Expression profiling of genes involved in starch synthesis in sink and source organs of rice.

A comprehensive analysis of the transcript levels of genes which encode starch-synthesis enzymes is fundamental for the assessment of the function of each enzyme and the regulatory mechanism for starch biosynthesis in source and sink organs. Using quantitative real-time RT-PCR, an examination was made of the expression profiles of 27 rice genes encoding six classes of enzymes, i.e. ADPglucose pyrophosphorylase (AGPase), starch synthase, starch branching enzyme, starch debranching enzyme, starch phosphorylase, and disproportionating enzyme in developing seeds and leaves. The modes of gene expression were tissue- and developmental stage-specific. Four patterns of expression in the seed were identified: group 1 genes, which are expressed very early in grain formation and are presumed to be involved in the construction of fundamental cell machineries, de novo synthesis of glucan primers, and initiation of starch granules; group 2 genes, which are highly expressed throughout endosperm development; group 3 genes, which have transcripts that are low at the onset but which rise steeply at the start of starch synthesis in the endosperm and are thought to play essential roles in endosperm starch synthesis; and group 4 genes, which are expressed scantly, mainly at the onset of grain development, and might be involved in synthesis of starch in the pericarp. The methodology also revealed that the defect in the cytosolic AGPase small subunit2b (AGPS2b) transcription from the AGPS2 gene in endosperm sharply enhanced the expressions of endosperm and leaf plastidial AGPS1, the endosperm cytosolic AGPase large subunit2 (AGPL2), and the leaf plastidial AGPL1.