Transcriptomic Analysis of Oxalate Accumulation in Sour-Type Star Fruit (Averrhoa carambola L.) During Fruit Development

Star fruit (Averrhoa carambola L.; A. carambola) is an important fruit crop cultivated mainly in tropical and subtropical regions. The high soluble oxalate content of star fruit has become a major obstacle to the further expansion of its consumer market. Elucidating the origin and accumulation mechanisms of oxalate from the perspective of star fruit physiology is particularly important, as this will provide the theoretical foundation for modern molecular breeding and the development of advanced cultivation and management practices. In this study, we quantified oxalate, glycolate, L-ascorbate, and oxaloacetate in star fruit at two developmental stages of fruitlet and fruit expansion and performed an integrated analysis combining targeted metabolomic and transcriptomic data. The results showed that, relative to fruitlets, the levels of oxalate and L-ascorbate in expanding fruits increased by 4-fold and 0.6-fold, respectively, whereas the glycolate level decreased by 0.5-fold. The comparative transcriptomic analysis identified a total of 2441 DEGs (|log2FC| > 0.3, padj < 0.05), of which 1136 were significantly up-regulated and 1305 were significantly down-regulated. Genes encoding several key enzymes involved in the oxidative degradation pathways of glycolate and L-ascorbate were significantly up-regulated. As oxalate is the dominant product of the oxidative breakdown of glycolate and L-ascorbate, our findings suggest oxalate accumulation in expansion fruits likely associated with oxidative breakdown of glycolate and L-ascorbate.