Salinity Stress Affects Assimilate Metabolism at the Gene-expression Level during Fruit Development and Improves Fruit Quality in Tomato (Solanum lycopersicum L.)

Irrigation with moderate salinity stress in a hydroponic system improves the inner and outer fruit quality of tomato (Solanum lycopersicon Mill. ‘House Momotaro’). We investigated the effects of 50 mM NaCl in a hydroponic solution on the levels of various metabolites, including soluble sugars, amino acids, and organic acids, and on the expression levels of salinity-responsive genes during fruit development. Under salinity, Brix (%), surface color density, and flesh firmness of the fruit were significantly enhanced, whereas fruit enlargement was suppressed. Salinity stress strongly promoted the accumulation of sucrose, citrate, malate, and glutamate, and slightly promoted glucose and γ-amino butyric acid in red fruit. At the transcriptional level, up-regulation of ethylene-synthetic 1-aminocyclopropane-1-carboxylate oxidase and down-regulation of photosynthetic chlorophyll a/b binding protein Cab-1B occurred earlier in stressed fruit than in control fruit. Additionally, the carotenoidbiosynthesis regulatory gene, Phytoene synthase 1, and phosphoenolpyruvate carboxykinase (PEPCK) were upregulated under moderate salinity in the red stage. The expression profiles of these genes in stress-treated fruit were consistent with the changes in fruit quality, including earlier ripening and a deeper red color. Furthermore, the up-regulation of PEPCK suggested that gluconeogenesis is involved in the accumulation of sugars in salinitystressed fruit.