SlbHLH113 Promotes Tomato Fruit Elongation by Restricting Radial Growth of the Columella and Interacting with SlIQD21a

Fruit shape is determined by patterns of cell division and expansion during early development, yet the upstream transcription factors coordinating cell wall dynamics and cytoskeletal organization remain largely unknown. Here, we report that SlbHLH113, a bHLH transcription factor, positively regulates tomato fruit elongation. Overexpression (OE) of SlbHLH113 produced elongated fruits with increased length/width ratio, whereas RNAi lines exhibited flattened fruits. Histological analysis revealed that SlbHLH113 alters columella cell polarity—promoting elongated cell morphology without affecting cell area—and reduces columella–placenta width and locule width, without altering pericarp thickness. Transcriptomic profiling identified 87 differentially expressed genes in OE lines, with enrichment in cell wall-related processes. Notably, a pectate lyase gene (PL5) and an expansin gene (EXT90) were down-regulated, while genes involved in oriented cellulose deposition (COBRA4) and ethylene signaling were up-regulated. Importantly, SlbHLH113 physically interacts with the microtubule-associated protein SlIQD21a, as demonstrated by yeast two-hybrid and luciferase complementation assays. Finally, SlbHLH113 did not affect major nutrient contents in red-ripe fruits. Collectively, our findings identify SlbHLH113 as a novel regulator of tomato fruit shape that might act through cell polarity control, cell wall remodeling, and interaction with a microtubule-associated protein, offering a potential target for improving fruit morphology without compromising nutritional quality.