Genome-Wide Identification, Characterization, Expression Analysis, and Interacting Protein Prediction of the GSK3/Shaggy-like Gene Family in Watermelon

Glycogen synthase kinase 3 (GSK3/Shaggy-like) is a highly conserved serine/threonine kinase that orchestrates growth, hormone signaling, and abiotic stress responses in both animals and plants, yet its role in watermelon remains unexplored. In this study, we conducted a whole-genome identification, identifying a total of eight members of the <i>GSK3</i> gene family (<i>ClGSK3</i>) distributed across seven chromosomes. Phylogenetic and synteny analyses resolved the eight <i>ClGSK3s</i> into four subfamilies that display one-to-one or one-to-many orthology with <i>Arabidopsis</i> and rice <i>GSK3</i> genes, indicating conserved genomic micro-collinearity across dicots and monocots. Predictions of cis-acting elements and transcriptome data analysis indicate that <i>ClGSK3s</i> may be involved in hormone- and stress-responsive conditions. Protein-protein interaction networks predicted 53 candidate partners for five ClGSK3 proteins; yeast two-hybrid assays subsequently confirmed that ClSK21 associates with three of them-orthologs of the core brassinosteroid (BR)-signaling components BKI1 and BZR1. qRT-PCR revealed that <i>ClSK21</i>, <i>ClSK31</i>, and <i>ClSK41</i> are rapidly and significantly reprogrammed by BR treatment. Collectively, our data suggest that <i>ClGSK3s</i> modulate fruit development and stress tolerance by integrating hormone-related pathways, especially BR signaling. Future studies are encouraged to integrate genetics and multi-omics approaches to systematically validate the roles of <i>ClGSK3s</i> in hormone signaling and abiotic stress responses.