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What is hybrid rice?

什么是杂交稻

Hybrids are produced by crossing two inbred- genetically fixed - varieties of a particular crop. Hybrids are special because they express what is called "heterosis" or hybrid vigor.

The idea is that if you cross two parents which are genetically distant from each other, the offspring will be "superior", particularly in terms of yield. However, the so-called heterosis effect disappears after the first (F1) generation, so it is pointless for farmers to save seeds produced from a hybrid crop.

This makes it very profitable to go into the seed business, since farmers need to purchase new F1 seeds every season to get the heterosis effect (high yield) each time. Rice is a mainly self-pollinated crop.(i) Each rice plant produces its own pollen which gets into an ovary and through fertilisation produces seed - what we eat as the rice grain. Rice has been a poor candidate for commercial hybridisation because you would have to find a way to sterilize some of the plants and then force them to cross with fertile plants. Going against nature can be hard work. For this reason, private industry never came into the rice seed business, and left farmers with their own seed supply. Only recently did this start to change.(ii)

In 1970, Chinese researchers discovered a male sterile rice plant growing naturally within a population of wild rice ( Oryza sativa f .spontanea ) on Hainan Island. This plant had a particular cytoplasm - the material surrounding the cell nucleus - that induces male sterility through interaction with the nucleus. The plant was named "wild rice with abortive pollen" or WA for short. Scientists in China then began crossing WA with other rice varieties to determine whether this male sterility could be passed on to subsequent generations. Those that came out male sterile, called maintainer lines, were then repeatedly backcrossed until a stable sterile plant was achieved. This plant is called a "cytoplasmic male sterile" or CMS line. CMS lines form one of the parental lines for producing hybrid rice seeds. The other is known as the restorer line, as it restores fertility to the CMS line when it is crossed. The seeds from this cross are the F1 hybrid seeds, which is what farmers can then sow. The plants grown from F1 seeds show hybrid vigour. The next generation (F2) will normally not perform as well. The CMS system is known as the three-line system since it requires three lines of rice: a CMS line, a maintainer line, and a restorer line. This system is what is being used in every country working on hybrid rice at present. However, it has several drawbacks. It is complicated, there are very few CMS lines and scientists are having a hard time identifying good maintainer and restorer lines.(iii) For example, no effective restorer lines have been identified for japonica rice, which is cultivated in temperate zones. Similarly, scientists have only found a few varieties with male sterility-inducing cytoplasm, such as WA. None are as effective as WA, and WA continues to account for over 90% of all the hybrid rice varieties produced commercially in China alone.(iv)

Researchers are now experimenting with new methods of hybrid rice production. One is called "environment-sensitive genetic male sterility", which uses either photoperiod-sensitive genetic male sterility (PGMS) or thermo-sensitive genetic male sterility (TGMS). PGMS lines are sterile lines that regain fertility with daylight fluctuations. Therefore, they can only be used in temperate zones. TGMS lines regain fertility when the temperature fluctuates, which means they can be used in the highlands of the tropics. These methods are known as two-line systems since they do not require maintainer lines and any fertile line can be used as a pollen parent. Proponents maintain that this offers a wider choice of parental lines, but both PGMS and TGMS suffer from similar limitations to the CMS lines. Sources of PGMS and TGMS are exceedingly rare and by 1994, only 12 had been identified.(v)

There is still another horizon, which the Chinese call the one-line system . This refers to the long-term goal of transferring apomixis into rice, in this case hybrid rice. Apomixis is the capacity of a plant to reproduce asexually. Apomictic plants develop seeds, but without the merger of male and female reproductive cells. So the seeds are clones of a single parent plant. Apomixis is common in weeds but rare in crop plants. It doesn't exist in the Oryza genus but it does exist in Pennisetum and scientists have been hoping to transfer the genes for apomixis from pearl millet.

Other methods of inducing sterility in rice are genetic engineering, for example the SeedLink system,(vi) and chemical approaches using gametocides.

References

(i) Rice can also cross-pollinate, naturally. However, this doesn't occur very often. In cultivated rice, the highest outcrossing rate observed is 6.8% while the average is around 5%. In wild rice, however, the outcrossing rate can be up to 100%. (S.S. Virmani, "Heterosis and Hybrid Rice Breeding", Monographs on Theoretical and Applied Genetics #22, Springer-Verlag, Berlin, 1994, p. 79.)

(ii) Much of the following information is derived from S.S. Virmani, "Hybrid Rice", Advances in Agronomy , Vol. 57, Academic Press, Inc, 1996, pp. 377-462.

(iii) For example, there is no effective restorer for japonica rice, which is the rice type grown in temperate zones.

(iv) .S.S. Virmani, "Hybrid Rice", op cit. , p. 395.

(v) Ibid. , p. 399.

(vi) Plant Genetic Systems, a biotech company in Belgium, developed the SeedLink system which involves a gene isolated from the tobacco plant, TA 29. The gene, which has been patented, is being transferred into rice to make it male sterile. PGS has since been bought by Bayer.

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