To study leaf development of rice (Oryza sativa L.), it is useful to characterize mutants exhibiting abnormal leaf shape or size. Although there is a vast inventory of morphological mutants on shape of leaf blades such as narrow and rolled, their phenotypes do not change during their growing period (Jones 1952, Mori et al. 1973). Accordingly, mutants whose leaf phenotype changes during development would be interesting materials for elucidating leaf development. In this paper, we describe the isolation and characterization of a rice mutant whose leaf blades are normal until the 12th leaf but the 13th and following leaves are abnormal (narrow and rolled).

When we screened less-tillering-mutants from over than 11,000 M₄ plants derived from mutagened Koshihikari seeds in a paddy field, we found a rice mutant with the normal leaf blades in the lower part and the abnormal leaf blades in the upper part including the flag leaf. The shape of narrow/rolled leaf blades of the mutant (designated KT95) resembled the rolled leaf blades of upland rice grown under extreme water stress in a dry field (Nemoto et al. 1998) (Fig. 1).

To know when and how the shape of leaf blades on KT95 plant change morphologically from normal to abnormal leaf blades, KT95 and Koshihikari were seeded and planted four times at two-week intervals in a paddy. The leaf phenotypes were observed once a

week. Although total numbers of leaf blades on the main stem of KT95 and Koshihikari ranged from 14 to 17 depending on date of transplanting, total numbers of leaves of both strains were almost equal. Leaf-emergence intervals of KT95 were longer than those of Koshihikari, and therefore the heading date of KT95 was 10 - 12 days later than that of Koshihikari. Koshihikari did not show any abnormal leaf blades in all four transplanting times. On the other hand, KT95 showed normal blades on the 1st to 12th leaves as those of Koshihikari, but abnormal blades on the 13th and following leaves regardless of the total leaf number (Fig. 1). Furthermore, the mode of inheritance of the character was analyzed by crossing KT95 with Koshihikari. Out of 299 F₂ plants, there were 226 normal and 71 abnormal plants (chi² for 3:1 = 0.25), indicating that this mutation was controlled by a single recessive gene.

There are several reports on mutants with abnormal leaf morphology, all the mutants, however, constantly showed abnormal phenotypes throughout their growing period (Jones 1952, Mori et al. 1973). On the contrary, the abnormal leaf blades of KT95 did appear on the 13th leaf and thereafter, suggesting that the leaf number was recognized (counted) in KT95. Therefore, the mutation on KT95 could be associated with perception and/or counting mechanism of the leaf number. Accordingly, this mutant should be useful in understanding morphological and genetic mechanisms of leaf development.

References

Nemoto, H., R. Suga, M. Ishihara and Y. Okutsu, 1998. Deep rooted rice varieties detected through the observation of root characteristics using the trench method. Breed. Sci. 48: 321-324.

Mori, K., T. Kinoshita and M. Takahashi, 1973. Linkage relationships of genes for some mutant characters of rice kept in Kyushu University. - Genetical studies on rice plant, LV. Mem. Fac. Agr. Hokkaido Univ. 8: 377-385. (in Japanese with English summary)

Jones, J.W., 1952. Inheritance of natural and induced mutations in Caloro rice and observations on sterile Caloro types. J. Hered. 43: 81-85.