Improving hybrid rice through anther culture and transgenic approaches

Among the available genetic means to break the yield barrier in rice, agreement is widespread that rice hybrids have the potential to increase yield by 1-1.5 t ha¯¹ in farmers' fields under irrigated conditions. However, rice hybrids are affected by as many pests and diseases as common rice varieties. For the large-scale adoption of this technology, hybrids need to be resistant to the major pests and diseases prevailing in the target areas. Chemical control of yellow stem borer has been either ineffective or expensive, besides leading to environmental pollution. As an effective plant protection strategy, genetic engineering approaches now enable the production of insecticidal protein within the rice plant itself, which offers environment-friendly protection against insect pests. The incorporation of resistance genes in CMS or maintainer or restorer lines is expected to make hybrids resistant to the target disease or pest. While it is difficult to product transgenic CMS plants every time, the transformed maintainer line with resistance gene will result in a resistant CMS line by backcrossing. The transgenic restorer line, in constrast, can be directly used for hybrid production. Recently, we have successfully transformed two transgenic maintainer lines (IR68899B and IR 68897B) and two restorer lines (MH-63 and BR-827-35R) with the truncated chimeric Bt gene, crylAb (driven by 35S and PEPC promoters), and/or the hybrid Bt gene crylAb/crylAc (driven by the actin I promoter). These lines showed a wide range of expression (low to high) of Bt proteins, which were stably inherited. For BR-827-35R (T0 plants), protein expression was 0.61 mg g¯¹ of fresh leaf vis-a-vis 20 mg mg¯¹ soluble protein found in the Bt MH-63 line. A selected homozygous MH-63 Bt was hybridized with CMS line Zhenshan 97A to produce the first-ever hybrid Bt rice (Shanyou 63). The hybrid rice was field-evaluated for the first time in Wuhan, China, in 1999-2000. The transgenic CMS restorer plant and its hybrid exhibited excellent protection against extremely high, repeated infestations of yellow stem borer and natural outbreaks of leaffolder. The transgenic hybrid not only showed high protection against both insects but also recorded 28.9% more yield than the non-Bt hybrid. Similarly, Xa21 has been incorporated in maintainer (B) and restorer (R) lines. The combinations of Xa21 and Bt will further enhance the yield of hybrid rice and ensure a pesticide-free environment and improved rice breeding. Further work on developing transgenic IR58025B with plant protection is in progress. Our recent research on nutrition (iron, protein, and provitamin-A) improvement and tolerance for abiotic stresses will probably diversify and improve the hybrid rice-breeding program.