A DUF21 domain–containing protein regulates plant dwarfing in watermelon

Dwarf or semidwarf plant structures are well suited for intensive farming, maximizing yield, and minimizing labor costs. Watermelon (Citrullus lanatus) is classified as an annual vine plant with elongated internodes, yet the mechanism governing watermelon dwarfing remains unclear. In this study, a compact watermelon mutant dwarf, induced by the insertion of transferred DNA (T-DNA), was discovered. Through resequencing, a gene named domain of unknown function 21 (ClDUF21), located downstream of the T-DNA insertion site, was identified as the candidate gene for the dwarf mutant, and its functionality was subsequently confirmed. Watermelon mutants generated through CRISPR/Cas9-mediated knockout of ClDUF21 revealed that homozygous mutants displayed a pronounced dwarfing phenotype, and protein-protein interaction analysis confirmed the direct interaction between ClDUF21 and ClDWF1. Subsequently, we employed CRISPR/Cas9 technology to precisely modify the homologous gene CsDUF21 in cucumber (Cucumis sativus) and performed protein interaction validation between CsDUF21 and CsDWF1, thereby demonstrating that the CsDUF21 gene also exhibits analogous functionality in plant dwarfing. These findings demonstrate that ClDUF21 governs plant dwarfism by modulating the brassinosteroid synthesis pathway via ClDWF1.