Yellowhorn (Xanthoceras sorbifolium) 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 the electrophoretic mobility shift assay, the yeast one hybrid assay and the dual-luciferase assay demonstrated that XsMYB30 could directly and $$ 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.

Drought is a key factor that limits plant growth and yield. Yellowhorn is an important and vigorously promoted oil tree in China. It can survive under certain drought conditions, but a lack of water severely restricts its growth and results in yield losses in arid and semi-arid areas. Therefore, it is important to identify the key pathways and genes to understand the mechanisms of its drought resistance. In this study, we evaluated drought resistance in four types of yellowhorn, and obtained 2669 and 2451 differentially expressed genes (DEGs) via the transcriptome analysis of the comparison of water-saving/water-consuming and fast-growing/slow-growing yellowhorn, respectively, under long-term drought conditions. The gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses of DEGs showed the key biological processes and metabolic pathways involved in drought resistance, which demonstrated that there are both the same and different biological processes involved in regulating water use efficiency (WUE) and growth in response to drought stress. Furthermore, the network analysis indicated hub genes (especially seven protein kinases) and potential co-expressed gene clusters in a greenyellow module associated with WUE and a blue module associated with growth. These identified hub genes and key biological processes can significantly enhance our knowledge about the molecular mechanisms of drought resistance in yellowhorn.

The β-ketoacyl CoA synthase encoded by the KCS genes is a rate-limiting enzyme for the synthesis of very-long-chain fatty acid (VLCFA), which catalyzes the VLCFA elongation. Yellow horn (Xanthoceras sorbifolium) is a horticultural tree species known for its kernel oil, which has strong resistance to drought, cold, high temperature, and saline-alkali. The conserved domain FAE1-CUT1-RPPA and ACP-syn-III_C of the KCS gene family were used to search the KCS sequences across the whole genomic sequence of yellow horn; a total of 20 XsKCS genes were identified and divided into four subfamilies. The conserved motif and transmembrane structure analysis revealed that most XsKCSs had a conserved transmembrane domain except XsKCS10 and XsKCS20. The prediction of cis-acting elements of XsKCS genes showed that XsKCS genes contained many stress and hormone response elements, such as ABRE, MBS, and LTR. Furthermore, XsKCS genes exhibited differential expression profiles under abiotic stress and stress-related hormone treatment conditions. Transcriptomic data showed that XsKCS1, XsKCS11, and XsKCS17 had higher expression in yellow horn with high leaf cuticular wax, indicating that they may function in the cuticular wax accumulation and drought response. This study laid a foundation for further functional verification of XsKCS genes in yellow horn stress response.