Development of Xa-gene pyramid rice lines with durable resistance to bacterial blight

Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the major diseases of rice worldwide and it causes substantial damage on rice yields. Utilization of host-plant resistance in reducing the disease already proven effective and is extensively used for reducing damage due to bacterial blight. An effective resistant gene has to be durable after its deployment for a long time over a large area, this is essential to attain a sustainable disease management for the disease. At present, there are 28 known BB resistant genes designated Xa-genes. Only a few of these genes have been actively used in breeding genes tagged with molecular markers have been pyramided into IR24 as a resistant donor in breeding program and also an experimental population for studying pathogen adaptation in nature. However, none of the pyramid lines contain Xa7, a resistant gene predicted to provide durable resistance in rice based on a 6-year study in farmers' fields. The Xa7 gene imposed a fitness penalty on Xoo carrying the corresponding virulence gene, avrXa7. A cross between IRBB7 (carrying Xa7 gene) and IRBB60 (carrying Xa4, xa5, xa13 and Xa21) was made to develop pyramids of Xa7 in various combination with Xa4, xa5, xa13, and Xa21. Backcrossing and progeny selection was based on the presence of the molecular markers linked to the resistance genes. The presence of resistance genes was confirmed at each backcross generation. Lines containing the different gene combinations were selected and advanced until all genes are in homozygous condition. Lines with different gene combinations was evaluated at the F6 generation against 10 Xoo races. Reaction of the Xa7 pyramids showed shorter lesions than the NILs and comparable with IRBB60 carrying four genes for BB resistance. Incorporation of Xa7 and other Xa-genes into pyramid lines having extended into maintainer lines of hybrid rice from PhilRice and IRRI's new plant type (NPT) from tropical japonica. Currently the pyramid lines carrying Xa4, xa5, xa13, and Xa21 in various combination are also grown in farmers' fields to continue the study of pathogen adaptation and fitness changes in the pathogen population