Fig. 1. Mutant (right) and control (left).
National Institute of Genetics, Mishima, 411 Japan
Different stature mutants have been reported in rice. Most of them are dwarf or semi-dwarf, but a few of them are taller than the original strain. Overgrowth mutations twice as tall as the original plants have been described in barley, tomato and pea, but none in rice. Some of the overgrowth mutations were found to be associated with a genetic lesion, in signal transduction pathway from phytochrome in aurea tomato (Kendrick and Nagatani 1991), or in regulation of seed biosynthetic pathway in slender barley (Pollock et al. 1992). These mutants provide us with a system to study the final phenotypic expression as a consequence of the basic metabolic pathway.
A rice mutant named "Awa-odori" showing overgrowth characteristics, has been isolated from a japonica cultivar Koshihikari. About 40,000 M2 seeds derived from 25 or 30 kR gamma-ray seed irradiation were kindly made available by Professor H. Nakai of Shizuoka University. These seeds were sown directly into a paddy field with a high density, and the plants were grown without fertilizers. A total of 21,190 panicles each from a plant were collected, and 10 seeds were taken randomly from each panicle. About 200,000 M3 seeds thus obtained were germinated in plastic trays and were screened for seedling stature. Only one plant showed an extreme extension of internodes and leaf blades at the seedling stage
Fig. 1. Mutant (right) and control (left).
(Fig. 1). Although it resembled seedlings infected by "bakanae" disease caused by Gibberella fujikuroi, it showed no overproduction of endogenous gibberellic acid. The Awa-odori mutant showed a stature resembling that of slender mutant of barley (Foster 1977), but its developmental pattern differed from that of the latter. The slender mutant of barley was four times taller than the normal at the first leaf stage; the Awa-odori mutant showed no particular elongation at the first leaf stage. Its elongation doubling the height started from the third leaf stage and continued throughout the growing period. In the slender mutant of barley, the leaf blades were rather normal in shape and ears were extended abnormally, while in the Awa-odori mutant the leaf blades were extremely extended and narrow in shape and no morphological indication of panicle formation was seen.
The Awa-odori mutant produced no seeds, but two heterozygous plants were found in the same panicle from which the mutant was derived. The progeny test of the heterozygotes gave 1,459 normal and 461 mutant seedlings (including lethal ones), and the segregation ratio agreed with 3 normal: 1 mutant (X2= 1.0, P>0.3) indicating that the mutant was controlled by a recessive gene. The gene causing heterochronic internode elongation was named accelerated internode overgrowth 1 and symboled ao-l(t) tentatively.
References
Kendrick, R. E. and A. Nagatani, 1991. Phytochrome mutants. Plant J. 1: 133-139.
Pollock, C. P., H. J. Ougham and J. L. Stodaart, 1992. The slender mutation of barley. Biotechnology No. 5. Barley: genetics, biochemistry, molecular biology and biotechnology. Ed. P. R. Shewry, C. A. B. pp. 265-276.
Foster, C. A., 1977. Slender: an accelerated extension growth mutant of barley. Barley Genetics Newsletter 7: 24-27.