Cytoplasmic sucrolytic activity controls tomato fruit growth under salinity

Tomato (Lycopersicon esculentum (L.) Mill cv. Volgogradskij) crop yield reduction under salinity is mainly due to a decrease in dry matter accumulation rate during the early fruit-growing period (15-35 days after anthesis, DAA). However, partial restoration of growth occurred during the late phase of fruit development (35-45 DAA). Salt stress provoked sucrose accumulation in both apoplastic and symplastic spaces of up to 15 mM, as well as acidification and Na+ accumulation in the apoplast and a transient increase in starch content. According to the hypothesis stated, the decrease in apoplastic pH imposed by salinity caused a loss of insoluble acid invertase activity, which was associated with sucrose accumulation, and resulted in a limiting step for fruit growth under moderate and high salinities. As a consequence of sucrose accumulation in the symplast, cytoplasmic sucrolytic activity (neutral invertase + sucrose synthase) was induced. It acted as a metabolical adaptive response by regulating the sucrose homeostasis and controlling the assimilate import, dry matter accumulation and fruit growth under salt conditions. Although the compartmentation of the hexose excess into starch could be considered as another adaptive response, the key enzyme ADP-glucose pyrophosphorylase does not seem to be important in the control of fruit growth under salinity conditions.