Mild low-phosphate stress induced by coral sand increases the sugar content in tomato fruits

Sugar content is among the most important agronomic traits in tomatoes. High-sugar tomatoes are usually produced by applying water stress, which results in a significant reduction in fruit yield. We developed a hydroponic method involving the use of coral sand as a solid medium and optimized a nutrient management protocol to produce high-sugar tomatoes while largely maintaining yield. In this study, we analyzed the mechanism by which coral sand increases sugar content. Transcriptome analysis revealed that the expression of phosphorus deficiency-inducible genes increased in the leaves of tomato plants grown on coral sand. The phosphate concentration in both the nutrient solution and leaves decreased when the plants were grown on the coral medium, suggesting that the response to mild low-phosphate conditions may be associated with the increase in sugar content. In addition, under mild low phosphate in hydroponic culture, the sugar content in the fruit increased, even though the fruit yield decreased. Interestingly, comparisons of gene expression levels under these conditions with those from previously reported mild drought experiments showed that homologs of genes that are induced by coral sand were also upregulated in response to mild drought. Collectively, our findings reveal a previously unrecognized ability of hydroponic cultivation using coral sand to create mild low-phosphate conditions that trigger sugar accumulation pathways associated with mild drought. This study not only provides a practical strategy for producing high-sugar tomatoes using a coral sand cultivation system with minimal impact on yield, but also suggests a role for low-phosphate responses in regulating sugar accumulation in fruits.