Sugar signaling-mediated hormonal regulatory networks: molecular mechanisms underlying fruit abscission and development in seedless litchi

Seedless litchi (Litchi chinensis ‘Wu He’) exhibits an excellent parthenocarpic trait, the physiological regulation of which during early fruit development is critical for ultimate yield. Sugar availability serves as the predominant factor governing both fruit abscission and development. However, the molecular mechanisms by which sugar signaling mediates hormonal networks to regulate these processes in seedless litchi remain unclear. This study explored the regulatory patterns in seedless litchi during the massive abscission stage (MA stage) and subsequent developmental stages (SD stages), through integrated physiological and transcriptomic analyses. At the MA stage (15 days after flowering, 15 DAF), the concentrations of glucose and fructose in fruits dropped to their lowest levels. Concurrently, auxin and abscisic acid (ABA) levels increased significantly, while the ethylene (ET) precursor (1-aminocyclopropane-1-carboxylic acid, ACC) content declined. Tissue section revealed that both cell size and cell number were at their lowest, indicating delayed fruit development. Upon entering the SD stages (22 DAF, 29 DAF, 36 DAF), fruit abscission progressively decreased as sugar availability improved. During these stages, cytokinin (iP-type-CK) and gibberellin (GA3) levels increased, accompanied by synchronized increases in cell volume and number, leading to continuous fruit enlargement. Transcriptomic analysis demonstrated that differentially expressed genes were primarily enriched in pathways related to starch and sucrose metabolism, hormone metabolism and signal transduction. During the MA stage, expression of genes such as MAT, ACS, ACO, CTR1, ZEP,NCED, PP2C, and SnRK2 was upregulated, whereas EIN2 was downregulated. In contrast, the SD stages exhibited upregulation of IPT,B-ARR, KAO, GID1, GID2, TFs and DELLA genes, alongside downregulation of CKX, A-ARR, and GA2ox. These gene expression patterns were consistent with fluctuations in hormone levels and collectively regulated fruit abscission and development. RT-qPCR validation confirmed the accuracy of the RNA-Seq results. Deficiencies in fructose and glucose mediate ABA and ET accumulation while disrupting polar auxin transport, thereby exacerbating fruit abscission. Following massive fruit abscission, improved sugar availability enhanced the effects of CK and GA, which promoted cell division and expansion, resulting in gradual fruit enlargement. These findings elucidate the sugar signaling-mediated hormonal regulatory networks during seedless litchi abscission and development, providing a theoretical foundation for achieving high and stable yields in orchards.