• Achievements

1. On Three-line System Hybrid Rice
2. On Two-line System Hybrid Rice
3. On Utilization of Heterosis between Rice Subspecies

• Constraints and Challenges

1. The Yield Level of Three-line Hybrids Has Been Stagnating for Years
2. The Area Has Been at a Standstill for Years
3. Grain Quality of Hybrid Rice Needs ImprovingThe Sources of Male Sterility Inducing Cytoplasm for Developing Better CMS Lines Are Poor

• Strategie

In 1964, Yuan Long ping first put forward the idea of utilizing the heterosis in rice and initiated the research on hybrid rice in China. In November 1970, a pollen abortive wild rice plant (shortly called wild abortive, i.e., WA, thereafter) was discovered among the plants of common wild rice at Nanhong Farm of Ya County of Hainan Island, the south most province of China.

The discovery resulted in the subsequent breakthrough in success of hybrid rice development. After the discovery of WA, a large amount of nationwide cooperative work was immediately conducted to testcross and screen for its maintainers and restorers. Soon in 1972, the first group of CMS lines such as Erjiunan 1A, Zhenshan 97A, V20A were developed by using WA as the donor of male sterile genes by way of successive backcrossing method. In 1973, the first group of restorer lines such as Taiyin 1, IR24, IR661 were screened out through direct test crossing method and in 1974 the hybrids with strong heterosis such as Nanyou 2, Nanyou 3 were released. Such combinations showed very significant yield increasing ability in the experimental production.

In the meantime, the technology of hybrid rice seed production was basically developed in 1975. Subsequently, hybrid rice was rapidly and widely extended in China and its planting area rose steeply to 139 000 ha in 1976 from 373 ha in 1975 and 7 ha in 1974. After that, the area under hybrid rice was increasing and increasing with its per unit area grain yield higher and higher. In 1991, the area of hybrid rice reached 17.6 million ha, accounting for 54% of total rice area; the national average grain yield of hybrid rice increased from 4.2 t/ha in 1976, the very beginning of hybrid rice commercialization, to 6. 7 t/ha in 1990 (Table 1 & Fig. 1).

In terms of the national average yield in commercial production, it has been proven that hybrid rice has a yield advantage of more than 20% over conventional rice. In recent years, the yearly area under hybrid rice has been about 15.5 million ha, which accounts for 50% of the total rice area, and the production of hybrid rice occupies nearly 60% of the total rice production in China. From 1976 to 1999, the accumulated planting area of hybrid rice is 233 million ha and the increased paddy from hybrid rice is as high as 350 million tons. Up to now, hybrid rice has covered indica and japonica types with different maturity, which made hybrid rice production throughout the whole China from Hainan in the south to Liaoning in the north and from Shanghai in the east to Yunnan in the west. Hybrid rice shows not only a high-yielding ability but also a wide adaptability.

In the meantime, the seed yield has also been greatly increased with the development of technology in hybrid rice seed production. In 1976, the average yield was only 274.5 kg/ha in China, in 1982 rose to 909 kg/ha, in 1985 to 1655 kg/ha and in 1990 the yield was increased to 2 250 kg/ha (Fig. 2). In Hunan Province, the yield was over 1500 kg/ha in 1983 and higher than 2 250 kg/ha in 1986. In 1990s, the yield on a large scale was increased to 3 000-4 500 kg/ha. In 1993, the highest record yield of 7.386 t/ha in hybrid rice seed production was made on a small plot (0.113 ha) at Zixing City, Hunan Province. With the increase of yield in multiplication and seed production, the area ratio of A line multiplication to F1 hybrid seed production to F1 hybrid commercial production has changed from 1:30:1000 in 1970s to 1:50:6 000 in recent years, which greatly lowers the cost of hybrid seeds.

In 1973, Chinese scientist Shi Mingsun discovered a natural male sterile plant in the field of Nongken 58, a commercial late japonica rice variety, at Shahu Farm of Mianyang County, Hubei Province. After eight years of survey on this material, he reported his discovery of the dual-purpose rice line Nongken 58S in 1981, and suggested a new way to utilize heterosis in rice by two-line system on the basis of his research results. Nongken 58S, selected from the original sterile plant, is the first dual-purpose line in rice and possesses the characteristics of fertility alteration, i.e., under long day length and high temperature conditions it is completely sterile, while under short day length and low temperature conditions it is partially fertile. Therefore, it can be used to produce hybrid seeds in its sterile period and it can also be used to multiply itself in its fertile period. The dual-purpose line is also called photoperiod- and thermo-sensitive genic male sterile (PTGMS) line for its fertility expression is regulated by day length and temperature.

After the discovery of Nongken 58S, a nationwide research was conducted on the mechanism of PTGMS and its application, soon afterwards a group of japonica and indica PTGMS lines were transferred from the original Nongken 58S. Furthermore, some other germplasms with fertility alteration such as Annong S-1, 5460 S and Hengnong S-1 were found. Up to now, tens of viable PTGMS lines in rice which possess the characteristics of low critical sterility inducing temperature (CSIT) and safe hybrid seed production have been technically identified and approved. In 1995, the success in the two-line hybrid rice research was declared and the two-line hybrid rice began to be extended in commercial production in China.

The area planted to two-line hybrid rice has been increasing year after year from 4 300 ha in 1991 to 704 400 ha in 1999 (Fig. 3). It is expected that, up to 2000, the acreage under two-line hybrid rice would have been more than 3 million ha in China. In general, the two-line hybrid rice shows a yield increase of 5%-10% compared with the current three-line hybrid rice.

With the reports on the discovery of PTGMS gene by Chinese scientist Shi Mingsun in 1981 and the discovery of a wide compatibility (WC) gene by Japanese scientists Ikehashi et al in 1984, Prof. Yuan put forward the strategic breeding idea on utilization of intersubspecific heterosis between indica and japonica rice by two-line system. Generally speaking, the yielding potential of intersubspecific hybrid rice is more than 20% higher than that of intrasubspecific hybrid rice or intervarietal hybrid rice. Since the research on two-line intersubspecific hybrid rice was included in the key national programme "863" Project in 1987, through more than ten years of research, some great progresses have been achieved in two-line intersubspecific hybrid rice. Some elite WC lines and some promising intersubspecific hybrid rice combinations have been developed and these hyrids are now under various trials and experimental productions.?Recently, the super-high yield hybrid rice program by way of utilization of intersubspecific heterosis, proposed by Prof. Yuan in 1997, has made a breakthrough in greatly increasing rice yield. In 1999, Liangyou Peijiu (Pei'ai 64S/9311), a two-line intersubspecific hybrid with a very high yielding ability in its demonstrative production, was registered and released to farmers' production in Jiangsu Province.

Although tremendous achievements have been made in three-line hybrid rice, there also exist some constraints and challenges in its development. To sum up, the major problems are as follows.

Since the yield of hybrid rice reached 6.6 t/ha in 1986, it has been remaining at the same level for many years (Fig. 1). It seems that it is very difficult to further increase the yield of hybrid rice if no new genetic resources are utilized and no new methods are exploited in terms of hybrid rice breeding.

In 1991, the acreage of hybrid rice reached its peak at 17.6 million ha, but after that the area decreased and retained at about 15.5 million ha. The main reasons are considered to be the cease and even decrease in the acreage of double cropping early hybrid rice and japonica hybrid rice. In recent years, only 20% of early cropping rice area in South of China is covered by hybrid rice, while over 90% of late cropping rice area is under hybrid rice in the same region. The limit to the development of early cropping hybrid rice is lack of elite high yielding combinations with shorter growth duration and acceptable grain quality. The planting area of japonica hybrid rice has been limited to around 0.1 million ha, accounting for only 1%-2% of total japonica rice in China for many years because of its relatively poor heterosis (about 10% over conventional japonica rice) and the sterility of its CMS lines not stable enough to produce high purity of F1 seeds and poorer grain quality.

With the increasing living standards of rice consumers in China, it is required that the grain quality of rice should be improved. In comparison with conventional rice, hybrid rice shows poorer grain quality in the traits of head rice recovery and chalkiness. How to develop rice hybrids with both high yield and good grain quality is still a challenge for breeders.

Currently, more than 85% of the CMS lines used in commercial production belong to WA types. This dominant cytoplasm situation in the existing three-line hybrid rice may have a latent crisis of leading to incidence of a destructive pest.

Based on the studies of Chinese scientists, the breeding strategy for the development of hybrid rice has been put forward by Prof. Yuan. It involves the following phases.

1. Three-line method or CMS system.
2. Two-line method or PTGMS system.
3. One-line method or apomixis system.

Second, from the view point of increasing the degree of heterosis, the exploitation of heterosis in rice could also be divided into three levels:

1. Inter-varietal hybrids.
2. Inter-sub-specific hybrids.
3. Inter-specific or intergeneric hybrids (distant hybrids).

The existing rice hybrids dominantly used in commercial production belong to the category of inter-varietal hybrids by three-line method. It has been proven that the three-line method is an effective way to develop hybrid combinations and it will continue to play an important role in hybrid rice development. However, because of its limits mentioned above, the new two-line method may become a more effective way to further promote the development of hybrid rice.

Compared with three-line method, two-line method has some advantages. Firstly, the maintainer line is not needed. PTGMS lines can either be used to conduct hybrid seed production in sterile period under higher temperature and longer day length conditions, or multiply themselves by selfing in fertile period under lower temperature and shorter day length conditions. Secondly, the choice of parents in developing heterotic hybrids is greatly broadened. Studies have shown that over 90% varieties within the same subspecies can restore the sterility of PTGMS lines in their F1 hybrids. Thirdly, because of the genic sterility inheritance of PTGMS lines, there is no negative effects caused by sterile cytoplasms and the dominant cytoplasm status occurring in CMS system would be easily avoided.

Apparently, it is easier to develop rice hybrids that possess higher yield, earlier maturity, better grain quality and improved pest resistance by two-line method than by three-line method. The research findings and production experiences have also proven that two-line hybrid rice outyields three-line hybrid rice by 5%-10%. Furthermore, it is promising to develop elite hybrids for the early cropping rice with both high yield and early maturity, and to develop heterotic japonica hybrids by two-line method, which would very likely break the deadlock of stagnant yield and area in hybrid rice. For example, Xiangliangyou 68, an early-cropping two-line hybrid rice combination with high yield, fine grain quality and early maturity, was successfully released to commercial production in 1998. It shows a very promising prospect in overcoming the great difficult long existing in developing high-yield, good-quality and early-maturity early-cropping hybrid rice in China.

However, it should be pointed out that two-line hybrid rice also faces the risk of seed purity in case of lower temperature occurred in thermo-sensitive stage of PTGMS lines in hybrid rice seed production, for the fertility alteration of PTGMS lines is conditioned by temperature and even in hot season like summer, low temperature may occur and even last for a few days. Therefore, the most important to guarantee the seed purity in hybrid seed production is to develop viable PTGMS lines with their CSIT low enough, generally 23 for temperate zones and 24 for subtropical zone.

The more advanced technology in hybrid rice development is the exploitation of heterosis of inter-sub-specific hybrids because of their very strong heterosis. However, there are some barriers such as low seed setting rate, too tall plant, too late maturity, poorly filled grains and poor grain quality commonly found in indica/japonica inter-sub-specific hybrids. For the first three problems, effective ways have been found to solve them, viz., use of wide compatibility (WC) genes, transference of allelic dwarf gene and careful choice of crossing parental lines with suitable maturity, while for the last two problems, the idea to put emphasis on developing indica/javanica hybrids in indica rice growing region and japonica/javanica hybrids in japonica rice growing area rather than developing indica/japonica hybrids has been put forward.

Based on the great progress achieved in two-line intersubspecific hybrid rice breeding, Prof. Yuan proposed the Chinese super hybrid rice breeding program in 1997. The ultimate target is to develop the hybrid rice with a daily grain yield of 100 kg/ha, i.e., 12 t/ha for hybrids with a medium maturity of 120 days. He put forward a new model of plant type for super hybrid rice and the technical approaches to realize the target. The most important in super hybrid rice breeding is to combine ideal plant type with utilization of heterosis of intersubspecific hybrids. Since the program was initiated, a great progress has been achieved. Up to now, a series of promising hybrids with super-high yielding potential has been developed. For instance, in the demonstrative productions of super hybrid rice in 1999, the average yield was over 10.5 t/ha under an area of more than 6.7 ha at 14 locations and under an area of more than 66.7 ha at one location in the provinces of Hunan and Jiangsu. Especially, a record yield of 17.07 t/ha for single-cropping rice was made under a small plot (487 m2) at Yongsheng, Yunnan Province. Based on the results from the large-scale demonstrative productions, super hybrid rice shows a great yield-increasing capacity, generally 2.25 t/ha higher than the existing three-line hybrid rice. It is expected that super hybrid rice will play an important role in making a great leap in increasing rice yield in the new century.

The farmost target in hybrid rice is to develop one-line system hybrid rice, which is to breed a true breeding heterotic F1 hybrid, i.e., the non-segregating F1 hybrid, so it is not necessary to conduct hybrid seed production year after year, which means the fixation of heterosis. At present, it seems that use of apomixis to develop true breeding F1 hybrid may be a more promising way. The research programme on apomixis was started in late 1980s in China, but it is still in tentative stage. The development of apomictic rice may require biotechnology as well as traditional breeding methods, which might take a long run to succeed in.

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