BAR TODAY || Official Quarterly Publication of the Bureau of Agricultural Research

Archives (2003) : Rice

Developing a rice hybrid resistant to bacterial blight
by Junelyn de la Rosa

January-March 2003
Volume 5 No. 1

Bacterial blight (BB) is one of the most devastating diseases in rice and reports show that it could easily wipe out more than a fourth of the total harvest in rice hybrids.

Leaf lesions caused by bacterial blight

Hybrid rice is gaining popularity as the government's Hybrid Commercialization Rice Program (HCRP) reaches the countryside. It is more high yielding than the inbreds. Mestizo, a hybrid yields 15% to 20% higher than the local inbred varieties.

However, the downside of hybrid rice is, they are more susceptible to pests and diseases. Hybrids exhibit "cytoplasmic uniformity". The cytoplasm is the material that surrounds the nucleus, and carries extra-nuclear genes that can provide resistance to pests and diseases. Widespread use of only one or two types of cytoplasm can lead to massive crop failures, as when a fungus wiped out 15 percent of a corn crop in the US in 1970. Almost the entire crop was derived from an identical source of susceptible cytoplasm.

In response, scientists from the Philippine Rice Research Institute (PhilRice) in collaboration with some hybrid research institutions in China worked on diversifying cytoplasmic sources. They introduced more Cytoplasmic Male Sterility (CMS) types such as the E-shan-ta-bei-gu (STB) and the Zhao-tong-bei-zi-gu (ZTB) types, however both types were found susceptible to bacterial blight.

The scientists were concerned about the findings since bacterial blight (BB) is one of the most devastating diseases in rice and reports show that it could easily wipe out more than a fourth of the total harvest in rice hybrids. Bacterial blight is caused by the bacteria Xanthomonas oryzae pv.orysae (Xoo). There are nine kinds of bacteria-causing bacterial blight in the Philippines, thus the Philippines is known as the "capital of bacterial blight".

The scientists enhanced genetic resistance as a sure way of controlling bacterial blight. Using marker-aided selection (MAS), they incorporated any or a combination of the three BB resistance genes (Xa4, Xa7 and Xa21) to five maintainer lines of rice hybrids. They reported that lines that contain a pyramid of at least two genes are more durable since it would be more difficult for the pathogen to overcome more than one resistance gene.

Scientists explained that the incorporation of resistance genes into cultivars is the most economical and effective method to control bacterial blight. The availability of the cloned resistance genes, particularly the complete dominant gene Xa21, that has a wide spectrum of resistance to the pathogen provides an important opportunity for improving bacterial blight resistance in hybrid rice.

With the new improved lines, hybrid rice breeders can develop rice hybrids that are resistant to bacterial blight. The scientists are optimistic that hybrid rice breeders will utilize the improved lines and solve one of the country's endemic pest problems--bacterial blight. Finally, scientists say that this technology can help hybrid rice i.e.Mestizo reach its yield potential and fill all rice bowls in the country.

Source: Marker-aided Pyramiding of Bacterial-Blight Resistance Genes in Maintainer Lines of Rice (Oryza sativa L.) Hybrids by Lucia M. Borines of Leyte State University, Edilberto D. Redoña of the Philippine Rice Research Institute, Marina P. Natural of Department of Plant Pathology of UPLB, Casiana M. Vera Cruz and Hei Leung of IRRI. Winner of AFMA R&D Paper Award and Best Poster Award during the 2002 National Research Symposium