38. Elongated uppermost internode (eui) is a recurring mutation

1) USDA-ARS, Dept. of Agronomy and Range Science, University of California, Davis, CA 95616, U.S.A.

2) USDA-ARS, Jamie Whitten Delta States Research Center, P.O. Box 225, Stoneville, MS 38776-0225, U.S.A.

3) California Cooperative Rice Research Foundation, Rice Experiment Station, P.O. Box 306, Biggs, CA 95717, U.S.A.

Hybrid seed production currently relies on cytoplasmic male sterility in the female parent. Low outcrossing reduces the seed-set on the sterile plants. Seed set can be increased by practices such as clipping the flag leaves of the females, applying GA to promote panicle exsertion, pulling ropes across the field to increase pollination and selecting pollinator parents that are 10-20 cm taller than the sterile parent to increase pollen shedding on the female panicles (Virmani and Edwards 1983).

Rutger and Carnahan (1981) isolated a recessive-tall mutant which they designated 'elongated uppermost internode' (eui). They noted that this gene would be a useful trait in a pollinator parent for hybrid rice seed production; the taller height would increase the amount of pollen shed on the female. As the trait is recessive, the F1 plants would have normal (semidwarf) height. In China, this gene has been introduced into cytoplasmic male sterile plants to increase panicle exsertion (Shen and He 1989, He and Shen 1991). The cms plants with the eui gene do not need GA for promotion of panicle exsertion.

The original eui mutant was identified in 1976 in California, and this mutant has served as a source for this trait in other breeding programs including China. We are aware of only one report of this mutant from another source (Maekawa et al. 1989). In contrast, eui mutants have been repeatedly identified in California cultivars since the original report. We tested the allelism of three eui mutants isolated independently from the California cultivars M-202 and CM-101, and the breeding line 83Y45, with the original eui mutant. All F2 and F3 plants from crosses with these mutants were eui, indicating that they are allelic to the original eui gene. This indicates that eui is a recurring mutation in California.

He, Z. and Z. Shen, 1991. Inheritance of panicle exsertion and improvement of male sterile line in rice [In Chinese, English summary]. Chinese J. Rice Sci. 5: 1-6.

Maekawa, M., T. Maekawa, N. Shinbashi, and T. Kinoshita, 1989. Allelism of genes for elongation of uppermost internode from two sources. RGN 6: 101-103.

Rutger, J.N. and H.L. Carnahan, 1981. A fourth genetic element for facilitating hybrid seed production in cereals-a recessive tall in rice. Crop Sci. 21: 373-376.

Shen, Z. and Z. He, 1989. Interaction between eui gene and WAMS cytoplasm of rice and improvement of panicle exsertion of MS line. p. 753-756. In Proceedings from the SABRAO Congress, 21-25 August 1989, Tsukuba, Japan.

Virmani, S.S. and I.B. Edwards, 1983. Current status and future prospects for breeding hybrid rice and wheat. Advances in Agronomy 36: 145-214.