Strawberry Fruit Softening Driven by Cell Wall Metabolism, Gene Expression, Enzyme Activity, and Phytohormone Dynamics

Texture is a critical quality attribute of strawberry fruit, and phytohormones play a pivotal role in fruit softening, which mainly results from cell wall metabolism, which is governed by genes and enzymes. To gain further insights into strawberry (Fragaria × ananassa, Duch. cv. Akihime ) softening, our study investigated changes across five stages in fruits in their firmness, soluble solids content (SSC), cell microstructure, cell wall materials, activities of cell wall-modifying enzymes, gene expression, endogenous phytohormone levels, and their correlation. During strawberry ripening, firmness decreased, while SSC, intercellular space, and separation of the cell wall from the plasma membrane increased. Meanwhile, the contents of ionic pectin (ISP) and cellulose (CE), pectin methylesterase (PME) activity, FaPME expression, and the levels of zeatin (Z) and strigolactone (SL) decreased, showing a positive correlation with firmness. In contrast, the activities of pectate lyase (PL) and cellulase (Cx), the expression of FaPL and FaCx, and the contents of gibberellin A4 (GA4), GA9, and abscisic acid (ABA) increased during ripening, and these were negatively correlated with firmness. These results suggest that Z and SL are associated with the maintenance of cell wall integrity and firmness, whereas increases in GA4, GA9, and ABA are linked to enhanced cell wall disassembly and fruit softening.