The F-box protein SlSAP1 and SlSAP2 redundantly control leaf and fruit size by modulating the stability of SlKIX8 and...

Tomato fruit size is a crucial trait in domestication, determined by cell division and cell expansion. Despite the identification of several quantitative trait loci associated with fruit size in tomatoes, the underlying molecular mechanisms that govern cell division and expansion to control fruit size remain unclear. CRISPR/Cas9 gene editing was used to generate single and double loss-of-function mutants of the tomato STERILE APETALA1 (c) and SlSAP2. The results demonstrate that the two SlSAP genes function redundantly in regulating leaf and fruit size by positively regulating cell proliferation and expansion, with SlSAP1 having the predominant effect. Consistently, overexpression of either SlSAP1 or SlSAP2 leads to enlarged fruits due to an increase in both cell layers and cell size in the pericarp. Biochemical evidence suggests that both SlSAP1 and SlSAP2 can form an SCF complex and physically interact with SlKIX8 and SlKIX9, which are crucial negative regulators of fruit size. Further results reveal that SlSAP1 and SlSAP2 target them for degradation. This study uncovers that the ubiquitination pathway plays an important role in the determination of tomato fruit size, and offers new genetic loci for improving fruit yield and biomass by manipulating pericarp thickness.