Many studies have been conducted to understand plant stress responses to salinity because irrigation-dependent salt accumulation compromises crop productivity and also to understand the mechanism ...through which some plants thrive under saline conditions. As mechanistic understanding has increased during the last decades, discovery-oriented approaches have begun to identify genetic determinants of salt tolerance. In addition to osmolytes, osmoprotectants, radical detoxification, ion transport systems, and changes in hormone levels and hormone-guided communications, the Salt Overly Sensitive (SOS) pathway has emerged to be a major defense mechanism. However, the mechanism by which the components of the SOS pathway are integrated to ultimately orchestrate plant-wide tolerance to salinity stress remains unclear. A higher-level control mechanism has recently emerged as a result of recognizing the involvement of GIGANTEA (GI), a protein involved in maintaining the plant circadian clock and control switch in flowering. The loss of GI function confers high tolerance to salt stress via its interaction with the components of the SOS pathway. The mechanism underlying this observation indicates the association between GI and the SOS pathway and thus, given the key influence of the circadian clock and the pathway on photoperiodic flowering, the association between GI and SOS can regulate growth and stress tolerance. In this review, we will analyze the components of the SOS pathways, with emphasis on the integration of components recognized as hallmarks of a halophytic lifestyle.
Abiotic stress, a serious threat to plants, occurs for extended periods in nature. Abscisic acid (ABA) plays a critical role in abiotic stress responses in plants. Therefore, stress responses ...mediated by ABA have been studied extensively, especially in short‐term responses. However, long‐term stress responses mediated by ABA remain largely unknown. To elucidate the mechanism by which plants respond to prolonged abiotic stress, we used long‐term ABA treatment that activates the signalling against abiotic stress such as dehydration and investigated mechanisms underlying the responses. Long‐term ABA treatment activates constitutive photomorphogenic 1 (COP1). Active COP1 mediates the ubiquitination of golden2‐like1 (GLK1) for degradation, contributing to lowering expression of photosynthesis‐associated genes such as glutamyl‐tRNA reductase (HEMA1) and protochlorophyllide oxidoreductase A (PORA), resulting in the suppression of chloroplast development. Moreover, COP1 activation and GLK1 degradation upon long‐term ABA treatment depend on light intensity. Additionally, plants with COP1 mutation or exposed to higher light intensity were more sensitive to salt stress. Collectively, our results demonstrate that long‐term treatment of ABA leads to activation of COP1 in a light intensity‐dependent manner for GLK1 degradation to suppress chloroplast development, which we propose to constitute a mechanism of balancing normal growth and stress responses upon the long‐term abiotic stress.
Here, we show that long‐term ABA promotes GLK1 degradation through COP1 to suppress chloroplast development dependent on light intensity, revealing that balancing normal growth and stress response in long‐term stress.
SUMMARY
Many ubiquitin E3 ligases function in plant immunity. Here, we show that Oryza sativa (rice) DDB1 binding WD (OsDWD1) suppresses immune responses by targeting O. sativa non‐expresser of ...pathogenesis‐related gene 1 (OsNPR1) for degradation. Knock‐down and overexpression experiments in rice plants showed that OsDWD1 is a negative regulator of the immune response and that OsNPR1 is a substrate of OsDWD1 and a substrate receptor of OsCRL4. After constructing the loss‐of‐function mutant OsDWD1R239A, we showed that the downregulation of OsNPR1 seen in rice lines overexpressing wild‐type (WT) OsDWD1 (OsDWD1WT‐ox) was compromised in OsDWD1R239A‐ox lines, and that OsNPR1 upregulation enhanced resistance to pathogen infection, confirming that OsCRL4OsDWD1 regulates OsNPR1 protein levels. The enhanced disease resistance seen in OsDWD1 knock‐down (OsDWD1‐kd) lines contrasted with the reduced disease resistance in double knock‐down (OsDWD1/OsNPR1‐kd) lines, indicating that the enhanced disease resistance of OsDWD1‐kd resulted from the accumulation of OsNPR1. Moreover, an in vivo heterologous protein degradation assay in Arabidopsis thaliana ddb1 mutants confirmed that the CUL4‐based E3 ligase system can also influence OsNPR1 protein levels in Arabidopsis. Although OsNPR1 was degraded by the OsCRL4OsDWD1‐mediated ubiquitination system, the phosphodegron‐motif‐mutated NPR1 was partially degraded in the DWD1‐ox protoplasts. This suggests that there might be another degradation process for OsNPR1. Taken together, these results indicate that OsDWD1 regulates OsNPR1 protein levels in rice to suppress the untimely activation of immune responses.
Significance Statement
NPR1 (non‐expresser of pathogenesis‐related gene 1), master regulator in plant immunity and systemic acquired resistance, was regulated by a ubiquitin‐proteasome system (UPS) that has been well studied in the model plant Arabidopsis. In this study, we discovered that Oryza sativa CRL4‐DDB binding WD1 (OsDWD1) was a substrate receptor of the CRL4 complex and that OsNPR1 could be targeted and degraded by the OsDWD1‐mediated ubiquitination system.
Circadian clock systems help establish the correct daily phasing of the behavioral, developmental, and molecular events needed for the proper coordination of physiology and metabolism. The circadian ...oscillator comprises transcription-translation feedback loops but also requires post-translational processes that regulate clock protein homeostasis. GIGANTEA is a unique plant protein involved in the maintenance and control of numerous facets of plant physiology and development. Through an unknown mechanism GIGANTEA stabilizes the F-box protein ZEITLUPE, a key regulator of the circadian clock. Here, we show that GIGANTEA has general protein chaperone activity and can act to specifically facilitate ZEITLUPE maturation into an active form in vitro and in planta. GIGANTEA forms a ternary complex with HSP90 and ZEITLUPE and its co-chaperone action synergistically enhances HSP90/HSP70 maturation of ZEITLUPE in vitro. These results identify a molecular mechanism for GIGANTEA activity that can explain its wide-ranging role in plant biology.The plant-specific GIGANTEA protein regulates the circadian clock by stabilizing the F-box protein ZEITLUPE via an unknown mechanism. Here Cha et al. show that GIGANTEA has intrinsic chaperone activity and can facilitate ZEITLUPE maturation by acting synergistically with HSP90.
Summary
Plants have developed multilayered defense strategies to adapt and acclimate to the kaleidoscopic environmental changes that rapidly produce reactive oxygen species (ROS) and induce redox ...changes. Thiol‐based redox sensors containing the redox‐sensitive cysteine residues act as the central machinery in plant defense signaling. Here, we review recent research on thiol‐based redox sensors in plants, which perceive the changes in intracellular H2O2 levels and activate specific downstream defense signaling. The review mainly focuses on the molecular mechanism of how the thiol sensors recognize internal/external stresses and respond to them by demonstrating several instances, such as cold‐, drought‐, salinity‐, and pathogen‐resistant signaling pathways. Also, we introduce another novel complex system of thiol‐based redox sensors operating through the liquid–liquid phase separation.
Switching from repressed to active status in chromatin regulation is part of the critical responses that plants deploy to survive in an ever-changing environment. We previously reported that HOS15, a ...WD40-repeat protein, is involved in histone deacetylation and cold tolerance in Arabidopsis. However, it remained unknown how HOS15 regulates cold responsive genes to affect cold tolerance. Here, we show that HOS15 interacts with histone deacetylase 2C (HD2C) and both proteins together associate with the promoters of cold-responsive COR genes, COR15A and COR47. Cold induced HD2C degradation is mediated by the CULLIN4 (CUL4)-based E3 ubiquitin ligase complex in which HOS15 acts as a substrate receptor. Interference with the association of HD2C and the COR gene promoters by HOS15 correlates with increased acetylation levels of histone H3. HOS15 also interacts with CBF transcription factors to modulate cold-induced binding to the COR gene promoters. Our results here demonstrate that cold induces HOS15-mediated chromatin modifications by degrading HD2C. This switches the chromatin structure status and facilitates recruitment of CBFs to the COR gene promoters. This is an apparent requirement to acquire cold tolerance.
Summary
C‐repeat binding factors (CBFs) are key cold‐responsive transcription factors that play pleiotropic roles in the cold acclimation, growth, and development of plants. Cold‐sensitive cbf ...knockout mutants and cold‐tolerant CBF overexpression lines exhibit abnormal phenotypes at warm temperatures, suggesting that CBF activity is precisely regulated, and a critical threshold level must be maintained for proper plant growth under normal conditions. Cold‐inducible CBFs also exist in warm‐climate plants but as inactive disulfide‐bonded oligomers. However, upon translocation to the nucleus under a cold snap, the h2‐isotype of cytosolic thioredoxin (Trx‐h2), reduces the oxidized (inactive) CBF oligomers and the newly synthesized CBF monomers, thus producing reduced (active) CBF monomers. Thus, the redox‐dependent structural switching and functional activation of CBFs protect plants under cold stress.
Cellular proteins undergo denaturation and oxidative damage under heat stress, forming insoluble aggregates that are toxic to cells. Plants possess versatile mechanisms to deal with insoluble protein ...aggregates. Denatured proteins are either renatured to their native conformations or removed from cellular compartments; these processes are often referred to as protein quality control. Heat shock proteins (HSPs) act as molecular chaperones that assist in the renaturation-degradation process. However, how protein aggregates are cleared from cells in plants is largely unknown. Here, we demonstrate that heat-induced protein aggregates are removed by a protein quality control system that includes the ZEITLUPE (ZTL) E3 ubiquitin ligase, a central circadian clock component in Arabidopsis thaliana. ZTL mediates the polyubiquitination of aggregated proteins, which leads to proteasomal degradation and enhances the thermotolerance of plants growing at high temperatures. The ZTL-defective ztl-105 mutant exhibited reduced thermotolerance, which was accompanied by a decline in polyubiquitination but an increase in protein aggregate formation. ZTL and its interacting partner HSP90 were cofractionated with insoluble aggregates under heat stress, indicating that ZTL contributes to the thermoresponsive protein quality control machinery. Notably, the circadian clock was hypersensitive to heat in ztl-105. We propose that ZTL-mediated protein quality control contributes to the thermal stability of the circadian clock.
Excess soil salinity significantly impairs plant growth and development. Our previous reports demonstrated that the core circadian clock oscillator GIGANTEA (GI) negatively regulates salt stress ...tolerance by sequestering the SALT OVERLY SENSITIVE (SOS) 2 kinase, an essential component of the SOS pathway. Salt stress induces calcium‐dependent cytoplasmic GI degradation, resulting in activation of the SOS pathway; however, the precise molecular mechanism governing GI degradation during salt stress remains enigmatic. Here, we demonstrate that salt‐induced calcium signals promote the cytoplasmic partitioning of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1), leading to the 26S proteasome‐dependent degradation of GI exclusively in the roots. Salt stress‐induced calcium signals accelerate the cytoplasmic localization of COP1 in the root cells, which targets GI for 26S proteasomal degradation. Align with this, the interaction between COP1 and GI is only observed in the roots, not the shoots, under salt‐stress conditions. Notably, the gi‐201 cop1‐4 double mutant shows an enhanced tolerance to salt stress similar to gi‐201, indicating that GI is epistatic to COP1 under salt‐stress conditions. Taken together, our study provides critical insights into the molecular mechanisms governing the COP1‐mediated proteasomal degradation of GI for salt stress tolerance, raising new possibilities for developing salt‐tolerant crops.
Summary statement
Salt‐induced Ca2+ signal triggers E3 ubiquitin ligase CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) dependent proteasomal degradation of cytosolic GIGANTEA (GI) in the roots under salt‐stress conditions. Consequently, COP1‐regulated GI degradation positively regulates salt stress tolerance in Arabidopsis thaliana.
Agriculture is largely dependent on climate and is highly vulnerable to climate change. The global mean surface temperatures are increasing due to global climate change. Temperature beyond the ...physiological optimum for growth induces heat stress in plants causing detrimental and irreversible damage to plant development, growth, as well as productivity. Plants have evolved adaptive mechanisms in response to heat stress. The classical plant hormones, such as auxin, abscisic acid (ABA), brassinosteroids (BRs), cytokinin (CK), salicylic acid (SA), jasmonate (JA), and ethylene (ET), integrate environmental stimuli and endogenous signals to regulate plant defensive response to various abiotic stresses, including heat. Exogenous applications of those hormones prior or parallel to heat stress render plants more thermotolerant. In this review, we summarized the recent progress and current understanding of the roles of those phytohormones in defending plants against heat stress and the underlying signal transduction pathways. We also discussed the implication of the basic knowledge of hormone-regulated plant heat responsive mechanism to develop heat-resilient plants as an effective and efficient way to cope with global warming.
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