22. Mutant genes controlling starch synthesis in rice endosperms

Kazutoshi OKUNO¹ and Masahiro YANO'²

1) Hokuriku Natl. Agric. Exp. Station, Joetsu, Niigata, 943-01, and

2) Faculty of Agriculture, Kyushu University, Hakozaki, Fukuoka, 812 Japan

Starch is generally composed of two kinds of polysaccaride, amylose and amylopectin. In rice, genes controlling the proportion of amylose to amylopectin have not been identified although the wx is known to suppress amylose production. In maize, several genes which change the proportion of the two types of starches are known. The present authors found genes for low and high amylose contents among lines induced from Japanese cultivars; the low amylose mutant was obtained from the progeny of Norin 8 treated with ³²P (beta ray), and the high amylose mutant from Kinmaze treated with NHU.

The low amylose mutant was characterized by 'dull' endosperm. The starch was analyzed by gel filtration on a Sephadex G-75 column of starch components after debranching with Pseudomonas isoamylase. The content of amylose (Fr. I) in the dull mutant was about half of that of normal Norin 8. The contents of 'intermediate fraction' and Fr. II (longer unit chains of amylopectin) were similar to those in the waxy (glutinous) mutant and normal, and the content of Fr. III (shorter unit chains of amylopectin) was higher than that of normal, covering the reduction of amylose. However, the Fr. III/Fr, II ratios did not differ much among the normal, dull, and glutinous endosperms, suggesting that the distribution of unit-chain length of amylopectin remained unchanged in the mutant lines (Okuno et ai. 1983).

The amylose content in F\2\ and B\1\F\1\ endosperms was determined by Technicon Autoanalyzer with single grains. The F\2\ endosperms from a cross between normal and dull segregated into 3 normal : 1 dull, and the B\1\F\1\ endosperms from dull/normal//dull segregated into 1 normal: 1 dull. The F\2\ of a cross between dull and waxy mutants segregated into waxy, dull and normal types and the ratio was assumable to be 4 : 3 : 9 although the variation was continuous. The data thus indicated that amylose production in the dull mutant was controlled by a recessive gene, du, which was independent of wx. The normal, dull and waxy lines were intercrossed in different combinations, and the parental and reciprocal F\1\ seeds were compared. The wx gene decreased amylose content in proportion to dosage in the endosperm, but du showed no such dosage effect.

Table 1. Properties of isoamylase-debranched endosperm starches from a high amylose mutant (EM-16) and control (Kinmaze)

=============================================================================
Line      Distribution of components (%)     Fr. III/ Chain length at peak of
         Fr.I    Int.Fr.   Fr.II    FR.III    Fr.II      Fr.II    FR.III
=============================================================================
Kinmaze  20.4      3.7     16.7      59.2      3.5         38        14
EM-16    27.7     10.1     26.7      36.0      1.4         42        16
=============================================================================

Furthermore, five mutant lines with an increased amylose content were found among those with floury endosperms, which had been induced from Kinmaze. Their amylose contents were 29.4 to 35.4 %, about twice as high as that of the normal line. One of them, EM-16, was used for analysis by gel filtration on a Sephadex G-75 column of starch components after debranching with Pseudomonas isoamylase. It showed an increased proportion of longer unit chains of amylopectin (Fr. II) as compared with the control (Table 1). The X-ray diffractogram of starch granules from the normal showed a type-A pattern which was typical of cereal starches, while that from the high amylose mutant showed a type-B pattern. The onset temparature of gelatinization of the mutant was much higher (63 - 69degC) than that of the normal line (52deg C). The endosperm cells of the mutant were loosely packed by irregularly round-shaped starch granules, while those of the normal line were densely packed by polyhedral starch granules. The high amylose mutant seemed to have similar starch properties to those of the amylose-extender (ae) lines of maize. The F\2\ seeds from a cross between normal and high amylose lines segregated into 3 normal : 1 high. The high amylose content was controlled by a recessive gene, which was symboled ae tentatively.

Reference

Okuno, K., H. Fuwa and M. Yano, 1983. A new mutant gene lowering amylose content in endosperm starch of rice, Oryza sativa L. Jpn. J. Breed 33: 387-394.