to abiotic stresses such as drought, salt, and temperature. Abscisic acid (ABA), a plant endogenous hormone, is essential to regulating plant growth, development, disease resistance, and defense against abiotic stresses, and acts through a complex ABA signaling pathway. Hormone signaling transduction relies on protein regulation, and E3 ubiquitin ligases play important parts in regulating the ABA pathway. Therefore, this paper reviews the ubiquitin-proteasome-mediated protein degradation pathway, ABA-related signaling pathways, and the regulation of ABA-signaling-pathway-related genes by E3 ubiquitin ligases, aiming to provide references for further exploration of the relevant research on how plant E3 ubiquitin ligases regulate the ABA pathway.

ABA INSENSITIVE 2 promotes flowering by inhibiting OST1/ABI5-dependent FLOWERING LOCUS C transcription in Arabidopsis

The plant hormone abscisic acid (ABA) is an important regulator of plant growth and development and plays a crucial role in both biotic and abiotic stress responses. ABA modulates flowering time, but the precise molecular mechanism remains poorly understood. Here we report that ABA INSENSITIVE 2 (ABI2) is the only phosphatase from the ABA-signaling core that positively regulates the transition to flowering in Arabidopsis. Loss-of-function abi2-2 mutant shows significantly delayed flowering both under long day and short day conditions. Expression of floral repressor genes such as FLOWERING LOCUS C (FLC) and CYCLING DOF FACTOR 1 (CDF1) was significantly up-regulated in abi2-2 plants while expression of the flowering promoting genes FLOWERING LOCUS T (FT) and SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) was down-regulated. Through genetic interactions we further found that ost1-3 and abi5-1 mutations are epistatic to abi2-2, as both of them individually rescued the late flowering phenotype of abi2-2. Interestingly, phosphorylation and protein stability of ABA INSENSITIVE 5 (ABI5) were enhanced in abi2-2 plants suggesting that ABI2 dephosphorylates ABI5, thereby reducing protein stability and the capacity to induce FLC expression. Our findings uncovered the unexpected role of ABI2 in promoting flowering by inhibiting ABI5-mediated FLC expression in Arabidopsis.

Integrative regulatory mechanisms of stomatal movements under changing climate

Global climate change-caused drought stress, high temperatures and other extreme weather profoundly impact plant growth and development, restricting sustainable crop production. To cope with various environmental stimuli, plants can optimize the opening and closing of stomata to balance CO2 uptake for photosynthesis and water loss from leaves. Guard cells perceive and integrate various signals to adjust stomatal pores through turgor pressure regulation. Molecular mechanisms and signaling networks underlying the stomatal movements in response to environmental stresses have been extensively studied and elucidated. This review focuses on the molecular mechanisms of stomatal movements mediated by abscisic acid, light, CO2 , reactive oxygen species, pathogens, temperature, and other phytohormones. We discussed the significance of elucidating the integrative mechanisms that regulate stomatal movements in helping design smart crops with enhanced water use efficiency and resilience in a climate-changing world.

Genome-wide identification and characterization of the RZFP gene family and analysis of its expression pattern under stress in Populus trichocarpa

Forest trees face many abiotic stressors during their lifetime, including drought, heavy metals, high salinity, and chills, affecting their quality and yield. The RING-type ubiquitin ligase E3 is an invaluable component of the ubiquitin-proteasome system (UPS) and participates in plant growth and environmental interactions. Interestingly, only a few studies have explored the RING ZINC FINGER PROTEIN (RZFP) gene family. This study identified eight PtrRZFPs genes in the Populus genome, and their molecular features were analyzed. Gene structure analysis revealed that all PtrRZFPs genes contained >10 introns. Evolutionarily, the RZFPs were separated into four categories, and segmental replication events facilitated their amplification. Notably, many stress-related elements have been identified in the promoters of PtrRZFPs using Cis-acting element analysis. Moreover, some PtrRZFPs were significantly induced by drought and sorbitol, revealing their potential roles in regulating stress responses. Particularly, overexpression of the PtrRZFP1 gene in poplars conferred excellent drought tolerance; however, PtrRZFP1 knockdown plants were drought-sensitive. We identified the potential upstream transcription factors of PtrRZFPs and revealed the possible biological functions of RZFP1/4/7 in resisting osmotic and salt stress, laying the foundation for subsequent biological function studies and providing genetic resources for genetic engineering breeding for drought resistance in forest trees. This study offers crucial information for the further exploration of the functions of RZFPs in poplars.

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

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

RING finger ubiquitin E3 ligase CsCHYR1 targets CsATAF1 for degradation to modulate the drought stress response of cucumber through the ABA-dependent pathway

CsCHYR1 (CHY ZINC-FINGER AND RING PROTEIN1) encodes a RING (Really Interesting New Gene) finger E3 ubiquitin ligase involved in ubiquitin-mediated protein degradation and plays an important role for cucumber to resist drought stress. Here, we obtain one of the candidate proteins CsCHYR1 that probably interacts with CsATAF1 by yeast-two hybrid screening. Subsequently, it is verified that CsCHYR1 interacts with CsATAF1 and has self-ubiquitination activity. When the cysteine residue at 180 in the RING domain of CsCHYR1 is replaced by serine or alanine, ubiquitin could not be transported from E2 to the substrate. CsCHYR1 ubiquitinates CsATAF1 and affects the stability of CsATAF1 when plants are subjected to drought stress. The expression level of CsCHYR1 is increased by 4-fold after ABA treatment at 9 h. The Atchyr1 mutants perform an ABA-hyposensitive phenotype and have a lower survival rate than Col-0 and CsCHYR1 Atchyr1 lines. In addition, CsCHYR1 interacts with CsSnRK2.6. Therefore, our study reveals a CsSnRK2.6-CsCHYR1-CsATAF1 complex to promote the drought stress response by decreasing CsATAF1 protein accumulation and inducing stomatal closure. Those findings provide new ideas for cucumber germplasm innovation from the perspective of biochemistry and molecular biology.

Heterologous expression of bacterial dehydrin gene in Arabidopsis thaliana promotes abiotic stress tolerance

Salinity, low temperature, and drought are major environmental factors in agriculture leading to reduced crop yield. Dehydrins (DHNs) are induced transcriptionally during cellular dehydration and accumulate in different tissues during abiotic stresses. Here we isolated and characterized a bacterial gene BG757 in Arabidopsis, encoding a putative dehydrin type protein. ABA induces the expression of various dehydrins in plants, therefore, to elucidate the potential role, ABA sensitivity was examined in Arabidopsis transgenic lines expressing BG757. Interestingly, BG757-expressing plants showed hypersensitivity towards NaCl and ABA during seed germination. In addition to germination, BG757-expressing plants also showed root growth retardation in the presence of ABA and NaCl when compared with wild type (WT), suggesting that BG757 positively regulate salt stress and ABA response. Furthermore, BG757-expressing plants showed significant drought tolerance compared with WT. Consistent with drought tolerance, expression levels of stress inducible genes (DREB2A, RD22, RD26, LEA7 and SOS1) were strongly upregulated in transgenic plants compared with WT. All together these results suggest that heterologous expression of bacterial gene, BG757 in plants promotes resistance to environmental stresses.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12298-023-01358-w.

Tyrosylprotein sulfotransferase suppresses ABA signaling via sulfation of SnRK2.2/2.3/2.6

Phytohormone abscisic acid (ABA) plays vital roles in stress tolerance, while long-term overactivation of ABA signaling suppresses plant growth and development. However, the braking mechanism of ABA responses is not clear. Protein tyrosine sulfation catalyzed by tyrosylprotein sulfotransferase (TPST) is a critical post-translational modification. Through genetic screening, we identified a tpst mutant in Arabidopsis that was hypersensitive to ABA. In-depth analysis revealed that TPST could interact with and sulfate SnRK2.2/2.3/2.6, which accelerated their degradation and weakened the ABA signaling. Taken together, these findings uncovered a novel mechanism of desensitizing ABA responses via protein sulfation.

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.

MIW1 participates in ABA signaling through the regulation of MYB30 in Arabidopsis

Seed germination and seedling establishment are critical biological processes, and their underlying molecular mechanisms have practical implications. The ABA signaling during seed germination and early seedling development is negatively regulated by transcription factor MYB30, but its interaction partners and downstream targets are not fully understood. In this study, we identified MIW1 (MYB30-interacting WD40 protein 1), a WD40 protein that could interact with MYB30 and promote its degradation. In the miw1 mutant, the MYB30 protein became more stable. MIW1 enhanced the ABA-mediated inhibition of postgerminative development. The miw1 mutants became hyposensitive to exogenous ABA, and this effect was suppressed by mutations in MYB30. Furthermore, we found that MYB30 negatively regulated the expression of the ABA receptor genes PYR1/PYL/RCARs. The changes in PYLs expression during early seedling development or under ABA treatment became more pronounced in the myb30 mutant. ChIP-qPCR analyses showed MYB30 could directly bind to the promoters of PYL11 and PYL12. Our study reveals that the WD40 protein MIW1 promotes the expression of PYLs by destabilizing MYB30, thus positively regulating the ABA signaling during postgermination in Arabidopsis.

Advances in Receptor-like Protein Kinases in Balancing Plant Growth and Stress Responses

Accompanying the process of growth and development, plants are exposed to ever-changing environments, which consequently trigger abiotic or biotic stress responses. The large protein family known as receptor-like protein kinases (RLKs) is involved in the regulation of plant growth and development, as well as in the response to various stresses. Understanding the biological function and molecular mechanism of RLKs is helpful for crop breeding. Research on the role and mechanism of RLKs has recently received considerable attention regarding the balance between plant growth and environmental adaptability. In this paper, we systematically review the classification of RLKs, the regulatory roles of RLKs in plant development (meristem activity, leaf morphology and reproduction) and in stress responses (disease resistance and environmental adaptation). This review focuses on recent findings revealing that RLKs simultaneously regulate plant growth and stress adaptation, which may pave the way for the better understanding of their function in crop improvement. Although the exact crosstalk between growth constraint and plant adaptation remains elusive, a profound study on the adaptive mechanisms for decoupling the developmental processes would be a promising direction for the future research.

Negative regulation of floral transition in Arabidopsis by HOS15-PWR-HDA9 complex

Arabidopsis HOS15/PWR/HDA9 repressor complex, which is similar to the TBL1/NcoR1/HDAC complex in animals, plays a well-known role in epigenetic regulation. PWR and HDA9 have been reported to interact with each other and modulate the flowering time by repressing AGL19 expression, whereas HOS15 and HDA9, together with the photoperiodic evening complex, regulate flowering time through repression of GI transcription. However, the role of the HOS15/PWR/HDA9 core repressor complex as a functional unit in the regulation of flowering time is yet to be explored. In this study, we reported that the loss-of-function hos15-2/pwr/hda9 triple mutant accumulates higher transcript levels of AGL19 and exhibits an early flowering phenotype similar to those of hos15, pwr, and hda9 single mutants. Interestingly, the accumulation of HOS15 in the nucleus was drastically reduced in pwr and hda9 mutants. As a result, HOS15 could not perform its role in histone deacetylation or interaction with H3 in the nucleus. Furthermore, HOS15 is also associated with the same region of the AGL19 promoter known for PWR-HDA9 binding. The acetylation level of the AGL19 promoter was increased in the hos15-2 mutant, similar to the pwr and hda9 mutants. Therefore, our findings reveal that the HOS15/PWR/HDA9 repressor complex deacetylates the promoter region of AGL19, thereby negatively regulating AGL19 transcription, which leads to early flowering in Arabidopsis.

NUCLEAR PORE ANCHOR and EARLY IN SHORT DAYS 4 negatively regulate abscisic acid signaling by inhibiting Snf1-related protein kinase2 activity and stability in Arabidopsis

Abscisic acid (ABA) is a key regulator of plant responses to abiotic stresses, such as drought. Abscisic acid receptors and coreceptors perceive ABA to activate Snf1-related protein kinase2s (SnRK2s) that phosphorylate downstream effectors, thereby activating ABA signaling and the stress response. As stress responses come with fitness penalties for plants, it is crucial to tightly control SnRK2 kinase activity to restrict ABA signaling. However, how SnRK2 kinases are inactivated remains elusive. Here, we show that NUCLEAR PORE ANCHOR (NUA), a nuclear pore complex (NPC) component, negatively regulates ABA-mediated inhibition of seed germination and post-germination growth, and drought tolerance in Arabidopsis thaliana. The role of NUA in response to ABA depends on SnRK2.2 and SnRK2.3 for seed germination and on SnRK2.6 for drought. NUA does not directly inhibit the phosphorylation of these SnRK2s or affects their abundance. However, the NUA-interacting protein EARLY IN SHORT DAYS 4 (ESD4), a SUMO protease, negatively regulates ABA signaling by directly interacting with and inhibiting SnRK2 phosphorylation and protein levels. More importantly, we demonstrated that SnRK2.6 can be SUMOylated in vitro, and ESD4 inhibits its SUMOylation. Taken together, we identified NUA and ESD4 as SnRK2 kinase inhibitors that block SnRK2 activity, and reveal a mechanism whereby NUA and ESD4 negatively regulate plant responses to ABA and drought stress possibly through SUMOylation-dependent regulation of SnRK2s.

NUA and ESD4 negatively regulate ABA signaling during seed germination

The phytohormone abscisic acid (ABA) plays important roles in plant growth, development and adaptative responses to abiotic stresses. SNF1-related protein kinase 2s (SnRK2) are key components that activate the ABA core signaling pathway. NUCLEAR PORE ANCHOR (NUA) is a component of the nuclear pore complex (NPC) that involves in deSUMOylation through physically interacting with the EARLY IN SHORT DAYS 4 (ESD4) SUMO protease. However, it is not clear how NUA functions with SnRK2 and ESD4 to regulate ABA signaling. In our study, we found that nua loss-of-function mutants exhibited pleiotropic ABA-hypersensitive phenotype. We also found that ABA-responsive genes remarkably up-regulated in nua by exogenous ABA. The nua snrk2.2 snrk2.3 triple mutant and nua abi5 double mutant partially rescued the ABA-hypersensitive phenotype of nua, thereby suggesting that NUA is epistatic to SnRK2s. Additionally, we observed that esd4-3 mutant was also ABA-hypersensitive. NUA and ESD4 were further demonstrated to physically interact with SnRK2s and negatively regulate ABA signaling by reducing SnRK2s stability. Taken together, our findings uncover a new regulatory mechanism that can modulate ABA signaling.

The Combined Effect of Heat and Osmotic Stress on Suberization of Arabidopsis Roots

The simultaneous occurrence of heat stress and drought is becoming more regular as a consequence of climate change, causing extensive agricultural losses. The application of either heat or osmotic stress increase cell-wall suberization in different tissues, which may play a role in improving plant resilience. In this work, we studied how the suberization process is affected by the combination of drought and heat stress by following the expression of suberin biosynthesis genes, cell-wall suberization and the chemical composition in Arabidopsis roots. The Arabidopsis plants used in this study were at the onset of secondary root development. At this point, one can observe a developmental gradient in the main root, with primary development closer to the root tip and secondary development, confirmed by the suberized phellem, closer to the shoot. Remarkably, we found a differential response depending on the root zone. The combination of drought and heat stress increased cell wall suberization in main root segments undergoing secondary development and in lateral roots (LRs), while the main root zone, at primary development stage, was not particularly affected. We also found differences in the overall chemical composition of the cell walls in both root zones in response to combined stress. The data gathered showed that, under combined drought and heat stress, Arabidopsis roots undergo differential cell wall remodeling depending on developmental stage, with modifications in the biosynthesis and/or assembly of major cell wall components.

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

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

The deubiquitinases UBP12 and UBP13 integrate with the E3 ubiquitin ligase XBAT35.2 to modulate VPS23A stability in ABA signaling

Ubiquitination-mediated protein degradation in both the 26S proteasome and vacuole is an important process in abscisic acid (ABA) signaling. However, the role of deubiquitination in this process remains elusive. Here, we demonstrate that two deubiquitinating enzymes (DUBs), ubiquitin-specific protease 12 (UBP12) and UBP13, modulate ABA signaling and drought tolerance by deubiquitinating and stabilizing the endosomal sorting complex required for transport-I (ESCRT-I) component vacuolar protein sorting 23A (VPS23A) and thereby affect the stability of ABA receptors in Arabidopsis thaliana. Genetic analysis showed that VPS23A overexpression could rescue the ABA hypersensitive and drought tolerance phenotypes of ubp12-2w or ubp13-1. In addition to the direct regulation of VPS23A, we found that UBP12 and UBP13 also stabilized the E3 ligase XB3 ortholog 5 in A. thaliana (XBAT35.2) in response to ABA treatment. Hence, we demonstrated that UBP12 and UBP13 are previously unidentified rheostatic regulators of ABA signaling and revealed a mechanism by which deubiquitination precisely monitors the XBAT35/VPS23A ubiquitination module in the ABA response.

The MIEL1-ABI5/MYB30 regulatory module fine tunes abscisic acid signaling during seed germination

The transcription factor ABSCISIC ACID INSENSITIVE5 (ABI5) plays a crucial role in abscisic acid (ABA) signaling during seed germination. However, how ABI5 is regulated during this process is poorly understood. Here, we report that the ubiquitin E3 ligase MIEL1 and its target transcription factor MYB30 modulate ABA responses in Arabidopsis thaliana during seed germination and seedling establishment via the precise regulation of ABI5. MIEL1 interacts with and ubiquitinates ABI5 to facilitate its degradation during germination. The transcription factor MYB30, whose turnover is mediated by MIEL1 during seed germination, also interacts with ABI5 to interfere with its transcriptional activity. MYB30 functions downstream of MIEL1 in the ABA response, and both are epistatic to ABI5 in ABA-mediated inhibition of seed germination and postgerminative growth. ABA treatment induces the degradation of MIEL1 and represses the interaction between MIEL1 and ABI5/MYB30, thus releasing both ABI5 and MYB30. Our results demonstrate that MIEL1 directly mediates the proteasomal degradation of ABI5 and inhibits its activity via the release of its target protein MYB30, thus ensuring precise ABA signaling during seed germination and seedling establishment.

Non-Expresser of PR-Genes 1 Positively Regulates Abscisic Acid Signaling in Arabidopsis thaliana

The plant hormone, abscisic acid (ABA), is not only important for promoting abiotic stress responses but also plays a versatile and crucial role in plant immunity. The pathogen infection-induced dynamic accumulation of ABA mediates the degradation of non-expresser of PR genes 1 (NPR1) through the CUL3^NPR3NPR4 proteasome pathway. However, the functional significance of NPR1 degradation by other E3 ligases in response to ABA remains unclear. Here, we report that NPR1 is induced transcriptionally by ABA and that npr1-1 mutation results in ABA insensitivity during seed germination and seedling growth. Mutants lacking NPR1 downregulate the expression of ABA-responsive transcription factors ABA INSENSITIVE4 (ABI4) and ABA INSENSITIVE5 (ABI5), and that of their downstream targets EM6, RAB18, RD26, and RD29B. The npr1-1 mutation also affects the transcriptional activity of WRKY18, which activates WRKY60 in the presence of ABA. Furthermore, NPR1 directly interacts with and is degraded by HOS15, a substrate receptor for the DDB1-CUL4 ubiquitin E3 ligase complex. Collectively, our findings demonstrate that NPR1 acts as a positive regulator of ABA-responsive genes, whereas HOS15 promotes NPR1 degradation in a proteasome-dependent manner.

The Transcriptional Corepressor HOS15 Mediates Dark-Induced Leaf Senescence in Arabidopsis

Multiple endogenous and environmental signals regulate the intricate and highly complex processes driving leaf senescence in plants. A number of genes have been identified in a variety of plant species, including Arabidopsis, which influence leaf senescence. Previously, we have shown that HOS15 is a multifunctional protein that regulates several physiological processes, including plant growth and development under adverse environmental conditions. HOS15 has also been reported to form a chromatin remodeling complex with PWR and HDA9 and to regulate the chromatin structure of numerous genes. However, unlike PWR and HDA9, the involvement of HOS15 in leaf senescence is yet to be identified. Here, we report that HOS15, together with PWR and HDA9, promotes leaf senescence via transcriptional regulation of SAG12/29, senescence marker genes, and CAB1/RCBS1A, photosynthesis-related genes. The expression of ORE1, SAG12, and SAG29 was downregulated in hos15-2 plants, whereas the expression of photosynthesis-related genes, CAB1 and RCBS1A, was upregulated. HOS15 also promoted senescence through dark stress, as its mutation led to a much greener phenotype than that of the WT. Phenotypes of double and triple mutants of HOS15 with PWR and HDA9 produced phenotypes similar to those of a single hos15-2. In line with this observation, the expression levels of NPX1, APG9, and WRKY57 were significantly elevated in hos15-2 and hos15/pwr, hos15/hda9, and hos15/pwr/hda9 mutants compared to those in the WT. Surprisingly, the total H3 acetylation level decreased in age-dependent manner and under dark stress in WT; however, it remained the same in hos15-2 plants regardless of dark stress, suggesting that dark-induced deacetylation requires functional HOS15. More interestingly, the promoters of APG9, NPX1, and WRKY57 were hyperacetylated in hos15-2 plants compared to those in WT plants. Our data reveal that HOS15 acts as a positive regulator and works in the same repressor complex with PWR and HDA9 to promote leaf senescence through aging and dark stress by repressing NPX1, APG9, and WRKY57 acetylation.

Systematic modeling-driven experiments identify distinct molecular clockworks underlying hierarchically organized pacemaker neurons

In metazoan organisms, circadian (∼24 h) rhythms are regulated by pacemaker neurons organized in a master-slave hierarchy. Although it is widely accepted that master pacemakers and slave oscillators generate rhythms via an identical negative feedback loop of transcription factor CLOCK (CLK) and repressor PERIOD (PER), their different roles imply heterogeneity in their molecular clockworks. Indeed, in Drosophila, defective binding between CLK and PER disrupts molecular rhythms in the master pacemakers, small ventral lateral neurons (sLN_vs), but not in the slave oscillator, posterior dorsal neuron 1s (DN1_ps). Here, we develop a systematic and expandable approach that unbiasedly searches the source of the heterogeneity in molecular clockworks from time-series data. In combination with in vivo experiments, we find that sLN_vs exhibit higher synthesis and turnover of PER and lower CLK levels than DN1_ps. Importantly, light shift analysis reveals that due to such a distinct molecular clockwork, sLN_vs can obtain paradoxical characteristics as the master pacemaker, generating strong rhythms that are also flexibly adjustable to environmental changes. Our results identify the different characteristics of molecular clockworks of pacemaker neurons that underlie hierarchical multi-oscillator structure to ensure the rhythmic fitness of the organism.

Effective Use of Water in Crop Plants in Dryland Agriculture: Implications of Reactive Oxygen Species and Antioxidative System

Under dryland conditions, annual and perennial food crops are exposed to dry spells, severely affecting crop productivity by limiting available soil moisture at critical and sensitive growth stages. Climate variability continues to be the primary cause of uncertainty, often making timing rather than quantity of precipitation the foremost concern. Therefore, mitigation and management of stress experienced by plants due to limited soil moisture are crucial for sustaining crop productivity under current and future harsher environments. Hence, the information generated so far through multiple investigations on mechanisms inducing drought tolerance in plants needs to be translated into tools and techniques for stress management. Scope to accomplish this exists in the inherent capacity of plants to manage stress at the cellular level through various mechanisms. One of the most extensively studied but not conclusive physiological phenomena is the balance between reactive oxygen species (ROS) production and scavenging them through an antioxidative system (AOS), which determines a wide range of damage to the cell, organ, and the plant. In this context, this review aims to examine the possible roles of the ROS-AOS balance in enhancing the effective use of water (EUW) by crops under water-limited dryland conditions. We refer to EUW as biomass produced by plants with available water under soil moisture stress rather than per unit of water (WUE). We hypothesize that EUW can be enhanced by an appropriate balance between water-saving and growth promotion at the whole-plant level during stress and post-stress recovery periods. The ROS-AOS interactions play a crucial role in water-saving mechanisms and biomass accumulation, resulting from growth processes that include cell division, cell expansion, photosynthesis, and translocation of assimilates. Hence, appropriate strategies for manipulating these processes through genetic improvement and/or application of exogenous compounds can provide practical solutions for improving EUW through the optimized ROS-AOS balance under water-limited dryland conditions. This review deals with the role of ROS-AOS in two major EUW determining processes, namely water use and plant growth. It describes implications of the ROS level or content, ROS-producing, and ROS-scavenging enzymes based on plant water status, which ultimately affects photosynthetic efficiency and growth of plants.

Core Components of Abscisic Acid Signaling and Their Post-translational Modification

Abscisic acid (ABA) is a major phytohormone that regulates plant growth, development, and abiotic/biotic stress responses. Under stress, ABA is synthesized in various plant organs, and it plays roles in diverse adaptive processes, including seed dormancy, growth inhibition, and leaf senescence, by modulating stomatal closure and gene expression. ABA receptor, clade A protein phosphatase 2C (PP2C), and SNF1-related protein kinase 2 (SnRK2) proteins have been identified as core components of ABA signaling, which is initiated via perception of ABA with receptor and subsequent activation or inactivation by phosphorylation/dephosphorylation. The findings of several recent studies have established that the post-translational modification of these components, including phosphorylation and ubiquitination/deubiquitination, play important roles in regulating their activity and stability. In this review, we discuss the functions of the core components of ABA signaling and the regulation of their activities via post-translational modification under normal and stress conditions.

Histone modifications and chromatin remodelling in plants in response to salt stress

In the face of global food security crises, it is necessary to boost agricultural production. One factor hampering the attempts to increase food production is elevated soil salinity, which can be due to salt that is naturally present in the soil or a consequence of excessive or prolonged irrigation or application of fertiliser. In response to environmental stresses, plants activate multiple molecular mechanisms, including the timely activation of stress-responsive transcriptional networks. However, in the case of salt stress, the combined effects of the initial osmotic shock and the subsequent ion-specific stress increase the complexity in the selective regulation of gene expressions involved in restoring or maintaining osmotic balance, ion homeostasis and reactive oxygen species scavenging. Histone modifications and chromatin remodelling are important epigenetic processes that regulate gene expressions by modifying the chromatin status and recruiting transcription regulators. In this review, we have specifically summarised the currently available knowledge on histone modifications and chromatin remodelling in relation to plant responses to salt stress. Current findings have revealed the functional importance of chromatin modifiers in regulating salt tolerance and identified the effector genes affected by epigenetic modifications, although counteraction between modifiers within the same family may occur. Emerging evidence has also illustrated the crosstalk between epigenetic modifications and hormone signalling pathways which involves formation of protein complexes. With an improved understanding of these processes, plant breeders will be able to develop alternative strategies using genome editing technologies for crop improvement.

Genome-wide identification and expression analysis of sucrose nonfermenting-1-related protein kinase (SnRK) genes in Triticum aestivum in response to abiotic stress

The SnRK gene family is a key regulator that plays an important role in plant stress response by phosphorylating the target protein to regulate subsequent signaling pathways. This study was aimed to perform a genome-wide analysis of the SnRK gene family in wheat and the expression profiling of SnRKs in response to abiotic stresses. An in silico analysis identified 174 SnRK genes, which were then categorized into three subgroups (SnRK1/2/3) on the basis of phylogenetic analyses and domain types. The gene intron-exon structure and protein-motif composition of SnRKs were similar within each subgroup but different amongst the groups. Gene duplication and synteny between the wheat and Arabidopsis genomes was also investigated in order to get insight into the evolutionary aspects of the TaSnRK family genes. The result of cis-acting element analysis showed that there were abundant stress- and hormone-related cis-elements in the promoter regions of 129 SnRK genes. Furthermore, quantitative real-time PCR data revealed that heat, salt and drought treatments enhanced TaSnRK2.11 expression, suggesting that it might be a candidate gene for abiotic stress tolerance. We also identified eight microRNAs targeting 16 TaSnRK genes which are playing important role across abiotic stresses and regulation in different pathways. These findings will aid in the functional characterization of TaSnRK genes for further research.

Abscisic acid receptors are involves in the Jasmonate signaling in Arabidopsis

The phytohormones jasmonates (JAs) act as important molecules of elicitors for the chlorophyll degradation and anthocyanin biosynthesis. JAs do usually not act independently but integrate in complex networks linking to other hormonal signaling transduction. Here, the crosstalk was detected between the JAs (jasmonic acid) and abscisic acid (ABA) signaling pathways in the mediation of chlorophyll degradation and anthocyanin biosynthesis. In this study, we found that the ABA receptor mutants, pyr1pyl1pyl2pyl4 (1124) and pyr1pyl1ply2pyl4pyl5pyl8 (112458) showed less level of chlorophyll and anthocyanin than the wild-type plants, while gain-of-function of RCAR13 transgenic lines inhibited chlorophyll degradation and enhanced anthocyanin accumulation after MeJA treatment. The amidohydrolases, including ILL6 and IAR3 and cytochrome P450 (CYP94B3), encoding JA-Ile catabolism were markedly depressed by ABA receptors. While transcripts of the enzymes for activation of anthocyanin biosynthesis pathway were analyzed, the results indicating that JA biosynthetic genes, including allene oxide synthase (AOS), LOX3 and LOX4 were enhanced by the link of JAs and ABA receptors. Moreover, the ABA receptors are also involved in JAs signal transduction through the regulation of COI, JAZ and MYC2 transcripts. These findings elucidate a connection between a core component of the ABA signaling pathway and JA responses.

Exploitation of Drought Tolerance-Related Genes for Crop Improvement

Drought has become a major threat to food security, because it affects crop growth and development. Drought tolerance is an important quantitative trait, which is regulated by hundreds of genes in crop plants. In recent decades, scientists have made considerable progress to uncover the genetic and molecular mechanisms of drought tolerance, especially in model plants. This review summarizes the evaluation criteria for drought tolerance, methods for gene mining, characterization of genes related to drought tolerance, and explores the approaches to enhance crop drought tolerance. Collectively, this review illustrates the application prospect of these genes in improving the drought tolerance breeding of crop plants.

The Multifaceted Regulation of SnRK2 Kinases

SNF1-related kinases 2 (SnRK2s) are central regulators of plant responses to environmental cues simultaneously playing a pivotal role in the plant development and growth in favorable conditions. They are activated in response to osmotic stress and some of them also to abscisic acid (ABA), the latter being key in ABA signaling. The SnRK2s can be viewed as molecular switches between growth and stress response; therefore, their activity is tightly regulated; needed only for a short time to trigger the response, it has to be induced transiently and otherwise kept at a very low level. This implies a strict and multifaceted control of SnRK2s in plant cells. Despite emerging new information concerning the regulation of SnRK2s, especially those involved in ABA signaling, a lot remains to be uncovered, the regulation of SnRK2s in an ABA-independent manner being particularly understudied. Here, we present an overview of available data, discuss some controversial issues, and provide our perspective on SnRK2 regulation.

Pepper ubiquitin-specific protease, CaUBP12, positively modulates dehydration resistance by enhancing CaSnRK2.6 stability

Abscisic acid (ABA) is a plant hormone that activates adaptive mechanisms to environmental stress conditions. Plant adaptive mechanisms are complex and highly modulated processes induced by stress-responsive proteins; however, the precise mechanisms by which these processes function under adverse conditions remain unclear. Here, we isolated CaUBP12 (Capsicum annuum ubiquitin-specific protease 12) from pepper (C. annuum) leaves. We show that CaUBP12 expression is significantly induced after exposure to abiotic stress treatments. We conducted loss-of-function and gain-of-function genetic studies to elucidate the biological functions of CaUBP12 in response to ABA and dehydration stress. CaUBP12-silenced pepper plants and CaUBP12-overexpressing Arabidopsis plants displayed dehydration-sensitive and dehydration-tolerant phenotypes, respectively; these phenotypes were characterized by regulation of transpirational water loss and stomatal aperture. Under dehydration stress conditions, CaUBP12-silenced pepper plants and CaUBP12-overexpressing Arabidopsis plants exhibited lower and higher expression levels of stress-related genes, respectively, than the control plants. We isolated a CaUBP12 interaction protein, CaSnRK2.6, which is a homolog of Arabidopsis OST1; degradation of this protein was partially inhibited by CaUBP12. Similar to CaUBP12-silenced pepper plants and CaUBP12-overexpressing Arabidopsis plants, CaSnRK2.6-silenced pepper plants and CaSnRK2.6-overexpressing Arabidopsis displayed dehydration-sensitive and dehydration-tolerant phenotypes, respectively. Our findings suggest that CaUBP12 positively modulates the dehydration stress response by suppressing CaSnRK2.6 protein degradation.

Growth Promotion or Osmotic Stress Response: How SNF1-Related Protein Kinase 2 (SnRK2) Kinases Are Activated and Manage Intracellular Signaling in Plants

Reversible phosphorylation is a major mechanism for regulating protein function and controls a wide range of cellular functions including responses to external stimuli. The plant-specific SNF1-related protein kinase 2s (SnRK2s) function as central regulators of plant growth and development, as well as tolerance to multiple abiotic stresses. Although the activity of SnRK2s is tightly regulated in a phytohormone abscisic acid (ABA)-dependent manner, recent investigations have revealed that SnRK2s can be activated by group B Raf-like protein kinases independently of ABA. Furthermore, evidence is accumulating that SnRK2s modulate plant growth through regulation of target of rapamycin (TOR) signaling. Here, we summarize recent advances in knowledge of how SnRK2s mediate plant growth and osmotic stress signaling and discuss future challenges in this research field.

Ubiquitylation of ABA Receptors and Protein Phosphatase 2C Coreceptors to Modulate ABA Signaling and Stress Response

Post-translational modifications play a fundamental role in regulating protein function and stability. In particular, protein ubiquitylation is a multifaceted modification involved in numerous aspects of plant biology. Landmark studies connected the ATP-dependent ubiquitylation of substrates to their degradation by the 26S proteasome; however, nonproteolytic functions of the ubiquitin (Ub) code are also crucial to regulate protein interactions, activity, and localization. Regarding proteolytic functions of Ub, Lys-48-linked branched chains are the most common chain type for proteasomal degradation, whereas promotion of endocytosis and vacuolar degradation is triggered through monoubiquitylation or Lys63-linked chains introduced in integral or peripheral plasma membrane proteins. Hormone signaling relies on regulated protein turnover, and specifically the half-life of ABA signaling components is regulated both through the ubiquitin-26S proteasome system and the endocytic/vacuolar degradation pathway. E3 Ub ligases have been reported that target different ABA signaling core components, i.e., ABA receptors, PP2Cs, SnRK2s, and ABFs/ABI5 transcription factors. In this review, we focused specifically on the ubiquitylation of ABA receptors and PP2C coreceptors, as well as other post-translational modifications of ABA receptors (nitration and phosphorylation) that result in their ubiquitination and degradation.

HOS15-PWR chromatin remodeling complex positively regulates cold stress in Arabidopsis

Cold stress is a major environmental constraint that restrains plant growth and productivity. To cope with cold stress, plants must be able to perceive a cold signal and regulate the expression of cold-regulated (COR) genes. In our recent study, we showed that Arabidopsis HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE GENE 15 (HOS15) acts as a substrate receptor for CULLIN4-based ubiquitin E3 ligase complex to promote cold-induced histone deacetylase 2 C (HD2C) degradation that allows the activation of COR genes. Additionally, we found that POWERDRESS (PWR), a HOS15-interacting protein, is required for the association of HOS15 with COR gene chromatin and HD2C degradation. The HOS15/PWR complex interacts with and recruits CBF transcription factors to the promoters of COR genes. Collectively, our previous findings suggest that HOS15 and PWR function as positive regulators for the expression of COR genes, and promote cold tolerance. Accordingly, we herein discuss the role of PWR in cold tolerance.

Control of ABA Signaling and Crosstalk with Other Hormones by the Selective Degradation of Pathway Components

A rapid and appropriate genetic and metabolic acclimation, which is crucial for plants' survival in a changing environment, is maintained due to the coordinated action of plant hormones and cellular degradation mechanisms influencing proteostasis. The plant hormone abscisic acid (ABA) rapidly accumulates in plants in response to environmental stress and plays a pivotal role in the reaction to various stimuli. Increasing evidence demonstrates a significant role of autophagy in controlling ABA signaling. This field has been extensively investigated and new discoveries are constantly being provided. We present updated information on the components of the ABA signaling pathway, particularly on transcription factors modified by different E3 ligases. Then, we focus on the role of selective autophagy in ABA pathway control and review novel evidence on the involvement of autophagy in different parts of the ABA signaling pathway that are important for crosstalk with other hormones, particularly cytokinins and brassinosteroids.

Cellular Phosphorylation Signaling and Gene Expression in Drought Stress Responses: ABA-Dependent and ABA-Independent Regulatory Systems

Drought is a severe and complex abiotic stress that negatively affects plant growth and crop yields. Numerous genes with various functions are induced in response to drought stress to acquire drought stress tolerance. The phytohormone abscisic acid (ABA) accumulates mainly in the leaves in response to drought stress and then activates subclass III SNF1-related protein kinases 2 (SnRK2s), which are key phosphoregulators of ABA signaling. ABA mediates a wide variety of gene expression processes through stress-responsive transcription factors, including ABA-RESPONSIVE ELEMENT BINDING PROTEINS (AREBs)/ABRE-BINDING FACTORS (ABFs) and several other transcription factors. Seed plants have another type of SnRK2s, ABA-unresponsive subclass I SnRK2s, that mediates the stability of gene expression through the mRNA decay pathway and plant growth under drought stress in an ABA-independent manner. Recent research has elucidated the upstream regulators of SnRK2s, RAF-like protein kinases, involved in early responses to drought stress. ABA-independent transcriptional regulatory systems and ABA-responsive regulation function in drought-responsive gene expression. DEHYDRATION RESPONSIVE ELEMENT (DRE) is an important cis-acting element in ABA-independent transcription, whereas ABA-RESPONSIVE ELEMENT (ABRE) cis-acting element functions in ABA-responsive transcription. In this review article, we summarize recent advances in research on cellular and molecular drought stress responses and focus on phosphorylation signaling and transcription networks in Arabidopsis and crops. We also highlight gene networks of transcriptional regulation through two major regulatory pathways, ABA-dependent and ABA-independent pathways, that ABA-responsive subclass III SnRK2s and ABA-unresponsive subclass I SnRK2s mediate, respectively. We also discuss crosstalk in these regulatory systems under drought stress.

Arabidopsis U-box E3 ubiquitin ligase PUB11 negatively regulates drought tolerance by degrading the receptor-like protein kinases LRR1 and KIN7

Both plant receptor-like protein kinases (RLKs) and ubiquitin-mediated proteolysis play crucial roles in plant responses to drought stress. However, the mechanism by which E3 ubiquitin ligases modulate RLKs is poorly understood. In this study, we showed that Arabidopsis PLANT U-BOX PROTEIN 11 (PUB11), an E3 ubiquitin ligase, negatively regulates abscisic acid (ABA)-mediated drought responses. PUB11 interacts with and ubiquitinates two receptor-like protein kinases, LEUCINE RICH REPEAT PROTEIN 1 (LRR1) and KINASE 7 (KIN7), and mediates their degradation during plant responses to drought stress in vitro and in vivo. pub11 mutants were more tolerant, whereas lrr1 and kin7 mutants were more sensitive, to drought stress than the wild type. Genetic analyses show that the pub11 lrr1 kin7 triple mutant exhibited similar drought sensitivity as the lrr1 kin7 double mutant, placing PUB11 upstream of the two RLKs. Abscisic acid and drought treatment promoted the accumulation of PUB11, which likely accelerates LRR1 and KIN7 degradation. Together, our results reveal that PUB11 negatively regulates plant responses to drought stress by destabilizing the LRR1 and KIN7 RLKs.

Identification and analysis of a differentially expressed wheat RING-type E3 ligase in spike primordia development during post-vernalization

We identified a RING-type E3 ligase (TaBAH1) protein in winter wheat that targets TaSAHH1 for degradation and might be involved in primordia development by regulating targeted protein degradation. Grain yield per spike in wheat (Triticum aestivum), is mainly determined prior to flowering during mature primordia development; however, the genes involved in primordia development have yet to be characterized. In this study, we demonstrated that, after vernalization for 50 days at 4 °C, there was a rapid acceleration in primordia development to the mature stages in the winter wheat cultivars Keumgang and Yeongkwang compared with the Chinese Spring cultivar. Although Yeongkwang flowers later than Keumgang under normal condition, it has the same heading time and reaches the WS9 stage of floral development after vernalization for 50 days. Using RNA sequencing, we identified candidate genes associated with primordia development in cvs. Keumgang and Yeongkwang, that are differentially expressed during wheat reproductive stages. Among these, the RING-type E3 ligase TaBAH1 (TraesCS5B01G373000) was transcriptionally upregulated between the double-ridge (WS2.5) stage and later stages of floret primordia development (WS10) after vernalization. Transient expression analysis indicated that TaBAH1 was localized to the plasma membrane and nucleus and was characterized by self-ubiquitination activity. Furthermore, we found that TaBAH1 interacts with TaSAHH1 to mediate its polyubiquitination and degradation through a 26S proteasomal pathway. Collectively, the findings of this study indicate that TaBAH1 might play a prominent role in post-vernalization floret primordia development.

Protein kinases in plant responses to drought, salt, and cold stress

Protein kinases are major players in various signal transduction pathways. Understanding the molecular mechanisms behind plant responses to biotic and abiotic stresses has become critical for developing and breeding climate-resilient crops. In this review, we summarize recent progress on understanding plant drought, salt, and cold stress responses, with a focus on signal perception and transduction by different protein kinases, especially sucrose nonfermenting1 (SNF1)-related protein kinases (SnRKs), mitogen-activated protein kinase (MAPK) cascades, calcium-dependent protein kinases (CDPKs/CPKs), and receptor-like kinases (RLKs). We also discuss future challenges in these research fields.

Role of Raf-like kinases in SnRK2 activation and osmotic stress response in plants

Environmental drought and high salinity impose osmotic stress, which inhibits plant growth and yield. Thus, understanding how plants respond to osmotic stress is critical to improve crop productivity. Plants have multiple signalling pathways in response to osmotic stress in which the phytohormone abscisic acid (ABA) plays important roles. However, since little is known concerning key early components, the global osmotic stress-signalling network remains to be elucidated. Here, we review recent advances in the identification of osmotic-stress activated Raf-like protein kinases as regulators of ABA-dependent and -independent signalling pathways and discuss the plant stress-responsive kinase network from an evolutionary perspective.

A better understanding of how plants respond to osmotic stress could potentially help improve crop yields. Here Fàbregas et al. review the recent characterization of Raf-like kinases that act in both in ABA-dependent and -independent responses to osmotic stress.

HOS15 is a transcriptional corepressor of NPR1-mediated gene activation of plant immunity

Immune responses protect organisms against biotic challenges but can also produce deleterious effects, such as inflammation and necrosis. This growth-defense trade-off necessitates fine control of immune responses, including the activation of defense gene expression. The transcriptional coactivator NPR1 is a key regulatory hub of immune activation in plant cells. Surprisingly, full activation of NPR1-activated defense genes requires proteasome-mediated degradation of NPR1 induced by a CUL3-based E3 ubiquitin ligase complex. Our work demonstrates that HOS15 is the specificity determinant of a CUL1-based E3 ubiquitin ligase complex that limits defense gene expression by targeting NPR1 for proteasome-mediated degradation. Thus, distinct ubiquitin-based degradation pathways coordinately modulate the timing and amplitude of transcriptional outputs during plant defense.

Transcriptional regulation is a complex and pivotal process in living cells. HOS15 is a transcriptional corepressor. Although transcriptional repressors generally have been associated with inactive genes, increasing evidence indicates that, through poorly understood mechanisms, transcriptional corepressors also associate with actively transcribed genes. Here, we show that HOS15 is the substrate receptor for an SCF/CUL1 E3 ubiquitin ligase complex (SCF^HOS15) that negatively regulates plant immunity by destabilizing transcriptional activation complexes containing NPR1 and associated transcriptional activators. In unchallenged conditions, HOS15 continuously eliminates NPR1 to prevent inappropriate defense gene expression. Upon defense activation, HOS15 preferentially associates with phosphorylated NPR1 to stimulate rapid degradation of transcriptionally active NPR1 and thus limit the extent of defense gene expression. Our findings indicate that HOS15-mediated ubiquitination and elimination of NPR1 produce effects contrary to those of CUL3-containing ubiquitin ligase that coactivate defense gene expression. Thus, HOS15 plays a key role in the dynamic regulation of pre- and postactivation host defense.

ESCRT-I Component VPS23A Is Targeted by E3 Ubiquitin Ligase XBAT35 for Proteasome-Mediated Degradation in Modulating ABA Signaling

A myriad of abiotic stress responses in plants are controlled by abscisic acid (ABA) signaling. ABA receptors can be degraded by both the 26S proteasome pathway and vacuolar degradation pathway after processing via the endosomal sorting complex required for transport (ESCRT) proteins. Despite being essential for ABA signaling, the upstream regulators of ESCRTs remain unknown. Here, we report that the ESCRT-I component VPS23A is an unstable protein that is degraded via the ubiquitin-proteasome system (UPS). The UEV domain of VPS23A physically interacts with the two PSAP motifs of XBAT35, an E3 ubiquitin ligase, and this interaction results in the deposition of K48 polyubiquitin chains on VPS23A, marking it for degradation by 26S proteasomes. We showed that XBAT35 in plants is a positive regulator of ABA responses that acts via the VPS23A/PYL4 complex, specifically by accelerating VPS23A turnover and thereby increasing accumulation of the ABA receptor PYL4. This work deciphers how an ESCRT component is regulated in plants and deepens our understanding of plant stress responses by illustrating a mechanism whereby crosstalk between the UPS and endosome-vacuole-mediated degradation pathways controls ABA signaling.

AtPPRT3, a novel E3 ubiquitin ligase, plays a positive role in ABA signaling

KEY MESSAGE

The RING-type E3 ligase AtPPRT3 participates in the plant ABA responding as a positive regulator. E3 ubiquitin ligase, alike of classic plant stress resistance proteins, plays a vital role in regulating the degradation of stress-related proteins. In this study, we investigated the function of the RING-type E3 ubiquitin ligase AtPPRT3 in the ABA signaling pathway. AtPPRT3, located in the endoplasmic reticulum membrane, is involved in ABA signaling. The transcriptional expression of AtPPRT3 was induced by ABA, and the promoter region upstream of AtPPRT3 contains the ABA-responsive element (ABRE). Additionally, the β-glucuronidase (GUS) gene driven by the AtPPRT3 promoter was up-regulated in transgenic plants after ABA treated. We obtained AtPPRT3 function-deficient mutants atpprt3-1, atpprt3-2, and AtPPRT3 over-expressing lines (OE4 and OE5). In this study, atpprt3-1 and atpprt3-2 were less sensitive to exogenous ABA compared to Col-0, whereas OE4 and OE5 were more sensitive. Moreover, AtPPRT3 promotes ABA-mediated stomatal closure and inhibits water loss in Arabidopsis thaliana. After exogenous ABA treated, the transcriptional expression levels of AtDREB2A, AtKIN1, AtRD29A, AtERD10 and AtRD29B were up-regulated to greater extents in OEs and lower extents in atpprt3-1 and atpprt3-2 compared to Col-0. These results suggest that AtPPRT3 positively regulates ABA signaling in A. thaliana.

Chromatin remodeling complex HDA9-PWR-ABI4 epigenetically regulates drought stress response in plants

Among all the major environmental challenges, drought stress causes considerable damage to plant growth and agricultural productivity. Drought stress directly promotes the accumulation of abscisic acid (ABA) via the activation of genes that encode enzymes involved in ABA biosynthesis, which protect the plant against water-limiting conditions. At the same time, the expression of genes that encode ABA-hydroxylases that inactivate the newly synthesized ABA, is repressed by drought stress. These phenomena occur through epigenetic modifications via the reversible processes of histone acetylation and deacetylation, also known as chromatin remodeling, which is an important regulatory mechanism that responds to various environmental stresses. Recently, we had reported that the chromatin remodeling complex HDA9-PWR-ABI4 promotes the development of drought tolerance through the deacetylation of CYP707A1/2 genes that encode the major enzymes involved in ABA catabolism. Here, we discuss the role of HDA9 and PWR in regulating drought stress by modulating the acetylation status of the CYP707A genes.

Counteraction of ABA-Mediated Inhibition of Seed Germination and Seedling Establishment by ABA Signaling Terminator in Arabidopsis

Seed germination and seedling establishment are important for the reproductive success of plants, but seeds and seedlings typically encounter constantly changing environmental conditions. By inhibiting seed germination and post-germinative growth through the PYR1/PYL/RCAR ABA receptors and PP2C co-receptors, the phytohormone abscisic acid (ABA) prevents premature germination and seedling growth under unfavorable conditions. However, little is known about how the ABA-mediated inhibition of seed germination and seedling establishment is thwarted. Here, we report that ABA Signaling Terminator (ABT), a WD40 protein, efficiently switches off ABA signaling and is critical for seed germination and seedling establishment. ABT is induced by ABA in a PYR1/PYL/RCAR-PP2C-dependent manner. Overexpression of ABT promotes seed germination and seedling greening in the presence of ABA, whereas knockout of ABT has the opposite effect. We found that ABT interacts with the PYR1/PYL/RCAR and PP2C proteins, interferes with the interaction between PYR1/PYL4 and ABI1/ABI2, and hampers the inhibition of ABI1/ABI2 by ABA-bound PYR1/PYL4, thereby terminating ABA signaling. Taken together, our results reveal a core mechanism of ABA signaling termination that is critical for seed germination and seedling establishment in Arabidopsis.

Linking signaling pathways to histone acetylation dynamics in plants

As sessile organisms, plants face versatile environmental challenges and require proper responses at multiple levels for survival. Epigenetic modification of DNA and histones is a conserved gene-regulatory mechanism and plays critical roles in diverse aspects of biological processes, ranging from genome defense and imprinting to development and physiology. In recent years, emerging studies have revealed the interplay between signaling transduction pathways, epigenetic modifications, and chromatin cascades. Specifically, histone acetylation and deacetylation dictate plant responses to environmental cues by modulating chromatin dynamics to regulate downstream gene expression as signaling outputs. In this review, we summarize current understandings of the link between plant signaling pathways and epigenetic modifications with a focus on histone acetylation and deacetylation.

HOS15: A missing link that fine-tunes ABA signaling and drought tolerance in Arabidopsis

Among the phytohormones, abscisic acid (ABA) specifically regulates plant adaptation to osmotic stresses, such as drought and high salinity, by controlling the internal water status in plants. A significant accumulation of ABA occurs in response to conditions of water deficit; this is followed by a sophisticated signaling relay, known as the ABA signaling pathway, which decreases the rate of transpiration through stomatal closure, thereby suppressing photosynthetic activity. Snf1-related kinases (SnRK2s) are the major components regulating the ABA signaling pathway. Of these, SnRK2.6 (OST1) and SnRK2.3 are negatively regulated by HOS15 (HIGH EXPRESSION OF OSMOTICALLY RESPONSIVE15), in an ABA-dependent manner, to cease the signaling relay. HOS15 is a WD40-repeat protein that regulates several physiological processes, including plant growth and development, freezing stress responses, and ABA signaling. Here, we provide a brief overview of the functional importance of HOS15 in the regulation of ABA signaling and drought stress.

Genome-wide identification and characterization of SnRK family genes in Brassica napus

BACKGROUND

Sucrose non-fermenting 1 related protein kinases (SnRK) play crucial roles in responding to biotic and abiotic stresses through activating protein phosphorylation pathways. However, little information of SnRK genes was available in Brassica napus, one of important oil crops. Recently, the released sequences of the reference genome of B.napus provide a good chance to perform genome-wide identification and characterization of BnSnRK gene family in the rapeseed.

RESULTS

Totally 114 SnRK genes distributed on 19 chromosomes were identified in the genome of B.napus and classified into three subfamilies on the basis of phylogenetic analysis and the domain types. According to gene structure and motif composition analysis, the BnSnRK sequences showed obvious divergence among three subfamilies. Gene duplication and synteny between the genomes of the rapeseed and Arabidopsis were also analyzed to provide insights into the evolutionary characteristics of BnSnRK family genes. Cis-element analysis revealed that BnSnRKs may response to diverse environmental stresses. Moreover, the expression patterns of BnSnRKs in various tissues and under diverse abiotic stresses were distinct difference. Besides, Single Nucleotide Polymorphisms (SNP) distribution analysis suggests the function disparity of BnSnRK family genes in different genotypes of the rapeseed.

CONCLUSION

We examined genomic structures, evolution features, expression patterns and SNP distribution of 114 BnSnRKs. The results provide valuable information for functional characterization of BnSnRK genes in future studies.

Plant abiotic stress response and nutrient use efficiency

Abiotic stresses and soil nutrient limitations are major environmental conditions that reduce plant growth, productivity and quality. Plants have evolved mechanisms to perceive these environmental challenges, transmit the stress signals within cells as well as between cells and tissues, and make appropriate adjustments in their growth and development in order to survive and reproduce. In recent years, significant progress has been made on many fronts of the stress signaling research, particularly in understanding the downstream signaling events that culminate at the activation of stress- and nutrient limitation-responsive genes, cellular ion homeostasis, and growth adjustment. However, the revelation of the early events of stress signaling, particularly the identification of primary stress sensors, still lags behind. In this review, we summarize recent work on the genetic and molecular mechanisms of plant abiotic stress and nutrient limitation sensing and signaling and discuss new directions for future studies.