Genome-Wide Survey and Expression Analysis of B-Box Family Genes in Cucumber Reveal Their Potential Roles in Response to Diverse Abiotic and Biotic Stresses

As a class of zinc finger transcription factors, B-box (BBX) proteins play diverse roles in numerous biological processes, and they have been identified in a series of plant species in recent years. However, the roles of BBX genes in regulating cucumber growth regulation and stress response have not yet been established. Here, a total of 22 BBX family genes were identified via an analysis of the latest cucumber genome data, which were classified into five groups (I–V) on the basis of their phylogenetic features and number of B-box domains and CCT domains. The CsBBX genes were unevenly distributed across the seven cucumber chromosomes, and segmental duplication was found to play a significant role in the expansion of the cucumber BBX gene family. Gene structure and motif composition analysis suggested that the evolutionarily close CsBBXs have similar conserved motif composition and gene structure. Most CsBBX genes possessed 1–3 introns, and intron gain rather than intron loss could contribute to the different structures of CsBBX genes across different groups during their evolution. Promoter analysis revealed the presence of 13 kinds of hormone-related and nine kinds of stress-related cis -regulatory elements in the promoter regions of these CsBBX genes. Expression analysis via RNA-seq and qRT-PCR suggested that the CsBBX genes exhibit differential expression in different tissues and in response to various abiotic and biotic stresses. This work constitutes a starting point for further revealing the function of the CsBBX genes and sheds light on the potential molecular mechanism of stress resistance in cucumber.