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

A review on ubiquitin ligases: Orchestrators of plant resilience in adversity

Ubiquitin- proteasome system (UPS) is universally present in plants and animals, mediating many cellular processes needed for growth and development. Plants constantly defend themselves against endogenous and exogenous stimuli such as hormonal signaling, biotic stresses such as viruses, fungi, nematodes, and abiotic stresses like drought, heat, and salinity by developing complex regulatory mechanisms. Ubiquitination is a regulatory mechanism involving selective elimination and stabilization of regulatory proteins through the UPS system where E3 ligases play a central role; they can bind to the targets in a substrate-specific manner, followed by poly-ubiquitylation, and subsequent protein degradation by 26 S proteasome. Increasing evidence suggests different types of E3 ligases play important roles in plant development and stress adaptation. Herein, we summarize recent advances in understanding the regulatory roles of different E3 ligases and primarily focus on protein ubiquitination in plant-environment interactions. It also highlights the diversity and complexity of these metabolic pathways that enable plant to survive under challenging conditions. This reader-friendly review provides a comprehensive overview of E3 ligases and their substrates associated with abiotic and biotic stresses that could be utilized for future crop improvement.

Coronatine orchestrates ABI1-mediated stomatal opening to facilitate bacterial pathogen infection through importin β protein SAD2

Stomatal immunity plays an important role during bacterial pathogen invasion. Abscisic acid (ABA) induces plants to close their stomata and halt pathogen invasion, but many bacterial pathogens secrete phytotoxin coronatine (COR) to antagonize ABA signaling and reopen the stomata to promote infection at early stage of invasion. However, the underlining mechanism is not clear. SAD2 is an importin β family protein, and the sad2 mutant shows hypersensitivity to ABA. We discovered ABI1, which negatively regulated ABA signaling and reduced plant sensitivity to ABA, was accumulated in the plant nucleus after COR treatment. This event required SAD2 to import ABI1 to the plant nucleus. Abolition of SAD2 undermined ABI1 accumulation. Our study answers the long-standing question of how bacterial COR antagonizes ABA signaling and reopens plant stomata during pathogen invasion.

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.

Genome-Wide Identification of CHYR Gene Family in Sophora alopecuroides and Functional Analysis of SaCHYR4 in Response to Abiotic Stress

Sophora alopecuroides has important uses in medicine, wind breaking, and sand fixation. The CHY-zinc-finger and RING-finger (CHYR) proteins are crucial for plant growth, development, and environmental adaptation; however, genetic data regarding the CHYR family remain scarce. We aimed to investigate the CHYR gene family in S. alopecuroides and its response to abiotic stress, and identified 18 new SaCHYR genes from S. alopecuroides whole-genome data, categorized into 3 subclasses through a phylogenetic analysis. Gene structure, protein domains, and conserved motifs analyses revealed an exon-intron structure and conserved domain similarities. A chromosome localization analysis showed distribution across 12 chromosomes. A promoter analysis revealed abiotic stress-, light-, and hormone-responsive elements. An RNA-sequencing expression pattern analysis revealed positive responses of SaCHYR genes to salt, alkali, and drought stress. SaCHYR4 overexpression considerably enhanced alkali and drought tolerance in Arabidopsis thaliana. These findings shed light on SaCHYR's function and the resistance mechanisms of S. alopecuroides, presenting new genetic resources for crop resistance breeding.

Genomic analyses reveal the stepwise domestication and genetic mechanism of curd biogenesis in cauliflower

Cauliflower (Brassica oleracea L. var. botrytis) is a distinctive vegetable that supplies a nutrient-rich edible inflorescence meristem for the human diet. However, the genomic bases of its selective breeding have not been studied extensively. Herein, we present a high-quality reference genome assembly C-8 (V2) and a comprehensive genomic variation map consisting of 971 diverse accessions of cauliflower and its relatives. Genomic selection analysis and deep-mined divergences were used to explore a stepwise domestication process for cauliflower that initially evolved from broccoli (Curd-emergence and Curd-improvement), revealing that three MADS-box genes, CAULIFLOWER1 (CAL1), CAL2 and FRUITFULL (FUL2), could have essential roles during curd formation. Genome-wide association studies identified nine loci significantly associated with morphological and biological characters and demonstrated that a zinc-finger protein (BOB06G135460) positively regulates stem height in cauliflower. This study offers valuable genomic resources for better understanding the genetic bases of curd biogenesis and florescent development in crops.

The Role of Plant Ubiquitin-like Modifiers in the Formation of Salt Stress Tolerance

The climate-driven challenges facing Earth necessitate a comprehensive understanding of the mechanisms facilitating plant resilience to environmental stressors. This review delves into the crucial role of ubiquitin-like modifiers, particularly focusing on ATG8-mediated autophagy, in bolstering plant tolerance to salt stress. Synthesising recent research, we unveil the multifaceted contributions of ATG8 to plant adaptation mechanisms amidst salt stress conditions, including stomatal regulation, photosynthetic efficiency, osmotic adjustment, and antioxidant defence. Furthermore, we elucidate the interconnectedness of autophagy with key phytohormone signalling pathways, advocating for further exploration into their molecular mechanisms. Our findings underscore the significance of understanding molecular mechanisms underlying ubiquitin-based protein degradation systems and autophagy in salt stress tolerance, offering valuable insights for designing innovative strategies to improve crop productivity and ensure global food security amidst increasing soil salinisation. By harnessing the potential of autophagy and other molecular mechanisms, we can foster sustainable agricultural practices and develop stress-tolerant crops resilient to salt stress.

The F-box protein RhSAF destabilizes the gibberellic acid receptor RhGID1 to mediate ethylene-induced petal senescence in rose

Roses are among the most popular ornamental plants cultivated worldwide for their great economic, symbolic, and cultural importance. Nevertheless, rapid petal senescence markedly reduces rose (Rosa hybrida) flower quality and value. Petal senescence is a developmental process tightly regulated by various phytohormones. Ethylene accelerates petal senescence, while gibberellic acid (GA) delays this process. However, the molecular mechanisms underlying the crosstalk between these phytohormones in the regulation of petal senescence remain largely unclear. Here, we identified SENESCENCE-ASSOCIATED F-BOX (RhSAF), an ethylene-induced F-box protein gene encoding a recognition subunit of the SCF-type E3 ligase. We demonstrated that RhSAF promotes degradation of the GA receptor GIBBERELLIN INSENSITIVE DWARF1 (RhGID1) to accelerate petal senescence. Silencing RhSAF expression delays petal senescence, while suppressing RhGID1 expression accelerates petal senescence. RhSAF physically interacts with RhGID1s and targets them for ubiquitin/26S proteasome-mediated degradation. Accordingly, ethylene-induced RhGID1C degradation and RhDELLA3 accumulation are compromised in RhSAF-RNAi lines. Our results demonstrate that ethylene antagonizes GA activity through RhGID1 degradation mediated by the E3 ligase RhSAF. These findings enhance our understanding of the phytohormone crosstalk regulating petal senescence and provide insights for improving flower longevity.

Interspecific grafting promotes poplar growth and drought resistance via regulating phytohormone signaling and secondary metabolic pathways

Populus cathayana (C) grafted onto P. deltoides (D) (C/D) can promote growth better than self-grafting (C/C and D/D). However, the mechanisms underlying growth and resistance to drought stress are not clear. In this study, we performed physiological and RNA-seq analysis on the different grafted combinations. It was found that C/D plants exhibited higher growth, net photosynthetic rate, IAA content and intrinsic water use efficiency (WUEi) than C/C and D/D plants under both well-watered and drought-stressed conditions. However, most growth, photosynthetic indices, and IAA content were decreased less in C/D, whereas ABA content, WUEi and root characteristics (e.g., root length, volume, surface area and vitality) were increased more in C/D than in other grafting combinations under drought-stressed conditions. Transcriptomic analysis revealed that the number of differentially expressed genes (DEGs) in leaves of C/D vs C/C (control, 181; drought, 121) was much lower than that in the roots of C/D vs D/D (control, 1639; drought, 1706), indicating that the rootstocks were more responsive to drought resistance. KEGG and GO functional enrichment analysis showed that the enhanced growth and drought resistance of C/D were mainly related to DEGs involved in the pathways of ABA and IAA signaling, and secondary metabolite biosynthesis, especially the pathways for lignin and dopamine synthesis and metabolism. Therefore, our results further demonstrated the dominant role of rootstock in drought resistance, and enriched our knowledge on the mechanism of how interspecific grafting enhanced the growth and drought resistance in poplar.

GhUBC10-2 mediates GhGSTU17 degradation to regulate salt tolerance in cotton (Gossypium hirsutum)

Ubiquitin-conjugating enzyme (UBC) is a crucial component of the ubiquitin-proteasome system, which contributes to plant growth and development. While some UBCs have been identified as potential regulators of abiotic stress responses, the underlying mechanisms of this regulation remain poorly understood. Here, we report a cotton (Gossypium hirsutum) UBC gene, GhUBC10-2, which negatively regulates the salt stress response. We found that the gain of function of GhUBC10-2 in both Arabidopsis (Arabidopsis thaliana) and cotton leads to reduced salinity tolerance. Additionally, GhUBC10-2 interacts with glutathione S-transferase (GST) U17 (GhGSTU17), forming a heterodimeric complex that promotes GhGSTU17 degradation. Intriguingly, GhUBC10-2 can be self-polyubiquitinated, suggesting that it possesses E3-independent activity. Our findings provide new insights into the PTM of plant GST-mediated salt response pathways. Furthermore, we found that the WRKY transcription factor GhWRKY13 binds to the GhUBC10-2 promoter and suppresses its expression under salt conditions. Collectively, our study unveils a regulatory module encompassing GhWRKY13-GhUBC10-2-GhGSTU17, which orchestrates the modulation of reactive oxygen species homeostasis to enhance salt tolerance.

Carbon Nanomaterials for Plant Priming through Mechanostimulation: Emphasizing the Role of Shape

The use of nanomaterials to improve plant immunity for sustainable agriculture is gaining increasing attention; yet, the mechanisms involved remain unclear. In contrast to metal-based counterparts, carbon-based nanomaterials do not release components. Determining how these carbon-based nanomaterials strengthen the resistance of plants to diseases is essential as well as whether shape influences this process. Our study compared single-walled carbon nanotubes (SWNTs) and graphene oxide (GO) infiltration against the phytopathogen Pseudomonas syringae pv tomato DC3000. Compared with plants treated with GO, plants primed with SWNTs showed a 29% improvement in the pathogen resistance. Upon nanopriming, the plant displayed wound signaling with transcriptional regulation similar to that observed under brushing-induced mechanostimulation. Compared with GO, SWNTs penetrated more greatly into the leaf and improved transport, resulting in a heightened wound response; this effect resulted from the tubular structure of SWNTs, which differed from the planar form of GO. The shape effect was further demonstrated by wrapping SWNTs with bovine serum albumin, which masked the sharp edges of SWNTs and resulted in a significant decrease in the overall plant wound response. Finally, we clarified how the local wound response led to systemic immunity through increased calcium ion signaling in distant plant areas, which increased the antimicrobial efficacy. In summary, our systematic investigation established connections among carbon nanomaterial priming, mechanostimulation, and wound response, revealing recognition patterns in plant immunity. These findings promise to advance nanotechnology in sustainable agriculture by strengthening plant defenses, enhancing resilience, and reducing reliance on traditional chemicals.

Arabidopsis HECT and RING-type E3 Ligases Promote MAPKKK18 Degradation to Regulate Abscisic Acid Signaling

Mitogen-activated protein kinase (MAPK) cascades are conserved signaling pathways that transduce extracellular signals into diverse cellular responses. Arabidopsis MAPKKK18 is a component of the MAPKKK17/18-MKK3-MPK1/2/7/14 cascades, which play critical roles in abscisic acid (ABA) signaling, drought tolerance and senescence. A very important aspect of MAP kinase signaling is both its activation and its termination, which must be tightly controlled to achieve appropriate biological responses. Recently, the ubiquitin-proteasome system (UPS) has received increasing attention as a key mechanism for maintaining the homeostasis of MAPK cascade components and other ABA signaling effectors. Previous studies have shown that the stability of MAPKKK18 is regulated by the UPS via the ABA core pathway. Here, using multiple proteomic approaches, we found that MAPKKK17/18 turnover is tightly controlled by three E3 ligases, UPL1, UPL4 and KEG. We also identified lysines 154 and 237 as critical for MAPKKK18 stability. Taken together, this study sheds new light on the mechanism that controls MAPKKK17/18 activity and function.

Mitogen-activated protein kinases MPK3 and MPK6 phosphorylate receptor-like cytoplasmic kinase CDL1 to regulate soybean basal immunity

Soybean cyst nematode (SCN; Heterodera glycines Ichinohe), one of the most devastating soybean (Glycine max) pathogens, causes significant yield loss in soybean production. Nematode infection triggers plant defense responses; however, the components involved in the upstream signaling cascade remain largely unknown. In this study, we established that a mitogen-activated protein kinase (MAPK) signaling module, activated by nematode infection or wounding, is crucial for soybeans to establish SCN resistance. GmMPK3 and GmMPK6 directly interact with CDG1-LIKE1 (GmCDL1), a member of the receptor-like cytoplasmic kinase (RLCK) subfamily VII. These kinases phosphorylate GmCDL1 at Thr-372 to prevent its proteasome-mediated degradation. Functional analysis demonstrated that GmCDL1 positively regulates immune responses and promotes SCN resistance in soybeans. GmMPK3-mediated and GmMPK6-mediated phosphorylation of GmCDL1 enhances GmMPK3 and GmMPK6 activation and soybean disease resistance, representing a positive feedback mechanism. Additionally, 2 L-type lectin receptor kinases, GmLecRK02g and GmLecRK08g, associate with GmCDL1 to initiate downstream immune signaling. Notably, our study also unveils the potential involvement of GmLecRKs and GmCDL1 in countering other soybean pathogens beyond nematodes. Taken together, our findings reveal the pivotal role of the GmLecRKs-GmCDL1-MAPK regulatory module in triggering soybean basal immune responses.

A transposon insertion in the promoter of OsUBC12 enhances cold tolerance during japonica rice germination

Low-temperature germination (LTG) is an important agronomic trait for rice (Oryza sativa). Japonica rice generally has greater capacity for germination at low temperatures than the indica subpopulation. However, the genetic basis and molecular mechanisms underlying this complex trait are poorly understood. Here, we report that OsUBC12, encoding an E2 ubiquitin-conjugating enzyme, increases low-temperature germinability in japonica, owing to a transposon insertion in its promoter enhancing its expression. Natural variation analysis reveals that transposon insertion in the OsUBC12 promoter mainly occurs in the japonica lineage. The variation detected in eight representative two-line male sterile lines suggests the existence of this allele introgression by indica-japonica hybridization breeding, and varieties carrying the japonica OsUBC12 locus (transposon insertion) have higher low-temperature germinability than varieties without the locus. Further molecular analysis shows that OsUBC12 negatively regulate ABA signaling. OsUBC12-regulated seed germination and ABA signaling mainly depend on a conserved active site required for ubiquitin-conjugating enzyme activity. Furthermore, OsUBC12 directly associates with rice SUCROSE NON-FERMENTING 1-RELATED PROTEIN KINASE 1.1 (OsSnRK1.1), promoting its degradation. OsSnRK1.1 inhibits LTG by enhancing ABA signaling and acts downstream of OsUBC12. These findings shed light on the underlying mechanisms of UBC12 regulating LTG and provide genetic reference points for improving LTG in indica rice.

N6-methyladenosine-mediated feedback regulation of abscisic acid perception via phase-separated ECT8 condensates in Arabidopsis

N6-methyladenosine (m6A) is the most abundant internal modification in eukaryotic mRNAs, yet how plants recognize this chemical modification to swiftly adjust developmental plasticity under environmental stresses remains unclear. Here we show that m6A mRNA modification and its reader protein EVOLUTIONARILY CONSERVED C-TERMINAL REGION 8 (ECT8) act together as a key checkpoint for negative feedback regulation of abscisic acid (ABA) signalling by sequestering the m6A-modified ABA receptor gene PYRABACTIN RESISTANCE 1-LIKE 7 (PYL7) via phase-separated ECT8 condensates in stress granules in response to ABA. This partially depletes PYL7 mRNA from its translation in the cytoplasm, thus reducing PYL7 protein levels and compromising ABA perception. The loss of ECT8 results in defective sequestration of m6A-modified PYL7 in stress granules and permits more PYL7 transcripts for translation. This causes overactivation of ABA-responsive genes and the consequent ABA-hypersensitive phenotypes, including drought tolerance. Overall, our findings reveal that m6A-mediated sequestration of PYL7 by ECT8 in stress granules negatively regulates ABA perception, thereby enabling prompt feedback regulation of ABA signalling to prevent plant cell overreaction to environmental stresses.

Integrative regulatory mechanisms of stomatal movements under changing climate

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

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

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

An ERAD-related ubiquitin-conjugating enzyme boosts broad-spectrum disease resistance and yield in rice

Rice is a staple crop for over half of the global population. However, blast disease caused by Magnaporthe orzae can result in more than a 30% loss in rice yield in epidemic years. Although some major resistance genes bolstering blast resistance have been identified in rice, their stacking in elite cultivars usually leads to yield penalties. Here we report that OsUBC45, a ubiquitin-conjugating enzyme functioning in the endoplasmic reticulum-associated protein degradation system, promotes broad-spectrum disease resistance and yield in rice. OsUBC45 is induced upon infection by M. oryzae, and its overexpression enhances resistance to blast disease and bacterial leaf blight by elevating pathogen-associated molecular pattern-triggered immunity (PTI) while nullifying the gene-attenuated PTI. The OsUBC45 overexpression also increases grain yield by over 10%. Further, OsUBC45 enhances the degradation of glycogen synthase kinase 3 OsGSK3 and aquaporin OsPIP2;1, which negatively regulate the grain size and PTI, respectively. The OsUBC45 reported in our study has the potential for improving yield and disease resistance for sustainable rice production.

Dry side of the core: a meta-analysis addressing the original nature of the ABA signalosome at the onset of seed imbibition

The timing of seedling emergence is a major agricultural and ecological fitness trait, and seed germination is controlled by a complex molecular network including phytohormone signalling. One such phytohormone, abscisic acid (ABA), controls a large array of stress and developmental processes, and researchers have long known it plays a crucial role in repressing germination. Although the main molecular components of the ABA signalling pathway have now been identified, the molecular mechanisms through which ABA elicits specific responses in distinct organs is still enigmatic. To address the fundamental characteristics of ABA signalling during germination, we performed a meta-analysis focusing on the Arabidopsis dry seed proteome as a reflexion basis. We combined cutting-edge proteome studies, comparative functional analyses, and protein interaction information with genetic and physiological data to redefine the singular composition and operation of the ABA core signalosome from the onset of seed imbibition. In addition, we performed a literature survey to integrate peripheral regulators present in seeds that directly regulate core component function. Although this may only be the tip of the iceberg, this extended model of ABA signalling in seeds already depicts a highly flexible system able to integrate a multitude of information to fine-tune the progression of germination.

The ubiquitin-binding protein MdRAD23D1 mediates drought response by regulating degradation of the proline-rich protein MdPRP6 in apple (Malus domestica)

RAD23 (RADIATION SENSITIVE23) proteins are a group of UBL-UBA (ubiquitin-like-ubiquitin-associated) proteins that shuttle ubiquitylated proteins to the 26S proteasome for breakdown. Drought stress is a major environmental constraint that limits plant growth and production, but whether RAD23 proteins are involved in this process is unclear. Here, we demonstrated that a shuttle protein, MdRAD23D1, mediated drought response in apple plants (Malus domestica). MdRAD23D1 levels increased under drought stress, and its suppression resulted in decreased stress tolerance in apple plants. Through in vitro and in vivo assays, we demonstrated that MdRAD23D1 interacted with a proline-rich protein MdPRP6, resulting in the degradation of MdPRP6 by the 26S proteasome. And MdRAD23D1 accelerated the degradation of MdPRP6 under drought stress. Suppression of MdPRP6 resulted in enhanced drought tolerance in apple plants, mainly because the free proline accumulation is changed. And the free proline is also involved in MdRAD23D1-mediated drought response. Taken together, these findings demonstrated that MdRAD23D1 and MdPRP6 oppositely regulated drought response. MdRAD23D1 levels increased under drought, accelerating the degradation of MdPRP6. MdPRP6 negatively regulated drought response, probably by regulating proline accumulation. Thus, "MdRAD23D1-MdPRP6" conferred drought stress tolerance in apple plants.

The safflower MBW complex regulates HYSA accumulation through degradation by the E3 ligase CtBB1

The regulatory mechanism of the MBW (MYB-bHLH-WD40) complex in safflower (Carthamus tinctorius) remains unclear. In the present study, we show that the separate overexpression of the genes CtbHLH41, CtMYB63, and CtWD40-6 in Arabidopsis thaliana increased anthocyanin and procyanidin contents in the transgenic plants and partially rescued the trichome reduction phenotype of the corresponding bhlh41, myb63, and wd40-6 single mutants. Overexpression of CtbHLH41, CtMYB63, or CtWD40-6 in safflower significantly increased the content of the natural pigment hydroxysafflor yellow A (HYSA) and negatively regulated safflower petal size. Yeast-two-hybrid, functional, and genetic assays demonstrated that the safflower E3 ligase CtBB1 (BIG BROTHER 1) can ubiquitinate CtbHLH41, marking it for degradation through the 26S proteasome and negatively regulating flavonoid accumulation. CtMYB63/CtWD40-6 enhanced the transcriptional activity of CtbHLH41 on the CtDFR (dihydroflavonol 4-reductase) promoter. We propose that the MBW-CtBB1 regulatory module may play an important role in coordinating HYSA accumulation with other response mechanisms.

A molecular framework underlying low-nitrogen-induced early leaf senescence in Arabidopsis thaliana

Nitrogen (N) deficiency causes early leaf senescence, resulting in accelerated whole-plant maturation and severely reduced crop yield. However, the molecular mechanisms underlying N-deficiency-induced early leaf senescence remain unclear, even in the model species Arabidopsis thaliana. In this study, we identified Growth, Development and Splicing 1 (GDS1), a previously reported transcription factor, as a new regulator of nitrate (NO₃^-) signaling by a yeast-one-hybrid screen using a NO₃^- enhancer fragment from the promoter of NRT2.1. We showed that GDS1 promotes NO₃^- signaling, absorption and assimilation by affecting the expression of multiple NO₃^- regulatory genes, including Nitrate Regulatory Gene2 (NRG2). Interestingly, we observed that gds1 mutants show early leaf senescence as well as reduced NO₃^- content and N uptake under N-deficient conditions. Further analyses indicated that GDS1 binds to the promoters of several senescence-related genes, including Phytochrome-Interacting Transcription Factors 4 and 5 (PIF4 and PIF5) and represses their expression. Interestingly, we found that N deficiency decreases GDS1 protein accumulation, and GDS1 could interact with Anaphase Promoting Complex Subunit 10 (APC10). Genetic and biochemical experiments demonstrated that Anaphase Promoting Complex or Cyclosome (APC/C) promotes the ubiquitination and degradation of GDS1 under N deficiency, resulting in loss of PIF4 and PIF5 repression and consequent early leaf senescence. Furthermore, we discovered that overexpression of GDS1 could delay leaf senescence and improve seed yield and N-use efficiency (NUE) in Arabidopsis. In summary, our study uncovers a molecular framework illustrating a new mechanism underlying low-N-induced early leaf senescence and provides potential targets for genetic improvement of crop varieties with increased yield and NUE.

Identification of AREB/ABF Gene Family Involved in the Response of ABA under Salt and Drought Stresses in Jute (Corchorus olitorius L.)

The abscisic acid (ABA)-responsive element binding protein/ABRE-binding factor (AREB/ABF) subfamily members are essential to ABA signaling pathways and plant adaptation to various environmental stresses. Nevertheless, there are no reports on AREB/ABF in jute (Corchorus L.). Here, eight AREB/ABF genes were identified in the C. olitorius genome and classified into four groups (A-D) based on their phylogenetic relationships. A cis-elements analysis showed that CoABFs were widely involved in hormone response elements, followed by light and stress responses. Furthermore, the ABRE response element was involved in four CoABFs, playing an essential role in the ABA reaction. A genetic evolutionary analysis indicated that clear purification selection affects jute CoABFs and demonstrated that the divergence time was more ancient in cotton than in cacao. A quantitative real-time PCR revealed that the expression levels of CoABFs were upregulated and downregulated under ABA treatment, indicating that CoABF3 and CoABF7 are positively correlated with ABA concentration. Moreover, CoABF3 and CoABF7 were significantly upregulated in response to salt and drought stress, especially with the application of exogenous ABA, which showed higher intensities. These findings provide a complete analysis of the jute AREB/ABF gene family, which could be valuable for creating novel jute germplasms with a high resistance to abiotic stresses.

Ubiquitin-Conjugating Enzyme OsUBC11 Affects the Development of Roots via Auxin Pathway

Rice has 48 ubiquitin-conjugating enzymes, and the functions of most of these enzymes have not been elucidated. In the present study, a T-DNA insertional mutant named R164, which exhibited a significant decrease in the length of primary and lateral roots, was used as the experimental material to explore the potential function of OsUBC11. Analysis using the SEFA-PCR method showed that the T-DNA insertion was present in the promoter region of OsUBC11 gene, which encodes ubiquitin-conjugating enzyme (E2), and activates its expression. Biochemical experiments showed that OsUBC11 is a lysine-48-linked ubiquitin chain-forming conjugase. OsUBC11 overexpression lines showed the same root phenotypes. These results demonstrated that OsUBC11 was involved in root development. Further analyses showed that the IAA content of R164 mutant and OE3 line were significantly lower compared with wild-type Zhonghua11. Application of exogenous NAA restored the length of lateral and primary roots in R164 and OsUBC11 overexpression lines. Expression of the auxin synthesis regulating gene OsYUCCA4/6/7/9, the auxin transport gene OsAUX1, auxin/indole-3-acetic acid (Aux/IAA) family gene OsIAA31, auxin response factor OsARF16 and root regulator key genes, including OsWOX11, OsCRL1, OsCRL5 was significantly down-regulated in OsUBC11 overexpressing plants. Collectively, these results indicate that OsUBC11 modulates auxin signaling, ultimately affecting root development at the rice seedling stage.

The interaction of ABA and ROS in plant growth and stress resistances

The plant hormone ABA (abscisic acid) plays an extremely important role in plant growth and adaptive stress, including but are not limited to seed germination, stomatal closure, pathogen infection, drought and cold stresses. Reactive oxygen species (ROS) are response molecules widely produced by plant cells under biotic and abiotic stress conditions. The production of apoplast ROS is induced and regulated by ABA, and participates in the ABA signaling pathway and its regulated plant immune system. In this review, we summarize ABA and ROS in apoplast ROS production, plant response to biotic and abiotic stresses, plant growth regulation, ABA signal transduction, and the regulatory relationship between ABA and other plant hormones. In addition, we also discuss the effects of protein post-translational modifications on ABA and ROS related factors.

Grafting enhances plants drought resistance: Current understanding, mechanisms, and future perspectives

Drought, one of the most severe and complex abiotic stresses, is increasingly occurring due to global climate change and adversely affects plant growth and yield. Grafting is a proven and effective tool to enhance plant drought resistance ability by regulating their physiological and molecular processes. In this review, we have summarized the current understanding, mechanisms, and perspectives of the drought stress resistance of grafted plants. Plants resist drought through adaptive changes in their root, stem, and leaf morphology and structure, stomatal closure modulation to reduce transpiration, activating osmoregulation, enhancing antioxidant systems, and regulating phytohormones and gene expression changes. Additionally, the mRNAs, miRNAs and peptides crossing the grafted healing sites also confer drought resistance. However, the interaction between phytohormones, establishment of the scion-rootstock communication through genetic materials to enhance drought resistance is becoming a hot research topic. Therefore, our review provides not only physiological evidences for selecting drought-resistant rootstocks or scions, but also a clear understanding of the potential molecular effects to enhance drought resistance using grafted plants.

Arabidopsis Ubiquitin-Conjugating Enzymes UBC4, UBC5, and UBC6 Have Major Functions in Sugar Metabolism and Leaf Senescence

The ubiquitin-conjugating enzyme (E2) is required for protein ubiquitination. Arabidopsis has 37 E2s grouped into 14 subfamilies and the functions for many of them are unknown. We utilized genetic and biochemical methods to study the roles of Arabidopsis UBC4, UBC5, and UBC6 of the E2 subfamily IV. The Arabidopsis ubc4/5/6 triple mutant plants had higher levels of glucose, sucrose, and starch than the control plants, as well as a higher protein level of a key gluconeogenic enzyme, cytosolic fructose 1,6-bisphosphatase 1 (cyFBP). In an in vitro assay, the proteasome inhibitor MG132 inhibited the degradation of recombinant cyFBP whereas ATP promoted cyFBP degradation. In the quadruple mutant ubc4/5/6 cyfbp, the sugar levels returned to normal, suggesting that the increased sugar levels in the ubc4/5/6 mutant were due to an increased cyFBPase level. In addition, the ubc4/5/6 mutant plants showed early leaf senescence at late stages of plant development as well as accelerated leaf senescence using detached leaves. Further, the leaf senescence phenotype remained in the quadruple ubc4/5/6 cyfbp mutant. Our results suggest that UBC4/5/6 have two lines of important functions, in sugar metabolism through regulating the cyFBP protein level and in leaf senescence likely through a cyFBP-independent mechanism.

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.

The transcription factor OsMYBc and an E3 ligase regulate expression of a K+ transporter during salt stress

Sodium (Na+) and potassium (K+) homeostasis is essential for plant survival in saline soils. A member of the High-Affinity K+ Transporter (HKT) family in rice (Oryza sativa), OsHKT1;1, is a vital regulator of Na+ exclusion from shoots and is bound by a MYB transcription factor (OsMYBc). Here, we generated transgenic rice lines in the oshkt1;1 mutant background for genetic complementation using genomic OsHKT1;1 containing a native (Com) or mutated (mCom) promoter that cannot be bound by OsMYBc. In contrast to wild-type (WT) or Com lines, the mCom lines were not able to recover the salt-sensitive phenotype of oshkt1;1. The OsMYBc-overexpressing plants were more tolerant to salt stress than WT plants. A yeast two-hybrid screen using the OsMYBc N-terminus as bait identified a rice MYBc stress-related RING finger protein (OsMSRFP). OsMSRFP is an active E3 ligase that ubiquitinated OsMYBc in vitro and mediated 26S proteasome-mediated degradation of OsMYBc under semi-in vitro and in vivo conditions. OsMSRFP attenuated OsMYBc-mediated OsHKT1;1 expression, and knockout of OsMSRFP led to rice salt tolerance. These findings uncover a regulatory mechanism of salt response that fine-tunes OsHKT1;1 transcription by ubiquitination of OsMYBc.

The ABI3 Transcription Factor Interaction and Antagonism with Ubiquitin E3 Ligase ScPRT1 in Syntrichia caninervis

The ubiquitination pathway has been found to regulate plant responses to environmental stress. However, the role of E3 ubiquitin ligase in desiccation tolerant moss has not yet been elucidated. Previous research has shown that the abscisic acid (ABA) signaling factor ScABI3 can significantly increase desiccation tolerance and reduce ABA sensitivity in the desert moss Syntrichia caninervis. In this study, we identified a RING-type E3 ubiquitin ligase, ScPRT1, and showed that ScABI3 can directly interact with ScPRT1 in vitro and in vivo. Furthermore, we found that the high expression of ScPRT1 can interfere with the transcription of ScABI3 under ABA treatment. Therefore, we speculate that ScPRT1 may degrade ScABI3 through the ubiquitin-26S proteasome system and participate in ABA-dependent signaling in response to ABA-insensitivity or desiccation tolerance in S. caninervis. The findings from our study may enrich our knowledge of the role of E3 ubiquitin ligase in desiccation tolerance and lay a theoretical foundation for an in-depth study of the relationship between ubiquitination modification and ABA signal transduction under environmental stress.

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

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

Hydrogen Sulfide-Linked Persulfidation Maintains Protein Stability of ABSCISIC ACID-INSENSITIVE 4 and Delays Seed Germination

Hydrogen sulfide (H₂S) is an endogenous gaseous molecule that plays an important role in the plant life cycle. The multiple transcription factor ABSCISIC ACID INSENSITIVE 4 (ABI4) was precisely regulated to participate in the abscisic acid (ABA) mediated signaling cascade. However, the molecular mechanisms of how H₂S regulates ABI4 protein level to control seed germination and seedling growth have remained elusive. In this study, we demonstrated that ABI4 controls the expression of L-CYSTEINE DESULFHYDRASE1 (DES1), a critical endogenous H₂S-producing enzyme, and both ABI4 and DES1-produced H₂S have inhibitory effects on seed germination. Furthermore, the ABI4 level decreased during seed germination while H₂S triggered the enhancement of the persulfidation level of ABI4 and alleviated its degradation rate, which in turn inhibited seed germination and seedling establishment. Conversely, the mutation of ABI4 at Cys250 decreased ABI4 protein stability and facilitated seed germination. Moreover, ABI4 degradation is also regulated via the 26S proteasome pathway. Taken together, these findings suggest a molecular link between DES1 and ABI4 through the post-translational modifications of persulfidation during early seedling development.

Long-distance control of pumpkin rootstock over cucumber scion under drought stress as revealed by transcriptome sequencing and mobile mRNAs identifications

Grafting with pumpkin rootstock is commonly used not only to improve the quality of cucumber fruits but also to confer biotic or abiotic stress tolerance. However, the molecular mechanism of grafted cucumbers to drought stress and the possible roles of mobile mRNAs to improve stress tolerance have remained obscure. Hence, we conducted transcriptome sequencing and combined it with morpho-physiological experiments to compare the response of homografts (cucumber as scion and rootstock) (C) and heterografts (cucumber as scion and pumpkin as rootstock) (P) to drought stress. After applying drought stress, homografts and heterografts expressed 2960 and 3088 genes in response to drought stress, respectively. The identified DEGs in heterografts under drought stress were categorized into different stress-responsive groups, such as carbohydrate metabolism (involved in osmotic adjustment by sugar accumulation), lipid and cell wall metabolism (involved in cell membrane integrity by a reduction in lipid peroxidation), redox homeostasis (increased antioxidant enzymes activities), phytohormone (increased ABA content), protein kinases and transcription factors (TFs) using MapMan software. Earlier and greater H2O2 accumulation in xylem below the graft union was accompanied by leaf ABA accumulation in heterografts in response to drought stress. Greater leaf ABA helped heterografted cucumbers to sense and respond to drought stress earlier than homografts. The timely response of heterografts to drought stress led to maintain higher water content in the leaves even in the late stage of drought stress. The identified mobile mRNAs (mb-mRNAs) in heterografts were mostly related to photosynthesis which would be the possible reason for improved chlorophyll content and maximum photochemical efficiency of PSII (Fv/Fm). The existence of some stress-responsive pumpkin (rootstock) mRNAs in cucumber (scion), such as heat shock protein (HSP70, a well-known stress-responsive gene), led to the higher proline accumulation than homografts. The expression of the mobile and immobile stress-responsive mRNAs and timely response of heterografts to drought stress could improve drought tolerance in pumpkin-rooted plants.

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

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

Roles of Glutathione in Mediating Abscisic Acid Signaling and Its Regulation of Seed Dormancy and Drought Tolerance

Plant growth and development and interactions with the environment are regulated by phytohormones and other signaling molecules. During their evolution, plants have developed strategies for efficient signal perception and for the activation of signal transduction cascades to maintain proper growth and development, in particular under adverse environmental conditions. Abscisic acid (ABA) is one of the phytohormones known to regulate plant developmental events and tolerance to environmental stresses. The role of ABA is mediated by both its accumulated level, which is regulated by its biosynthesis and catabolism, and signaling, all of which are influenced by complex regulatory mechanisms. Under stress conditions, plants employ enzymatic and non-enzymatic antioxidant strategies to scavenge excess reactive oxygen species (ROS) and mitigate the negative effects of oxidative stress. Glutathione (GSH) is one of the main antioxidant molecules playing a critical role in plant survival under stress conditions through the detoxification of excess ROS, maintaining cellular redox homeostasis and regulating protein functions. GSH has recently emerged as an important signaling molecule regulating ABA signal transduction and associated developmental events, and response to stressors. This review highlights the current knowledge on the interplay between ABA and GSH in regulating seed dormancy, germination, stomatal closure and tolerance to drought.

ERAD-related E2 and E3 enzymes modulate the drought response by regulating the stability of PIP2 aquaporins

Endoplasmic reticulum-associated degradation (ERAD) is known to regulate plant responses to diverse stresses, yet its underlying molecular mechanisms and links to various stress signaling pathways are poorly understood. Here, we show that the ERAD component ubiquitin-conjugating enzyme UBC32 positively regulates drought tolerance in Arabidopsis thaliana by targeting the aquaporins PIP2;1 and PIP2;2 for degradation. Furthermore, we demonstrate that the RING-type ligase Rma1 acts together with UBC32 and that the E2 activity of UBC32 is essential for the ubiquitination of Rma1. This complex ubiquitinates a phosphorylated form of PIP2;1 at Lys276 to promote its degradation, thereby enhancing plant drought tolerance. Extending these molecular insights into crops, we show that overexpression of Arabidopsis UBC32 also improves drought tolerance in rice (Oryza sativa). Thus, beyond uncovering the molecular basis of an ERAD-regulated stress response, our study suggests multiple potential strategies for engineering crops with improved drought tolerance.

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.

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.

The Ubiquitin Switch in Plant Stress Response

Ubiquitin is a 76 amino acid polypeptide common to all eukaryotic organisms. It functions as a post-translationally modifying mark covalently linked to a large cohort of yet poorly defined protein substrates. The resulting ubiquitylated proteins can rapidly change their activities, cellular localization, or turnover through the 26S proteasome if they are no longer needed or are abnormal. Such a selective modification is essential to many signal transduction pathways particularly in those related to stress responses by rapidly enhancing or quenching output. Hence, this modification system, the so-called ubiquitin-26S proteasome system (UPS), has caught the attention in the plant research community over the last two decades for its roles in plant abiotic and biotic stress responses. Through direct or indirect mediation of plant hormones, the UPS selectively degrades key components in stress signaling to either negatively or positively regulate plant response to a given stimulus. As a result, a tightly regulated signaling network has become of much interest over the years. The ever-increasing changes of the global climate require both the development of new crops to cope with rapid changing environment and new knowledge to survey the dynamics of ecosystem. This review examines how the ubiquitin can switch and tune plant stress response and poses potential avenues to further explore this system.