Ploidy levels influence cold tolerance of Cyclocarya paliurus: insight into the roles of WRKY genes.

Cold stress in winter is one of the most severe abiotic stresses on plant growth and flourishing, and the selection of cold tolerant genotypes is an important strategy to ensure the safety of plant growth and development. Cyclocarya paliurus, a diclinous and versatile tree species originally in subtropical regions, has been introduced and cultivated in the warm temperate zone of China to meet the increasing market demand for its leaf yield. However, information regarding its cold tolerance remains limited. Based on the ploidy identification of tested materials, an imitation experiment was conducted to investigate the variation in freezing injury index and expression of the CpaWRKY family members in diploid and tetraploid C. paliurus seedlings. The results indicated a significant difference in freezing injury index between diploids and tetraploids under the imitating temperature of southern warm temperate zone, with diploids showing better cold tolerance than the tetraploids. A total of 88 CpaWRKY genes were identified from the C. paliurus genome, and RNA-Seq results showed significant differences in WRKY gene expression in C. paliurus under cold stress. Correlation analysis between differentially expressed genes and freezing injury index suggested that CpaWRKY14, CpaWRKY26 and CpaWRKY86 play essential roles in the diploids to respond to cold stress. In contrast, the major genes involved in the cold stress response in tetraploids were CpaWRKY14, CpaWRKY60, CpaWRKY63 and CpaWRKY81. Moreover, CpaWRKY14 expression was considerably higher in diploids compared to tetraploids. The results from this study not only enhance our comprehension of the role of the CpaWRKY genes in cold stress, but also provide a foundation for the genetic improvement of C. paliurus.