Altered tissue-specific expression at the Wx gene of the opaque mutants in rice

Amylose content is a major determinant of the eating quality in rice. To elucidate the allelic diversity at the Waxy (Wx) gene which controls the amylose synthesis, two cultivated strains having opaque endosperms were studied. The gene responsible for opaque endosperms was introduced into the genetic background of the Japonica type of rice by successive backcrosses, and the two near-isogenic lines (NILs) were selected from the B-5 generation. The genetic experiments revealed that an allele, Wx(op), controls opaque endosperms which show chalky as wx endosperms in spite of the production of amylose. Immunoblotting analysis was carried out to compare the gene expression by using the NILs with 4 different alleles (Wx(a), Wx(b), Wx(op) and wx). The level of the gene product bound to starch granules was slightly lower in the NILs with Wx(op) than that with Wx(b), showing a positive correlation with amylose content in the endosperm. Extracts from mature anthers indicated that the gene product was markedly reduced in the NILs with Wx(op) as well as that with wx, showing an altered expression in the tissue specificity in the Wx(op) lines. Sequence analysis suggested that the Wx(op) had been derived from Wx(a), independently of the origin of Wx(b). The importance of the gene regulation was discussed in relation to diversified phenotypes established during the domestication process.