• Rgn13

The two opaque cultivars (ARC6622, ARC 10818 from India) were crossed to T65 and T65wx which is a NIL carrying wx. The opaque endosperms were waxy in appearance but stained blue with iodine. Amylose contents of the two strains were 8.6% and 6.3%, respectively. Pollen and seed fertilities were low in the hybrids of ARC6622 (50.7% and 4.0%) compared to those of ARC10818 (59.3% and 32.2%). As expected, all endosperms of the F2 seeds were waxy in crosses with T65wx showing that the both genes are allelic to wx although a ratio of 3:1 was observed by I2-KI staining. On the other hand, pollen grains in the F1 plants showed only red by I2-KI staining in crosses with T65wx but segregated into 1 blue : 1 red grains in crosses with T65. Then, the hybrids with T65 were backcrossed to T65 since the heterozygotes are distinguished at the flowering stage. Backcrosses were continued up to the BC5 generation and the resultant plants were fully fertile. The homozygotes were selected as the NILs with opaque endosperms from BC5F2.

The amylose contents in the endosperms were 7.8% (from ARC6622) and 6,5% (from ARC10818) being consistent with those of the original cultivars. Segregation patterns were reconfirmed by using the near-isogenic lines (Table 1). The two opaque alleles were designated Wx op (6622) and Wx op (10818), and the near-isogenic lines were

Note: 1) Segregation in appearance shows transparent: waxy.

According to the amounts of the gene product and amylose, 4 different alleles (Wxa, Wx in, Wx b and Wx op) were identified among rice cultivars so far. In addition, wx alleles might be regarded as a null allele (Sano et al. 1991; Sano 1991). It should be noted in plants with Wx op, the reaction to I2-KI greatly differed between endosperm and pollen grains. As far as known, low amylose mutants due to du show no difference between endosperms and pollen grains in their reaction to iodine. Accordingly, the opaque gene appears to have altered expressions in tissue specificities.

The waxy locus of rice is responsible for the synthesis of amylose in the endosperm. It encodes ADP-glucose starch glycosyl transferase which is detected as a major protein bound to starch granules (Sano 1984; Hirano and Sano 1991). The gene product (WX protein) was examined to compare among the NILs with different alleles at

the wx locus (Fig. 1). Immunoblotting analysis was carried out by using a polyclonal antiserum against the purified WX protein (Hirano and Sano 1991). WX proteins were extracted from starch granules in the endosperms as well as from pollen grains. The levels of WX proteins bound to starch granules were higher in T65Wx op than in T65Wx b (Fig. 1A). In addition, the levels of WX proteins from endosperms of T65Wx op were almost the same as that from T65Wx a despite the fact that the amylose contents were lower in the opaque NILs (Fig.lB), showing that the amout of WX proteins is not correlated with the amylose content. Immunoblotting analysis in mature anthers (Fig. 1C) indicated that no gene product was detectable in the opaque NILs as well as in T65 wx, which is consistent with the finding that pollen grains with Wxop are stained reddish as those with wx. The present results confirmed that the expression of Wx op is different between endosperms and pollen grains. To understand the mechanism of the tissue specificity, molecular characterizations of Wxop alleles are now in progress. (Gene symbol: Old system)

References 

Heu, M.H., 1986. Inheritance of chalkiness of brown rice found in a non glutinous cultivar "Pokhareli

cultivar "Pokhareli Machine". In Breeding research: the key to the survival of the earth. Proceedings of

locus of rice. In Rice Genetics II, (ed. IRRI), p. 11-20, IRRI, Los Banos.
 
 

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