<i>BhSUN</i> coordinates calcium signaling, microtubule organization, and hormone metabolism to drive directional fruit growth in wax gourd

Abstract Fruit shape is a critical determinant of market value in wax gourd (Benincasa hispida), yet its genetic and mechanistic basis remains largely unknown. Here, we identify BhSUN, an IQD family protein containing an IQ67 domain, as a major regulator of fruit shape through quantitative trait locus mapping in a recombinant inbred line population. BhSUN harbors two non-synonymous single-nucleotide polymorphisms that define haplotypes strongly associated with long cylindrical (BhSUNCC-TT) versus spherical (BhSUNGG-GG) fruits. Near-isogenic line analysis, F₂ segregation, marker-assisted selection, and CRISPR/Cas9-mediated knockout consistently demonstrate that loss of BhSUN function causes a transition from long cylindrical to spherical fruits. Histological analyses reveal that BhSUN controls fruit morphology by coordinating oriented cell division and cell expansion. Protein interaction assays show that BhSUN protein interacts with calcium-bound calmodulin, calmodulin-like proteins, and the microtubule-associated protein MAP65–1, thereby linking calcium signaling to microtubule organization. Concurrently, Phytohormone analysis revealed changes in auxin, cytokinin, and gibberellin pathways during BhSUN-mediated fruit shape regulation. Our findings identify BhSUN as a key regulator that integrates calcium signaling, microtubule dynamics, and hormone metabolism to direct directional fruit morphogenesis.