3.6.5S
NETWORKING SCIENCE: PARTNERSHIP OF RICE DISEASE RESEARCH

H LEUNGl, GL WANG2 and JE LEACH3

1International Rice Research Institute, Los Banos, Philippines; 2Institute of Molecular Agrobiology, Singapore; 3Kansas State University, Manhattan, Kansas, USA

Background
With advancing tools and techniques, much information has accumulated in various aspects of rice disease research, including genetics, physiology, epidemiology, molecular biology, and population biology. Each sub-discipline requires specialization that is often beyond the research capability of a single institute. There is a growing need to combine complementary expertise to maximize the impact of individual research efforts. This paper uses three research projects to illustrate the coordinated efforts towards the development and deployment of durable resistance in rice.

Developing broad-spectrum disease resistance
Recognizing the importance of non-race specific resistance in conferring durable resistance, this project aims at identifying genes controlling the common defense pathway against multiple pathogens. Our approach is to dissect the defense pathway using induced and spontaneous mutants that show common susceptibility to bacterial blight and blast. We screened 4,500 M3 mutants of IRBB21 (which contains the bacterial blight resistance gene Xa21 in IR24 background) generated from chemical and irradiation mutagenesis, and identified mutants susceptible to both diseases. At least one class of mutants contains deletions at the Xa21 locus. In addition, natural lesion mimic mutants show enhanced resistance to both blast and blight. Preliminary data suggests that the lesion mimic mutants constitutively express defense response genes. Genetic mapping and biochemical analyses of these mutants are underway.

Predicting durability of resistance genes
This project aims at developing a proactive approach to assessing the durability of resistance genes based on analysis of pathogen adaptation. We hypothesize that resistance genes that impose a high penalty for adaptation will likely be durable. We are using bacterial blight to test this hypothesis. A three-year field experiment showed that despite the presence of virulent strains in the field, an isogenic line containing Xa7 does not show severe disease. Laboratory strains with a deleted aviruience gene avrXa7 show a fitness penalty as measured by bactedal multiplication and lesion development in the plant. Molecular analysis of Xa7-adapted strains from the field showed that those without the avrXa7 DNA fragment do not persist in the population. We are determining the region of the aviruience gene responsible for pathogenic fitness. Validation of this concept of "adaptation cost" may lead to a systematic ranking of durability of resistance genes.

integrating pathogen analysis into cultivar development
Through the Asian Rice Biotechnology Network (ARBN), we are integrating pathogen population analysis and host genetics in developing durable bacterial blight resistance in commercial cultivars. The ARBN program involves research teams from PhilRice-Philippines, Punjab Agricultural University-India, Central Rice Research Institute-India, and Central Research Institute for Food Crops-Indonesia, who share a mandate of developing bacterial blight resistant cultivars. Each team has characterized extensively the local pathogen populations using pathogenicity and molecular markers. Representative pathogen strains are then used to assess the effectiveness of single and multiple resistance genes in near-isogenic lines. This parallel pathogen-host analysis has enabled each country/region to incorporate appropriate genes into high-yielding cultivars with resistance against local pathogen populations.

Perspectives
Identification and use of durable resistance is a major goal of rice pathology research. Advances in our understanding of host-pathogen interactions have offered new approaches to design better resistance at the genotypic and population level. The above examples illustrate a possible pathway in converting research results in the laboratory to impact in the field. Given the broadness of the disciplines involved, more collaborative projects designed to capture both basic and applied research will be needed in the future.