Transcription Factor TaMYB30 Activates Wheat Wax Biosynthesis

Wheat WW Domain-Containing Protein TaCFL1 Negatively Regulates Cuticular Wax Biosynthesis

Waxy cuticle covers plant aerial organs and protects plants against environmental challenges. Although improved cuticle-associated traits are aimed at the wheat breeding programs, the mechanism governing wheat cuticular wax biosynthesis remains to be elucidated. Herein, wheat WW domain-containing protein TaCFL1 is characterized as a negative regulator of wax biosynthesis. The knockdown of TaCFL1 expression results in a 15% increase in wax accumulation and decreased leaf cuticle permeability in bread wheat. Furthermore, wheat class IV homeodomain transcription factors TaHDG1.1 and TaHDG1.2 are identified as partially redundant activators of wax biosynthesis. The silencing of TaHDG1.1 or TaHDG1.2 expression leads to an 11% reduction in epidermal wax accumulation and an increase in leaf cuticle permeability wax, while the co-silencing of TaHDG1.1 and TaHDG1.2 results in a 31% reduction in epidermal wax accumulation and a further increase in wax in the leaf cuticle permeability. Moreover, wheat 3-Ketoacyl-CoA synthase TaKCS10 is isolated as an essential component of the wax biosynthetic machinery. The silencing of TaKCS10 expression results in a 22% reduction in wax accumulation and increased leaf cuticle permeability. In addition, we demonstrated that the TaKCS10 expression is activated by TaHDG1.1 and TaHDG1.2, and that TaCFL1 attenuates the TaHDG1-mediated transcriptional activation of TaKCS10. This evidence supports that the WW domain-containing protein TaCFL1 negatively regulates wax biosynthesis via attenuating the transcriptional activation of the TaKCS10 gene mediated by HD-ZIP IV transcription factor TaHDG1.

Cuticular wax in wheat: biosynthesis, genetics, and the stress response

All terrestrial plants possess a hydrophobic cuticle in the outermost layer of their aerial organs that is composed of cutin and wax. The cuticle serves as the first barrier between the plant and the surrounding environment and plays a key role in the resistance of plants to abiotic and biotic stressors. Additionally, they are closely associated with plant growth and development. Cuticular wax has attracted considerable attention as the main mediator of cuticular functions. In this review, we summarize the advances in the research investigating wheat cuticular wax, focusing on three aspects that include biosynthesis, genetics, and stress responses. Additionally, we discuss the applications of cuticular wax in wheat breeding.

Wheat Topoisomerase VI Positively Regulates the Biosynthesis of Cuticular Wax and Cutin

Lipophilic cuticles mainly composed of wax mixtures and cutin matrices seal the plant epidermis and control plant development and environmental adaptation. Although cuticle-associated traits have been selected in the breeding of agronomically important cereal bread wheat, the biosynthesis of wheat cuticular wax and cutin remains poorly understood. Herein, wheat topoisomerase VI was characterized as an essential activator of cuticular wax and cutin biosynthesis. Knock-down of wheat TaTOP6A, TaTOP6B, TaRHL1, or TaBIN4 gene encoding component of topoisomerase VI resulted in decreased loads of leaf cuticular wax and cutin, as well as increased leaf cuticle permeability. Moreover, TaCYP86A2 was identified as a key component of the wheat cutin biosynthetic machinery. Reduction of wheat TaCYP86A2 expression led to decreased cutin accumulation and enhanced cuticle permeability. In addition, TaTOP6A, TaTOP6B, TaRHL1, or TaBIN4 was shown to enrich at the promoter regions of the wax biosynthesis gene TaKCS1 and the cutin biosynthesis gene TaCYP86A2. Importantly, chromatin at TaKCS1 and TaCYP86A2 promoters is marked by high nucleosome occupancy and low histone acetylation in TaTOP6A-, TaTOP6B-, TaRHL1-, or TaBIN4-silenced wheat leaves. These results collectively support that wheat topoisomerase VI positively regulates the biosynthesis of cuticular wax and cutin probably via maintaining a permissive chromatin state at TaKCS1 and TaCYP86A2 genes.

The yellowhorn MYB transcription factor MYB30 is required for wax accumulation and drought tolerance

Yellowhorn (Xanthoceras sorbifolium Bunge) is an economically important tree species in northern China, mainly distributed in arid and semi-arid areas where water resources are scarce. Drought affects its yield and the expansion of its suitable growth area. It was found that the wax content in yellowhorn leaves varied significantly among different germplasms, which had a strong correlation with the drought resistance of yellowhorn. In this study, XsMYB30 was isolated from 'Zhongshi 4' of yellowhorn, a new highly waxy variety. DAP-Seq technology revealed that the pathways associated with fatty acids were significantly enriched in the target genes of XsMYB30. Moreover, the results of electrophoretic mobility shift assay, yeast one hybrid assay and dual-luciferase assay demonstrated that XsMYB30 could directly and specifically bind with the promoters of genes involved in wax biosynthesis (XsFAR4, XsCER1 and XsKCS1), lipid transfer (XsLTPG1 and XsLTP1) and fatty acid synthesis (XsKASIII), thus enhancing their expression. In addition, the overexpression of XsMYB30 in poplar promoted the expression levels of these target genes and increased the wax deposition on poplar leaves leading to a notable improvement in the plant's ability to withstand drought. These findings indicate that XsMYB30 is an important regulatory factor in cuticular wax biosynthesis and the drought resistance of yellowhorn.

Wheat Transcription Factor TaMYB60 Modulates Cuticular Wax Biosynthesis by Activating TaFATB and TaCER1 Expression

Cuticular wax mixtures cover the epidermis of land plants and shield plant tissues from abiotic and biotic stresses. Although cuticular wax-associated traits are employed to improve the production of bread wheat, regulatory mechanisms underlying wheat cuticular wax biosynthesis remain poorly understood. In this research, partially redundant transcription factors TaMYB60-1 and TaMYB60-2 were identified as positive regulators of wheat cuticular wax biosynthesis. Knock-down of wheat TaMYB60-1 and TaMYB60-2 genes by virus-induced gene silencing resulted in attenuated wax accumulation and enhanced cuticle permeability. The roles of wheat fatty acyl-ACP thioesterase genes TaFATB1 and TaFATB2 in cuticular wax biosynthesis were characterized. Silencing wheat TaFATB1 and TaFATB2 genes led to reduced wax accumulation and increased cuticle permeability, suggesting that TaFATB1 and TaFATB2 genes positively contribute to wheat cuticular wax biosynthesis. Importantly, transcription factors TaMYB60-1 and TaMYB60-2 exhibit transcriptional activation ability and could stimulate the expression of wax biosynthesis genes TaFATB1, TaFATB2, and ECERIFERUM 1 (TaCER1). These findings support that transcription factor TaMYB60 positively regulates wheat cuticular wax biosynthesis probably by activating transcription of TaFATB1, TaFATB2, and TaCER1 genes.

Wheat MIXTA-like Transcriptional Activators Positively Regulate Cuticular Wax Accumulation

MIXTA-like transcription factors AtMYB16 and AtMYB106 play important roles in the regulation of cuticular wax accumulation in dicot model plant Arabidopsis thaliana, but there are very few studies on the MIXTA-like transcription factors in monocot plants. Herein, wheat MIXTA-like transcription factors TaMIXTA1 and TaMIXTA2 were characterized as positive regulators of cuticular wax accumulation. The virus-induced gene silencing experiments showed that knock-down of wheat TaMIXTA1 and TaMIXTA2 expressions resulted in the decreased accumulation of leaf cuticular wax, increased leaf water loss rate, and potentiated chlorophyll leaching. Furthermore, three wheat orthologous genes of ECERIFERUM 5 (TaCER5-1A, 1B, and 1D) and their function in cuticular wax deposition were reported. The silencing of TaCER5 by BSMV-VIGS led to reduced loads of leaf cuticular wax and enhanced rates of leaf water loss and chlorophyll leaching, indicating the essential role of the TaCER5 gene in the deposition of wheat cuticular wax. In addition, we demonstrated that TaMIXTA1 and TaMIXTA2 function as transcriptional activators and could directly stimulate the transcription of wax biosynthesis gene TaKCS1 and wax deposition gene TaCER5. The above results strongly support that wheat MIXTA-Like transcriptional activators TaMIXTA1 and TaMIXTA2 positively regulate cuticular wax accumulation via activating TaKCS1 and TaCER5 gene transcription.

Chemical and Transcriptomic Analyses of Leaf Cuticular Wax Metabolism in Ammopiptanthus mongolicus under Osmotic Stress

Plant cuticular wax forms a hydrophobic structure in the cuticle layer covering epidermis as the first barrier between plants and environments. Ammopiptanthus mongolicus, a leguminous desert shrub, exhibits high tolerances to multiple abiotic stress. The physiological, chemical, and transcriptomic analyses of epidermal permeability, cuticular wax metabolism and related gene expression profiles under osmotic stress in A. mongolicus leaves were performed. Physiological analyses revealed decreased leaf epidermal permeability under osmotic stress. Chemical analyses revealed saturated straight-chain alkanes as major components of leaf cuticular wax, and under osmotic stress, the contents of total wax and multiple alkane components significantly increased. Transcriptome analyses revealed the up-regulation of genes involved in biosynthesis of very-long-chain fatty acids and alkanes and wax transportation under osmotic stress. Weighted gene co-expression network analysis identified 17 modules and 6 hub genes related to wax accumulation, including 5 enzyme genes coding KCS, KCR, WAX2, FAR, and LACS, and an ABCG transporter gene. Our findings indicated that the leaf epidermal permeability of A. mongolicus decreased under osmotic stress to inhibit water loss via regulating the expression of wax-related enzyme and transporter genes, further promoting cuticular wax accumulation. This study provided new evidence for understanding the roles of cuticle lipids in abiotic stress tolerance of desert plants.

Wheat Transcriptional Corepressor TaTPR1 Suppresses Susceptibility Genes TaDND1/2 and Potentiates Post-Penetration Resistance against Blumeria graminis forma specialis tritici

The obligate biotrophic fungal pathogen Blumeria graminis forma specialis tritici (B.g. tritici) is the causal agent of wheat powdery mildew disease. The TOPLESS-related 1 (TPR1) corepressor regulates plant immunity, but its role in regulating wheat resistance against powdery mildew remains to be disclosed. Herein, TaTPR1 was identified as a positive regulator of wheat post-penetration resistance against powdery mildew disease. The transient overexpression of TaTPR1.1 or TaTPR1.2 confers wheat post-penetration resistance powdery mildew, while the silencing of TaTPR1.1 and TaTPR1.2 results in an enhanced wheat susceptibility to B.g. tritici. Furthermore, Defense no Death 1 (TaDND1) and Defense no Death 2 (TaDND2) were identified as wheat susceptibility (S) genes facilitating a B.g. tritici infection. The overexpression of TaDND1 and TaDND2 leads to an enhanced wheat susceptibility to B.g. tritici, while the silencing of wheat TaDND1 and TaDND2 leads to a compromised susceptibility to powdery mildew. In addition, we demonstrated that the expression of TaDND1 and TaDND2 is negatively regulated by the wheat transcriptional corepressor TaTPR1. Collectively, these results implicate that TaTPR1 positively regulates wheat post-penetration resistance against powdery mildew probably via suppressing the S genes TaDND1 and TaDND2.

Comparative Transcriptome Analysis Provides Insights into the Effect of Epicuticular Wax Accumulation on Salt Stress in Coconuts

The coconut is an important tropical economical crop and exhibits high tolerance to various types of salinity stress. However, little is known about the molecular mechanism underlying its salt tolerance. In this study, RNA-Seq was applied to examine the different genes expressed in four coconut varieties when exposed to a salt environment, resulting in the generation of data for 48 transcriptomes. Comparative transcriptome analysis showed that some genes involved in cutin and wax biosynthesis were significantly upregulated in salt treatment compared to the control, including CYP86A4, HTH, CER1, CER2, CER3, DCR, GPAT4, LTP3, LTP4, and LTP5. In particular, the expression of CER2 was induced more than sixfold, with an RPKM value of up to 205 ten days after salt treatment in Hainan Tall coconut, demonstrating superior capacity in salt tolerance compared to dwarf coconut varieties. However, for yellow dwarf and red dwarf coconut varieties, the expression level of the CER2 gene was low at four different time points after exposure to salt treatment, suggesting that this gene may contribute to the divergence in salt tolerance between tall and dwarf coconut varieties. Cytological evidence showed a higher abundance of cuticle accumulation in tall coconut and severe damage to cuticular wax in dwarf coconut.