Type I H+-Pyrophosphatase Regulates the Vacuolar Storage of Sucrose in Citrus Fruit.

The aim of this work was to evaluate the general role of pyrophosphatase proton pump (V-PPase) in sucrose accumulation in citrus species. Firstly, three citrus V-PPase genes, designated CsVPP-1, CsVPP-2, and CsVPP-4 were identified in the citrus genome. CsVPP-1 and CsVPP-2 belonging to citrus type I V-PPase genes are targeted to the tonoplast and CsVPP-4 belonging to citrus type II V-PPase genes is located in the Golgi bodies. Moreover, there was a significant positive correlation between transcript levels of type I V-PPase genes and sucrose, rather than hexose content, in fruits of seven citrus cultivars. Drought and ABA treatments significantly induced the CsVPP-1 and CsVPP-2 transcript levels, as well as sucrose content. The overexpression of type I V-PPase genes significantly increased PPase activity, decreased pyrophosphate contents, and increased sucrose contents whereas V-PPase inhibition produced the opposite effect in both citrus fruits and leaves. Furthermore, altering the expression levels of type I V-PPase genes significantly influenced the transcript levels of sucrose transporter genes. Taken together, this study demonstrated that CsVPP-1 and CsVPP-2 play key roles in sucrose storage in the vacuole both by regulating pyrophosphate homeostasis and ultimately also transcript levels associated with sucrose biosynthesis and sucrose transporter genes, providing a novel lead for engineering or breeding modified taste in citrus and other fruits.