The Arabidopsis RING E3 ubiquitin ligase AtAIRP2 plays combinatory roles with AtAIRP1 in abscisic acid-mediated drought stress responses

Molecular Mechanism of GmSNE3 Ubiquitin Ligase-Mediated Inhibition of Soybean Nodulation by Halosulfuron Methyl

In this study, the role of E3 ubiquitin ligase GmSNE3 in halosulfuron methyl (HSM) inhibiting soybean nodulation was investigated. GmSNE3 was strongly induced by HSM stress, and the overexpression of GmSNE3 significantly reduced the number of soybean nodules. Further investigation found that GmSNE3 could interact with a nodulation signaling pathway 1 protein (GmNSP1a) and GmSNE3 could mediate the degradation of GmNSP1a. Importantly, GmSNE3-mediated degradation of GmNSP1a could be promoted by HSM stress. Moreover, HSM stress and the overexpression of GmSNE3 resulted in a substantial decrease in the expression of the downstream target genes of GmNSP1a. These results revealed that HSM promotes the ubiquitin-mediated degradation of GmNSP1a by inducing GmSNE3, thereby inhibiting the regulatory effect of GmNSP1a on its downstream target genes and ultimately leading to a reduction in nodulation. Our findings will promote a better understanding of the toxic mechanism of herbicides on the symbiotic nodulation between legumes and rhizobia.

Deciphering the mechanism of E3 ubiquitin ligases in plant responses to abiotic and biotic stresses and perspectives on PROTACs for crop resistance

With global climate change, it is essential to find strategies to make crops more resistant to different stresses and guarantee food security worldwide. E3 ubiquitin ligases are critical regulatory elements that are gaining importance due to their role in selecting proteins for degradation in the ubiquitin-proteasome proteolysis pathway. The role of E3 Ub ligases has been demonstrated in numerous cellular processes in plants responding to biotic and abiotic stresses. E3 Ub ligases are considered a class of proteins that are difficult to control by conventional inhibitors, as they lack a standard active site with pocket, and their biological activity is mainly due to protein-protein interactions with transient conformational changes. Proteolysis-targeted chimeras (PROTACs) are a new class of heterobifunctional molecules that have emerged in recent years as relevant alternatives for incurable human diseases like cancer because they can target recalcitrant proteins for destruction. PROTACs interact with the ubiquitin-proteasome system, principally the E3 Ub ligase in the cell, and facilitate proteasome turnover of the proteins of interest. PROTAC strategies harness the essential functions of E3 Ub ligases for proteasomal degradation of proteins involved in dysfunction. This review examines critical advances in E3 Ub ligase research in plant responses to biotic and abiotic stresses. It highlights how PROTACs can be applied to target proteins involved in plant stress response to mitigate pathogenic agents and environmental adversities.

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.

E3 ubiquitin ligase TaSDIR1-4A activates membrane-bound transcription factor TaWRKY29 to positively regulate drought resistance

Drought is a deleterious abiotic stress factor that constrains crop growth and development. Post-translational modification of proteins mediated by the ubiquitin-proteasome system is an effective strategy for directing plant responses to stress, but the regulatory mechanisms in wheat remain unclear. In this study, we showed that TaSDIR1-4A is a positive modulator of the drought response. Overexpression of TaSDIR1-4A increased the hypersensitivity of stomata, root length and endogenous abscisic acid (ABA) content under drought conditions. TaSDIR1-4A encodes a C3H2C3-type RING finger protein with E3 ligase activity. Amino acid mutation in its conserved domain led to loss of activity and altered the subcellular localization. The membrane-bound transcription factor TaWRKY29 was identified by yeast two-hybrid screening, and it was confirmed as interacting with TaSDIR1-4A both in vivo and in vitro. TaSDIR1-4A mediated the polyubiquitination and proteolysis of the C-terminal amino acid of TaWRKY29, and its translocation from the plasma membrane to the nucleus. Activated TaWRKY29 bound to the TaABI5 promoter to stimulate its expression, thereby positively regulating the ABA signalling pathway and drought response. Our findings demonstrate the positive role of TaSDIR1-4A in drought tolerance and provide new insights into the involvement of UPS in the wheat stress response.

Comprehensive Characterization of the C3HC4 RING Finger Gene Family in Potato (Solanum tuberosum L.): Insights into Their Involvement in Anthocyanin Biosynthesis

The C3HC4 RING finger gene (RING-HC) family is a zinc finger protein crucial to plant growth. However, there have been no studies on the RING-HC gene family in potato. In this study, 77 putative StRING-HCs were identified in the potato genome and grouped into three clusters based on phylogenetic relationships, the chromosome distribution, gene structure, conserved motif, gene duplication events, and synteny relationships, and cis-acting elements were systematically analyzed. By analyzing RNA-seq data of potato cultivars, the candidate StRING-HC genes that might participate in tissue development, abiotic stress, especially drought stress, and anthocyanin biosynthesis were further determined. Finally, a StRING-HC gene (Soltu.DM.09G017280 annotated as StRNF4-like), which was highly expressed in pigmented potato tubers was focused on. StRNF4-like localized in the nucleus, and Y2H assays showed that it could interact with the anthocyanin-regulating transcription factors (TFs) StbHLH1 of potato tubers, which is localized in the nucleus and membrane. Transient assays showed that StRNF4-like repressed anthocyanin accumulation in the leaves of Nicotiana tabacum and Nicotiana benthamiana by directly suppressing the activity of the dihydroflavonol reductase (DFR) promoter activated by StAN1 and StbHLH1. The results suggest that StRNF4-like might repress anthocyanin accumulation in potato tubers by interacting with StbHLH1. Our comprehensive analysis of the potato StRING-HCs family contributes valuable knowledge to the understanding of their functions in potato development, abiotic stress, hormone signaling, and anthocyanin biosynthesis.

Transcription factor PbMYB80 regulates lignification of stone cells and undergoes RING finger protein PbRHY1-mediated degradation in pear fruit

The Chinese white pear (Pyrus bretschneideri) fruit carries a high proportion of stone cells, adversely affecting fruit quality. Lignin is a main component of stone cells in pear fruit. In this study, we discovered that a pear MYB transcription factor, PbMYB80, binds to the promoters of key lignin biosynthesis genes and inhibits their expression. Stable overexpression of PbMYB80 in Arabidopsis showed that lignin deposition and secondary wall thickening were inhibited, and the expression of the lignin biosynthesis genes in transgenic Arabidopsis was decreased. Transient overexpression of PbMYB80 in pear fruit inhibited lignin metabolism and stone cell development, and the expression of some genes in the lignin metabolism pathway was reduced. In contrast, silencing PbMYB80 with VIGS increased the lignin and stone cell content in pear fruit, and increased expression of genes in the lignin metabolism pathway. By screening a pear fruit cDNA library in yeast, we found that PbMYB80 binds to a RING finger (PbRHY1) protein. We also showed that PbRHY1 exhibits E3 ubiquitin ligase activity and degrades ubiquitinated PbMYB80 in vivo and in vitro. This investigation contributes to a better understanding of the regulation of lignin biosynthesis in pear fruit, and provides a theoretical foundation for increasing pear fruit quality at the molecular level.

ATL Protein Family: Novel Regulators in Plant Response to Environmental Stresses

Plants actively develop intricate regulatory mechanisms to counteract the harmful effects of environmental stresses. The ubiquitin-proteasome pathway, a crucial mechanism, employs E3 ligases (E3s) to facilitate the conjugation of ubiquitin to specific target substrates, effectively marking them for proteolytic degradation. E3s play critical roles in many biological processes, including phytohormonal signaling and adaptation to environmental stresses. Arabidopsis Toxicosa en Levadura (ATL) proteins, belonging to a subfamily of RING-H2 E3s, actively modulate diverse physiological processes and plant responses to environmental stresses. Despite studies on the functions of certain ATL family members in rice and Arabidopsis, most ATLs still need more comprehensive study. This review presents an overview of the ubiquitin-proteasome system (UPS), specifically focusing on the pivotal role of E3s and associated enzymes in plant development and environmental adaptation. Our study seeks to unveil the active modulation of plant responses to environmental stresses by E3s and ATLs, emphasizing the significance of ATLs within this intricate process. By emphasizing the importance of studying the roles of E3s and ATLs, our review contributes to developing more resilient plant varieties and promoting sustainable agricultural practices while establishing a research roadmap for the future.

Phosphate Transporter OsPT4, Ubiquitinated by E3 Ligase OsAIRP2, Plays a Crucial Role in Phosphorus and Nitrogen Translocation and Consumption in Germinating Seed

Phosphorus (P) and nitrogen (N) are essential macronutrients necessary for plant growth and development. OsPT4 is a high-affinity phosphate (Pi) transporter that has a positive impact on nutrient uptake and seed development. In this study, the expression patterns of different Pi transporter genes in germinating seeds were determined, and the relative expression of OsPT4 was induced in Pi-deficient seeds and gradually increased with the passage of germination time. The analysis of P, N, Pi, and amino acid concentrations in germinating seeds of OsPT4 mutants showed that the OsPT4 mutation caused P and N retention and a continuous reduction in multiple amino acid concentrations in germinating seeds. Transcriptome analysis and qRT-PCR results also indicated that the OsPT4 mutation inhibits the expression of genes related to P and N transportation and amino acid synthesis in germinating seeds. In addition, the paraffin section and TUNEL assay of OsPT4 mutant germinating seeds suggests that OsPT4 mutation causes programmed cell death (PCD) delayed in the aleurone layer and inhibition of leaf outgrowth. Moreover, we also found that OsPT4 was ubiquitinated by OsAIRP2, which is a C3HC4-type RING E3 Ub ligase. Our studies illustrate that OsPT4 plays a crucial role in P and N collaborative translocation and consumption in germinating seeds. It also provides a theoretical basis for the molecules and physiological mechanisms of P and N cross-talk under suppressed Pi uptake conditions.

RING finger E3 ubiquitin ligase gene TaAIRP2-1B controls spike length in wheat

E3 ubiquitin ligase genes play important roles in the regulation of plant development. They have been well studied in plants, but have not been sufficiently investigated in wheat. Here, we identified a highly expressed RING finger E3 ubiquitin ligase gene TaAIRP2-1B (ABA-insensitive RING protein 2) in wheat spike. Sequence polymorphism and association analysis showed that TaAIRP2-1B is significantly associated with spike length under various conditions. The genotype with haplotype Hap-1B-1 of TaAIRP2-1B has a longer spike than that of Hap-1B-2, and was positively selected in the process of wheat breeding in China. Moreover, the TaAIRP2-1B-overexpressing rice lines have longer panicles compared with wild-type plants. The expression levels of TaAIRP2-1B in Hap-1B-1 accessions were higher than in Hap-1B-2 accessions. Further study revealed that the expression of TaAIRP2-1B was negatively regulated by TaERF3 (ethylene-responsive factor 3) via binding to the Hap-1B-2 promoter, but not via binding of Hap-1B-1. Additionally, several candidate genes interacting with TaAIRP2-1B were obtained by screening the cDNA library of wheat in yeast cells. It was found that TaAIRP2-1B interacted with TaHIPP3 (heavy metal-associated isoprenylated protein 3) and promoted TaHIPP3 degradation. Our study demonstrates that TaAIRP2-1B controls spike length, and the haplotype Hap-1B-1 of TaAIRP2-1B is a favorable natural variation for spike length enhancement in wheat. This work also provides genetic resources and functional markers for wheat molecular breeding.

Ring the yield: regulation of spike architecture by an E3 ubiquitin ligase in crops

This article comments on:

Zhang J, Li C, Li L, Xi Y, Wang J, Mao X, Jing R. 2023. RING finger E3 ubiquitin ligase gene TaAIRP2-1B controls spike length in wheat. Journal of Experimental Botany 74, 5014–5025.

PIF1, a phytochrome-interacting factor negatively regulates drought tolerance and carotenoids biosynthesis in tobacco

The phytochrome-interacting factors (PIFs) function crucially in multiple physiological processes, but the biological functions of some PIFs remain elusive in some species. Here, a PIF transcription factor NtPIF1 was cloned and characterized in tobacco (Nicotiana tabacum L.). The transcript of NtPIF1 was significantly induced by drought stress treatments, and it localized in the nuclear. Knockout of NtPIF1 by CRISPR/Cas9 system led to the improved drought tolerance of tobacco with increased osmotic adjustment, antioxidant activity, photosynthetic efficiency and decreased water loss rate. On the contrary, NtPIF1-overexpression plants displays drought-sensitive phenotypes. In addition, NtPIF1 reduced the biosynthesis of abscisic acid (ABA) and its upstream carotenoids by regulating the expression of genes involved in ABA and carotenoids biosynthetic pathway upon drought stress. Electrophoretic mobility shift and dual-luciferase assays illustrated that, NtPIF1 directly bind to the E-box elements within the promoters of NtNCED3, NtABI5, NtZDS and Ntβ-LCY to repress their transcription. Overall, these data suggested that NtPIF1 negatively regulate tobacco adaptive response to drought stress and carotenoids biosynthesis; moreover, NtPIF1 has the potential to develop drought-tolerant tobacco plants using CRISPR/Cas9 system.

Cosuppression of AtGELP22 and AtGELP23, two ubiquitinated target proteins of RING E3 ligase AtAIRP5, increases tolerance to drought stress in Arabidopsis

AtAIRP5 RING E3 ubiquitin ligase was recently identified as a positive regulator of the abscisic acid (ABA)-mediated drought stress response by stimulating the degradation of serine carboxypeptidase-like 1. Here, we identified GDSL-type esterase/lipase 22 (AtGELP22) and AtGELP23 as additional interacting partners of AtAIRP5. Yeast two-hybrid, pull-down, co-immunoprecipitation, and ubiquitination analyses verified that AtGELP22 and AtGELP23 are ubiquitinated target proteins of AtAIRP5. AtGELP22 and AtGELP23 were colocalized with AtAIRP5 to punctate-like structures in the cytosolic fraction, in which PYK10 and NAI2, two ER body marker proteins, are localized. T-DNA insertion atgelp22 and atgelp23 single knockout mutant plants showed phenotypes indistinguishable from those of wild-type plants under ABA treatment. In contrast, RNAi-mediated cosuppression of AtGELP22 and AtGELP23 resulted in hypersensitive ABA-mediated stomatal movements and higher tolerance to drought stress than that of the single mutant and wild-type plants. Taken together, our results suggest that the putative GDSL-type esterases/lipases AtGELP22 and AtGELP23 act as redundant negative regulators of the ABA-mediated drought stress response in Arabidopsis.

Biotinylated Tn5 transposase-mediated CUT&Tag efficiently profiles transcription factor-DNA interactions in plants

In contrast to CUT&Tag approaches for profiling bulk histone modifications, current CUT&Tag methods for analysing specific transcription factor (TF)-DNA interactions remain technically challenging due to TFs having relatively low abundance. Moreover, an efficient CUT&Tag strategy for plant TFs is not yet available. Here, we first applied biotinylated Tn5 transposase-mediated CUT&Tag (B-CUT&Tag) to produce high-quality libraries for interrogating TF-DNA interactions. B-CUT&Tag combines streptavidin-biotin-based DNA purification with routine CUT&Tag, optimizing the removal of large amounts of intact chromatin not targeted by specific TFs. The biotinylated chromatin fragments are then purified for construction of deep sequencing libraries or qPCR analysis. We applied B-CUT&Tag to probe genome-wide DNA targets of Squamosa promoter-binding-like protein 9 (SPL9), a well-established TF in Arabidopsis; the resulting profiles were efficient and consistent in demonstrating its well-established target genes in juvenile-adult transition/flowering, trichome development, flavonoid biosynthesis, wax synthesis and branching. Interestingly, our results indicate functions of AtSPL9 in modulating growth-defence trade-offs. In addition, we established a method for applying qPCR after CUT&Tag (B-CUT&Tag-qPCR) and successfully validated the binding of SPL9 in Arabidopsis and PHR2 in rice. Our study thus provides a convenient and highly efficient CUT&Tag strategy for profiling TF-chromatin interactions that is widely applicable to the annotation of cis-regulatory elements for crop improvement.

Pepper SnRK2.6-activated MEKK protein CaMEKK23 is directly and indirectly modulated by clade A PP2Cs in response to drought stress

The phytohormone abscisic acid (ABA) is important for the plant growth and development, in which it plays a key role in the responses to drought stress. Among the core components of ABA signaling, SnRK2s interact with a range of proteins, including Raf-like MAP3Ks. In this study, we isolated the pepper MEKK subfamily member CaMEKK23 that interacts with CaSnRK2.6. CaMEKK23 has kinase activity and is specifically trans-phosphorylated by CaSnRK2.6. Compared with control plants, CaMEKK23-silenced pepper were found to be sensitive to drought stress and insensitive to ABA, whereas overexpression of CaMEKK23 in both pepper and Arabidopsis plants induced the opposite phenotypes. These altered phenotypes were established to be dependent on the kinase activity of CaMEKK23, which was also shown to interact with CaPP2Cs, functioning upstream of CaSnRK2.6. In addition to inhibiting the kinase activity of CaMEKK23, these CaPP2Cs were found to have inhibitory effects on CaSnRK2.6. Using CaMEKK23-, CaAITP1/CaMEKK23-, CaSnRK2.6-, and CaAITP1/CaSnRK2.6-silenced pepper, we revealed that CaMEKK23 and CaSnRK2.6 function downstream of CaAITP1. Collectively, our findings indicate that CaMEKK23 plays a positive regulatory role in the ABA-mediated drought stress responses in pepper plants, and that its phosphorylation status is modulated by CaSnRK2.6 and CaPP2Cs, functioning as core components of ABA signaling.

Genome-wide identification and analysis of the evolution and expression pattern of the HVA22 gene family in three wild species of tomatoes

Wild tomato germplasm is a valuable resource for improving biotic and abiotic stresses in tomato breeding. The HVA22 is widely present in eukaryotes and involved in growth and development as well as stress response, such as cold, salt, drought, and biotic stress. In the present study, we identified 45 HVA22 genes in three wild species of tomatoes. The phylogenetic relationships, gene localization to chromosomes, gene structure, gene collinearity, protein interactions, and cis-acting element prediction of all 45 HVA22 genes (14 in Solanum pennellii, 15 in S. pimpinellifolium, and 16 in S. lycopersicoides) were analyzed. The phylogenetic analysis showed that the all HVA22 proteins from the family Solanaceae were divided into three branches. The identified 45 HVA22 genes were grouped into four subfamilies, which displayed similar number of exons and expanded in a fragmentary replication manner. The distribution of HVA22 genes on the chromosomes of the three wild tomato species was also highly similar. RNA-seq and qRT-PCR revealed that HVA22 genes were expressed in different tissues and induced by drought, salt, and phytohormone treatments. These results might be useful for explaining the evolution, expression patterns, and functional divergence of HVA22 genes in Lycopersicon.

E3 ligase AtAIRP5/GARU regulates drought stress response by stimulating SERINE CARBOXYPEPTIDASE-LIKE1 turnover

Ubiquitination is a major mechanism of eukaryotic posttranslational protein turnover that has been implicated in abscisic acid (ABA)-mediated drought stress response. Here, we isolated T-DNA insertion mutant lines in which ABA-insensitive RING protein 5 (AtAIRP5) was suppressed, resulting in hyposensitive ABA-mediated germination compared to wild-type Arabidopsis (Arabidopsis thaliana) plants. A homology search revealed that AtAIRP5 is identical to gibberellin (GA) receptor RING E3 ubiquitin (Ub) ligase (GARU), which downregulates GA signaling by degrading the GA receptor GID1, and thus AtAIRP5 was renamed AtAIRP5/GARU. The atairp5/garu knockout progeny were impaired in ABA-dependent stomatal closure and were markedly more susceptible to drought stress than wild-type plants, indicating a positive role for AtAIRP5/GARU in the ABA-mediated drought stress response. Yeast two-hybrid, pull-down, target ubiquitination, and in vitro and in planta degradation assays identified serine carboxypeptidase-like1 (AtSCPL1), which belongs to the clade 1A AtSCPL family, as a ubiquitinated target protein of AtAIRP5/GARU. atscpl1 single and atairp5/garu-1 atscpl1-2 double mutant plants were more tolerant to drought stress than wild-type plants in an ABA-dependent manner, suggesting that AtSCPL1 is genetically downstream of AtAIRP5/GARU. After drought treatment, the endogenous ABA levels in atscpl1 and atairp5/garu-1 atscpl1-2 mutant leaves were higher than those in wild-type and atairp5/garu leaves. Overall, our results suggest that AtAIRP5/GARU RING E3 Ub ligase functions as a positive regulator of the ABA-mediated drought response by promoting the degradation of AtSCPL1.

Genome-Wide Identification and Analysis of Ariadne Gene Family Reveal Its Genetic Effects on Agronomic Traits of Brassica napus

E3 ligases promote protein ubiquitination and degradation, which regulate every aspect of eukaryotic life. The Ariadne (ARI) proteins of RBR (ring between ring fingers) protein subfamily has been discovered as a group of potential E3 ubiquitin ligases. Only a few available research studies show their role in plant adaptations processes against the external environment. Presently, the functions of ARI proteins are largely unknown in plants. Therefore, in this study, we performed genome-wide analysis to identify the ARI gene family and explore their potential importance in B. napus. A total of 39 ARI genes were identified in the B. napus genome and were classified into three subfamilies (A, B and C) based on phylogenetic analysis. The protein–protein interaction networks and enrichment analysis indicated that BnARI genes could be involved in endoreduplication, DNA repair, proteasome assembly, ubiquitination, protein kinase activity and stress adaptation. The transcriptome data analysis in various tissues provided us an indication of some BnARI genes’ functional importance in tissue development. We also identified potential BnARI genes that were significantly responsive towards the abiotic stresses. Furthermore, eight BnARI genes were identified as candidate genes for multiple agronomic traits through association mapping analysis in B. napus; among them, BnaA02g12100D, which is the ortholog of AtARI8, was significantly associated with ten agronomic traits. This study provided useful information on BnARI genes, which could aid targeted functional research and genetic improvement for breeding in B. napus.

MaMYB4 is a negative regulator and a substrate of RING-type E3 ligases MaBRG2/3 in controlling banana fruit ripening

Fruit ripening is a complex developmental process, which is modulated by both transcriptional and post-translational events. Control of fruit ripening is important in maintaining moderate quality traits and minimizing postharvest deterioration. In this study, we discovered that the transcription factor MaMYB4 acts as a negative regulator of fruit ripening in banana. The protein levels of MaMYB4 decreased gradually with banana fruit ripening, paralleling ethylene production, and decline in firmness. DNA affinity purification sequencing combined with RNA-sequencing analyses showed that MaMYB4 preferentially binds to the promoters of various ripening-associated genes including ethylene biosynthetic and cell wall modifying genes. Furthermore, ectopic expression of MaMYB4 in tomato delayed tomato fruit ripening, which was accompanied by downregulation of ethylene biosynthetic and cell wall modifying genes. Importantly, two RING finger E3 ligases MaBRG2/3, whose protein accumulation increased progressively with fruit ripening, were found to interact with and ubiquitinate MaMYB4, contributing to decreased accumulation of MaMYB4 during fruit ripening. Transient overexpression of MaMYB4 and MaBRG2/3 in banana fruit ripening delayed or promoted fruit ripening by inhibiting or stimulating ethylene biosynthesis, respectively. Taken together, we demonstrate that MaMYB4 negatively modulates banana fruit ripening, and that MaMYB4 abundance could be regulated by protein ubiquitination, thus providing insights into the role of MaMYB4 in controlling fruit ripening at both transcriptional and post-translational levels.

RING Zinc Finger Proteins in Plant Abiotic Stress Tolerance

RING zinc finger proteins have a conserved RING domain, mainly function as E3 ubiquitin ligases, and play important roles in plant growth, development, and the responses to abiotic stresses such as drought, salt, temperature, reactive oxygen species, and harmful metals. RING zinc finger proteins act in abiotic stress responses mainly by modifying and degrading stress-related proteins. Here, we review the latest progress in research on RING zinc finger proteins, including their structural characteristics, classification, subcellular localization, and physiological functions, with an emphasis on abiotic stress tolerance. Under abiotic stress, RING zinc finger proteins on the plasma membrane may function as sensors or abscisic acid (ABA) receptors in abiotic stress signaling. Some RING zinc finger proteins accumulate in the nucleus may act like transcription factors to regulate the expression of downstream abiotic stress marker genes through direct or indirect ways. Most RING zinc finger proteins usually accumulate in the cytoplasm or nucleus and act as E3 ubiquitin ligases in the abiotic stress response through ABA, mitogen-activated protein kinase (MAPK), and ethylene signaling pathways. We also highlight areas where further research on RING zinc finger proteins in plants is needed.

Plant E3 Ligases and Their Role in Abiotic Stress Response

Plants, as sessile organisms, have limited means to cope with environmental changes. Consequently, they have developed complex regulatory systems to ameliorate abiotic stresses im-posed by environmental changes. One such system is the ubiquitin proteasome pathway, which utilizes E3 ligases to target proteins for proteolytic degradation via the 26S proteasome. Plants ex-press a plethora of E3 ligases that are categorized into four major groups depending on their structure. They are involved in many biological and developmental processes in plants, such as DNA repair, photomorphogenesis, phytohormones signaling, and biotic stress. Moreover, many E3 ligase targets are proteins involved in abiotic stress responses, such as salt, drought, heat, and cold. In this review, we will provide a comprehensive overview of E3 ligases and their substrates that have been connected with abiotic stress in order to illustrate the diversity and complexity of how this pathway enables plant survival under stress conditions.

Roles of E3 Ubiquitin Ligases in Plant Responses to Abiotic Stresses

Plants are frequently exposed to a variety of abiotic stresses, such as those caused by salt, drought, cold, and heat. All of these stressors can induce changes in the proteoforms, which make up the proteome of an organism. Of the many different proteoforms, protein ubiquitination has attracted a lot of attention because it is widely involved in the process of protein degradation; thus regulates many plants molecular processes, such as hormone signal transduction, to resist external stresses. Ubiquitin ligases are crucial in substrate recognition during this ubiquitin modification process. In this review, the molecular mechanisms of plant responses to abiotic stresses from the perspective of ubiquitin ligases have been described. This information is critical for a better understanding of plant molecular responses to abiotic stresses.

The soybean CBL-interacting protein kinase, GmCIPK2, positively regulates drought tolerance and ABA signaling

Calcineurin B-like protein (CBL) and CBL-interacting protein kinase (CIPK) play important roles in plant environmental stress responses. However, the biological functions of the CBL-CIPK signaling pathway in the tolerance of soybean (Glycine max) to drought stress remain elusive. Here, we characterized the GmCIPK2 gene in soybean, and its expression was induced by drought stress and exogenous abscisic acid (ABA) treatments. The overexpression of GmCIPK2 enhanced drought tolerance in transgenic Arabidopsis and soybean hairy roots, whereas downregulation of GmCIPK2 expression in soybean hairy roots by RNA interference resulted in increased drought sensitivity. Further analysis showed that GmCIPK2 was involved in ABA-mediated stomatal closure in plants under drought stress conditions. GmCIPK2 increased the expression of ABA- and drought-responsive genes during drought stress. Additionally, yeast two-hybrid, pull-down, and bimolecular fluorescence complementation assays demonstrated that a positive regulator of drought stress, GmCBL1, physically interacted with GmCIPK2 on the plasma membrane. Collectively, our results demonstrated that GmCIPK2 positively regulates drought tolerance and ABA signaling in plants, providing new insights into the underlying mechanisms of how the CBL-CIPK signaling pathway contributes to drought tolerance in soybean.

Identification of C3H2C3-type RING E3 ubiquitin ligase in grapevine and characterization of drought resistance function of VyRCHC114

BACKGROUND

RING is one of the largest E3 ubiquitin ligase families and C3H2C3 type is the largest subfamily of RING, which plays an important role in plant growth and development, and growth and responses to biotic and abiotic stresses.

RESULTS

A total of 143 RING C3H2C3-type genes (RCHCs) were discovered from the grapevine genome and separated into groups (I-XI) according to their phylogenetic analysis, and these genes named according to their positions on chromosomes. Gene replication analysis showed that tandem duplications play a predominant role in the expansion of VvRCHCs family together. Structural analysis showed that most VvRCHCs (67.13 %) had no more than 2 introns, while genes clustered together based on phylogenetic trees had similar motifs and evolutionarily conserved structures. Cis-acting element analysis showed the diversity of VvRCHCs regulation. The expression profiles of eight DEGs in RNA-Seq after drought stress were like the results of qRT-PCR analysis. In vitro ubiquitin experiment showed that VyRCHC114 had E3 ubiquitin ligase activity, overexpression of VyRCHC114 in Arabidopsis improved drought tolerance. Moreover, the transgenic plant survival rate increased by 30 %, accompanied by electrolyte leakage, chlorophyll content and the activities of SOD, POD, APX and CAT were changed. The quantitative expression of AtCOR15a, AtRD29A, AtERD15 and AtP5CS1 showed that they participated in the response to drought stress may be regulated by the expression of VyRCHC114.

CONCLUSIONS

This study provides valuable new information for the evolution of grapevine RCHCs and its relevance for studying the functional characteristics of grapevine VyRCHC114 genes under drought stress.

Potential stress tolerance roles of barley germins and GLPs

Germins and germin-like proteins (GLPs) known as germination markers are encoded by multigene families in several plant species, including barley. To date, functional analysis has revealed germins and GLPs are involved in diverse processes such as embryonic development and stress responses. The aim of this study was the analysis of barley germins and GLPs. In this study, 80 putative germins and GLPs have been identified in barley by using known 17 germins and GLP sequences. Analysis of germins and GLPs showed all germins and GLPs are distributed on all seven chromosomes that most of them spread through chromosome 3, 4 and 7 with 16, 18 and 12 proteins on each, respectively. The protein sizes varied between 185 and 335 amino acids, with an average length of 225 aa. Twelve conserved motifs were found. While germin motifs 1 and 3 were detected in all germins and GLPs, some motifs were found to be related to signalization. Interestingly, protein-protein interaction analysis demonstrated some GLPs are associated with RAB6-interacting golgin, oligopeptide transmembrane transporter activity, beta-glucuronidase activity, protein N-linked glycosylation, multi-pass membrane protein, and proteins containing zinc finger (Znf) domain and RING (really interesting new gene)-type zinc finger domains. Our findings suggest that barley germins and GLPs may have diverse functions that make them important candidates for crop improvement.

Comparative transcriptome analysis reveals function of TERF1 in promoting seed germination

Seed germination marks a new life cycle of a plant. Although ethylene promotes seed germination, the underlying molecular mechanism is poorly understood. Ethylene Responsive Factors (ERFs) play an essential role in ethylene signaling. Here we show that overexpression of Tomato Ethylene Responsive Factor 1 (TERF1), an ERF transcription factor isolated from tomato, can promote tobacco seed germination at 23 °C in darkness. Hormones analysis showed that salicylic acid (SA), 3-indoleacetic acid (IAA), abscisic acid (ABA) and gibberellic acids (GAs) were significantly increased by TERF1, while jasmonic acid (JA) was significantly reduced in TERF1 seeds. Transcriptome analysis identified 7,961 differentially expressed genes (DEGs), including 6,213 mRNAs, 25 miRNAs, 1,581 lncRNAs and 141 circRNAs. Gene Ontology (GO) enrichment analysis showed that cell cycles, sugar metabolism, microtubule-based processes were activated by TERF1, while DNA repair, lipid metabolism were repressed by TERF1. We also identified differentially expressed regulatory genes for ABA and GA biosynthesis or signaling in TERF1 seed, including transcription factors, kinases, phosphatases and ubiquitin protein ligases, non-coding RNAs (ncRNAs). At posttranscriptional level TERF1 also regulates gene expression through alternative splicing (AS). Protein-protein interaction (PPI) network analysis revealed three key biological processes regulated by TERF1, including nitrogen metabolism, light related processes and mitosis. Pheynotype and gene expression analysis showed that TERF1 significantly reduced seed sensitivity to ABA and auxin during germination through repressing key components of ABA signaling pathway. Our results unraveled the function of TERF1 in promoting seed germination.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-021-01049-4.

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.

Suppression of DRR1 results in the accumulation of insoluble ubiquitinated proteins, which impairs drought stress tolerance

Drought stress has detrimental effects on plants. Although the abscisic acid (ABA)-mediated drought response is well established, defensive mechanisms to cope with dehydration-induced proteotoxicity have been rarely studied. DRR1 was identified as an Arabidopsis drought-induced gene encoding an ER-localized RING-type E3 Ub ligase. Suppression of DRR1 markedly reduced tolerance to drought and proteotoxic stress without altering ABA-mediated germination and stomatal movement. Proteotoxicity- and dehydration-induced insoluble ubiquitinated protein accumulation was more obvious in DRR1 loss-of-function plants than in wild-type plants. These results suggest that DRR1 is involved in an ABA-independent drought stress response possibly through the mitigation of dehydration-induced proteotoxic stress.

AtKPNB1, an Arabidopsis importin-β protein, is downstream of the RING E3 ubiquitin ligase AtAIRP1 in the ABA-mediated drought stress response

AtKPNB1, an Arabidopsis importin-β protein, was regulated by AtAIRP1 E3 ubiquitin ligase, which intensified the ABA-mediated drought stress response. As an early step in the abscisic acid (ABA)-mediated drought response, the ABA signal is transduced into the nucleus, and thus the nuclear transport system is crucially involved in the drought stress response. AtKPNB1, an importin-β protein, which is a core component of nuclear transport, was previously reported to be a negative factor in the ABA-mediated drought stress response (Luo et al. Luo et al., Plant J 75:377-389, 2013). Here, we report that AtAIPR1, an Arabidopsis RING-type E3 ubiquitin (Ub) ligase, interacted with and ubiquitinated AtKPNB1. A null mutation of AtKPNB1 suppressed the ABA-insensitive germination phenotype of atairp1 mutant seedlings as compared to that of the wild-type plants. Furthermore, the ABA-insensitive stomatal closure and drought-susceptible phenotypes of atairp1 were rescued in atairp1atkpnb1 double mutant progeny, indicating that AtKPNB1 functions downstream of AtAIRP1. These data suggest that AtAIRP1 regulates the ABA-mediated drought response in Arabidopsis via ubiquitination of AtKPNB1.

Detailed Genetic Analysis for Identifying QTLs Associated with Drought Tolerance at Seed Germination and Seedling Stages in Barley

Drought induces several challenges for plant development, growth, and production. These challenges become more severe, in particular, in arid and semiarid countries like Egypt. In terms of production, barley ranks fourth after wheat, maize, and rice. Seed germination and seedling stages are critical stages for plant establishment and growth. In the current study, 60 diverse barley genotypes were tested for drought tolerance using two different treatments: control (0-PEG) and drought (20%-PEG). Twenty-two traits were estimated for seed germination and seedling parameters. All traits were reduced under drought stress, and a significant variation was found among genotypes under control and stress conditions. The broad-sense heritability estimates were very high under both control and drought for all traits. It ranged from 0.63 to 0.97 under the control condition and from 0.89 to 0.97 under drought, respectively. These high heritabilities suggested that genetic improvement of drought tolerance in barley at both stages is feasible. The principal component analysis revealed that root-related parameters account for the largest portion of phenotypic variation in this collection. The single-marker analysis (SMA) resulted in 71 quantitative trait loci (QTLs) distributed across the seven chromosomes of barley. Thirty-three QTLs were detected for root-length-related traits. Many hotspots of QTLs were detected for various traits. Interestingly, some markers controlled many traits in a pleiotropic manner; thus, they can be used to control multiple traits at a time. Some QTLs were constitutive, i.e., they are mapped under control and drought, and targeting these QTLs makes the selection for drought tolerance a single-step process. The results of gene annotation analysis revealed very potential candidate genes that can be targeted to select for drought tolerance.

Arabidopsis RING E3 ubiquitin ligase JUL1 participates in ABA-mediated microtubule depolymerization, stomatal closure, and tolerance response to drought stress

Ubiquitination is a critical post-translational protein modification that has been implicated in diverse cellular processes, including abiotic stress responses, in plants. In the present study, we identified and characterized a T-DNA insertion mutant in the At5g10650 locus. Compared to wild-type Arabidopsis plants, at5g10650 progeny were hyposensitive to ABA at the germination stage. At5g10650 possessed a single C-terminal C3HC4-type Really Interesting New Gene (RING) motif, which was essential for ABA-mediated germination and E3 ligase activity in vitro. At5g10650 was closely associated with microtubules and microtubule-associated proteins in Arabidopsis and tobacco leaf cells. Localization of At5g10650 to the nucleus was frequently observed. Unexpectedly, At5g10650 was identified as JAV1-ASSOCIATED UBIQUITIN LIGASE1 (JUL1), which was recently reported to participate in the jasmonate signaling pathway. The jul1 knockout plants exhibited impaired ABA-promoted stomatal closure. In addition, stomatal closure could not be induced by hydrogen peroxide and calcium in jul1 plants. jul1 guard cells accumulated wild-type levels of H₂ O₂ after ABA treatment. These findings indicated that JUL1 acts downstream of H₂ O₂ and calcium in the ABA-mediated stomatal closure pathway. Typical radial arrays of microtubules were maintained in jul1 guard cells after exposure to ABA, H₂ O₂ , and calcium, which in turn resulted in ABA-hyposensitive stomatal movements. Finally, jul1 plants were markedly more susceptible to drought stress than wild-type plants. Overall, our results suggest that the Arabidopsis RING E3 ligase JUL1 plays a critical role in ABA-mediated microtubule disorganization, stomatal closure, and tolerance to drought stress.

The AlRabring7 E3-Ub-ligase mediates AlRab7 ubiquitination and improves ionic and oxidative stress tolerance in Saccharomyces cerevisiae

The maintenance of ROS homeostasis, membrane biogenesis and recycling of molecules are common stress responses involving specific and complex regulatory network. Ubiquitination is an important and common mechanism which facilitates environmental adaptation in eukaryotes. In the present study we have cloned the AlRabring7, an E3-Ub-ligase, previously identified as AlRab7 interacting partner. The role of AlRabring7 for ubiquitinating AlRab7 and facilitating stress tolerance is analysed. The AlRabring7, with an open-reading frame of 702 bp encodes a protein of 233 amino acids, with RING-HC domain of 40 amino acids. In silico analysis shows that AlRabring7 is a C3HC4-type RING E3 Ub ligase. The protein - protein docking show interaction dynamics between AlRab7-AlRabring7-Ubiquitin proteins. The AlRab7 and AlRabring7 transcript showed up-regulation in response to different salts i.e: NaCl, KCl, CaCl₂, NaCl + KCl, NaCl + CaCl₂, imposing ionic as well as hyperosmotic stress, and also with oxidative stress by H₂O₂ treatment. Interestingly, the AlRabring7 showed early transcript expression with maximum expression in shoots on combinatorial stresses. The AlRab7 showed delayed and maximum expression with NaCl + CaCl₂ stress treatment. The AlRab7 complements yeast ypt7Δ mutants and restored the fragmented vacuole. The in vitro ubiquitination assay revealed that AlRabring7 function as E3 ubiquitin ligase and mediates AlRab7 ubiquitination. Overexpression of AlRab7 and AlRabring7 independently and when co-transformed enhanced the growth of yeast cells during stress conditions. Further, the bimolecular fluorescence complementation assay shows the in planta interaction of the two proteins. Our results suggest that AlRab7 and AlRabring7 confers enhanced stress tolerance in yeast.

The Ubiquitin Conjugating Enzyme: An Important Ubiquitin Transfer Platform in Ubiquitin-Proteasome System

Owing to a sessile lifestyle in nature, plants are routinely faced with diverse hostile environments such as various abiotic and biotic stresses, which lead to accumulation of free radicals in cells, cell damage, protein denaturation, etc., causing adverse effects to cells. During the evolution process, plants formed defense systems composed of numerous complex gene regulatory networks and signal transduction pathways to regulate and maintain the cell homeostasis. Among them, ubiquitin-proteasome pathway (UPP) is the most versatile cellular signal system as well as a powerful mechanism for regulating many aspects of the cell physiology because it removes most of the abnormal and short-lived peptides and proteins. In this system, the ubiquitin-conjugating enzyme (E2) plays a critical role in transporting ubiquitin from the ubiquitin-activating enzyme (E1) to the ubiquitin-ligase enzyme (E3) and substrate. Nevertheless, the comprehensive study regarding the role of E2 enzymes in plants remains unexplored. In this review, the ubiquitination process and the regulatory role that E2 enzymes play in plants are primarily discussed, with the focus particularly put on E2's regulation of biological functions of the cell.

The grape ubiquitin ligase VpRH2 is a negative regulator in response to ABA treatment

Ubiquitin ligase VpRH2 is a negative regulator in the grape ABA pathway by inhibiting ABL1, PYR1 and GRP2A expressions, and its promoter is inhibited by ABA treatment. In higher plants, ubiquitin ligases play key roles in various cellular processes. As in our previous study (Wang et al. in J Exp Bot 68:1669-1687, 2017), grape RING-H2-type ubiquitin ligase gene VpRH2 and its promoter was induced by powdery mildew and showed resistance to the disease. Diverse small-molecule hormones, like salicylic acid (SA), methyl jasmonate (MeJA) or abscisic acid (ABA), play pivotal roles in plant resistance. Here we found that VpRH2 expression could be induced by SA and MeJA treatment, but inhibited by ABA treatment. The promoter of VpRH2 revealed a similar variation trend under exogenous hormone treatments as the gene expression by GUS activity assay. By a series of deletion fragments, the promoter fragment of VpRH2-P656 to VpRH2-P513 was necessary in response to MeJA treatment, and the inhibition of ABA treatment to the VpRH2 promoter was independent of the ABRE motif. Over-expression of VpRH2 in Arabidopsis thaliana plants displayed ABA-insensitive phenotypes at the germination stage compared to wild type plants. In VpRH2 over-expressing Vitis vinifera cv. Thompson Seedless plants after ABA treatments, the expression of the ABA pathway related genes ABL1 and PYR1 showed a suppresive trend. Moreover, VpGRP2A (an VpRH2-interacting protein) also showed a suppresive trend in response to ABA treatment in VpRH2-overexpressing plants. Our results demonstrate that VpRH2 is a negative regulator in the grape ABA signal pathway by inhibiting ABL1, PYR1 and GRP2A expressions, and its promoter was also inhibited by ABA treatment.

Molecular Functions of Rice Cytosol-Localized RING Finger Protein 1 in Response to Salt and Drought and Comparative Analysis of Its Grass Orthologs

In higher plants, the post-translational modification of target proteins via the attachment of molecules such as ubiquitin (Ub) mediates a variety of cellular functions via the Ub/26S proteasome system. Here, a really interesting new gene (RING)-H2 type E3 ligase, which regulates target proteins via the Ub/26S proteasome system, was isolated from a rice plant, and its other grass orthologs were examined to determine the evolution of its molecular function during speciation. The gene encoding Oryza sativa cytoplasmic-localized RING finger protein 1 (OsCLR1) was highly expressed under salt and drought stresses. By contrast, the three grass orthologs, SbCLR1 from Sorghum bicolor, ZmCLR1 from Zea mays and TaCLR1 from Triticum aestivum, showed different responses to these stresses. Despite these differences, all four orthologs exhibited E3 ligase activity with cytosol-targeted localization, demonstrating conserved molecular functions. Although OsCLR1-overexpressing plants showed higher survival rates under both salt and drought stresses than that of the wild type (WT) plants, this pattern was not observed in the other orthologs. In addition, OsCLR1-overexpressing plants exhibited lower germination rates in ABA than that of WT plants, whereas the three ortholog CLR1-overexpressing plants showed rates similar to the WT plants. These results indicate the positive regulation of OsCLR1 in response to salt and drought in an ABA-dependent manner. Despite the molecular functions of the three CLR1 orthologs remaining largely unknown, our results provide an insight into the evolutionary fate of CLR1 grass orthologs during speciation after the divergence from a common ancestor.

The RING finger E3 ligases PIR1 and PIR2 mediate PP2CA degradation to enhance abscisic acid response in Arabidopsis

Recent work has established a core ABA signaling pathway in which A-type PP2C protein phosphatases act as central negative modulators. Although ABA signaling inhibits PP2C activity through ABA-receptor complex, it remains unknown if other mechanisms exist to modulate the level of PP2Cs. Here, we identified a RING domain ubiquitin E3 ligase, PIR1 (PP2CA interacting RING finger protein 1), that interacted with PP2CA. Of the two splicing isoforms, PIR1.2 was isolated from leaf tissue. The PIR1.2 exhibited E3 ligase activity and determined PP2CA stability in the presence of ABA. Consistent with the conclusion that PIR1 promotes ABA signaling by removing PP2CA, a negative modulator, the pir1 knockout mutant displayed an ABA-hyposensitive phenotype. We further showed that PIR2, the closest homologue of PIR1.2, also interacted with PP2CA. Although the pir2 knockout mutant did not display altered ABA response, the pir1-1/pir2 double mutant became more insensitive to ABA than the wild-type or pir1-1 and pir2 single mutants. Using a cell-free degradation assay, ABA promoted degradation of PP2CA, however, such degradation was delayed when incubated with protein extract prepared from the pir1-1/pir2 double mutant. Our data suggest that PIR1 and PIR2 positively modulate ABA signaling by targeting PP2CA for degradation.

The ubiquitin-proteasome system in plant responses to environments

The ubiquitin-proteasome system (UPS) is a rapid regulatory mechanism for selective protein degradation in plants and plays crucial roles in growth and development. There is increasing evidence that the UPS is also an integral part of plant adaptation to environmental stress, such as drought, salinity, cold, nutrient deprivation and pathogens. This review focuses on recent studies illustrating the important functions of the UPS components E2s, E3s and subunits of the proteasome and describes the regulation of proteasome activity during plant responses to environment stimuli. The future research hotspots and the potential for utilization of the UPS to improve plant tolerance to stress are discussed.

Genome-Wide Identification, Evolution, and Expression Analysis of RING Finger Gene Family in Solanum lycopersicum

RING domain proteins generally have E3 ubiquitin ligase activity and are involved in degrading their substrate proteins. The roles of these proteins in growth, development, and responses to different abiotic stresses have been described well in various plant species, but little is available on tomatoes. Here, we identified 474 RING domains in 469 potential proteins encoded in the tomato genome. These RING genes were found to be located in 12 chromosomes and could be divided into 51 and 11 groups according to the conserved motifs outside the RING domain and phylogenetic analysis, respectively. Segmental duplication could be the major driver in the expansion of the tomato RING gene family. Further comparative syntenic analysis suggested that there have been functional divergences of RING genes during plant evolution and most of the RING genes in various species are under negative selection. Expression profiles derived from a transcriptomic analysis showed that most tomato RING genes exhibited tissue-specific expression patterning. Further RT-qPCR validation showed that almost all genes were upregulated by salt treatment, which was consistent with the microarray results. This study provides the first comprehensive understanding of the RING gene family in the tomato genome. Our results pave the way for further investigation of the classification, evolution, and potential functions of the RING domain genes in tomato.

LOWER TEMPERATURE 1 Enhances ABA Responses and Plant Drought Tolerance by Modulating the Stability and Localization of C2-Domain ABA-Related Proteins in Arabidopsis

Plasma membrane-associated abscisic acid (ABA) signal transduction is an integral part of ABA signaling. The C2-domain ABA-related (CAR) proteins play important roles in the recruitment of ABA receptors to the plasma membrane to facilitate ABA signaling. However, how CAR proteins are regulated remains unclear. In this study, we conducted a genetic screen for mutants with altered leaf transpiration and identified an uncharacterized protein, LOWER TEMPERATURE 1 (LOT1), which regulates the dynamic localization and stability of CAR proteins. The lot1 mutant had a lower leaf temperature as compared with the wild type due to higher transpiration. We found that LOT1 physically interacts with CAR9 , and ABA reduces LOT1-CAR9 interaction in the nucleus, likely via Ca²⁺, resulting in increased localization of CAR9 to the plasma membrane. We further found that the stability of CAR9 is affected by LOT1 less CAR9 proteins were accumulated and more were ubiquitinated in lot1. While the lot1, car9 and lot1 car9 mutants were hyposensitive to ABA, the hyposensitive phenotype of lot1 could be rescued by CAR9 overexpression. Collectively, our study reveals that LOT1 regulates plant tolerance to drought stress by affecting ABA signaling through regulating the stability and dynamic localization of CAR9.

Inverse Correlation Between MPSR1 E3 Ubiquitin Ligase and HSP90.1 Balances Cytoplasmic Protein Quality Control

MISFOLDED PROTEIN SENSING RING1 (MPSR1) is a chaperone-independent E3 ubiquitin ligase that participates in protein quality control by eliminating misfolded proteins in Arabidopsis (Arabidopsis thaliana). Here, we report that in the early stages of proteotoxic stress, cellular levels of MPSR1 increased immediately, whereas levels of HEAT SHOCK PROTEIN90.1 (AtHSP90.1) were unaltered despite massively upregulated transcription. At this stage, the gene-silencing pathway mediated by microRNA 414 (miR414) suppressed AtHSP90.1 translation. By contrast, under prolonged stress, AtHSP90.1 was not suppressed, and instead competed with MPSR1 to act on misfolded proteins, promoting the destruction of MPSR1. Deficiency or excess of MPSR1 significantly abolished or intensified the suppression of AtHSP90.1, respectively. Similar to the MPSR1-overexpressing transgenic plants, the miR414-overexpressing plants showed an increased tolerance to proteotoxic stress as compared to the wild-type plants. Although the functional relationship between MPSR1 and miR414 remains unclear, both MPSR1 and miR414 demonstrated negative modulation of the expression of AtHSP90.1. The inverse correlation between MPSR1 and AtHSP90.1 via miR414 may adjust the set-point of the HSP90-mediated protein quality control process in response to increasing stress intensity in Arabidopsis.

OsDIRP1, a Putative RING E3 Ligase, Plays an Opposite Role in Drought and Cold Stress Responses as a Negative and Positive Factor, Respectively, in Rice (Oryza sativa L.)

As higher plants are sessile organisms, they are unable to move to more favorable places; thus, they have developed the ability to survive under potentially detrimental conditions. Ubiquitination is a crucial post-translational protein modification and participates in abiotic stress responses in higher plants. In this study, we identified and characterized OsDIRP1 (Oryza sativa Drought-Induced RING Protein 1), a nuclear-localized putative RING E3 ubiquitin (Ub) ligase in rice (Oryza sativa L.). OsDIRP1 expression was induced by drought, high salinity, and abscisic acid (ABA) treatment, but not by low temperature (4°C) stress, suggesting that OsDIRP1 is differentially regulated by different abiotic stresses. To investigate its possible role in abiotic stress responses, OsDIRP1-overexpressing transgenic rice plants (Ubi:OsDIRP1-sGFP) were generated, and their phenotypes were analyzed. The T4 Ubi:OsDIRP1-sGFP lines showed decreased tolerance to drought and salt stress as compared to wild-type rice plants. Moreover, Ubi:OsDIRP1-sGFP progeny were less sensitive to ABA than the wild-type during both germination and post-germination growth. In contrast, Ubi:OsDIRP1-sGFP plants exhibited markedly higher tolerance to prolonged cold (4°C) treatment. These results suggest that OsDIRP1 acts as a negative regulator during drought and salt stress, whereas it functions as a positive factor during the cold stress response in rice.

Repertoire of plant RING E3 ubiquitin ligases revisited: New groups counting gene families and single genes

E3 ubiquitin ligases of the ubiquitin proteasome system (UPS) mediate recognition of substrates and later transfer the ubiquitin (Ub). They are the most expanded components of the system. The Really Interesting New Gene (RING) domain contains 40-60 residues that are highly represented among E3 ubiquitin ligases. The Arabidopsis thaliana E3 ubiquitin ligases with a RING finger primarily contain RING-HC or RING-H2 type domains or less frequently RING-v, RING-C2, RING-D, RING-S/T and RING-G type domains. Our previous work on three E3 ubiquitin ligase families with a RING-H2 type domain, ATL, BTL, and CTL, suggested that a phylogenetic distribution based on the RING domain allowed for the creation a catalog of known domains or unknown conserved motifs. This work provided a useful and comprehensive view of particular families of RING E3 ubiquitin ligases. We updated the annotation of A. thaliana RING proteins and surveyed RING proteins from 30 species across eukaryotes. Based on domain architecture profile of the A. thaliana proteins, we catalogued 4711 RING finger proteins into 107 groups, including 66 previously described gene families or single genes and 36 novel families or undescribed genes. Forty-four groups were specific to a plant lineage while 41 groups consisted of proteins found in all eukaryotic species. Our present study updates the current classification of plant RING finger proteins and reiterates the importance of these proteins in plant growth and adaptation.

RING E3 ligases: key regulatory elements are involved in abiotic stress responses in plants

Plants are constantly exposed to a variety of abiotic stresses, such as drought, heat, cold, flood, and salinity. To survive under such unfavorable conditions, plants have evolutionarily developed their own resistant-mechanisms. For several decades, many studies have clarified specific stress response pathways of plants through various molecular and genetic studies. In particular, it was recently discovered that ubiquitin proteasome system (UPS), a regulatory mechanism for protein turn over, is greatly involved in the stress responsive pathways. In the UPS, many E3 ligases play key roles in recognizing and tethering poly-ubiquitins on target proteins for subsequent degradation by the 26S proteasome. Here we discuss the roles of RING ligases that have been defined in related to abiotic stress responses in plants.

The Ubiquitin-Conjugating Enzyme Gene Family in Longan (Dimocarpus longan Lour.): Genome-Wide Identification and Gene Expression during Flower Induction and Abiotic Stress Responses

Ubiquitin-conjugating enzymes (E2s or UBC enzymes) play vital roles in plant development and combat various biotic and abiotic stresses. Longan (Dimocarpus longan Lour.) is an important fruit tree in the subtropical region of Southeast Asia and Australia; however the characteristics of the UBC gene family in longan remain unknown. In this study, 40 D. longan UBC genes (DlUBCs), which were classified into 15 groups, were identified in the longan genome. An RNA-seq based analysis showed that DlUBCs showed distinct expression in nine longan tissues. Genome-wide RNA-seq and qRT-PCR based gene expression analysis revealed that 11 DlUBCs were up- or down-regualted in the cultivar “Sijimi” (SJ), suggesting that these genes may be important for flower induction. Finally, qRT-PCR analysis showed that the mRNA levels of 13 DlUBCs under SA (salicylic acid) treatment, seven under methyl jasmonate (MeJA) treatment, 27 under heat treatment, and 16 under cold treatment were up- or down-regulated, respectively. These results indicated that the DlUBCs may play important roles in responses to abiotic stresses. Taken together, our results provide a comprehensive insight into the organization, phylogeny, and expression patterns of the longan UBC genes, and therefore contribute to the greater understanding of their biological roles in longan.

Overexpression of the maize E3 ubiquitin ligase gene ZmAIRP4 enhances drought stress tolerance in Arabidopsis

Ubiquitin-mediated protein degradation plays a crucial role in enabling plants to effectively and efficiently cope with environmental stresses. The E3 ligases have emerged as a central component of the ubiquitination pathway and modulate plant response to abiotic stresses. However, few such studies have been reported in maize. In this study, a C3HC4-type RING finger E3 ligase in maize, ZmAIRP4 (Zea mays Abscisic acid [ABA]-Insensitive RING Protein 4), which is an ortholog of AtAIRP4, was isolated by reverse transcription polymerase chain reaction with specific primers, and its functions in tolerance to drought stress were described. ZmAIRP4 was upregulated by ABA, polyethylene glycol and sodium chloride. In vitro ubiquitination assays and subcellular localization indicated that ZmAIRP4 was an active E3 ligase predominantly localized in the cytoplasm and nucleus. Compared to wild type, ZmAIRP4-overexpressing Arabidopsis plants were hypersensitive to ABA during early seedling development, and showed enhanced drought tolerance. Moreover, the transcript levels of several drought-related downstream genes in transgenic plants were dramatically increased compared with wild type plants. Our results suggested that E3 ligase ZmAIRP4 is a positive regulator in the drought tolerance response pathway.

AtAIRP2 E3 Ligase Affects ABA and High-Salinity Responses by Stimulating Its ATP1/SDIRIP1 Substrate Turnover

AtAIRP2 is a cytosolic RING-type E3 ubiquitin ligase that positively regulates an abscisic acid (ABA) response in Arabidopsis (Arabidopsis thaliana). Yeast two-hybrid screening using AtAIRP2 as bait identified ATP1 (AtAIRP2 Target Protein1) as a substrate of AtAIRP2. ATP1 was found to be identical to SDIRIP1, which was reported recently to be a negative factor in ABA signaling and a target protein of the RING E3 ligase SDIR1. Accordingly, ATP1 was renamed ATP1/SDIRIP1. A specific interaction between AtAIRP2 and ATP1/SDIRIP1 and ubiquitination of ATP1/SDIRIP1 by AtAIRP2 were demonstrated in vitro and in planta. The turnover of ATP1/SDIRIP1 was regulated by AtAIRP2 in cell-free degradation and protoplast cotransfection assays. The ABA-mediated germination assay of 35S:ATP1/SDIRIP1-RNAi/atairp2 double mutant progeny revealed that ATP1/SDIRIP1 acts downstream of AtAIRP2. AtAIRP2 and SDIR1 reciprocally complemented the ABA- and salt-insensitive germination phenotypes of sdir1 and atairp2 mutants, respectively, indicating their combinatory roles in seed germination. Subcellular localization and bimolecular fluorescence complementation experiments in the presence of MG132, a 26S proteasome inhibitor, showed that AtAIRP2 and ATP1/SDIRIP1 were colocalized to the cytosolic spherical body, which lies in close proximity to the nucleus, in tobacco (Nicotiana benthamiana) leaf cells. The 26S proteasome subunits RPN12a and RPT1 and the molecular chaperones HSP70 and HSP101 were colocalized to these discrete punctae-like structures. These results raised the possibility that AtAIRP2 and ATP1/SDIRIP1 interact in the cytosolic spherical compartment. Collectively, our data suggest that the down-regulation of ATP1/SDIRIP1 by AtAIRP2 and SDIR1 RING E3 ubiquitin ligases is critical for ABA and high-salinity responses during germination in Arabidopsis.

Tissue-Specific Ubiquitination by IPA1 INTERACTING PROTEIN1 Modulates IPA1 Protein Levels to Regulate Plant Architecture in Rice

Plant architecture, a collection of genetically controlled agronomic traits, is one of the decisive factors that determine grain production. IDEAL PLANT ARCHITECTURE1 (IPA1) encodes a key transcription factor with pleiotropic effects on regulating plant architecture in rice (Oryza sativa), and IPA1 expression is controlled at the posttranscriptional level by microRNA156 and microRNA529. Here, we report the identification and characterization of IPA1 INTERACTING PROTEIN1 (IPI1), a RING-finger E3 ligase that can interact with IPA1 in the nucleus. IPI1 promotes the degradation of IPA1 in panicles, while it stabilizes IPA1 in shoot apexes. Consistent with these findings, the ipi1 loss-of-function mutants showed markedly altered plant architecture, including more tillers, enlarged panicles, and increased yield per plant. Moreover, IPI1 could ubiquitinate the IPA1-mediated complex with different polyubiquitin chains, adding K48-linked polyubiquitin chains in panicles and K63-linked polyubiquitin chains in the shoot apex. These results demonstrate that IPI1 affects plant architecture through precisely tuning IPA1 protein levels in different tissues in rice and provide new insight into the tissue-specific regulation of plant architecture and important genetic resources for molecular breeding.

Post-translational control of ABA signalling: the roles of protein phosphorylation and ubiquitination

The plant phytohormone abscisic acid (ABA) plays significant roles in integrating environmental signals with embryogenesis, germination, seedling establishment, the floral transition and the adaptation of plants to stressful environments by modulating stomatal movement and stress-responsive gene expression. ABA signalling consists of ABA perception, signal transduction and ABA-induced responses. ABA receptors such as members of the PYR/PYL family, group A type 2C protein phosphatases (as negative regulators), SnRK2 protein kinases (as positive regulators), bZIP transcription factors and ion channels are key components of ABA signalling. Post-translational modifications, including dephosphorylation, phosphorylation and ubiquitination, play important roles in regulating ABA signalling. In this review, we focus on the roles of post-translational modifications in ABA signalling. The studies presented provide a detailed picture of the ABA signalling network.

Posttranslational Modifications and Plant-Environment Interaction

Posttranslational modifications (PTMs) of proteins such as phosphorylation and ubiquitination are crucial for controlling protein stability, localization, and conformation. Genetic information encoded in DNA is transcribed, translated, and increases its complexity by multiple PTMs. Conformational change introduced by PTMs affects interacting partners of each proteins and their downstream signaling; therefore, PTMs are the major level of modulations of total outcome of living cells. Plants are living in harsh environment that requires unremitting physiological modulation to survive, and the plant response to various environment stresses is regulated by PTMs of proteins. This review deals with the novel knowledge of PTM-focused proteomic studies on various life conditions. PTMs are focused that mediate plant-environment interaction such as stress perception, protein homeostasis, control of energy shift, and defense by immune system. Integration of diverse signals on a protein via multiple PTMs is discussed as well, considering current situation where signal integration became an emerging area approached by systems biology into account.

Genome scans for divergent selection in natural populations of the widespread hardwood species Eucalyptus grandis (Myrtaceae) using microsatellites

Identification of loci or genes under natural selection is important for both understanding the genetic basis of local adaptation and practical applications, and genome scans provide a powerful means for such identification purposes. In this study, genome-wide simple sequence repeats markers (SSRs) were used to scan for molecular footprints of divergent selection in Eucalyptus grandis, a hardwood species occurring widely in costal areas from 32° S to 16° S in Australia. High population diversity levels and weak population structure were detected with putatively neutral genomic SSRs. Using three FST outlier detection methods, a total of 58 outlying SSRs were collectively identified as loci under divergent selection against three non-correlated climatic variables, namely, mean annual temperature, isothermality and annual precipitation. Using a spatial analysis method, nine significant associations were revealed between FST outlier allele frequencies and climatic variables, involving seven alleles from five SSR loci. Of the five significant SSRs, two (EUCeSSR1044 and Embra394) contained alleles of putative genes with known functional importance for response to climatic factors. Our study presents critical information on the population diversity and structure of the important woody species E. grandis and provides insight into the adaptive responses of perennial trees to climatic variations.