CsPH8, a P-type proton pump gene, plays a key role in the diversity of citric acid accumulation in citrus fruits

Citric acid homeostasis patterns and its content are diversified among the fruits of citrus cultivars, but the cause remained unclear. In this study we showed that changes of citric acid content were highly associated with the expression profiles of a P-type proton pump gene (CsPH8) in the fruits of six citrus cultivars; moreover, analysis of 21 different fruit samples indicated that the correlation coefficient between titratable acid content and CsPH8 transcript level was 0.5837 with a significant level (P 0.05). Overexpression of CsPH8 in acidless pumelo juice sacs, strawberry fruit, and tomato fruit significantly increased the titratable acid or citric acid content besides the gene transcript level. On another hand, RNA interference of CsPH8 in acidic pumelo juice sacs significantly decreased the CsPH8 transcript level and the titratable acid or citric acid content as well. In addition, severe drought significantly increased the CsPH8 transcript level besides the titratable acid content. Taken together, these findings address the function of CsPH8 in citrus vacuolar acidification, confirm that CsPH8 plays a key role in the variation of citric acid content, and supported that the acid fluctuation influenced by drought, is at least partly due to the change of CsPH8 transcript level.