Genome-wide identification of small heat shock protein (HSP20) homologs in three cucurbit species and the expression profiles of CsHSP20s under several abiotic stresses

Small heat shock protein (HSP20) genes play important roles in biological processes of plants. In this study, a total of 47 CsHSP20 genes, 45 CmHSP20 genes, and 47 ClHSP20 genes were genome-wide identified by 'hmmsearch' and BLASTP using the latest versions of cucumber, melon, and watermelon genomes, respectively. According to the phylogenetic relationships and predicted subcellular localizations, HSP20s of these three cucurbit species were divided into 8 subfamilies (CI-CIV, CP, ER, M, and PX), in which some HSP20s were closely related with each other based on the collinearity analysis. Specific expression patterns of CsHSP20s were checked in 10 different tissues of cucumber plants. RNA-seq analysis of transcript levels, combined with cis-acting elements and GO enrichment analysis suggested that CsHSP20s were responsive to several different types of abiotic stresses, including chilling, temperature and photoperiod, high temperature and high humidity, and salinity. In conclusion, results of this work not only provided valuable information for exploring the regulating mechanisms of CsHSP20s in responding to abiotic stresses in cucumber, but also shed light on the potentially evolutional relations among cucumber, melon, and watermelon from a perspective of comparative genomics that specified on HSP20 gene families.