The Arabidopsis F-box E3 ligase RIFP1 plays a negative role in abscisic acid signalling by facilitating ABA receptor RCAR3 degradation

An F-box Kelch repeat protein, PmFBK2, from Persicaria minor interacts with GID1b to modulate gibberellin signalling

The F-box protein (FBP) family plays diverse functions in the plant kingdom, with the function of many members still unrevealed. In this study, a specific FBP called PmFBK2, containing Kelch repeats from Persicaria minor, was functionally investigated. Employing the yeast two-hybrid (Y2H) assay, PmFBK2 was found to interact with Skp1-like proteins from P. minor, suggesting its potential to form an E3 ubiquitin ligase, known as the SCF complex. Y2H and co-immunoprecipitation tests revealed that PmFBK2 interacts with full-length PmGID1b. The interaction marks the first documented binding between these two protein types, which have never been reported in other plants before, and they exhibited a negative effect on gibberellin (GA) signal transduction. The overexpression of PmFBK2 in the kmd3 mutant, a homolog from Arabidopsis, demonstrated the ability of PmFBK2 to restore the function of the mutated KMD3 gene. The function restoration was supported by morphophysiological and gene expression analyses, which exhibited patterns similar to the wild type (WT) compared to the kmd3 mutant. Interestingly, the overexpression of PmFBK2 or PmGID1b in Arabidopsis had opposite effects on rosette diameter, seed weight, and plant height. This study provides new insights into the complex GA signalling. It highlights the crucial roles of the interaction between FBP and the GA receptor (GID1b) in regulating GA responses. These findings have implications for developing strategies to enhance plant growth and yield by modulating GA signalling in crops.

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.

The ubiquitin-proteasome system in the plant response to abiotic stress: Potential role in crop resilience improvement

The post-translational modification (PTM) of proteins by ubiquitination modulates many physiological processes in plants. As the major protein degradation pathway in plants, the ubiquitin-proteasome system (UPS) is considered a promising target for improving crop tolerance drought, high salinity, extreme temperatures, and other abiotic stressors. The UPS also participates in abiotic stress-related abscisic acid (ABA) signaling. E3 ligases are core components of the UPS-mediated modification process due to their substrate specificity. In this review, we focus on the abiotic stress-associated regulatory mechanisms and functions of different UPS components, emphasizing the participation of E3 ubiquitin ligases. We also summarize and discuss UPS-mediated modulation of ABA signaling. In particular, we focus our review on recent research into the UPS-mediated modulation of the abiotic stress response in major crop plants. We propose that altering the ubiquitination site of the substrate or the substrate-specificity of E3 ligase using genome editing technology such as CRISPR/Cas9 may improve the resistance of crop plants to adverse environmental conditions. Such a strategy will require continued research into the role of the UPS in mediating the abiotic stress response in plants.

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.

Identification and Expression Analysis of the SKP1-Like Gene Family under Phytohormone and Abiotic Stresses in Apple (Malus domestica)

Ubiquitination participates in plant hormone signaling and stress response to adversity. SKP1-Like, a core component of the SCF (Skp1-Cullin-F-box) complex, is the final step in catalyzing the ubiquitin-mediated protein degradation pathway. However, the SKP1-Like gene family has not been well characterized in response to apple abiotic stresses and hormonal treatments. This study revealed that 17 MdSKP1-Like gene family members with the conserved domain of SKP1 were identified in apples and were unevenly distributed on eight chromosomes. The MdSKP1-Like genes located on chromosomes 1, 10, and 15 were highly homologous. The MdSKP1-like genes were divided into three subfamilies according to the evolutionary affinities of monocotyledons and dicotyledons. MdSKP1-like members of the same group or subfamily show some similarity in gene structure and conserved motifs. The predicted results of protein interactions showed that members of the MdSKP1-like family have strong interactions with members of the F-Box family of proteins. A selection pressure analysis showed that MdSKP1-Like genes were in purifying selection. A chip data analysis showed that MdSKP1-like14 and MdSKP1-like15 were higher in flowers, whereas MdSKP1-like3 was higher in fruits. The upstream cis-elements of MdSKP1-Like genes contained a variety of elements related to light regulation, drought, low temperature, and many hormone response elements, etc. Meanwhile, qRT-PCR also confirmed that the MdSKP1-Like gene is indeed involved in the response of the apple to hormonal and abiotic stress treatments. This research provides evidence for regulating MdSKP1-Like gene expression in response to hormonal and abiotic stresses to improve apple stress resistance.

Comparative time-course transcriptome analysis of two contrasting alfalfa (Medicago sativa L.) genotypes reveals tolerance mechanisms to salt stress

Salt stress is a major abiotic stress affecting plant growth and crop yield. For the successful cultivation of alfalfa (Medicago sativa L.), a key legume forage, in saline-affected areas, it's essential to explore genetic modifications to improve salt-tolerance.Transcriptome assay of two comparative alfalfa genotypes, Adina and Zhaodong, following a 4 h and 8 h's 300 mM NaCl treatment was conducted in this study in order to investigate the molecular mechanism in alfalfa under salt stress conditions. Results showed that we obtained 875,023,571 transcripts and 662,765,594 unigenes were abtained from the sequenced libraries, and 520,091 assembled unigenes were annotated in at least one database. Among them, we identified 1,636 differentially expression genes (DEGs) in Adina, of which 1,426 were up-regulated and 210 down-regulated, and 1,295 DEGs in Zhaodong, of which 565 were up-regulated and 730 down-regulated. GO annotations and KEGG pathway enrichments of the DEGs based on RNA-seq data indicated that DEGs were involved in (1) ion and membrane homeostasis, including ABC transporter, CLC, NCX, and NHX; (2) Ca²⁺ sensing and transduction, including BK channel, EF-hand domain, and calmodulin binding protein; (3) phytohormone signaling and regulation, including TPR, FBP, LRR, and PP2C; (4) transcription factors, including zinc finger proteins, YABBY, and SBP-box; (5) antioxidation process, including GST, PYROX, and ALDH; (6) post-translational modification, including UCH, ubiquitin family, GT, MT and SOT. The functional roles of DEGs could explain the variations in salt tolerance performance observed between the two alfalfa genotypes Adina and Zhaodong. Our study widens the understanding of the sophisticated molecular response and tolerance mechanism to salt stress, providing novel insights on candidate genes and pathways for genetic modification involved in salt stress adaptation in alfalfa.

A Novel R2R3-MYB Transcription Factor FtMYB22 Negatively Regulates Salt and Drought Stress through ABA-Dependent Pathway

Tartary buckwheat (Fagopyrum tataricum Gaertn.) is a coarse cereal with strongly abiotic resistance. The MYB family plays a regulatory role in plant growth, development, and responses to biotic and abiotic stresses. However, the characteristics and regulatory mechanisms of MYB transcription factors in Tartary buckwheat remain unclarified. Here, this study cloned the FtMYB22 gene from Tartary buckwheat, and investigated its involvement in responding to individual water deficit and salt stress in Arabidopsis. Sequence analysis highlighted that the N-termini of FtMYB22 contained two highly conserved SANT domains and one conserved domain from the SG20 subfamily. Nucleus-localized FtMYB22 did not have individual transcriptional activation activity. Water deficiency and salt stress induced the high expression of the GUS gene, which was driven by the promoter of FtMYB22. Yeast stress experiments showed that the overexpression of FtMYB22 significantly reduced the growth activity of transgenic yeast under water deficit or salt stress. Consistently, the overexpression of FtMYB22 reduced the salt and water deficit stress resistance of the transgenic plants. In addition, physiological parameters showed that transgenic plants had lower proline and antioxidant enzyme activity under stress conditions. Compared to the wild-type (WT), transgenic plants accumulated more malondialdehyde (MDA), H2O2, and O2−; they also showed higher ion permeability and water loss rates of detached leaves under stress treatments. Notably, FtMYB22 was involved in plant stress resistance through an ABA-dependent pathway. Under stress conditions, the expression of RD29A, RD29B, PP2CA, KIN1, COR15A, and other genes in response to plant stress in transgenic lines was significantly lower than that in the WT (p < 0.05). Furthermore, yeast two-hybrid assay showed that there was a significant interaction between FtMYB22 and the ABA receptor protein RCAR1/2, which functioned in the ABA signal pathway. Altogether, FtMYB22, as a negative regulator, inhibited a variety of physiological and biochemical reactions, affected gene expression and stomatal closure in transgenic plants through the ABA-dependent pathway, and reduced the tolerance of transgenic Arabidopsis to water deficiency and salt stress. Based on these fundamental verifications, further studies would shed light on the hormone signal response mechanism of FtMYB22.

Emerging roles of the ubiquitin-proteasome pathway in enhancing crop yield by optimizing seed agronomic traits

Ubiquitin-proteasome pathway has the potential to modulate crop productivity by influencing agronomic traits. Being sessile, the plant often uses the ubiquitin-proteasome pathway to maintain the stability of different regulatory proteins to survive in an ever-changing environment. The ubiquitin system influences plant reproduction, growth, development, responses to the environment, and processes that control critical agronomic traits. E3 ligases are the major players in this pathway, and they are responsible for recognizing and tagging the targets/substrates. Plants have a variety of E3 ubiquitin ligases, whose functions have been studied extensively, ranging from plant growth to defense strategies. Here we summarize three agronomic traits influenced by ubiquitination: seed size and weight, seed germination, and accessory plant agronomic traits particularly panicle architecture, tillering in rice, and tassels branch number in maize. This review article highlights some recent progress on how the ubiquitin system influences the stability/modification of proteins that determine seed agronomic properties like size, weight, germination and filling, and ultimately agricultural productivity and quality. Further research into the molecular basis of the aforementioned processes might lead to the identification of genes that could be modified or selected for crop development. Likewise, we also propose advances and future perspectives in this regard.

Monomerization of abscisic acid receptors through CARKs-mediated phosphorylation

Cytosolic ABA Receptor Kinases (CARKs) play a pivotal role in abscisic acid (ABA)-dependent pathway in response to dehydration, but their regulatory mechanism in ABA signaling remains unexplored. In this study, we showed that CARK4/5 of CARK family physically interacted with ABA receptors (RCARs/PYR1/PYLs), including RCAR3, RCAR11-RCAR14, while CARK2/7/11 only interacted with RCAR11-RCAR14, but not RCAR3. It indicates that the members in CARK family function redundantly and differentially in ABA signaling. RCAR12 can form heterodimer with RCAR3 in vitro and in vivo. Moreover, the members of CARK family can form homodimer or heterodimer in a kinase activity dependent manner. ITC (isothermal titration calorimetry) analysis demonstrated that the phosphorylation of RCAR12 by CARK1 enhanced the ABA binding affinity. The phosphor-mimic RCAR12^T105D significantly displayed ABA-induced inhibition of the phosphatase ABI1 (ABA insensitive 1) activity, leading to upregulation of ABA-responsive genes RD29A and RD29B in cark157:RCAR12^T105D transgenic plants, which exhibited ABA hypersensitive phenotype. The transcription factor ABI5 (ABA insensitive 5) activates the transcriptions of CARK1 and CARK3 by binding to ABA-response elements (ABREs) of their promoters. Collectively, our data imply that the dimeric CARKs phosphorylate homodimer or heterodimer ABA receptors, leading to monomerization for triggering ABA responses in Arabidopsis.

Binding of 14-3-3κ to ADF4 is involved in the regulation of hypocotyl growth and response to osmotic stress in Arabidopsis

14-3-3 proteins, a family of conserved molecules in eukaryotes, target a number of protein clients through their ability to recognize well-defined phosphorylated motifs. ADF4, as one of Actin-Depolymerizing Factor (ADF) family of proteins, is involved in plant development, and response to biotic and abiotic stresses. Here, we show that 14-3-3κ specially interacted with ADF4 in vitro and in vivo. The 14-3-3κ×adf4 double mutant displayed less F-actin bundle and shorter hypocotyl compared with adf4 mutant, indicating that 14-3-3κ acts upstream of ADF4 to mediate the hypocotyl growth in the dark-grown seedlings. Under the osmotic stress, 14-3-3κ mutants displayed less survival rate than wild-type plants. The adf4 mutants exhibited markedly enhanced survival rate under osmotic treatment, while ADF4-overexpressing plants displayed the opposite results, indicating that ADF4 plays a negative role in response to osmotic stress in Arabidopsis. The interaction between ADF4 and 14-3-3κ inhibited the association of ADF4 with actin filament. Moreover, the in vitro phosphorylation assay demonstrates that the phosphorylation of ADF4 by CASEIN KINASE1-LIKE PROTEIN2 (CKL2) was enhanced by binding 14-3-3κ. Collectively, our data infer a fundamental role for the interaction between 14-3-3κ and ADF4 in regulating hypocotyl growth and osmotic tolerance of plants.

Orphan gene PpARDT positively involved in drought tolerance potentially by enhancing ABA response in Physcomitrium (Physcomitrella) patens

Almost all genomes have orphan genes, the majority of which are not functionally annotated. There is growing evidence showed that orphan genes may play important roles in the environmental stress response of Physcomitrium patens. We identified PpARDT (ABA-responsive drought tolerance) as a moss-specific and ABA-responsive orphan gene in P. patens. PpARDT is mainly expressed during the gametophytic stage of the life cycle, and the expression was induced by different abiotic stresses. A PpARDT knockout (Ppardt) mutant showed reduced dehydration-rehydration tolerance, and the phenotype could be rescued by exogenous ABA. Meanwhile, transgenic Arabidopsis lines exhibiting heterologous expression of PpARDT were more sensitive to exogenous ABA than wild-type (Col-0) plants and showed enhanced drought tolerance. These indicate that PpARDT confers drought tolerance among land plants potentially by enhancing ABA response. Further, we identified genes encoding abscisic acid receptor PYR/PYL family proteins, and ADP-ribosylation factors (Arf) as hub genes associated with the Ppardt phenotype. Given the lineage-specific characteristics of PpARDT, our results provide insights into the roles of orphan gene in shaping lineage-specific adaptation possibly by recruiting common pre-existed pathway components.

Genome-wide identification and characterization of F-box family proteins in sweet potato and its expression analysis under abiotic stress

In this study, genome-wide characterization of F-box proteins in sweet potato yielded 243 IbFBX genes, unevenly distributed on the 15 chromosomes of sweet potato. Gene duplication analysis suggested segmental duplication as the principal factor influencing the expansive evolution of IbFBX genes in sweet potato. Phylogenetic analysis clustered F-box proteins in sweet potato, Arabidopsis, and rice into six clades (I-VI). Gene structure analysis of the IbFBX genes revealed that most of the genes within the same clade were highly conserved. Expression profiles of IbFBX family genes in 9 different tissues and under stress conditions revealed that the IbFBXs were highly upregulated or downregulated in response to salt and drought stress, suggesting their significant roles in abiotic stress response and adaptation. Knowledge of the diverse functions and expression patterns of IbFBXs presents a solid theoretical basis for annotating the functions of IbFBXs and further facilitate the molecular breeding of sweet potato.

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.

A common F-box gene regulates the leucine homeostasis of Medicago truncatula and Arabidopsis thaliana

The F-box domain is a conserved structural protein motif that most frequently interacts with the SKP1 protein, the core of the SCFs (SKP1-CULLIN-F-box protein ligase) E3 ubiquitin protein ligases. As part of the SCF complexes, the various F-box proteins recruit substrates for degradation through ubiquitination. In this study, we functionally characterized an F-box gene (MtF-box) identified earlier in a population of Tnt1 retrotransposon-tagged mutants of Medicago truncatula and its Arabidopsis thaliana homolog (AtF-box) using gain- and loss-of-function plants. We highlighted the importance of MtF-box in leaf development of M. truncatula. Protein-protein interaction analyses revealed the 2-isopropylmalate synthase (IPMS) protein as a common interactor partner of MtF-box and AtF-box, being a key enzyme in the biosynthesis pathway of the branched-chain amino acid leucine. For further detailed analysis, we focused on AtF-box and its role during the cell division cycle. Based on this work, we suggest a mechanism for the role of the studied F-box gene in regulation of leucine homeostasis, which is important for growth.

The F-box E3 ubiquitin ligase AtSDR is involved in salt and drought stress responses in Arabidopsis

F-box protein genes have been shown to play vital roles in plant development and stress respones. In Arabidopsis, there are more than 600 F-box proteins, and most of their functions are unclear. The present study shows that the F-box (SKP1-Cullin/CDC53-F-box) gene At5g15710 (Salt and Drought Responsiveness, SDR) is involved in abiotic stress responses in Arabidopsis. SDR is expressed in all tissues of Arabidopsis and is upregulated by salt and heat stresses and ABA treatment but downregulated by drought stress. Subcellular localization analysis shows that the SDR protein colocalizes with the nucleus. 35S:AntiSDR plants are hypersensitive to salt stress, but 35S:SDR plants display a salt-tolerant phenotype. Furthermore, 35S:SDR plants are hypersensitive to drought stress, while 35S:AntiSDR plants are significantly more drought tolerant. Overall, our results suggest that SDR is involved in salt and drought stress responses in Arabidopsis.

Integrated physiological and transcriptomic analyses of two warm- and cool-season turfgrass species in response to heat stress

Warm- and cool-season turfgrasses were originated from different locations with contrasting heat tolerance. The molecular mechanisms of heat tolerance have not been extensively studied in turfgrass species. In this study, transcriptomic analysis showed that bermudagrass was more tolerant to heat stress as evidenced by lower contents of H₂O₂, proline and glutathione than those in tall fescue after heat treatment. RNA sequencing analysis revealed that 32.7% and 17.7% more genes were changed in tall fescue than in bermudagrass after 2 and 12h heat treatment, respectively. GO terms of redox were enriched in bermudagrass whereas metabolite transportation ones were over-represented in tall fescue after 2h treatment. Ubiquitin dependent degradation pathways were commonly regulated in both grass species. CdF-box and FaF-box transgenic Arabidopsis exhibited improved tolerance to heat stress. Regulatory elements analysis revealed that four ABA-responsive elements present in CdF-box promoter, indicating CdF-box could be potentially regulated by ABRE binding factors (ABFs). All these findings provide evidences for understanding heat stress response in warm- and cool-season grass species.

Wheat TaPUB1 protein mediates ABA response and seed development through ubiquitination

Abscisic acid (ABA) is an important regulator of plant growth, development, and biotic and abiotic stress responses. Ubiquitination plays important roles in regulating ABA signaling. E3 ligase, a key member in ubiquitination, actively participates in the regulation of biosynthesis, de-repression, and activation of ABA response and degradation of signaling components. In this study, we found that that overexpression of wheat E3 ligase TaPUB1 decreased the sensitivity of wheat seedlings to ABA, whereas TaPUB1-RNA interference (TaPUB1-RNAi) lines increased wheat sensitivity to ABA during germination, root growth, and stomatal opening. TaPUB1 influenced the expression of several ABA-responsive genes, and also interacted with TaPYL4 and TaABI5, which are involved in ABA signal transduction, and promoted their degradation. Additionally, we observed that TaPUB1-OE lines resulted in lower single-split grain numbers, larger seed size, and higher thousand kernel weight, when compared with the WT lines. Contrasting results were obtained for TaPUB1-RNAi lines. It suggests that TaPUB1 acts as a negative regulator in the ABA signaling pathway by interacting with TaPYL4 and TaABI5, subsequently affecting seed development in wheat. In addition, the enhanced abiotic tolerance of overexpression lines due to enhanced photosynthesis and root development may be related to the degradation of TaABI5 by TaPUB1.

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.

Transcriptome, degradome and physiological analysis provide new insights into the mechanism of inhibition of litchi fruit senescence by melatonin

Litchi fruit has high commercial value on the international market, but senesces rapidly after harvest. We used weighted gene co-expression network analysis (WGCNA) and degradome technology to investigate the molecular mechanisms of melatonin-mediated delay of litchi fruit senescence through application of exogenous melatonin and p-chlorophenylalanine (p-CPA, an inhibitor of melatonin biosynthesis) treatments. Results demonstrated that exogenous melatonin treatment delayed litchi fruit senescence while p-CPA accelerated senescence. Coupled analyses of transcriptome and physiological parameters of litchi fruit provided the correlation of network modules with dynamic changes in browning index during storage. Additionally, we found that microRNAs (miR858 and miR160a) and their targets were actively involved in melatonin-mediated delay of litchi fruit senescence. Melatonin treatment decreased abscisic acid (ABA) content but increased PP2C and F-box expression levels, suggesting the involvement of ABA signaling in melatonin-mediated antisenescence. The transcriptions of ZAT, NAC and DREB1 were activated by melatonin treatment. Moreover, the major functional genes involved in histone methylation, γ-aminobutyric acid (GABA) metabolism, energy production, reactive oxygen species (ROS) accumulation and cell death were identified in the melatonin-inhibited litchi pericarp browning. Taken together, we first constructed the global map of the important regulators and pathways to delay litchi senescence and pericarp browning mediated by melatonin.

Small kernel 501 (smk501) encodes the RUBylation activating enzyme E1 subunit ECR1 (E1 C-TERMINAL RELATED 1) and is essential for multiple aspects of cellular events during kernel development in maize

RUBylation plays essential roles in plant growth and development through regulating Cullin-RING ubiquitin E3 ligase (CRL) activities and the CRL-mediated protein degradations. However, the function of RUBylation in regulating kernel development remains unclear. Through genetic and molecular analyses of a small kernel 501 (smk501) mutant in maize (Zea mays), we cloned the smk501 gene, revealed its molecular function, and defined its roles in RUBylation pathway and seed development. Smk501 encodes a RUBylation activating enzyme E1 subunit ZmECR1 (E1 C-TERMINAL RELATED 1) protein. Destruction in RUBylation by smk501 mutation resulted in less embryo and endosperm cell number and smaller kernel size. The transcriptome and proteome profiling, hormone evaluation and cell proliferation observation revealed that disturbing ZmECR1 expression mainly affects pathways on hormone signal transduction, cell cycle progression and starch accumulation during kernel development. In addition, mutant in zmaxr1 (Auxin resistant 1), another RUB E1 subunit, also showed similar defects in kernel development. Double mutation of zmecr1 and zmaxr1 lead to empty pericarp kernel phenotype. RUBylation is a novel regulatory pathway affecting maize kernel development, majorly through its functions in modifying multiple cellular progresses.

An Update on Crop ABA Receptors

The hormone abscisic acid (ABA) orchestrates the plant stress response and regulates sophisticated metabolic and physiological mechanisms essential for survival in a changing environment. Plant ABA receptors were described more than 10 years ago, and a considerable amount of information is available for the model plant Arabidopsis thaliana. Unfortunately, this knowledge is still very limited in crops that hold the key to feeding a growing population. In this review, we summarize genomic, genetic and structural data obtained in crop ABA receptors. We also provide an update on ABA perception in major food crops, highlighting specific and common features of crop ABA receptors.

A ras-related small GTP-binding protein, RabE1c, regulates stomatal movements and drought stress responses by mediating the interaction with ABA receptors

Drought represents a leading constraint over crop productivity worldwide. The plant response to this stress is centered on the behavior of the cell membrane, where the transduction of abscisic acid (ABA) signaling occurs. Here, the Ras-related small GTP-binding protein RabE1c has been shown able to bind to an ABA receptor in the Arabidopsis thaliana plasma membrane, thereby positively regulating ABA signaling. RabE1c is highly induced by drought stress and expressed abundantly in guard cells. In the loss-of-function rabe1c mutant, both stomatal closure and the whole plant drought stress response showed a reduced sensitivity to ABA treatment, demonstrating that RabE1c is involved in the control over transpirative water loss through the stomata. Impairment of RabE1c's function suppressed the accumulation of the ABA receptor PYL4. The over-expression of RabE1c in A. thaliana enhanced the plants' ability to tolerate drought, and a similar phenotypic effect was achieved by constitutively expressing the gene in Chinese cabbage (Brassica rapassp. pekinensis). The leading conclusion was that RabE1c promotes the degradation of PYL4, suggesting a possible genetic strategy to engineer crop plants to better withstand drought stress.

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.

Role of ubiquitination enzymes in abiotic environmental interactions with plants

Ubiquitination, a post-translational modification, plays a crucial role in various aspects of plant development and stress responses. Protein degradation by ubiquitination is well established and ubiquitin is the main underlying component directing the turnover of proteins. Recent reports have also revealed the non-proteolytic roles of ubiquitination in plants. In the past decade, ubiquitination has emerged to be one of the most important players in modulating plant's responses to abiotic stresses, which led to identification of specific E3 ligases and their targets involved in the process. Most of the E3 ligases play regulatory roles by modifying the stability and accumulation of stress responsive regulatory proteins, such as transcription factors, thus, modifying the downstream responses, or by degrading the proteins involved in the downstream cascade itself. In this review, we summarize and highlight the recent advances in the field of ubiquitination-mediated regulation of plant's responses to various abiotic stresses including limited nutrient availability and metal toxicity. The non-proteolytic role of ubiquitination in epigenetic regulation of abiotic stress induced response has also been discussed.

Arabidopsis F-box protein At1g08710 interacts with transcriptional protein ADA2b and imparts drought stress tolerance by negatively regulating seedling growth

Plants experience abiotic stresses throughout their life cycle and accordingly respond to tide over the unfavorable conditions. Plants adopt to drought stress through various molecular, biochemical, physiological and cellular processes. F-box protein subunit of the Skp1-Cullin-F-box (SCF) E3 ubiquitin ligases plays crucial role in imparting specificity for selective degradation of target proteins. Here we report the function of Arabidopsis F-box protein At1g08710 in drought stress adaptation. F-box protein is a constituent of SCF complex as it is shown interacting with ASK1 and Cullin 1. F-box protein localizes in both nucleus and membrane. F-box gene transcript is highly accumulated in root and altered in response to drought stress conditions. F-box protein interacts with a transcriptional co-activator protein ADA2b. F-box mutant plants growth is better under drought stress conditions compared to the wild type. Accumulation of H₂O₂ and malondialdehyde (MDA) content is reduced in mutant plants. Drought responsive genes RD29A, RD22, ABI3 expression is induced in F-box mutant plants. These results indicate F-box protein At1g08710 role in drought stress adaptation in Arabidopsis thaliana.

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.

The UBC27-AIRP3 ubiquitination complex modulates ABA signaling by promoting the degradation of ABI1 in Arabidopsis

Abscisic acid (ABA) is the key phytohormone in plant drought tolerance and stress adaptation. The clade A protein phosphatase 2Cs (PP2Cs) like ABI1 (ABA-INSENSITIVE 1) work as coreceptors of ABA and regulate multiple ABA responses. Ubiquitination of ABI1 has been proven to play important regulatory roles in ABA signaling. However, the specific ubiquitin conjugating enzyme (E2) involved is unknown. Here, we report that UBC27 is an active E2 that positively regulates ABA signaling and drought tolerance. UBC27 forms the E2-E3 pair with the drought regulator RING E3 ligase AIRP3. Both UBC27 and AIRP3 interact with ABI1 and affect the ubiquitination and degradation of ABI1. ABA activates the expression of UBC27, inhibits the proteasome degradation of UBC27, and enhances the interaction between UBC27 and ABI1 to increase its activity. These findings uncover a regulatory mechanism in ABA signaling and drought response and provide a further understanding of the plant ubiquitination system and ABA signaling pathway.

Interplay between the Ubiquitin Proteasome System and Ubiquitin-Mediated Autophagy in Plants

All eukaryotes rely on the ubiquitin-proteasome system (UPS) and autophagy to control the abundance of key regulatory proteins and maintain a healthy intracellular environment. In the UPS, damaged or superfluous proteins are ubiquitinated and degraded in the proteasome, mediated by three types of ubiquitin enzymes: E1s (ubiquitin activating enzymes), E2s (ubiquitin conjugating enzymes), and E3s (ubiquitin protein ligases). Conversely, in autophagy, a vesicular autophagosome is formed that transfers damaged proteins and organelles to the vacuole, mediated by a series of ATGs (autophagy related genes). Despite the use of two completely different componential systems, the UPS and autophagy are closely interconnected and mutually regulated. During autophagy, ATG8 proteins, which are autophagosome markers, decorate the autophagosome membrane similarly to ubiquitination of damaged proteins. Ubiquitin is also involved in many selective autophagy processes and is thus a common factor of the UPS and autophagy. Additionally, the components of the UPS, such as the 26S proteasome, can be degraded via autophagy, and conversely, ATGs can be degraded by the UPS, indicating cross regulation between the two pathways. The UPS and autophagy cooperate and jointly regulate homeostasis of cellular components during plant development and stress response.

ABI5 modulates seed germination via feedback regulation of the expression of the PYR/PYL/RCAR ABA receptor genes

As abscisic acid (ABA) receptors, PYR1/PYL/RCAR (PYLs) play important roles in ABA-mediated seed germination, but the regulation of PYLs in this process, especially at the transcriptional level, remains unclear. In this study, we found that expression of 11 of 14 PYLs changes significantly during seed germination and is affected by exogenous ABA. Two PYLs, PYL11 and PYL12, both of which are expressed specifically in mature seeds, positively modulate ABA-mediated seed germination. However, ABI5 was found to modulate the PYL11- and PYL12-mediated ABA response. In the abi5-7 mutant, ABA hypersensitivity caused by PYL11 and PYL12 overexpression was totally or partially blocked. By contrast, ABI5 regulates the expression of PYL11 and PYL12 by directly binding to their promoters. Moreover, the expression of eight other PYLs is also affected during the germination of abi5 mutants. Promoter analysis revealed that an ABI5-binding region is present next to the TATA box or initiator box. Together, our data demonstrate the role of PYL11 and PYL12 in seed germination. In addition, the identification of PYLs as targets of ABI5 reveals a role of ABI5 in the feedback regulation of ABA-mediated seed germination.

Proteasome Inhibition in Brassica napus Roots Increases Amino Acid Synthesis to Offset Reduced Proteolysis

Cellular homeostasis is maintained by the proteasomal degradation of regulatory and misfolded proteins, which sustains the amino acid pool. Although proteasomes alleviate stress by removing damaged proteins, mounting evidence indicates that severe stress caused by salt, metal(oids), and some pathogens can impair the proteasome. However, the consequences of proteasome inhibition in plants are not well understood and even less is known about how its malfunctioning alters metabolic activities. Lethality causes by proteasome inhibition in non-photosynthetic organisms stem from amino acid depletion, and we hypothesized that plants respond to proteasome inhibition by increasing amino acid biosynthesis. To address these questions, the short-term effects of proteasome inhibition were monitored for 3, 8 and 48 h in the roots of Brassica napus treated with the proteasome inhibitor MG132. Proteasome inhibition did not affect the pool of free amino acids after 48 h, which was attributed to elevated de novo amino acid synthesis; these observations coincided with increased levels of sulfite reductase and nitrate reductase activities at earlier time points. However, elevated amino acid synthesis failed to fully restore protein synthesis. In addition, transcriptome analysis points to perturbed abscisic acid signaling and decreased sugar metabolism after 8 h of proteasome inhibition. Proteasome inhibition increased the levels of alternative oxidase but decreased aconitase activity, most sugars and tricarboxylic acid metabolites in root tissue after 48 h. These metabolic responses occurred before we observed an accumulation of reactive oxygen species. We discuss how the metabolic response to proteasome inhibition and abiotic stress partially overlap in plants.

Diverse and dynamic roles of F-box proteins in plant biology

The SCF complex is a widely studied multi-subunit ring E3 ubiquitin ligase that tags targeted proteins with ubiquitin for protein degradation by the ubiquitin 26S-proteasome system (UPS). The UPS is an important system that generally keeps cellular events tightly regulated by purging misfolded or damaged proteins and selectively degrading important regulatory proteins. The specificity of this post-translational regulation is controlled by F-box proteins (FBPs) via selective recognition of a protein-protein interaction motif at the C-terminal domain. Hence, FBPs are pivotal proteins in determining the plant response in multiple scenarios. It is not surprising that the FBP family is one of the largest protein families in the plant kingdom. In this review, the roles of FBPs, specifically in plants, are compiled to provide insights into their involvement in secondary metabolites, plant stresses, phytohormone signalling, plant developmental processes and miRNA biogenesis.

Desensitization of ABA-Signaling: The Swing From Activation to Degradation

Abscisic acid (ABA) is a key plant stress-signaling hormone that accumulates upon osmotic stresses such as drought and high salinity. Several proteins have been identified that constitute the ABA-signaling pathway. Among them ABA receptors (PYR/PYL/RCAR), co-receptor PP2Cs (protein phosphatases), SnRK2 kinases (SNF1-related protein kinases) and ABI5/ABFs (transcription factors) are the major components. Upon ABA signal, PYR/PYL receptors interact with and recruit PP2Cs, releasing SnRK2s kinases from sequestration with PP2Cs. This allows SnKR2s to promote the activation of downstream transcription factors of ABA pathway. However, apart from activation, ubiquitination and degradation of core proteins in the ABA pathway by the ubiquitin proteasome system is less explored. In this review we will focus on the recent findings about feedback regulation of ABA signaling core proteins through degradation, which is emerging as a critical step that modulates and eventually ceases the signal relay. Additionally, we also discuss the importance of the recently identified effector protein HOS15, which negatively regulate ABA-signaling through degradation of OST1.

Genome-wide analysis and characterization of F-box gene family in Gossypium hirsutum L

Background

F-box proteins are substrate-recognition components of the Skp1-Rbx1-Cul1-F-box protein (SCF) ubiquitin ligases. By selectively targeting the key regulatory proteins or enzymes for ubiquitination and 26S proteasome mediated degradation, F-box proteins play diverse roles in plant growth/development and in the responses of plants to both environmental and endogenous signals. Studies of F-box proteins from the model plant Arabidopsis and from many additional plant species have demonstrated that they belong to a super gene family, and function across almost all aspects of the plant life cycle. However, systematic exploration of F-box family genes in the important fiber crop cotton (Gossypium hirsutum) has not been previously performed. The genome-wide analysis of the cotton F-box gene family is now possible thanks to the completion of several cotton genome sequencing projects.

Results

In current study, we first conducted a genome-wide investigation of cotton F-box family genes by reference to the published F-box protein sequences from other plant species. 592 F-box protein encoding genes were identified in the Gossypium hirsutume acc.TM-1 genome and, subsequently, we were able to present their gene structures, chromosomal locations, syntenic relationships with their parent species. In addition, duplication modes analysis showed that cotton F-box genes were distributed to 26 chromosomes, with the maximum number of genes being detected on chromosome 5. Although the WGD (whole-genome duplication) mode seems play a dominant role during cotton F-box gene expansion process, other duplication modes including TD (tandem duplication), PD (proximal duplication), and TRD (transposed duplication) also contribute significantly to the evolutionary expansion of cotton F-box genes. Collectively, these bioinformatic analysis suggest possible evolutionary forces underlying F-box gene diversification. Additionally, we also conducted analyses of gene ontology, and expression profiles in silico, allowing identification of F-box gene members potentially involved in hormone signal transduction.

Conclusion

The results of this study provide first insights into the Gossypium hirsutum F-box gene family, which lays the foundation for future studies of functionality, particularly those involving F-box protein family members that play a role in hormone signal transduction.

Functional Analysis of Pepper F-box Protein CaDIF1 and Its Interacting Partner CaDIS1: Modulation of ABA Signaling and Drought Stress Response

Plant adaptive responses to environmental stress are coordinated by inhibition of plant growth and development. Abscisic acid (ABA) is a major phytohormone that regulates the stress response, and its sensitivity determines stress tolerance levels. In this study, we report the identification and functional role of a novel F-box protein, CaDIF1 (Capsicum annuum Drought-Induced F-box Protein 1). The expression of CaDIF1 in pepper leaves was induced by ABA, drought, H₂O₂, and NaCl treatments. In comparison with wild-type pepper plants, CaDIF1-silenced pepper plants exhibited a drought-sensitive phenotype, whereas CaDIF1-overexpressing (OX) plants exhibited ABA-sensitive and drought-tolerant phenotypes. Using a yeast two-hybrid assay, we identified CaDIS1 (C. annuum DIF1-Interacting SKP 1), which interacts with CaDIF1 in the cytoplasm and nucleus. Consistent with CaDIF1-silenced pepper plants, CaDIS1-silenced pepper plants displayed ABA insensitivity and reduced drought tolerance, and these were characterized by larger stomatal apertures and greater transpirational water loss. Taken together, our results indicate that CaDIF1 and its interacting partner CaDIS1 synergistically regulate the ABA-dependent defence signaling response to drought stress.

Wheat F-box Protein TaFBA1 Positively Regulates Plant Drought Tolerance but Negatively Regulates Stomatal Closure

The phytohormone abscisic acid (ABA) regulates plant growth and development, as well as responses to various stresses, such as salt and drought. The wheat TaFBA1 gene, which encodes an F-box protein, was previously identified in our laboratory by homologous cloning. We previously found that TaFBA1 expression was induced by ABA and drought stress. In this study, wild-type (WT), TaFBA1 over-expressing (OEs), TaFBA1 homologous gene mutants, and TaFBA1 recovery (Rs) Arabidopsis plants were used. We found that the germination rate, the cotyledon greening rate, the root length, and the photosynthetic performance of TaFBA1 OE plants were better than those of WT under drought and ABA conditions, but mutant plants showed the opposite trend, and overexpression of TaFBA1 in mutants can recover their phenotype. In addition, TaFBA1 was found to be a negative regulator of ABA-induced stoma movement; mRNA transcription of certain ABA signaling-related genes was lower in TaFBA1 OE plants than in WT plants following ABA treatment. Further, we found that TaFBA1 can interact with RCAR1 (an ABA receptor) and ABI5. BiFC assay showed that TaFBA1 may interact with RCAR1 in the plasma membrane. In addition, accumulation of ROS and MDA in TaFBA1 OE plants was lower than that in the WT plants after ABA and drought treatments. Based on these results, we suggest that TaFBA1-regulated ABA insensitivity may be dependent on regulating ABA-mediated gene expression through interacting with RCAR1 and ABI5. Increased antioxidant competence and decreased ROS accumulation may be an important mechanism that underlies improved drought tolerance in TaFBA1 OE plants.

Responses of PYR/PYL/RCAR ABA Receptors to Contrasting stresses, Heat and Cold in Arabidopsis

Plants growing in natural habitats have evolved a wide range of mechanisms to copy with environmental challenging, including biotic and abiotic stresses. Abiotic stresses-induced increases in Abscisic acid (ABA) levels in plants suffering from stresses, including drought, cold or heat stress. To explore the function of the core components in ABA signaling, we used the overexpression of RCARs transgenic plants to expose in heat or cold stress. In this study, overexpression of RCAR12 or RCAR13 (R12-OE or R13-OE) transgenic plants had higher germination and survival rate than the wild-type (WT) Arabidopsis, indicating that they are both positively responsive to the high temperature. And the heat shock genes HSP18.2 and HSP70 were significantly induced by RCAR12 or RCAR13. Further, the results inferred that the over-expression of RCAR12 or RCAR13 could tolerance the cold stress, through induction CBFs expressions, the cold-responsive genes when plants were challenged the cold tress. And when complementation of RCAR12 to the 1124 mutant (R12:1124), the results indicated that RCAR12 could recover the insensitivity of 1124 to heat and cold stresses. Hence, we propose that RCAR12 and RCAR13, the ABA receptors, may play the positive roles in regulating the extreme temperature, including cold and high temperature in Arabidopsis.

AtARRE, an E3 ubiquitin ligase, negatively regulates ABA signaling in Arabidopsis thaliana

The RING-type E3 ligase AtARRE participates in the plant ABA responding as a negative regulator. Ubiquitination protease system (UPS) is significant in post-transcriptional regulation. In UPS, E3 ligase recognizes the substrate protein and mediates the polyubiquitin chain onto the substrate. Here, we identified a new gene, named Arabidopsis thaliana ABA-related RING-type E3 ligase (AtARRE), which induced by ABA and NaCl. AtARRE encodes a functional RING-type E3 ligase protein localized in nucleus and plasma membrane of Arabidopsis. Physiological analysis demonstrated that mutation of AtARRE (T-DNA insert mutants atarre-1 and atarre-2) caused plants hypersensitivity to ABA, including enhanced stomatal closure, reduced root elongation and seed germination. However, overexpression of AtARRE transgenic lines caused plants hyposensitive to ABA compared with WT and mutant atarre plants. Under the treatment of ABA, the transcript abundances of ABA-responsive genes RD29A, RD29B, RD22 and ABI5 in atarre mutant plants were markedly higher than those of WT and AtARRE overexpression lines. Hence, these results indicate that AtARRE acts as a negative regulator of ABA-mediated stress responses in Arabidopsis.

ABA Receptor Subfamily III Enhances Abscisic Acid Sensitivity and Improves the Drought Tolerance of Arabidopsis

The phytohormone abscisic acid (ABA) regulates plant growth, the developmental process, and abiotic stresses. ABA signaling is induced in response to mediate plant acclimation to environmental challenges, including high salinity and drought. The ABA-binding receptors (RCAR/PYR1/PYL), composing of 14 members, are the core components of the ABA-signaling pathway. Here, we observed that the three subfamilies within the RCARs showed different expression patterns at the basal and exogenous ABA levels. Subsequently, we generated transgenic plants overexpressing subfamily III, RCAR11–RCAR14, respectively. The transgenic plants showed increased ABA sensitivity in seed germination and post-germination seedling establishment and root length. Further studies revealed that the overexpressing subfamily III transgenic plants enhanced drought resistance, increased water-use efficiency, and accelerated stress-responsive gene expression compared with the wild-type plants. These findings confirm that the subfamily III plays a key role in ABA-mediated developmental processes and, more importantly, is involved in drought tolerance in the ABA-dependent pathway.

Role of the Ubiquitin Proteasome System in Plant Response to Abiotic Stress

Ubiquitination is a prevalent post-translation modification system that is involved in almost all aspects of eukaryotic biology. It involves the attachment of ubiquitin, a small, highly conserved protein to selected substrates. The most notable function of ubiquitin is the targeting of modified proteins to the multi-proteolytic 26S proteasome complex for degradation. The ubiquitin proteasome system (UPS) regulates the abundance of numerous enzymes, structural and regulatory proteins ensuring proper cellular function. Plants utilize the UPS to facilitate cellular changes required to respond to and tolerate adverse growth conditions. In this review, the regulatory role of the UPS in responses to abiotic stress is discussed, particularly the function of ubiquitin-dependent degradation in the suppression, activation and attenuation or termination of stress signaling.

CARK1 mediates ABA signaling by phosphorylation of ABA receptors

The function of abscisic acid (ABA) is mediated by its receptors termed RCARs/PYR1/PYLs. Modulation of ABA signaling is vital for plant growth and development. The RCAR-PP2C-SnRK2 regulatory modules have been defined as the core components in ABA signaling. However, it is still not clear whether and how the ABA receptors could be modified at the initial post-translational stage to fine-tune ABA transduction pathway. Here we identify and characterize the putative receptor-like cytoplasmic kinase (RLCK) in Arabidopsis named CARK1, which interacts with RCAR3 (PYL8) and RCAR11 (PYR1) in the manner of phosphorylation. Structural studies of CARK1 revealed the critical active site, N204, which accounts for the kinase activity and the direct interaction with RCAR3/RCAR11. CARK1 phosphorylates RCAR3/RCAR11 at one conserved threonine site, T77/T78. Our genetic analyses further demonstrated that CARK1 positively regulates ABA-mediated physiological responses and overexpression of CARK1 in Arabidopsis distinctly promotes the drought resistance. Moreover, the phosphor-mimic form of RCAR11 in the cark1 mutant is able to functionally complement the ABA sensitivity. CARK1 positively regulates ABA-responsive gene expression and enhances RCAR3/RCAR11’s inhibition to Clade A PP2C. Taken together, our studies strongly support the functional significance of CARK1 in positively regulating ABA signaling via phosphorylation on RCAR3/RCAR11 in Arabidopsis.

AtRAE1 is involved in degradation of ABA receptor RCAR1 and negatively regulates ABA signalling in Arabidopsis

The phytohormone abscisic acid (ABA) plays an important role in regulating plant growth, development, and adaption to various environmental stresses. Regulatory components of ABA receptors (RCARs, also known as PYR/PYLs) sense ABA and initiate ABA signalling through inhibiting the activities of protein phosphatase 2C in Arabidopsis. However, the way in which ABA receptors are regulated is not well known. A DWD protein AtRAE1 (for RNA export factor 1 in Arabidopsis), which may act as a substrate receptor of CUL4-DDB1 E3 ligase, is an interacting partner of RCAR1/PYL9. The physical interaction between RCAR1 and AtRAE1 is confirmed in vitro and in vivo. Overexpression of AtRAE1 in Arabidopsis causes reduced sensitivity of plants to ABA, whereas suppression of AtRAE1 causes increased sensitivity to ABA. Analysis of protein stability demonstrates that RCAR1 is ubiquitinated and degraded in plant cells and AtRAE1 regulates the degradation speed of RCAR1. Our findings indicate that AtRAE1 likely participates in ABA signalling through regulating the degradation of ABA receptor RCAR1.

E3 Ubiquitin Ligases: Ubiquitous Actors in Plant Development and Abiotic Stress Responses

Understanding the precise regulatory mechanisms of plant development and stress responses at the post-translational level is currently a topic of intensive research. Protein ubiquitination, including the sequential performances of ubiquitin-activating (E1), ubiquitin-conjugating (E2) and ubiquitin ligase (E3) enzymes, is a refined post-translational modification ubiquitous in all eukaryotes. Plants are an integral part of our ecosystem and, as sessile organisms, the ability to perceive internal and external signals and to adapt well to various environmental challenges is crucial for their survival. Over recent decades, extensive studies have demonstrated that protein ubiquitination plays key roles in multiple plant developmental stages (e.g. seed dormancy and germination, root growth, flowering time control, self-incompatibility and chloroplast development) and several abiotic stress responses (e.g. drought and high salinity), by regulating the abundance, activities or subcellular localizations of a variety of regulatory polypeptides and enzymes. Importantly, diverse E3 ligases are involved in these regulatory pathways by mediating phytohormone and light signaling or other pathways. In this updated review, we mainly summarize recent advances in our understanding of the regulatory roles of protein ubiquitination in plant development and plant-environment interactions, and primarily focus on different types of E3 ligases because they play critical roles in determining substrate specificity.

Arabidopsis E3 Ubiquitin Ligases PUB22 and PUB23 Negatively Regulate Drought Tolerance by Targeting ABA Receptor PYL9 for Degradation

Drought causes osmotic stress and rapidly triggers abscisic acid (ABA) accumulation in plants. The roles of various ABA receptors in drought tolerance and molecular mechanisms regulating ABA receptor stability needs to be elucidated. Here, we report that Arabidopsis plants overexpressing PYL9, one of the 14 pyrabactin resistance (PYR)/pyrabactin resistance-like (PYL)/regulatory component of ABA receptors (RCAR) family ABA receptors, gained drought tolerance trait. Osmotic stress induced accumulation of the PYL9 protein, which was regulated by the 26S proteasome. PYL9 interacted with two highly homologous plant U-box E3 ubiquitin ligases PUB22 and PUB23. In the cell-free degradation assay, the degradation of GST-PYL9 was accelerated in protein extract from plants overexpressing PUB22 but slowed down in protein extract from the pub22 pub23 double mutant. The in vivo decay of Myc-PYL9 was significantly reduced in the pub22 pub23 double mutant as compared with the wild-type. Additionally, PUB22 also interacted with other ABA receptors such as PYL5, PYL7 and PYL8. Considering the improved drought tolerance in the pub22 pub23 double mutant in previous studies, our results suggest that PUB22 and PUB23 negatively regulate drought tolerance in part by facilitating ABA receptors degradation.

SCFAtPP2-B11 modulates ABA signaling by facilitating SnRK2.3 degradation in Arabidopsis thaliana

The phytohormone abscisic acid (ABA) is an essential part of the plant response to abiotic stressors such as drought. Upon the perception of ABA, pyrabactin resistance (PYR)/PYR1-like (PYL)/regulatory components of ABA receptor (RCAR) proteins interact with co-receptor protein phosphatase type 2Cs to permit activation Snf1-related protein kinase2 (SnRK2) kinases, which switch on ABA signaling by phosphorylating various target proteins. Thus, SnRK2 kinases are central regulators of ABA signaling. However, the mechanisms that regulate SnRK2 degradation remain elusive. Here, we show that SnRK2.3 is degradated by 26S proteasome system and ABA promotes its degradation. We found that SnRK2.3 interacts with AtPP2-B11 directly. AtPP2-B11 is an F-box protein that is part of a SKP1/Cullin/F-box E3 ubiquitin ligase complex that negatively regulates plant responses to ABA by specifically promoting the degradation of SnRK2.3. AtPP2-B11 was induced by ABA, and the knockdown of AtPP2-B11 expression markedly increased the ABA sensitivity of plants during seed germination and postgerminative development. Overexpression of AtPP2-B11 does not affect ABA sensitivity, but inhibits the ABA hypersensitive phenotypes of SnRK2.3 overexpression lines. These results reveal a novel mechanism through which AtPP2-B11 specifically degrades SnRK2.3 to attenuate ABA signaling and the abiotic stress response in Arabidopsis.

Fine mapping and identification of candidate genes for the sy-2 locus in a temperature-sensitive chili pepper (Capsicum chinense)

The sy - 2 temperature-sensitive gene from Capsicum chinense was fine mapped to a 138.8-kb region at the distal portion of pepper chromosome 1. Based on expression analyses, two putative F-box genes were identified as sy - 2 candidate genes. Seychelles-2 ('sy-2') is a temperature-sensitive natural mutant of Capsicum chinense, which exhibits an abnormal leaf phenotype when grown at temperatures below 24 °C. We previously showed that the sy-2 phenotype is controlled by a single recessive gene, sy-2, located on pepper chromosome 1. In this study, a high-resolution genetic and physical map for the sy-2 locus was constructed using two individual F2 mapping populations derived from a cross between C. chinense mutant 'sy-2' and wild-type 'No. 3341'. The sy-2 gene was fine mapped to a 138.8-kb region between markers SNP 5-5 and SNP 3-8 at the distal portion of chromosome 1, based on comparative genomic analysis and genomic information from pepper. The sy-2 target region was predicted to contain 27 genes. Expression analysis of these predicted genes showed a differential expression pattern for ORF10 and ORF20 between mutant and wild-type plants; with both having significantly lower expression in 'sy-2' than in wild-type plants. In addition, the coding sequences of both ORF10 and ORF20 contained single nucleotide polymorphisms (SNPs) causing amino acid changes, which may have important functional consequences. ORF10 and ORF20 are predicted to encode F-box proteins, which are components of the SCF complex. Based on the differential expression pattern and the presence of nonsynonymous SNPs, we suggest that these two putative F-box genes are most likely responsible for the temperature-sensitive phenotypes in pepper. Further investigation of these genes may enable a better understanding of the molecular mechanisms of low temperature sensitivity in plants.