Molecular Mechanisms of Hormonal Regulatory Networks Mediated by Sugar Signaling in Fruit Abscission and Development of Seedless Litchi

Background Litchi chinensis 'Wu He' (seedless litchi) exhibits excellent parthenocarpic trait, whose physiological regulation during early fruit development is critical for ultimate yield. Sugar supply is the predominant factor governing both fruit abscission and development. However the molecular mechanisms through which sugar signaling in seedless litchi induces hormonal networks to regulate these processes remain unclear. This study explored the regulatory patterns of seedless litchi during the massive abscission stage (MA stage) and subsequent developmental stages (SD stages), through integrated physiological and transcriptomic analyses.Results At MA stage (15 days after flowering, 15 DAF), the concentrations of sucrose and fructose in fruits dropped to lowest levels. Concurrently, auxin and abscisic acid (ABA) levels increased significantly, while ethylene (ET) precursor (ACC) content declined. Tissue section revealed that both cell size and number were at lowest, indicating delayed fruit development. Upon entering SD stages (22 DAF, 29 DAF and 36 DAF), fruit abscission progressively decreased as sugar supply improved. During this stage, the cytokinin (iP-type CK) and gibberellin (GA3) levels increased, accompanied by synchronized expansion in cell volume and number, leading to continuous fruit enlargement. Transcriptome analysis demonstrated that differentially expressed genes were primarily enriched in pathways related to starch and sucrose metabolism, hormone metabolism and signal transduction. During MA stage, gene expressions such as MAT, ACS, ACO, CTR1, ZEP, NCED, PP2C, and SnRK2 were 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 further verified the accuracy of RNA-Seq.Conclusions Deficiency in fructose and glucose may induce the accumulation of ABA and ET, while disrupting polar auxin transport, exacerbating fruit abscission. Following massive fruit abscission, the improved sugar supply enhanced the effects of CK and GA, which promoted cell division and expansion, resulting in gradual fruit enlargement. These findings elucidate the sugar signaling-induced hormonal regulatory network during seedless litchi abscission and development, which can provide a theoretical foundation for high and stable yield of orchard.