Molecular regulation of starch accumulation in rice seedling leaves in response to salt stress

In this report we show that the starch content decreased in NaCl-stressed rice (Oryza sativa L.) seedling leaves during the daytime. Because photosynthetic efficiency and starch degradation enzyme activity were not significantly affected by the high salt, it is likely that this effect results from repression of starch biosynthesis. To determine the regulatory mechanism, the activities of enzymes such as ADP-glucose pyrophosphorylase (AGPase), granule-bound starch synthase (GBSS), soluble starch synthase (SSS) and starch branching enzyme (SBE) involved in starch synthetic pathway were examined. Data suggest that NaCl-induced repression of GBSS activity was the most significant factor reducing starch accumulation. Based on real-time RT-PCR analysis, the effect of salinity on GBSS expression was primarily controlled on the transcriptional level. Furthermore, the salt-induced decrease of both GBSSI and GBSSII gene expressions could be mostly contributed by ion-specific effect and not by osmotic stress. Although the mRNA accumulation of GBSSI and GBSSII can be down-regulated by exogenous ABA, the negative influence of salt stress on GBSSI and II gene expression could be chiefly mediated via an ABA-independent pathway.