Abstract
Salinity stress enhances sugar accumulation in tomato (Solanum lycopersicum) fruits. Prior to the sugar accumulation, the stress promotes carbohydrate influx into fruit and starch accumulation at early developing stages. To elucidate the molecular mechanisms underlying this phenomenon, we examined expression manner of genes encoding ADP-glucose pyrophosphorylase (AGPase) in immature fruits of plants exposed to 160 mM of NaCl. Quantitative RT-PCR analyses revealed that among the AGPase genes, AgpL1 and AgpS1 were specifically up-regulated by the stress at the transcriptional level in developing fruits. Interestingly, only AgpL1 was obviously up-regulated by sugar but not by abscisic acid (ABA) and osmotic stress. These results indicate AgpL1 and AgpS1 are involved in the promotion of starch biosynthesis under the salinity stress in ABA- and osmotic stress-independent manners, and those two genes are differentially regulated by sugar-dependent and -independent ways in developing tomato fruit. Because the hexokinase inhibitor suppressed the sugar-response of AgpL1, a hexokinase-mediated sugar-signaling pathway should be involved in this event.
