Figure 1.

Model for the function of a PM-localized H⁺-PPase in Suc partitioning. 1, Suc is synthesized in chloroplast of mesophyll cells and transported to CCs, where it can be hydrolyzed or delivered to the sieve elements. 2, Detail of Suc transport and metabolism in CCs and sieve elements. An adequate supply of ATP for the maintenance of the transmembrane proton gradient generated by the PM H⁺-ATPase requires PPi-dependent Suc metabolism. The most relevant enzymes and transporters are indicated. 3, Suc travels through sieve elements from source to sink tissues such as roots. Its presence in roots activates the rhizosphere acidification instrumental for Pi uptake.