Expression Analysis of Brachypodium Genes Involved in the ADP-Glucose Production for Storage Starch Biosynthesis

Plant leaves produce sucrose from starch and transport it to the other organs via phloem. The transported sucrose is converted to ADP-glucose, and used as the initial material for storage starch at the plastids. However, only in the case of the endosperm cells of grasses, ADP-glucose is produced at cytosol. Therefore, understanding the cytosolic ADP-glucose production and establishing a method to regulate this pathway are important to improve the quality of grass seed biomass. Hence, we explored the Brachypodium, which is the model plants of Pooideae, genes involved in the reaction of ADP-glucose production [ADP-glucose pyrophosphorylase (AGPase)] and investigated the expression pattern of these genes in the shoot (10-, 20- and 30-day-old plants), and developing seeds (early-, middle- and late-developed stages).

Sequence database search results yielded 5 AGPase genes in the Brachypodium genome. Among them, 3 genes encoding the large subunit (APL) and 2 genes encoding the small subunit (APS) of AGPase, respectively. One APS gene had 2 types of transcripts derived from alternative splicing (E1a and E1b type). Phylogenetic analyses with putative orthologues of other plants revealed that the number and structure of these genes are well conserved in comparison with those in rice, except that 1 gene encoding the large subunit of AGPase is absent in Brachypodium. Detailed expression analyses of the AGPase suggested that duplicate copies of APS1 (E1a type) and APL3 mainly function as the cytosolic AGPase in the endosperm cells.