Analysis of Expansin genes in Cucurbitaceae and identification of candidate genes for stress resistance

Expansin (EXP) genes are critical for plant growth and abiotic stress adaptation, yet their evolutionary and functional dynamics in Cucurbitaceae remain poorly characterized. Here, we systematically identified 136 EXP genes across four Cucurbitaceae species (Cucurbita argyrosperma, Cucurbita pepo, Citrullus lanatus, Lagenaria siceraria) and the transcriptome of 'Yayao' (L. siceraria). Phylogenetic analysis classified these genes into four subfamilies (EXPA, EXPB, EXLA, EXLB), with EXPA predominating and EXPB notably absent in 'Yayao' (L. siceraria). Physicochemical profiling revealed EXP proteins as hydrophilic (84.66%) with isoelectric points (pI 8-10), indicative of basic properties. Cis-regulatory element analysis identified ABRE and TCA-elements as highly enriched, linking EXP genes to hormone signaling, cell wall integrity, and stress responses. Protein interaction networks highlighted functional associations with cell wall-modifying enzymes (e.g., XTHs, GH9) and developmental regulators. Tissue-specific expression profiling in 'Yayao' (L. siceraria) showed predominant EXP expression in leaves, with minimal activity in flowers. Under cold, drought, and salt stresses, four genes (LSEXPA2, LSEXPA3, LSEXPA7, LSEXPA13) exhibited marked upregulation, suggesting roles in stress tolerance. This study elucidates the evolutionary diversification of EXP genes in Cucurbitaceae and identifies candidate targets for enhancing stress resilience in L. siceraria breeding programs.