Differential expression of three key starch biosynthetic genes in developing grains of rice differing in glycemic index

Rices with low glycemic index (GI) values may be useful in the management of common metabolic conditions. The GI of five different rice lines (MR219, MRQ74, UKMRC10, BD192 and BRRI29) were assessed through eating trials. Both BD192 and BRRI29 were identified as low GI, MR219 as high GI and the other two as intermediate GI types. Analysis of starch biosynthesis in the grain of high and low GI rice lines 10 days after anthesis (DAA) showed a greater accumulation of granule-bound starch synthase (encoded by Wx) and starch synthase IIa (encoded by SSIIa) proteins in the two low GI compared to the high GI rice lines but this difference is not evident at later stages. However, a greater accumulation of soluble SBEIIb in high GI compared to low GI rice lines was observed. Analysis of RNA-seq data at 10 DAA indicated that Wx and SSIIa were differentially expressed between BD192 and BRRI29 on the one hand and MR219 on the other. Sequence differences between BD192, BRRI29 and MR219 in Wx, SSIIa and SBEIIb genes were uncovered which may be associated with mean GI values. These results could help to develop low GI rice lines with good eating quality. Selected indica rices were tested for GI. The two low GI rices accumulated relatively more GBSS early in development but later there was no difference with the high GI rice.