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 <i>BBX</i> genes in regulating cucumber growth regulation and stress response have not yet been established. Here, a total of 22 <i>BBX</i> 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 <i>CsBBX</i> 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 <i>BBX</i> gene family. Gene structure and motif composition analysis suggested that the evolutionarily close CsBBXs have similar conserved motif composition and gene structure. Most <i>CsBBX</i> genes possessed 1–3 introns, and intron gain rather than intron loss could contribute to the different structures of <i>CsBBX</i> genes across different groups during their evolution. Promoter analysis revealed the presence of 13 kinds of hormone-related and nine kinds of stress-related <i>cis</i>-regulatory elements in the promoter regions of these <i>CsBBX</i> genes. Expression analysis via RNA-seq and qRT-PCR suggested that the <i>CsBBX</i> 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 <i>CsBBX</i> genes and sheds light on the potential molecular mechanism of stress resistance in cucumber.