Phosphorus Concentration Modulates Locule Number in Tomato via Sugar Accumulation in the Shoot Apex

Abstract

Locule number, a key agronomic trait determining tomato yield and quality, is regulated by genetic determinants, low temperature, and nitrogen supply, and is also affected by phosphorus concentration during the seedling stage, although the underlying mechanism remains unclear. To address this, we treated ‘Mati’ tomato with three phosphorus concentrations (high, normal, and low) and systematically analyzed the physiological and molecular changes associated with locule formation. Results showed that high-P treatment significantly increased floral organ and locule number, increased the content of sucrose, soluble sugars, reducing sugars, fructose, soluble protein, and starch in the shoot apex, enhanced POD and APX activities, inhibited SOD activity, reduced O2− content, increased MDA content, and promoted the accumulation of active auxin and cytokinin. Based on RT-qPCR validation and their known roles in sucrose metabolism and signaling, we identified PMEI (a putative invertase/pectin methylesterase inhibitor) and TPS.1 (a trehalose-6-phosphate synthase) as candidate genes, with PMEI being upregulated and TPS.1 downregulated under high-P treatment. Consistently, TPS enzyme activity in the shoot apex was also significantly reduced under high-P treatment. In locule number mutants, their expression levels showed positive and negative correlations with locule number, respectively. These results suggest that PMEI and TPS.1 may participate in phosphorus-regulated locule number formation through coordinated modulation of sucrose accumulation and signaling in the shoot apex.