Genome-wide identification of HXK gene families in cucumber and watermelon: CsHXK1 and ClHXK6 play key roles in responding to multiple stresses

In this study, six HXK genes were identified in both the cucumber (CsHXK1-CsHXK6) and watermelon (ClHXK1-ClHXK6) genomes. Phylogenetic analysis classified these genes into two types (Type A and Type B). Analyses of gene structure and conserved motifs confirmed high conservation among family members. Collinearity analysis further revealed that segmental duplication was the main driver of HXK family expansion in these two crops. Additionally, cis-acting element analysis showed that the promoter regions of CsHXK and ClHXK genes contained abundant stress-responsive elements. Using 7 publicly available cucumber transcriptome datasets and 14 publicly available watermelon transcriptome datasets, we analyzed the tissue-specific expression patterns of CsHXK and ClHXK genes, as well as their expression responses to abiotic and biotic stresses. The results showed that CsHXK1 was highly expressed in cucumber roots and flowers, while ClHXK6 exhibited broad expression across multiple watermelon tissues. Functionally, CsHXK1 was significantly up-regulated under five stress conditions (salt, waterlogging, downy mildew, powdery mildew, and angular leaf spot), and ClHXK6 showed significant differential expression under six stress conditions (salt, drought, osmotic stress, Fusarium wilt-1, Fusarium wilt-3, and squash vein yellowing virus infection). qRT-PCR validation was performed for CsHXK1 under salt stress and for ClHXK6 under salt and drought stresses, and the results consistently supported the transcriptome data, confirming the critical roles of CsHXK1 and ClHXK6 in mediating stress responses in cucumber and watermelon, respectively. This study represents the first comprehensive and systematic identification of the HXK gene family in cucumber and watermelon, coupled with an analysis of their expression patterns under diverse abiotic and biotic stresses. It provides a theoretical basis for in-depth exploration of HXK gene functions and offers valuable candidate genes (CsHXK1 and ClHXK6) for improving stress resistance in cucumber and watermelon breeding programs.