Genome-wide identification, classification, and expression analysis of sHSP genes in Chinese cabbage (Brassica rapa...

Small heat shock proteins (sHSPs) are essential for the plant's normal development and stress responses, especially the heat stress response. The information regarding sHSP genes in Chinese cabbage (Brassica rapa ssp pekinensis) is sparse, hence we performed a genome-wide analysis to identify sHSP genes in this species. We identified 26 non-redundant sHSP genes distributed on all chromosomes, except chromosome A7, with one additional sHSP gene identified from an expressed sequence tag library. Chinese cabbage was found to contain more sHSP genes than Arabidopsis. The 27 sHSP genes were classified into 11 subfamilies. We identified 22 groups of sHSP syntenic orthologous genes between Chinese cabbage and Arabidopsis. In addition, eight groups of paralogous genes were uncovered in Chinese cabbage. Protein structures of the 27 Chinese cabbage sHSPs were modeled using Phyre2, which revealed that all of them contain several conserved β strands across different subfamilies. In general, gene structure was conserved within each subfamily between Chinese cabbage and Arabidopsis, except for peroxisome sHSP. Analysis of promoter motifs showed that most sHSP genes contain heat shock elements or variants. We also found that biased gene loss has occurred during the evolution of the sHSP subfamily in Chinese cabbage. Expression analysis indicated that the greatest transcript abundance of most Chinese cabbage sHSP genes was found in siliques and early cotyledon embryos. Thus, genome-wide identification and characterization of sHSP genes is a first and important step in the investigation of sHSPs in Chinese cabbage.