Stage-Specific Coordination of Auxin, Abscisic Acid, and Sucrose Signaling Controls Cell Expansion and Anthocyanin Biosynthesis in Strawberry

Strawberry (Fragaria × ananassa) is a typical non-climacteric fruit whose development and ripening are coordinately regulated by auxin (IAA), abscisic acid (ABA) and sucrose signaling. However, stage-specific interactions among these signals in controlling cell expansion and anthocyanin biosynthesis remain incompletely understood. In this study, ‘Sweet Charlie’ strawberry fruits at six developmental stages (small green, SG; medium green, MG; large green, LG; white, WT; initial red, IR; and full red, FR) were treated with exogenous IAA, ABA or sucrose while remaining attached to the plant. Cellular morphology, endogenous hormone profiles, fruit coloration and anthocyanin content, firmness and cell-wall enzyme activities, soluble sugar and organic acid composition, and the expression of key genes involved in anthocyanin biosynthesis, sugar metabolism and ripening were systematically analyzed. Results showed that IAA maintained compact cell arrangement, delayed tissue loosening and softening, suppressed anthocyanin accumulation and the expression of anthocyanin biosynthetic genes (FaCHS, FaCHI, FaF3H, FaDFR, FaANS, FaUFGT, etc.), and limited sugar accumulation across stages. In contrast, ABA and sucrose treatments promoted cell expansion and intercellular space enlargement (especially from WT to FR), accelerated softening by elevating polygalacturonase (PG), pectin methylesterase (PME) and cellulase activities, markedly increased anthocyanin content and CIRG values, enhanced soluble sugar accumulation, and upregulated anthocyanin pathway, sugar metabolism/transport (FaSPS1, FaSUTs) and ripening-related genes (FaNCEDs, FaSnRK2.6, etc.). Endogenous hormone dynamics further confirmed opposing patterns between IAA and ABA, with sucrose producing parallel effects with ABA. Collectively, these findings demonstrate that early high IAA levels drive cell expansion while repressing ABA and ripening programs, whereas the subsequent decline of IAA and rise of ABA and sucrose coordinately trigger anthocyanin biosynthesis, sugar accumulation and softening. This work elucidates the stage-specific coordination of IAA–ABA–sucrose signaling in strawberry fruit development and provides a theoretical basis for precise quality regulation of non-climacteric fruits.