CitSAR-mediated coordination of sucrose and citrate metabolism in citrus fruits

Soluble sugars and organic acids are the primary determinants of flavor quality and consumer appeal in fruits. In citrus, sucrose and citric acid dominate the sugar and acid profiles, directly influencing sensory attributes and market preference. Here, we identified the sugar and acid regulator (CitSAR) through transcriptome analysis by investigating sugar and acid levels at the fruit development stage of 2 mandarin orange (Citrus reticulata Blanco) cultivars: "Ponkan" (PK), and a bud sport mutant of PK (mPK) that exhibits a higher sugar and lower acid profile. Functional characterization revealed that CitSAR orchestrates a hierarchical transcriptional network governing sucrose and citrate metabolism, as demonstrated by overexpression assays in citrus callus and tomato. Mechanistic studies using dual-luciferase reporter assays and electrophoretic mobility shift assays confirmed that CitSAR binds directly to the GCC-box in the promoters of CitSPS4 and CitGAD4, activating their expression. Our findings corroborate the important role of CitSAR in modulating sucrose and citrate homeostasis and provide a molecular framework for enhancing citrus sensory quality.