Sorbitol source-sink distribution and S6PDH regulation modulate freezing tolerance in young loquat (Eriobotrya japonica) fruits

Loquat (Eriobotrya japonica) bears young fruits that overwinter and are susceptible to freezing damage. Sorbitol is a crucial metabolite for photosynthate synthesis and translocation in Rosaceae fruit trees, and contributes significantly to sugar accumulation and stress tolerance. This study characterized sorbitol biosynthesis and tissue distribution in loquat, demonstrating its protective role against freezing stress in young fruits through exogenous application and functional validation of sorbitol-6-phosphate dehydrogenase (S6PDH). Results revealed that sorbitol accumulation in leaves correlated strongly with sorbitol-6-phosphate dehydrogenase (S6PDH) activity governed by S6PDH expression, serving as a critical biomarker for the leaf's intrinsic source-to-sink transition. Annual profiling showed that leaf sorbitol content peaked in winter, coinciding with the highest sorbitol levels in overwintering young fruits. Low temperatures enhanced sorbitol synthesis by upregulating S6PDH1 and S6PDH4 expression and increasing S6PDH activity. Exogenous application of sorbitol (200mg/L) significantly reduced the freezing injury index in young fruits, increased endogenous sorbitol content, and activated S6PDH4 expression. Transient overexpression and silencing assays confirmed the critical roles of S6PDH1 and S6PDH4 in cold tolerance. Overexpression of S6PDH1 and S6PDH4 enhanced sorbitol content by 20.99 % and 39.84 % while reducing the freezing injury index by 8.53 % and 28.30 %, respectively; in contrast, silencing these genes significantly decreased sorbitol content and elevated the freezing injury index. These findings elucidate the tissue-specific and seasonally dynamic sorbitol accumulation in loquats, validate the regulatory function of sorbitol biosynthetic pathways in low-temperature stress responses, and provide a theoretical framework for developing frost-resistant cultivation strategies.