Wax gourd is a widely cultivated cucurbit crop valued for its culinary and medicinal uses, and fruit shape is a key trait influencing commercial quality and consumer preference. Long-term domestication and regional market selection have produced a wide spectrum of fruit shapes, from elongated cylinders to nearly perfect spheres. In cucumber and tomato, researchers have identified several genes that control fruit shape, including SUN genes that encode IQ67-domain proteins. However, the molecular mechanisms by which these genes translate genetic variation into distinct fruit morphologies remain poorly defined in wax gourd. Due to these challenges, there is a need for in-depth investigation into the regulatory networks that govern fruit morphogenesis in this crop.
A team led by researchers at Guangxi University in China has published (DOI: 10.1093/hr/uhag190) the findings in Horticulture Research, an open-access journal of Nanjing Agricultural University published by Oxford University Press. The study, available online in 2026, identifies the BhSUN gene as a central determinant of fruit shape and demonstrates how it integrates calcium signaling, microtubule organization, and hormone metabolism to direct directional fruit growth.
The researchers developed a recombinant inbred line (RIL) population by crossing a long-cylindrical-fruit variety with a spherical-fruit variety and mapped a major-effect quantitative trait locus (QTL) on chromosome 2. Within this region, they pinpointed BhSUN, a gene carrying two natural DNA changes that define two haplotypes: one associated with long cylindrical fruits and the other with spherical fruits. Using CRISPR/Cas9 genome editing, the team knocked out BhSUN in long-fruited plants and watched the fruits transform into a round shape—fruit length dropped by over 60% while diameter increased by more than 50%, flipping the shape index from 4.6 to nearly 1.0. At the cellular level, the mutant fruits showed drastically fewer cells along the long axis but maintained normal cell numbers in the transverse direction, while individual cells became larger overall—evidence that BhSUN controls both the orientation of cell division and the extent of cell expansion. At the molecular level, the BhSUN protein was found to interact with calcium-sensing proteins—calmodulin (CaM) and calmodulin-like (CML) proteins—and with a microtubule-associated protein called MAP65-1, directly linking calcium signals to the structural machinery that determines cell growth direction. The study also uncovered changes in auxin, cytokinin, and gibberellin pathways, with exogenous application of a cytokinin compound, cis-zeatin riboside (cZR), promoting fruit elongation.
The authors said that finding a single gene that controls such a dramatic switch in fruit shape was unexpected. “We were surprised that knocking out just one gene could flip a long cylinder into a near-perfect sphere,” they explained. “What makes this even more interesting is that BhSUNdoesn't work alone—it sits at the intersection of calcium signaling, the microtubule cytoskeleton, and hormone pathways, coordinating all three to decide which way the fruit grows.” They added that the two natural mutations they identified fall within a conserved domain of the protein, suggesting that this region is critical for function and may represent a common mechanism by which SUN genes shape fruits across different species.
The findings provide a precise genetic target for breeding wax gourd varieties with tailored fruit shapes to meet diverse market demands—from long fruits preferred for certain culinary uses to rounder shapes favored in other regions. The CRISPR/Cas9 validation demonstrates that targeted gene editing can efficiently alter fruit morphology, opening the door for rapid trait improvement without years of conventional crossing. Beyond wax gourd, the study advances the general understanding of how SUN-family genes control organ shape, with implications for other cucurbit crops such as cucumber, melon, and watermelon, where related genes have been implicated in fruit shape variation. The integration of calcium signaling, microtubule dynamics, and hormone pathways also provides a conceptual framework for studying shape determination in other plant organs.
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References
DOI
10.1093/hr/uhag190
Original Source URL
https://doi.org/10.1093/hr/uhag190
Funding information
National Natural Science Foundation of China (Grant No. 32560730); Guangxi Science and Technology Major Project (Guike AA23062048)
About Horticulture Research
Horticulture Research is an open access journal of Nanjing Agricultural University and ranked number one in the Horticulture category of the Journal Citation Reports ™ from Clarivate, 2023. The journal is committed to publishing original research articles, reviews, perspectives, comments, correspondence articles and letters to the editor related to all major horticultural plants and disciplines, including biotechnology, breeding, cellular and molecular biology, evolution, genetics, inter-species interactions, physiology, and the origination and domestication of crops.