Two-line hybrid rice breeding in and outside China

摘要

The two-line system for hybrid breeding is more efficient than the three-line system for exploiting heterosis in rice to increase the yield per unit area. Three PGMS genes and five TGMS genes have been identified by scientists from China, Japan, IRRI, Vietnam, and India. The PGMS genes pms1, pms2,and pms3 were located on chromosomes 7, 3, and 12, respectively. The TGMS genes tms1, tms2, tms3, tms4(t), and tms5 were located on chromosomes 8, 7, 6, 9, and 2, respectively. The molecular markers linked with these genes have also been found. In China, more than ten EGMS lines were used to breed commercial two-line hybrid rice. Thirty two-line rice hybrids were released in various rice-growing regions up to 2001. The area planted to two-line hybrid rice was 1.54 million ha in 2000. EGMS lines are multiplied in three ways: in the autumn season, in the winter season, and under cool-water irrigation. Yields of seed multiplication were 2.5 t ha.1. A technological package for seed production of two-line hybrid rice has been developed.The area of seed production was around 13,000 ha across China in 2001. The average yield was 2.5 t ha.1, which was similar to that of threeline hybrid rice seed production. The two-line hybrid rice breeding program at IRRI focuses on developing the technology for the tropics by deploying the TGMS system. TGMS genes tms2 (from Norin PL12) and tms3 (from IR32364Smutant) have been primarily deployed for breeding new TGMS lines. ID24, a TGMS line introduced from India, has also been used. TGMS lines IR73827-23S and IR73834S, derived from ID24 mutant, have stable sterility. Identification of such lines can be done in the field by evaluating them during the wet season when the mean temperature is 1.2 oC lower than in the dry season. These lines can be multiplied by growing them under high-altitude conditions. To facilitate incorporation of the tms2 gene, a simple sequence repeat (SSR) marker, RM11 located on chromosome 7, was identified and found to be useful in identifying heterozygous fertile plants in F2 populations and F3.F4 progenies for generation advance in selected crosses. The tms genes are transferred in indica and indica/TJ derivative lines from IRRI and elite lines from collaborating countries (Bangladesh, the Philippines, and Sri Lanka). TGMS lines are also being developed at IRRI through a shuttle breeding procedure for some temperate countries such as China, Iran, and Egypt. Using the genetic male sterility.facilitated recurrent selection procedure, a composite population is also being developed at IRRI for extracting TGMS lines. This population will be shared with national agricultural research and extension systems to enable them to extract locally adapted TGMS lines. In Vietnam and India, several TGMS lines with a relatively low critical sterility point were developed. Some of them are being evaluated in the field and used for developing two-line hybrid rice. Some two-line rice hybrids were tested in multilocation trials. Other countries, such as the Philippines,Bangladesh, Sri Lanka, Iran, Egypt, and Indonesia, were evaluating EGMS lines introduced from IRRI. In the tropics, TGMS lines can be multiplied by growing them under high-altitude conditions. Experimental seed production of two-line hybrid rice was carried out at IRRI and in the Philippines, Vietnam, and India. Two-line hybrid rice would be commercialized in 3.5 years in tropical countries. Future strategies and constraints to developing two-line hybrid rice worldwide are discussed in this paper.