Pepper SUMO E3 ligase CaDSIZ1 enhances drought tolerance by stabilizing the transcription factor CaDRHB1

Pepper SUMO protease CaDeSI2 positively modulates the drought responses via deSUMOylation of clade A PP2C CaAITP1

Posttranslational modification of multiple ABA signaling components is an essential process for the adaptation and survival of plants under stress conditions. In our previous study, we established that the pepper group A PP2C protein CaAITP1, one of the core components of ABA signaling, undergoes ubiquitination mediated by the RING-type E3 ligase CaAIRE1. In this study, we discovered an additional form of regulation mediated via the SUMOylation of CaAITP1. Pepper plants subjected to drought stress were characterized by reductions in both the stability and SUMOylation of CaAITP1 protein. Moreover, we identified a SUMO protease, Capsicum annuum DeSUMOylating Isopeptidase 2 (CaDeSI2), as a new interacting partner of CaAITP1. In vitro and in vivo analyses revealed that CaAITP1 is deSUMOylated by CaDeSI2. Silencing of CaDeSI2 in pepper plants led to drought-hypersensitive and ABA-hyposensitive phenotypes, whereas overexpression of CaDeSI2 in transgenic Arabidopsis plants resulted in the opposite phenotypes. Importantly, we found that the CaAITP1 protein was stabilized in response to the silencing of CaDeSI2, and CaDeSI2 and CaAITP1 co-silenced pepper plants were characterized by drought-tolerant phenotypes similar to those observed in CaAITP1-silenced pepper. Collectively, our findings indicate that CaDeSI2 reduces the stability of CaAITP1 via deSUMOylation, thereby positively regulating drought tolerance.

SpSIZ1 from hyperaccumulator Sedum plumbizincicola orchestrates SpABI5 to fine-tune cadmium tolerance

Sedum plumbizincicola is a renowned hyperaccumulator of cadmium (Cd), possesses significant potential for eco-friendly phytoremediation of soil contaminated with Cd. Nevertheless, comprehension of the mechanisms underpinning its Cd stress response remains constrained, primarily due to the absence of a comprehensive genome sequence and an established genetic transformation system. In this study, we successfully identified a novel protein that specifically responds to Cd stress through early comparative iTRAQ proteome and transcriptome analyses under Cd stress conditions. To further investigate its structure, we employed AlphaFold, a powerful tool for protein structure prediction, and found that this newly identified protein shares a similar structure with Arabidopsis AtSIZ1. Therefore, we named it Sedum plumbizincicola SIZ1 (SpSIZ1). Our study revealed that SpSIZ1 plays a crucial role in positively regulating Cd tolerance through its coordination with SpABI5. Overexpression of SpSIZ1 significantly enhanced plant resistance to Cd stress and reduced Cd accumulation. Expression pattern analysis revealed higher levels of SpSIZ1 expression in roots compared to stems and leaves, with up-regulation under Cd stress induction. Importantly, overexpressing SpSIZ1 resulted in lower Cd translocation factors (Tfs) but maintained relatively constant Cd levels in roots under Cd stress, leading to enhanced Cd stress resistance in plants. Protein interaction analysis revealed that SpSIZ1 interacts with SpABI5, and the expression of genes responsive to abscisic acid (ABA) through SpABI5-dependent signaling was significantly up-regulated in SpSIZ1-overexpressing plants with Cd stress treatment. Collectively, our results illustrate that SpSIZ1 interacts with SpABI5, enhancing the expression of ABA downstream stress-related genes through SpABI5, thereby increasing Cd tolerance in plants.

Insights into the role of SUMO in regulating drought stress responses in pigeonpea (Cajanus cajan)

KEY MESSAGE

We have identified and analyzed 28 SUMO-pathway proteins from pigeonpea. Enhanced transcripts of pathway genes and increased SUMO conjugation under drought signifies the role of SUMO in regulating stress. Being a protein-rich and nutrient-dense legume crop, pigeonpea (Cajanus cajan) holds a vital position in a vegetarian meal. It is a resilient crop capable of striving in harsh climates and provides a means of subsistence to small-holding farmers. Nevertheless, extremes of water scarcity and drought conditions, especially during seedling and reproductive stages, remains a major issue severely impacting the growth and overall productivity of pigeonpea. Small ubiquitin-like modifier (SUMO), a post-translational modification system, plays a pivotal role in fortifying plants against stressful conditions by rapid reprogramming of molecular events. In this study, we have scanned the entire pigeonpea genome and identified 28 candidates corresponding to SUMO machinery components of pigeonpea. qRT-PCR analysis of different SUMO machinery genes validated their presence under natural conditions. The analysis of the promoters of identified SUMO machinery genes revealed the presence of abiotic stress-related cis-regulatory elements highlighting the potential involvement of the genes in abiotic stress responses. The transcript level analysis of selected SUMO machinery genes and global SUMO status of pigeonpea proteins in response to drought stress suggests an integral role of SUMO in regulating drought stress conditions in pigeonpea. Collectively, the work puts forward a detailed in silico analysis of pigeonpea SUMO machinery candidates and highlights the essential role of SUMOylation in drought stress responses. Being the first report on a pulse crop, the study will serve as a resource for devising strategies for counteracting drought stress in pigeonpea that can be further extended to other pulse crops.

Small ubiquitin-like modifiers E3 ligases in plant stress

Plants regularly encounter various environmental stresses such as salt, drought, cold, heat, heavy metals and pathogens, leading to changes in their proteome. Of these, a post-translational modification, SUMOylation is particularly significant for its extensive involvement in regulating various plant molecular processes to counteract these external stressors. Small ubiquitin-like modifiers (SUMO) protein modification significantly contributes to various plant functions, encompassing growth, development and response to environmental stresses. The SUMO system has a limited number of ligases even in fully sequenced plant genomes but SUMO E3 ligases are pivotal in recognising substrates during the process of SUMOylation. E3 ligases play pivotal roles in numerous biological and developmental processes in plants, including DNA repair, photomorphogenesis, phytohormone signalling and responses to abiotic and biotic stress. A considerable number of targets for E3 ligases are proteins implicated in reactions to abiotic and biotic stressors. This review sheds light on how plants respond to environmental stresses by focusing on recent findings on the role of SUMO E3 ligases, contributing to a better understanding of how plants react at a molecular level to such stressors.

Exogenous strigolactone enhanced the drought tolerance of pepper (Capsicum chinense) by mitigating oxidative damage and altering the antioxidant mechanism

KEY MESSAGE

Exogenous SL positively regulates pepper DS by altering the root morphology, photosynthetic character, antioxidant enzyme activity, stomatal behavior, and SL-related gene expression. Drought stress (DS) has always been a problem for the growth and development of crops, causing significant negative impacts on crop productivity. Strigolactone (SL) is a newly discovered class of plant hormones that are involved in plants' growth and development and environmental stresses. However, the role of SL in response to DS in pepper remains unknown. DS considerably hindered photosynthetic pigments content, damaged root architecture system, and altered antioxidant machinery. In contrast, SL application significantly restored pigment concentration modified root architecture system, and increased relative chlorophyll content (SPAD). Additionally, SL treatment reduced oxidative damage by reducing hydrogen peroxide (H2O2) (24-57%) and malondialdehyde (MDA) (79-89%) accumulation in pepper seedlings. SL-pretreated pepper seedlings showed significant improvement in antioxidant enzyme activity, proline accumulation, and soluble sugar content. Furthermore, SL-related genes (CcSMAX2, CcSMXL6, and CcSMXL3) were down-regulated under DS. These findings suggest that the foliar application of SL can alleviate the adverse effects of drought tolerance by up-regulating chlorophyll content and activating antioxidant defense mechanisms.

HD-Zip I protein LlHOX6 antagonizes homeobox protein LlHB16 to attenuate basal thermotolerance in lily

Homeodomain-leucine zipper (HD-Zip) I transcription factors are crucial for plant responses to drought, salt, and cold stresses. However, how they are associated with thermotolerance remains mostly unknown. We previously demonstrated that lily (Lilium longiflorum) LlHB16 (HOMEOBOX PROTEIN 16) promotes thermotolerance, whereas the roles of other HD-Zip I members are still unclear. Here, we conducted a transcriptomic analysis and identified a heat-responsive HD-Zip I gene, LlHOX6 (HOMEOBOX 6). We showed that LlHOX6 represses the establishment of basal thermotolerance in lily. LlHOX6 expression was rapidly activated by high temperature, and its protein localized to the nucleus. Heterologous expression of LlHOX6 in Arabidopsis (Arabidopsis thaliana) and overexpression in lily reduced their basal thermotolerance. In contrast, silencing LlHOX6 in lily elevated basal thermotolerance. Cooverexpressing or cosilencing LlHOX6 and LlHB16 in vivo compromised their functions in modulating basal thermotolerance. LlHOX6 interacted with itself and with LlHB16, although heterologous interactions were stronger than homologous ones. Notably, LlHOX6 directly bounds DNA elements to repress the expression of the LlHB16 target genes LlHSFA2 (HEAT STRESS TRANSCRIPTION FACTOR A2) and LlMBF1c (MULTIPROTEIN BRIDGING FACTOR 1C). Moreover, LlHB16 activated itself to form a positive feedback loop, while LlHOX6 repressed LlHB16 expression. The LlHOX6-LlHB16 heterooligomers exhibited stronger DNA binding to compete for LlHB16 homooligomers, thus weakening the transactivation ability of LlHB16 for LlHSFA2 and LlMBF1c and reducing its autoactivation. Altogether, our findings demonstrate that LlHOX6 interacts with LlHB16 to limit its transactivation, thereby impairing heat stress responses in lily.

Multilayer omics landscape analyses reveal the regulatory responses of tea plants to drought stress

Adverse environments, especially drought conditions, deeply influence plant development and growth in all aspects, and the yield and quality of tea plants are largely dependent on favorable growth conditions. Although tea plant responses to drought stress (DS) have been studied, a comprehensive multilayer epigenetic, transcriptomic, and proteomic investigation of how tea responds to DS is lacking. In this study, we generated DNA methylome, transcriptome, proteome, and phosphoproteome data to explore multiple regulatory landscapes in the tea plant response to DS. An integrated multiomics analysis revealed the response of tea plants to DS at multiple regulatory levels. Furthermore, a set of DS-responsive genes involved in photosynthesis, transmembrane transportation, phytohormone metabolism and signaling, secondary metabolite pathways, transcription factors, protein kinases, posttranslational and epigenetic modification, and other key stress-responsive genes were identified for further functional investigation. These results reveal the multilayer regulatory landscape of the tea plant response to DS and provide insight into the mechanisms of these DS responses.

Roles of the pepper JAZ protein CaJAZ1-03 and its interacting partner RING-type E3 ligase CaASRF1 in regulating ABA signaling and drought responses

Plants have developed various defense mechanisms against environmental stresses by regulating hormone signaling. Jasmonic acid (JA) is a major phytohormone associated with plant defense responses. JASMONATE ZIM-DOMAIN (JAZ) proteins play a regulatory role in repressing JA signaling, impacting plant responses to both biotic and abiotic stresses. Here, we isolated 7 JAZ genes in pepper and selected CA03g31030, a Capsicum annuum JAZ1-03 (CaJAZ1-03) gene, for further study based on its expression level in response to abiotic stresses. Through virus-induced gene silencing (VIGS) in pepper and overexpression in transgenic Arabidopsis plants, we established the functional role of CaJAZ1-03. Functional studies revealed that CaJAZ1-03 dampens abscisic acid (ABA) signaling and drought stress responses. The cell-free degradation assay showed faster degradation of CaJAZ1-03 in drought- or ABA-treated pepper leaves compared to healthy leaves. Conversely, CaJAZ1-03 was completely preserved under MG132 treatment, indicating that CaJAZ1-03 stability is modulated via the ubiquitin-26s proteasome pathway. We also found that the pepper RING-type E3 ligase CaASRF1 interacts with and ubiquitinates CaJAZ1-03. Additional cell-free degradation assays revealed a negative correlation between CaJAZ1-03 and CaASRF1 expression levels. Collectively, these findings suggest that CaJAZ1-03 negatively regulates ABA signaling and drought responses and that its protein stability is modulated by CaASRF1.

When drought meets heat - a plant omics perspective

Changes in weather patterns with emerging drought risks and rising global temperature are widespread and negatively affect crop growth and productivity. In nature, plants are simultaneously exposed to multiple biotic and abiotic stresses, but most studies focus on individual stress conditions. However, the simultaneous occurrence of different stresses impacts plant growth and development differently than a single stress. Plants sense the different stress combinations in the same or in different tissues, which could induce specific systemic signalling and acclimation responses; impacting different stress-responsive transcripts, protein abundance and modifications, and metabolites. This mini-review focuses on the combination of drought and heat, two abiotic stress conditions that often occur together. Recent omics studies indicate common or independent regulators involved in heat or drought stress responses. Here, we summarize the current research results, highlight gaps in our knowledge, and flag potential future focus areas.

A functional intact SUMOylation machinery in Aspergillus flavus contributes to fungal and aflatoxin contamination of food

SUMO adducts occur in Aspergillus flavus, and are implicated in fungal biology, while the underlying mechanism and the SUMOylation apparatus components in this saprophytic food spoilage mould, remain undefined. Herein, genes encoding SUMOylation cascade enzymes in A. flavus, including two heterodimeric SUMO E1 activating enzymes, a unique SUMO E2 conjugating enzyme, and one of SUMO E3 ligases, were identified and functionally analyzed. Global SUMO adducts immunoassay, multiple morphological comparison, aflatoxin attributes test, fungal infection and transcriptomic analyses collectively revealed that: E1 and E2 were essential for intracellular SUMOylation, and contributed to both stress response and fungal virulence-related events, including sporulation, colonization, aflatoxins biosynthesis; the primary E3 in this fungus, AfSizA, might serve as the molecular linkage of SUMOylation pathway to fungal virulence rather than SUMOylation-mediated stress adaptation. These findings demonstrated that SUMOylation machinery in A. flavus was functionally intact and contributed to multiple pathobiological processes, hence offering ideas and targets to control food contamination by this mycotoxigenic fungus.

Current perspectives of ubiquitination and SUMOylation in abiotic stress tolerance in plants

Post-translational modification (PTM) is a critical and rapid mechanism to regulate all the major cellular processes through the modification of diverse protein substrates. Substrate-specific covalent attachment of ubiquitin and Small Ubiquitin-Like Modifier (SUMO) with the target proteins, known as ubiquitination and SUMOylation, respectively, are crucial PTMs that regulate almost every process in the cell by modulating the stability and fidelity of the proteins. Ubiquitination and SUMOylation play a very significant role to provide tolerance to the plants in adverse environmental conditions by activating/deactivating the pre-existing proteins to a great extent. We reviewed the importance of ubiquitination and SUMOylation in plants, implicating its prospects in various abiotic stress regulations. An exhaustive study of molecular mechanisms of ubiquitination and SUMOylation of plant proteins and their role will contribute to the understanding of physiology underlying mitigation of the abiotic stresses and survival in plants. It will be helpful to strategize the improvement of crops for abiotic stress tolerance.