15. A trans-acting regulatory gene controlling the Wx gene expression and its temperature responses

1) National Institute of Genetics, Mishima 411, 2) Experiment Farm, Faculty of Agriculutre, Sapporo, 060, 3) Hokkaido Kamikawa Experimental Station, Asahikawa, 078-02 Japan

To look into the gene regulation at the waxy locus and the effects of the gene product called "Wx protein" on amylose content in endosperm starch, two rice strains, Shiokari and its low amylose mutant (SM-1), were compared with regard to the levels of Wx protein and amylose content under different conditions. SM-1 gave a monogenic segregation for low amylose content in the F\2\ of its cross with Shiokari. The mutant showed a reduced level of Wx protein bound to starch granules as compared with that of Shiokari, showing that the recessive gene regulates the Wx protein level as well as amylose content. Since the recessive gene for low amylose content is inherited independently of wx, this mutant gene may be regarded as a trans-acting regulatory gene controlling the Wx gene expression. In addition, low temperatures during grain development increased the amount of Wx protein as well as the amylose content in the two strains, indicating that the gene expression at the wx locus is influenced by the temperature.

As to the gene regulation at the wx locus, two different alleles, Wx^a and Wx^b, which control different levels of the gene product, are known to exist among rice cultivars. The allelic difference would result from a mutation at a cis-acting regulatory site(s) near or within the structural gene of Wx protein (Sano 1984). Thus, the gene regulation at the waxy locus seems to be rather complex in rice. The present result suggests that regulatory changes controlling the gene expression might be important for variation in amylose content of rice endosperm rather than changes in the structural gene.

Y. Sano, 1984. Differential regulation of wxy gene expression in rice endosperm. Theo. Appl. Genet. 68:467-473.