BRs (Brassinosteroids) regulate many essential pathways related to growth, cell elongation, cell expansion, plant architecture, and fruit development. The potential exogenous application of BR-derivatives has been proven to stimulate plant growth and development, including quality attributes of fruits, whereas its biosynthesis inhibition has shown the opposite effect. In this study, BR-insensitive tomato mutants were used to reveal the potential function of BR signaling in the regulation of fruit development to elaborate the regulatory mechanism of BR signaling in tomato fruits. The BR-signaling mutant exhibited a typical dwarf phenotype and reduced vegetative growth, fruit size, and weight. Microscopic and transcriptional evaluation of the abs1 mutant fruits implies that reduced cell size and number are responsible for the phenotypic variations. Additionally, we also found that the altered content of phytohormones, such as auxin, gibberellin, cytokinin, and ethylene levels, contributed to altered fruit development. Moreover, fruit growth and cell development-specific gene expression levels were downregulated in BR-insensitive plants; culminating in reduced cell size, cell number, and cell layers. These findings provide insight into physio-chemical changes during fruit development in response to BR-insensitivity.

Sugar content is pivotal in determining the flavor quality of tomato, and numerous genes related to tomato fruit quality have been identified. The distribution of sugar sources in plants primarily relies on the functionality of sugar transporters. Despite this, the specific role of SUT2, a sucrose transporter family member, in sugar accumulation within tomato fruits is still unclear. This study demonstrates that SUT2 is localized to the plasma membrane and possesses the function of transporting sucrose from the extracellular side to the intracellular side of the plasma membrane. Its expression level progressively decreases during fruit development. SUT2 knockout resulted in a significant increase in sugar content in tomato fruits. Further investigation revealed that the elevated sugar levels in knockout lines were accompanied by alterations in the expression of the sugar accumulation related genes STP1 and CDPK26/27. These findings provide new insights into the biological role of SUT2 in regulating sugar content in tomato fruits, improve our understanding of sugar accumulation mechanisms in tomato fruits, and offer valuable perspectives for quality improvement in tomato.