20. An intermediate amylose content determined by an allele at the wx locus

Amylose content which is one of major determinants of eating quality greatly varies among non-glutinous rice cultivars. Although the inheritance of amylose content appeared to be rather complicated, two alleles Wx^a and Wx^b were found to regulate the quantitative level of the gene product called Wx protein as well as amylose content (Sano 1984). The level of Wx protein was readily detected by polyacrylamide gel electrophoresis of SDS extracts from starch granules. The allelic states at the wx locus were defined in terms of the intensity of starch- granule-boud Wx protein on the gel (Sano et al. 1986). The amount of Wx protein was about 10 times higher in strains with Wx^a than those with Wx^b , and the marked difference allowed to classify the Wx genes into Wx^a and Wx^b. However,some strains showed an amount of Wx protein intermediate between those given by Wx^a and Wx^b, and their allelic state remained to be studied (Sano et al. 1985a). The present study was conducted to look into whether the interniediate amont of Wx protein and amylose is caused by a modifier regulating the expression of Wx genes or a new allele at the wx locus. If a modifier were present, the intermediate strains would be considered to carry either Wx^a or Wx^b.

The allelic states of five strains were examined after their Wx genes were introduced into T65wx, an isogenic line of Taichung 65 having gene wx froni Kinoshita-mochi established by 14 successive backcrosses (by Dr. K. H. Tsai,

Fig. 1. Starch-granule-bound Wx protein in near-isogenic lines of Taichung 65 carrying alien Wx genes. 1: A line with Wx from W025 (BC\11\F\4\), 2: A line with Wx from Patpaku (BC\8\F\3\), 3: A line with Wx from W593 (BC\8\F\4\). 4: A line with Wx from Macan Coranglan (BC\4\F\3\), 5: a line with Wx from Nagaewase (BC\4\F\3\), 6: Taichung 65, 7: A line with Wx from Kinoshitamochi (BC\14\F\6\), 8: W025, 9: Patpaku, 10:W593, 11:Macan Coranglan, 12:Nagaewase, 13:Kinoshitamochi.

Taichung). Three strains (Patpaku, W593 and W025) were supposed to carry Wx^a, but the allelic state of the other two (Macan Coranglan and Nagaewase) with an intermediate amount of Wx protein and amylose was not known. In the backcross generations, heterozygous plants (Wx/wx) were selected after selling to obtain near-isogenic lines with alien Wx genes.

The SDS extracts of starch granules from the near-isogenic lines revealed the same tendency as found in those from the original strains (Fig. 1). The three near-isogenic lines with Wy genes from Patpaku, W593 and W025 showed an amount of Wx protein about 10 times higher than that from T65 carrying Wx^b. On the other hand, the near-isogenic lines, with the Wx genes from Macan Coranglan and Nagaewase showed an amount of Wx protein intermediate between the amounts from Wx^a and Wx^b. The two lines also showed an amylose content intermediate between the lines with Wx^a and Wx^b. Thus, amylose content was positively correlated with the level of Wx protein, supporting the previous hypothesis that amylose content in rice endosperm is controlled mainly by the amount of starch-granule-bound Wx protein (Sano et al. 1985b). The present result confirmed that Macan Coranglan and Nagaewase carry a new allele at the wx locus specifying an intermediate amount of Wx protein and amylose.

Sano, Y., 1984. Differential regulation of Waxy gene expression in rice endosperm, Theor. Appl. Genet. 64: 467-473.

____,M. Katsumata and E. Amano, 1985a. Correlation between the amount of amylose and Wx protein in rice endosperm. SABRAO J. 17(2): 121-127.

____, ____ and K. Okuno, 1986. Genetic studies of speciation in cultivated rice. 5. Inter- and intraspecific differentiation in the Waxy gene expression of rice, Euphytica 35: 1-9.

____, M. Maekawa and H. Kikuchi, 1985b. Temperature effects on the Wx protein level and amylose content in rice endosperm. J. Hered. 76: 221-222.