Salt Stress-Induced Ascorbic Acid Accumulation and Its Trade-Off with Mannan Content in Tomato

Salt stress causes osmotic stress and ion toxicity, often inhibiting plant growth and metabolism. However, salt-stressed tomato plants accumulate ascorbic acid, resulting in fruits with high commercial value. However, it was not well understood how mannose, the material for the synthesis of ascorbic acid, and its metabolism are affected under salt stress conditions. In this study, we found that tomatoes grown under salinity stress had increased levels of ascorbic acid, which correlated with decreased levels of mannan in the skin and seeds. Expression analysis of the ascorbic acid synthase gene showed increased expression in early ripening stages under salt stress. In addition, the expression of cellulose synthase-like A (CSLA), genes involved in mannan metabolism, increased significantly during mid-ripening in the control condition. Since ascorbic acid and mannan share mannose as a precursor, they are likely to compete for it. This suggests that salt-stressed tomatoes may be deficient in both ascorbic acid and mannose, thereby affecting mannan synthesis. To investigate this trade-off, we developed a culture system with added mannose. The results showed that in salt-stressed tomatoes supplemented with mannose, ascorbic acid levels in unripe green peels reached those of fully ripe fruit, highlighting the influence of mannose availability on ascorbic acid accumulation.