2. Rhizomatous individual was obtained from interspecific BC2F1 progenies between Oryza sativa and Oryza longistaminata

1)Food Crops Research Institute, Yunnan Academy of Agricultural sciences, Kunming 650205, P. R. China
2)Plant Breeding, Genetics and Biochemistry Division, International Rice Research Institute, Philippines
3)Agronomy Department, Kasetsart University, Bangkok 10900, Thailand

To breed for perennial rice, especially upland rice (Schmit 1996, Tao et al., 2000a), the most logical trait to select may be rhizomes, which are typically found in wild relatives such as O. longistaminata, O. rhizomatis, O. officinalis and O. australiensis. The development of perennial rice, especially perennial hybrid rice, has the potential to provide environmentally sound and economically viable alternatives for use on land where annual type production is not sustainable (Wagoner 1990). Perennial hybrid rice, like apomictic rice, would fix heterosis, but perennial hybrid rice would be easier to achieve (Tao et al., 2000a, Xiu 1995).

Donors of perenniality for perennial rice breeding could be O. longistaminata, O. rhizomatis, O. officinalis and O. australiensis (Vaughan, 1994). O. longistaminata should be an especially valuable source of perenniality because it has the same AA genome as O. sativa does. Introgression of rhizome from wild relatives to O. sativa is the prerequisite condition for breeding perennial rice or perennial hybrid rice.

RD23, an indica cultivar from Thailand, was pollinated by O. longistaminata. RD23 has no rhizomes and the O. longistaminata parent had long rhizomes (Photo 1). The F1 hybrid of RD23/O. longistaminata had rhizomes intermediate between the two parents (Photo 2)and had indehiscent anthers with 32.53% pollen fertility (Tao et al., 2000a).

Normally developed BC1F1 seeds were obtained when the F1 hybrid of RD23/O. longistaminata was pollinated by RD23. From 168BC1F1 seeds germinated on 1/4 MS media (3% sucrose+0.7% agar, pH 5.8), 162 plants were obtained and transplanted to an irrigated field. Among the progeny, only one individual had rhizomes like the F1 hybrid (Photo 2; Tao et al., 2000b).

When the rhizomatous BC1F1 plant was pollinated by RD23 in March, 2001, 719 normally developmed BC2F1 seeds were obtained. After the seeds were germinated on 1/4 MS media, 577 plantlets were obtained and transferred to an irrigated field. Among the BC2F1 progeny, 8 individuals with various lengths of rhizome were observed (Photo 3). This is the first report of a rhizomatous BC2F1 when O. sativa was used as recurrent parent.

Ghesquiere (1991, 1992) reported unilateral inheritance of the rhizomatous trait in crosses of O. longistaminata to O. sativa and thought that one of the two complementary lethal genes D1, located on chromosome 2, was closely linked to a rhizomatous gene or both were controlled by the same gene. Through linkage study by molecular markers, he could find that another complementary gene, D2, from O. sativa was located on chromosome 11. In contrast, Maekawa et al. (1998) reported that an incompletely dominant gene, designated as Rhz located on Chromosome 4, controlled the inheritance of rhizome. And the extremely low spikelet fertility (16.6%) of the F1 hybrid did not disturb segregation of the linked liguleless and rhizomatous traits in the F2 progenies.

In our population, the intermediate expression of rhizomatous traits in the RD23/O. longistaminata F1 indicated that the dominance was incomplete as reported in sorghum and O.

longistaminata (Makekawa et al. 1998, Yim et al. 1997). But the observation that there was only 1 rhizomatous individual out of 162 BC1F1 progenies and 8 rhizomatous individuals out of 577 BC2F1 progenies suggests that the inheritance of rhizomes was not simple. We consider two possible explanations. One is that rhizomatous gene(s) and embryo abortion gene(s) D1 are closely linkaged (Ghesquiere 1991), and the rhizomatous plant was a crossover product. Another explanation is that rhizomatous traits were controlled by several quantitative loci with dosage and/or epistatic effects. Thus, rhizomatous trait of F1 hybrids was intermediate between RD23 and O. longistaminata, and BC1F1 and BC2F1 progenies had few rhizomatous individuals. In order to test whether the embryo abortion gene(s) is linked to rhizomatous gene(s) or not, 220 young BC1 F1 embryos were cultured on 1/4 MS media 10-days after pollination, and 176 plantlets were obtained and transferred to an irrigated field. From this embryo rescued BC1F1 population, only one rhizomatous individual was observed, which indicated that there was no obvious linkage between embryo abortion genes and rhizome genes.

Financial support from National Natural Science Foundation of China, Yunnan Agricultural department, and Bundesministerium fur Wirtschaftliche Zusammenarbeit und Entwicklung (BMZ) is gratefully acknowledged.


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