Tomato (Solanum lycopersicum) is a typical fleshy fruit, and its fruit size is an important functional trait related to yield and quality. Although previous studies have identified that quantitative trait loci are related to fruit size in tomatoes, the specific molecular mechanisms governing cell division and expansion to control fruit size remain unidentified.

In a study published in New Phytologist, researchers from Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences discovered two homologs of AtSAP, designated as SlSAP1 and SlSAP2, and revealed the molecular mechanisms behind tomato fruit size regulation, shedding light on how two critical genes control cell proliferation and expansion to determine fruit dimensions.

Through a genome-wide analysis, researchers discovered SlSAP1 and SlSAP2 which belong to the STERILE APETALA1 (SAP) family. Genetic analysis indicated that both SlSAP1 and SlSAP2 have redundant roles in controlling the size of leaves, flowers, and fruits in tomatoes by facilitating cell division and expansion during development.

When SlSAP1 or SlSAP2 was overexpressed, researchers noted alterations in fruit size. Besides, they performed biochemical analyses to explore the formation of SKP1/Cullin1/F-box (SCF) complexes with SlKIX8 and SlKIX9, which are key negative regulators of fruit size, through their interaction with SlSAP1 and SlSAP2.

It was found that mutant lines devoid of both genes resulted in significantly smaller fruits and leaves, and overexpression of either gene produced larger fruits with thicker pericarps (fruit walls). The thickness of the pericarp, a major determinant of fruit size, was found to depend on SlSAP-mediated cell proliferation in the early stages of development and cell expansion in the later stages.

Furthermore. researchers found that the two SAP homologs are universally present in the Solanaceae family including potato, pepper, tobacco, and petunia, likely due to genome triplication events so as to ensure robust fruit development. They confirmed the critical role of the ubiquitination pathway in regulating tomato fruit size and yield, and identified new genetic loci associated with fruit yield and biomass, which can be manipulated by adjusting pericarp thickness.

“Our findings provide a genetic toolkit for precise breeding. Through careful modulation of SlSAP1 and SlSAP2 expression, we are able to genetically engineer tomatoes with improved yield and biomass,” said CHEN Jianghua from XTBG.