The MIEL1 E3 Ubiquitin Ligase Negatively Regulates Cuticular Wax Biosynthesis in Arabidopsis Stems

Research progress on the mechanisms of fruit glossiness in cucumber

Cucumber (Cucumis sativus L.) is an important horticultural crop in China. Consumer requirements for aesthetically pleasing appearances of horticultural crops are gradually increasing, and cucumbers having a good visual appearance, as well as flavor, are important for breeding and industry development. The gloss of cucumber fruit epidermis is an important component of its appeal, and the wax layer on the fruit surface plays important roles in plant growth and forms a powerful barrier against external biotic and abiotic stresses. The wax of the cucumber epidermis is mainly composed of alkanes, and the luster of cucumber fruit is mainly determined by the alkane and silicon contents of the epidermis. Several genes, transcription factors, and transporters affect the synthesis of ultra-long-chain fatty acids and change the silicon content, further altering the gloss of the epidermis. However, the specific regulatory mechanisms are not clear. Here, progress in research on the luster of cucumber fruit epidermis from physiological, biochemical, and molecular regulatory perspectives are reviewed. Additionally, future research avenues in the field are discussed.

Cuticle development and the underlying transcriptome-metabolome associations during early seedling establishment

The plant cuticle is a complex extracellular lipid barrier that has multiple protective functions. This study investigated cuticle deposition by integrating metabolomics and transcriptomics data gathered from six different maize seedling organs of four genotypes, the inbred lines B73 and Mo17, and their reciprocal hybrids. These datasets captured the developmental transition of the seedling from heterotrophic skotomorphogenic growth to autotrophic photomorphogenic growth, a transition that is highly vulnerable to environmental stresses. Statistical interrogation of these data revealed that the predominant determinant of cuticle composition is seedling organ type, whereas the seedling genotype has a smaller effect on this phenotype. Gene-to-metabolite associations assessed by integrated statistical analyses identified three gene networks associated with the deposition of different elements of the cuticle: cuticular waxes; monomers of lipidized cell wall biopolymers, including cutin and suberin; and both of these elements. These gene networks reveal three metabolic programs that appear to support cuticle deposition, including processes of chloroplast biogenesis, lipid metabolism, and molecular regulation (e.g. transcription factors, post-translational regulators, and phytohormones). This study demonstrates the wider physiological metabolic context that can determine cuticle deposition and lays the groundwork for new targets for modulating the properties of this protective barrier.

CaWRKY20 Negatively Regulates Plant Resistance to Colletotrichum scovillei in Pepper

Chili anthracnose, a fungal disease caused by Colletotrichum scovillei, is among the most devastating diseases affecting pepper (Capsicum annuum L.). Although WRKY transcription factors play important roles in plant immunity, it is unknown how WRKY gene family members contribute to pepper plant resistance to C. scovillei. Here, CaWRKY20 was found to negatively regulate pepper resistance to C. scovillei, which was demonstrated by virus-induced gene silencing and transient overexpression in pepper. Moreover, overexpression of CaWRKY20 enhanced susceptibility to C. scovillei in tomato. Additionally, our findings demonstrated that CaWRKY20 can indirectly regulate the expression of salicylic acid (SA)-related defense genes (CaPR1, CaPR10 and CaSAR8.2) as well as reactive oxygen species (ROS)-scavenging enzyme genes (CaCAT, CaPOD and CaSOD) in response to C. scovillei. In addition, CaWRKY20 was found to interact with CaMIEL1 in the nucleus to regulate the defense response to C. scovillei in pepper. Furthermore, CaWRKY20 directly bound to the W-box in the promoter of SYSTEMIC ACQUIRED RESISTANCE DEFICIENT 1 (CaSARD1) and suppressed its expression, resulting in reduced resistance to C. scovillei. These results will clarify the mechanism by which WRKY transcription factors are involved in pepper disease resistance and can thus facilitate molecular breeding for anthracnose-resistant varieties.

A novel three-layer module BoMYB1R1-BoMYB4b/BoMIEL1-BoDFR1 regulates anthocyanin accumulation in kale

Anthocyanin is an important pigment responsible for plant coloration and beneficial to human health. Kale (Brassica oleracea var. acephala), a primary cool-season flowers and vegetables, is an ideal material to study anthocyanin biosynthesis and regulation mechanisms due to its anthocyanin-rich leaves. However, the underlying molecular mechanism of anthocyanin accumulation in kale remains poorly understood. Previously, we demonstrated that BoDFR1 is a key gene controlling anthocyanin biosynthesis in kale. Here, we discovered a 369-bp InDel variation in the BoDFR1 promoter between the two kale inbred lines with different pink coloration, which resulted in reduced transcriptional activity of the BoDFR1 gene in the light-pink line. With the 369-bp insertion as a bait, an R2R3-MYB repressor BoMYB4b was identified using the yeast one-hybrid screening. Knockdown of the BoMYB4b gene led to increased BoDFR1 expression and anthocyanin accumulation. An E3 ubiquitin ligase, BoMIEL1, was found to mediate the degradation of BoMYB4b, thereby promoting anthocyanin biosynthesis. Furthermore, the expression level of BoMYB4b was significantly reduced by light signals, which was attributed to the direct repression of the light-signaling factor BoMYB1R1 on the BoMYB4b promoter. Our study revealed that a novel regulatory module comprising BoMYB1R1, BoMIEL1, BoMYB4b, and BoDFR1 finely regulates anthocyanin accumulation in kale. The findings aim to establish a scientific foundation for genetic improvement of leaf color traits in kale, meanwhile, providing a reference for plant coloration studies.

CsMIEL1 effectively inhibits the accumulation of anthocyanins under low temperatures in tea plants (Camelliasinensis)

Tea is one of the most prevalent non-alcoholic beverages. The leaves of tea plants hyperaccumulate anthocyanins under cold stress, resulting in enhanced bitterness. Previously, we determined that the RING-type E3 ubiquitin ligase CsMIEL1 from the tea plant (Camellia sinensis (L.) O. Kuntze) is involved in the response to stress conditions. This study aimed to determine the role of CsMIEL1 in anthocyanin accumulation at the post-translational modification level. The results showed that the heterologous expression of CsMIEL1 led to an 86% decrease in anthocyanin levels, resulting in a significant decrease in the mRNA levels of related genes in Arabidopsis at low temperatures but no significant differences in other phenotypes. Furthermore, multi-omics analysis and yeast two-hybrid library screening were performed to identify potential downstream targets of CsMIEL1. The results showed that the overexpression of CsMIEL1 resulted in 45% (448) of proteins being differentially expressed, of which 8% (36) were downregulated in A.thaliana, and most of these differentially expressed proteins (DEPs) were clustered in the plant growth and secondary metabolic pathways. Among the 71 potential targets that may interact with CsMIEL1, CsMYB90 and CsGSTa, which are related to anthocyanin accumulation, were selected. In subsequent analyses, these two proteins were verified to interact with CsMIEL1 via yeast two-hybrid (Y2H) and pull-down analyses in vitro. In summary, we explored the potential mechanism by which the E3 ligase relieves anthocyanin hyperaccumulation at low temperatures in tea plants. These results provide a new perspective on the mechanisms of anthocyanin regulation and the molecular breeding of tea plants.

Effect of Brucea javanica Oil on the Toxicity of β-Cypermethrin Emulsifiable Concentrate Formulation

Enhancing the performance of traditional pesticide formulations by improving their leaf surface wetting capabilities is a crucial approach for maximizing the pesticide efficiency. This study develops an emulsifiable concentrate (EC) of 4.5% β-cypermethrin containing Brucea javanica oil (BJO). The incorporation of BJO aims to improve the leaf-wetting properties of the EC formulation and enhance its insecticidal effectiveness. The droplet size and emulsion characteristics of β-CYP EC emulsion with varying concentrations of the emulsifier were evaluated, and changes after incorporating BJO were assessed to develop the optimal formulation. A comprehensive comparison was conducted among commercial 4.5% β-cypermethrin EC (β-CYP EC-1), 4.5% β-cypermethrin EC with BJO (β-CYP EC-2), and 4.5% β-cypermethrin EC without BJO (β-CYP EC-3). This comparison encompassed various factors including storage stability, insecticidal activity, cytotoxicity, and wetting performance on cabbage leaves. The results indicated that the ideal emulsifier concentration was 15% emulsifier 0201B. β-CYP EC-2 demonstrated superior wetting properties on cabbage leaves (the wetting performance of β-CYP EC-2 emulsion on cabbage leaves is 2.60 times that of the β-CYP EC-1 emulsion), heightened insecticidal activity against the third larvae of Plutella xylostella [diamondback moth (DBM)] [the insecticidal activity of the β-CYP EC-2 emulsion against the third larvae of DBM is 1.93 times that of the β-CYP EC-1 emulsion (12 h)], and more obvious inhibitory effects on the proliferation of DBM embryo cells than the other tested formulations. These findings have significant implications for advancing pest control strategies and promoting sustainable and effective agricultural practices.

MdDEWAX decreases plant drought resistance by regulating wax biosynthesis

Apple epidermal wax protects plants from environmental stresses, determines fruit gloss and improves postharvest storage quality. However, the molecular mechanisms underlying the biosynthesis and regulation of apple epidermal waxes are not fully understood. In this study, we isolated a MdDEWAX gene from apple, which localized in the nucleus, expressed mainly in apple fruit, and induced by drought. We transformed the MdDEWAX gene into Arabidopsis, and found that heterologous expression of MdDEWAX reduced the accumulation of cuticular waxes in leaves and stems, increased epidermal permeability, the rate of water loss, and the rate of chlorophyll extraction of leaves and stems, altered the sensitivity to ABA, and reduced drought tolerance. Meanwhile, overexpression or silencing of the gene in the epidermis of apple fruits decreased or increased wax content, respectively. This study provides candidate genes for breeding apple cultivars and rootstocks with better drought tolerance.

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

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

Integrating GWAS and TWAS to elucidate the genetic architecture of maize leaf cuticular conductance

The cuticle, a hydrophobic layer of cutin and waxes synthesized by plant epidermal cells, is the major barrier to water loss when stomata are closed. Dissecting the genetic architecture of natural variation for maize (Zea mays L.) leaf cuticular conductance (gc) is important for identifying genes relevant to improving crop productivity in drought-prone environments. To this end, we performed an integrated genome- and transcriptome-wide association studies (GWAS and TWAS) to identify candidate genes putatively regulating variation in leaf gc. Of the 22 plausible candidate genes identified, 4 were predicted to be involved in cuticle precursor biosynthesis and export, 2 in cell wall modification, 9 in intracellular membrane trafficking, and 7 in the regulation of cuticle development. A gene encoding an INCREASED SALT TOLERANCE1-LIKE1 (ISTL1) protein putatively involved in intracellular protein and membrane trafficking was identified in GWAS and TWAS as the strongest candidate causal gene. A set of maize nested near-isogenic lines that harbor the ISTL1 genomic region from eight donor parents were evaluated for gc, confirming the association between gc and ISTL1 in a haplotype-based association analysis. The findings of this study provide insights into the role of regulatory variation in the development of the maize leaf cuticle and will ultimately assist breeders to develop drought-tolerant maize for target environments.

Genome-wide analysis and functional characterization of CHYR gene family associated with abiotic stress tolerance in bread wheat (Triticum aestivum L.)

BACKGROUND

CHY zinc-finger and RING finger (CHYR) proteins have been functionally characterized in plant growth, development and various stress responses. However, the genome-wide analysis was not performed in wheat.

RESULTS

In this study, a total of 18 TaCHYR genes were identified in wheat and classified into three groups. All TaCHYR genes contained CHY-zinc finger, C3H2C3-type RING finger and zinc ribbon domains, and group III members included 1-3 hemerythrin domains in the N-terminus regions. TaCHYR genes in each group shared similar conserved domains distribution. Chromosomal location, synteny and cis-elements analysis of TaCHYRs were also analyzed. Real-time PCR results indicated that most of selected 9 TaCHYR genes exhibited higher expression levels in leaves during wheat seedling stage. All these TaCHYR genes were up-regulated after PEG treatment, and these TaCHYRs exhibited differential expression patterns in response to salt, cold and heat stress in seedling leaves. The growth of yeast cells expressing TaCHYR2.1, TaCHYR9.2 and TaCHYR11.1 were inhibited under salt and dehydration stress. Moreover, gene ontology (GO) annotation, protein interaction and miRNA regulatory network of TaCHYR genes were analyzed.

CONCLUSIONS

These results increase our understanding of CHYR genes and provide robust candidate genes for further functional investigations aimed at crop improvement.

Zinc-finger protein MdBBX7/MdCOL9, a target of MdMIEL1 E3 ligase, confers drought tolerance in apple

Water deficit is one of the main challenges for apple (Malus × domestica) growth and productivity. Breeding drought-tolerant cultivars depends on a thorough understanding of the drought responses of apple trees. Here, we identified the zinc-finger protein B-BOX 7/CONSTANS-LIKE 9 (MdBBX7/MdCOL9), which plays a positive role in apple drought tolerance. The overexpression of MdBBX7 enhanced drought tolerance, whereas knocking down MdBBX7 expression reduced it. Chromatin immunoprecipitation-sequencing (ChIP-seq) analysis identified one cis-element of MdBBX7, CCTTG, as well as its known binding motif, the T/G box. ChIP-seq and RNA-seq identified 1,197 direct targets of MdBBX7, including ETHYLENE RESPONSE FACTOR (ERF1), EARLY RESPONSIVE TO DEHYDRATION 15 (ERD15), and GOLDEN2-LIKE 1 (GLK1) and these were further verified by ChIP-qPCR and electronic mobility shift assays. Yeast two-hybrid screen identified an interacting protein of MdBBX7, RING-type E3 ligase MYB30-INTERACTING E3 LIGASE 1 (MIEL1). Further examination revealed that MdMIEL1 could mediate the ubiquitination and degradation of MdBBX7 by the 26S proteasome pathway. Genetic interaction analysis suggested that MdMIEL1 acts as an upstream factor of MdBBX7. In addition, MdMIEL1 was a negative regulator of the apple drought stress response. Taken together, our results illustrate the molecular mechanisms by which the MdMIEL1-MdBBX7 module influences the response of apple to drought stress.

The ARRE RING-Type E3 Ubiquitin Ligase Negatively Regulates Cuticular Wax Biosynthesis in Arabidopsis thaliana by Controlling ECERIFERUM1 and ECERIFERUM3 Protein Levels

The outer epidermal cell walls of plant shoots are covered with a cuticle, a continuous lipid structure that provides protection from desiccation, UV light, pathogens, and insects. The cuticle is mostly composed of cutin and cuticular wax. Cuticular wax synthesis is synchronized with surface area expansion during plant development and is associated with plant responses to biotic and abiotic stresses. Cuticular wax deposition is tightly regulated by well-established transcriptional and post-transcriptional regulatory mechanisms, as well as post-translationally via the ubiquitin-26S proteasome system (UPS). The UPS is highly conserved in eukaryotes and involves the covalent attachment of polyubiquitin chains to the target protein by an E3 ligase, followed by the degradation of the modified protein by the 26S proteasome. A large number of E3 ligases are encoded in the Arabidopsis genome, but only a few have been implicated in the regulation of cuticular wax deposition. In this study, we have conducted an E3 ligase reverse genetic screen and identified a novel RING-type E3 ubiquitin ligase, AtARRE, which negatively regulates wax biosynthesis in Arabidopsis. Arabidopsis plants overexpressing AtARRE exhibit glossy stems and siliques, reduced fertility and fusion between aerial organs. Wax load and wax compositional analyses of AtARRE overexpressors showed that the alkane-forming branch of the wax biosynthetic pathway is affected. Co-expression of AtARRE and candidate target proteins involved in alkane formation in both Nicotiana benthamiana and stable Arabidopsis transgenic lines demonstrated that AtARRE controls the levels of wax biosynthetic enzymes ECERIFERUM1 (CER1) and ECERIFERUM3 (CER3). CER1 has also been confirmed to be a ubiquitination substrate of the AtARRE E3 ligase by an in vivo ubiquitination assay using a reconstituted Escherichia coli system. The AtARRE gene is expressed throughout the plant, with the highest expression detected in fully expanded rosette leaves and oldest stem internodes. AtARRE gene expression can also be induced by exposure to pathogens. These findings reveal that wax biosynthesis in mature plant tissues and in response to pathogen infection is controlled post-translationally.

Alternative splicing regulation appears to play a crucial role in grape berry development and is also potentially involved in adaptation responses to the environment

BACKGROUND

Alternative splicing (AS) produces transcript variants playing potential roles in proteome diversification and gene expression regulation. AS modulation is thus essential to respond to developmental and environmental stimuli. In grapevine, a better understanding of berry development is crucial for implementing breeding and viticultural strategies allowing adaptation to climate changes. Although profound changes in gene transcription have been shown to occur in the course of berry ripening, no detailed study on splicing modifications during this period has been published so far. We report here on the regulation of gene AS in developing berries of two grapevine (Vitis vinifera L.) varieties, Gewurztraminer (Gw) and Riesling (Ri), showing distinctive phenotypic characteristics. Using the software rMATS, the transcriptomes of berries at four developmental steps, from the green stage to mid-ripening, were analysed in pairwise comparisons between stages and varieties.

RESULTS

A total of 305 differential AS (DAS) events, affecting 258 genes, were identified. Interestingly, 22% of these AS events had not been reported before. Among the 80 genes that underwent the most significant variations during ripening, 22 showed a similar splicing profile in Gw and Ri, which suggests their involvement in berry development. Conversely, 23 genes were subjected to splicing regulation in only one variety. In addition, the ratios of alternative isoforms were different in Gw and Ri for 35 other genes, without any change during ripening. This last result indicates substantial AS differences between the two varieties. Remarkably, 8 AS events were specific to one variety, due to the lack of a splice site in the other variety. Furthermore, the transcription rates of the genes affected by stage-dependent splicing regulation were mostly unchanged, identifying AS modulation as an independent way of shaping the transcriptome.

CONCLUSIONS

The analysis of AS profiles in grapevine varieties with contrasting phenotypes revealed some similarity in the regulation of several genes with developmental functions, suggesting their involvement in berry ripening. Additionally, many splicing differences were discovered between the two varieties, that could be linked to phenotypic specificities and distinct adaptive capacities. Together, these findings open perspectives for a better understanding of berry development and for the selection of grapevine genotypes adapted to climate change.

Transcriptome-Wide Analysis and Functional Verification of RING-Type Ubiquitin Ligase Involved in Tea Plant Stress Resistance

The ubiquitin/26S proteasome pathway is a critical protein-degradation pathway in plant growth and development as well as in nearly all biological and abiotic stress processes. Although as a member of the ubiquitin/26S proteasome pathway, the E3 ubiquitin ligase family has been shown to be essential for the selective degradation of downstream target proteins, it has been rarely reported in tea plants (Camellia sinensis). In this study, through database searches and extensive manual deduplication, 335 RING finger family proteins were selected from the Tea Plant Information Archive. These proteins were divided into six categories by the difference of RING finger domain: RING-H2, RING-HCa, RING-HCb, RING-C2, RING-v, and RING-G. Stress-induced differential gene expression analysis showed that 53 proteins in RING finger family can respond to selected exogenous stress. In vitro ubiquitination assays indicated that TEA031033, which was named CsMIEL1, exhibited the activity of E3 ubiquitin ligases. CsMIEL1-overexpressing transgenic Arabidopsis thaliana seedlings were resistant to some exogenous abiotic stresses, such as salt and drought stress but sensitive to exogenous methyl jasmonate treatment. Furthermore, CsMIEL1 reduced the accumulation of anthocyanin in transgenic plants in response to low temperature treatment. The results of this article provide basic date for studying the role of ubiquitin/26S proteasome pathway in tea plants response to stresses.

Early transcriptional responses in Solanum peruvianum and Solanum lycopersicum account for different acclimation processes during water scarcity events

Cultivated tomato Solanum lycopersicum (Slyc) is sensitive to water shortages, while its wild relative Solanum peruvianum L. (Sper), an herbaceous perennial small shrub, can grow under water scarcity and soil salinity environments. Plastic Sper modifies the plant architecture when suffering from drought, which is mediated by the replacement of leaf organs, among other changes. The early events that trigger acclimation and improve these morphological traits are unknown. In this study, a physiological and transcriptomic approach was used to understand the processes that differentiate the response in Slyc and Sper in the context of acclimation to stress and future consequences for plant architecture. In this regard, moderate (MD) and severe drought (SD) were imposed, mediating PEG treatments. The results showed a reduction in water and osmotic potential during stress, which correlated with the upregulation of sugar and proline metabolism-related genes. Additionally, the senescence-related genes FTSH6 protease and asparagine synthase were highly induced in both species. However, GO categories such as "protein ubiquitination" or "endopeptidase inhibitor activity" were differentially enriched in Sper and Slyc, respectively. Genes related to polyamine biosynthesis were induced, while several cyclins and kinetin were downregulated in Sper under drought treatments. Repression of photosynthesis-related genes was correlated with a higher reduction in the electron transport rate in Slyc than in Sper. Additionally, transcription factors from the ERF, WRKY and NAC families were commonly induced in Sper. Although some similar responses were induced in both species under drought stress, many important changes were detected to be differentially induced. This suggests that different pathways dictate the strategies to address the early response to drought and the consequent episodes in the acclimation process in both tomato species.

Apple B-box protein BBX37 regulates jasmonic acid mediated cold tolerance through the JAZ-BBX37-ICE1-CBF pathway and undergoes MIEL1-mediated ubiquitination and degradation

The plant hormone jasmonic acid (JA) is involved in the cold stress response, and the inducer of CBF expression 1 (ICE1)- C-repeat binding factor (CBF) regulatory cascade plays a key role in the regulation of cold stress tolerance. In this study, we showed that a novel B-box (BBX) protein MdBBX37 positively regulates JA-mediated cold-stress resistance in apple. We found that MdBBX37 bound to the MdCBF1 and MdCBF4 promoters to activate their transcription, and also interacted with MdICE1 to enhance the transcriptional activity of MdICE1 on MdCBF1, thus promoting its cold tolerance. Two JA signaling repressors, MdJAZ1 and MdJAZ2 (JAZ, JAZMONATE ZIM-DOMAIN), interacted with MdBBX37 to repress the transcriptional activity of MdBBX37 on MdCBF1 and MdCBF4, and also interfered with the interaction between MdBBX37 and MdICE1, thus negatively regulating JA-mediated cold tolerance. E3 ligase MdMIEL1 (MIEL1, MYB30-Interacting E3 Ligase1) reduced MdBBX37-improved cold resistance by mediating ubiquitination and degradation of the MdBBX37 protein. The data reveal that MIEL1 and JAZ proteins co-regulate JA-mediated cold stress tolerance through the BBX37-ICE1-CBF module in apple. These results will aid further examination of the post-translational modification of BBX proteins and the regulatory mechanism of JA-mediated cold stress tolerance.

Arabidopsis RING-type E3 ubiquitin ligase XBAT35.2 promotes proteasome-dependent degradation of ACD11 to attenuate abiotic stress tolerance

Plants employ multiple mechanisms to cope with a constantly changing and challenging environment, including using the ubiquitin proteasome system (UPS) to alter their proteome to assist in initiating, modulating and terminating responses to stress. We previously reported that the ubiquitin ligase XBAT35.2 mediates the proteasome-dependent degradation of Accelerated Cell Death 11 (ACD11) to promote pathogen defense. Here, we demonstrate roles for XBAT35.2 and ACD11 in abiotic stress tolerance. As seen in response to pathogen infection, abiotic stress stabilizes XBAT35.2 and the abundance of ACD11 rose consistently with increasing concentrations of abscisic acid (ABA) and salt. Surprisingly, exposure to ABA and salt increased the stability of ACD11, and the overexpression of ACD11 improves plant survival of salt and drought stress, suggesting a role for ACD11 in promoting tolerance. Prolonged exposure to high concentrations of ABA or salt resulted in ubiquitination and the proteasome-dependent degradation of ACD11, however. The stress-induced turnover of ACD11 requires XBAT35.2, as degradation is slowed in the absence of the E3 ubiquitin ligase. Consistent with XBAT35.2 mediating the proteasome-dependent degradation of ACD11, the loss of E3 ubiquitin ligase function enhances the tolerance of salt and drought stress, whereas overexpression increases sensitivity. A model is presented where, upon the perception of abiotic stress, ACD11 abundance increases to promote tolerance. Meanwhile, XBAT35.2 accumulates and in turn promotes the degradation of ACD11 to attenuate the stress response. The results characterize XBAT35.2 as an E3 ubiquitin ligase with opposing roles in abiotic and biotic stress.

Cuticular waxes-A shield of barley mutant in CBP20 (Cap-Binding Protein 20) gene when struggling with drought stress

CBP20 (Cap-Binding Protein 20) encodes a small subunit of nuclear Cap-Binding Complex (nCBC) that together with CBP80 binds mRNA cap. We previously described barley hvcbp20.ab mutant that demonstrated higher leaf water content and faster stomatal closure than the WT after drought stress. Hence, we presumed that the better water-saving mechanism in hvcbp20.ab may result from the lower permeability of epidermis that together with stomata action limit the water evaporation under drought stress. We asked whether hvcbp20.ab exhibited any differences in wax load on the leaf surface when subjected to drought in comparison to WT cv. 'Sebastian'. To address this question, we investigated epicuticular wax structure and chemical composition under drought stress in hvcbp20.ab mutant and its WT. We showed that hvcbp20.ab mutant exhibited the increased deposition of cuticular wax. Moreover, our gene expression results suggested a role of HvCBP20 as a negative regulator of both, the biosynthesis of waxes at the level of alkane-forming, and waxes transportation. Interestingly, we also observed increased wax deposition in Arabidopsis cbp20 mutant exposed to drought, which allowed us to describe the CBP20-regulated epicuticular wax accumulation under drought stress in a wider evolutionarily context.

An apple MYB transcription factor regulates cold tolerance and anthocyanin accumulation and undergoes MIEL1-mediated degradation

MYB transcription factors (TFs) have been demonstrated to play diverse roles in plant growth and development through interaction with basic helix-loop-helix (bHLH) TFs. MdbHLH33, an apple bHLH TF, has been identified as a positive regulator in cold tolerance and anthocyanin accumulation by activating the expressions of MdCBF2 and MdDFR. In the present study, a MYB TF MdMYB308L was found to also positively regulate cold tolerance and anthocyanin accumulation in apple. We found that MdMYB308L interacted with MdbHLH33 and enhanced its binding to the promoters of MdCBF2 and MdDFR. In addition, an apple RING E3 ubiquitin ligase MYB30-INTERACTING E3 LIGASE 1 (MdMIEL1) was identified to be an MdMYB308L-interacting protein and promoted the ubiquitination degradation of MdMYB308L, thus negatively regulated cold tolerance and anthocyanin accumulation in apple. These results suggest that MdMYB308L acts as a positive regulator in cold tolerance and anthocyanin accumulation in apple by interacting with MdbHLH33 and undergoes MdMIEL1-mediated protein degradation. The dynamic change in MYB-bHLH protein complex seems to play a key role in the regulation of plant growth and development.

AP2/DREB Transcription Factor RAP2.4 Activates Cuticular Wax Biosynthesis in Arabidopsis Leaves Under Drought

Drought is a critical environmental stress that limits growth and development of plants and reduces crop productivity. The aerial part of land plants is covered with cuticular waxes to minimize water loss. To understand the regulatory mechanisms underlying cuticular wax biosynthesis in Arabidopsis under drought stress conditions, we characterized the role of an AP2/DREB type transcription factor, RAP2.4. RAP2.4 expression was detected in one-week-old seedlings and rosette leaves, stems, stem epidermis, cauline leaves, buds, flowers, and siliques of 6-week-old Arabidopsis. The levels of RAP2.4 transcripts increased with treatments of abscisic acid (ABA), mannitol, NaCl, and drought stress. Under drought, total wax loads decreased by approximately 11% and 10%, and in particular, the levels of alkanes, which are a major wax component, decreased by approximately 11% and 12% in rap2.4-1 and rap2.4-2 leaves, respectively, compared with wild type (WT) leaves. Moreover, the transcript levels of cuticular wax biosynthetic genes, KCS2 and CER1, decreased by approximately 15-23% and 32-40% in rap2.4-1 and rap2.4-2 leaves, respectively, relative to WT 4 h after drought treatment, but increased by 2- to 12-fold and 3- to 70-fold, respectively, in three independent RAP2.4 OX leaves relative to WT. Epicuticular wax crystals were observed on the leaves of RAP2.4 OX plants, but not on the leaves of WT. Total wax loads increased by 1.5- to 3.3-fold in leaves of RAP2.4 OX plants relative to WT. Cuticular transpiration and chlorophyll leaching occurred slowly in the leaves of RAP2.4 OX plants relative to WT. Transcriptional activation assay in tobacco protoplasts showed that RAP2.4 activates the expression of KCS2 and CER1 through the involvement of the consensus CCGAC or GCC motifs present in the KCS2 and CER1 promoter regions. Overall, our results revealed that RAP2.4 is a transcription factor that activates cuticular wax biosynthesis in Arabidopsis leaves under drought stress conditions.

Transcriptomic and Gas Chromatography-Mass Spectrometry Metabolomic Profiling Analysis of the Epidermis Provides Insights into Cuticular Wax Regulation in Developing 'Yuluxiang' Pear Fruit

The layer of cuticular wax covering fruits plays important roles in protecting against disease, preventing non-stomatal water loss, and extending shelf life. However, the molecular basis of cuticular wax biosynthesis in pear (Pyrus) fruits remains elusive. Our study thoroughly investigates cuticular wax biosynthesis during pear fruit development from morphologic, transcriptomic, and gas chromatography-mass spectrometry metabolomic perspectives. Our results showed that cuticular wax concentrations increased during the early stage [20-80 days after full bloom (DAFB)] from 0.64 mg/cm² (50 DAFB) to 1.75 mg/cm² (80 DAFB) and then slightly decreased to 1.22 mg/cm² during the fruit ripening period (80-140 DAFB). Scanning electron microscopy imaging indicated that wax plate crystals increased and wax structures varied during the pear fruit development. The combined transcriptomic and metabolomic profiling analysis revealed 27 genes, including 12 genes encoding transcription factors and a new structural gene (Pbr028523) encoding β-amyrin synthase, participating in the biosynthesis, transport, and regulation of cuticular wax according to their expression patterns in pear fruit. The quantitative real-time polymerase chain reaction experiments of 18 differentially expressed genes were performed and confirmed the accuracy of the RNA-Seq-derived transcript expression. A model of VLCFAs and cuticular wax synthesis and transport in pear fruit is proposed, providing a mechanistic framework for understanding cuticular wax biosynthesis in pear fruit. These results and data sets provide a foundation for the molecular events related to cuticular wax in 'Yuluxiang' pear fruit and may also help guide the functional analyses of candidate genes important for improving the cuticular wax of pear fruit in the future.