Differential Regulation of Starch-synthetic Gene Expression in Endosperm Between Indica and Japonica Rice Cultivars

Grain filling rates (GFRs) of indica rice cultivars are often higher than those of japonica cultivars. Although GFR is mainly determined by the starch accumulation rate (SAR) in endosperm, the genetic basis for SAR during the ripening period has not been well studied in rice. To elucidate the factors influencing the differing SARs between typical indica and japonica cultivars, we focused on differences in sink potentials, especially on starch synthesis in the endosperm.

SAR in indica rice cultivar IR36 was significantly higher than in japonica cultivar T65. Although enzymes for both amylose and amylopectin syntheses had higher activity in IR36, amylopectin synthesis was seemingly more important for accelerating SAR because an elevation of amylose synthesis ability alone in the T65 genetic background did not result in the same level of SAR as IR36. In IR36, most starch-synthetic genes (SSGs) in the endosperm were more highly expressed during ripening than in T65. In panicle culture experiments, the SSGs in rice endosperm were regulated in either sucrose-dependent or -independent manners, or both. All SSGs except SSI and BEIIa were responsive to sucrose in both cultivars, and GBSSI, AGPS2b and PUL were more responsive to sucrose in IR36. Interestingly, the GBSSI gene (Wx ^a ) in IR36 was highly activated by sucrose, but the GBSSI gene (Wx ^b ) in T65 was insensitive. In sucrose-independent regulation, AGPL2, SSIIIa, BEI, BEIIb and ISA1 genes in IR36 were upregulated 1.5 to 2 times more than those in T65. Additionally, at least SSI and BEIIa might be regulated by unknown signals; that regulation pathway should be more activated in IR36 than T65.

In this study, at least three regulatory pathways seem to be involved in SSG expression in rice endosperm, and all pathways were more active in IR36. One of the factors leading to the high SAR of IR36 seemed to be an increase in the sink potential.