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Authors' Contribution ZC designed the study, performed experimental work and analyzed the data. LX performed the experiment, analyzed the data and wrote the manuscript. XY helped in DNA extraction and PCR. YZ helped in morphological evaluation. YY helped in paper writing.

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Fruit shape is an essential agronomic feature in many crops. We identified and functionally characterized an auxin pathway-related gene VvSUN. VvSUN, which belongs to the SUN/IQ67-DOMAIN (IQD) family, localizes to the plasma membrane and chloroplast and may be involved in controlling fruit shape through auxin. It is highly expressed in the ovary, and the expression level at one week before the anthesis stage is positively correlated with the fruit shape index. Functional analyses illustrated that VvSUN gene overexpression in tomato and tobacco plants changed fruit/pod shape. VvSUN promoter directly bound to VvARF6 in yeast and activated the GUS activity by IAA treatments in grapevine leaves, indicating VvSUN functions are in coordination with auxin. Further analysis of 35S::VvSUN transgenic tomato ovaries showed that the fruit shape changes caused by VvSUN were predominantly caused by variations in cell number along longitudinal directions by regulating endogenous auxin levels via polar transport and/or auxin signal transduction process variations. Moreover, an enrichment of the 35S::VvSUN transgenic tomato differentially expressed genes was found in a variety of biological processes, including primary metabolic process, transmembrane transport, calcium ion binding, cytoskeletal protein binding, tubulin binding, microtubule−based movement and so on. Using weighted gene co-expression network analysis (WGCNA), we confirmed that the plant hormone signal transduction may play a crucial role in controlling fruit shape. As a consequence, it is possible that VvSUN acts as a hub gene, altering cellular auxin levels and plant hormone signal transduction pathway, which plays a role in cell division patterns, leading to anisotropic growth of the ovary and, ultimately, an elongated fruit shape.