PAG-V: S9 - RECEPTOR KINASE MEDIATED DISEASE RESISTANCE IN RICE AND TOBACCO

Plant & Animal Genome V Conference

Town & Country Hotel, San Diego, CA, January 12-16, 1997.

S9

RECEPTOR KINASE MEDIATED DISEASE RESISTANCE IN RICE AND TOBACCO

RONALD, PAMELA CHRISTINE(1), Wen-Yuan Song(1), Guo-Liang Wang(1), Li Ya Pi(1), Veronique Szabo(2), randy Ruan(2), Alec Forsyth(2), Yujuan Song(2), Masa Yoshikawa(2), Lili Chen(3), ShePing Zhang(3), Roger Beachy(3), Claude Fauquet(3)
1. Dept Plant Pathology , UCDavis, Davis CA 95616
2. Center for Engineering Plants for Resitance Against Pathogens, University of California, Davis, 95616
3. International Laboratory for Tropical Agricultural Biotechnology, Scripps, San Diego

The rice Xa21 gene, which confers resistance to Xanthomonas oryzae pv. oryzae, was isolated by positional cloning. Transgenic plants containing the Xa21 gene showed resistance to 31 isolates of Xoo. The sequence of the predicted protein, which carries both a leucine-rich repeat motif and a serine-threonine kinase, suggests a role in cell surface recognition of a pathogen ligand and subsequent activation of an intracellular defense response. In support for this model, we have found that Xa21 encodes an active kinase with serine threonine specificity. The sequence of 6 Xa21 homologs in rice suggests that bp mutation, transposition, recombination and duplication have played an active role in evolution of the gene family. One member of the family, copy D, encodes a truncated protein due to the presence of a retrotransposon-like sequence in the LRR domain. Interestingly, although this sequence lacks a kinase, it confers partial resistance to Xoo in transgenic plants, suggesting that the receptor-like domain is capable of pathogen recognition. Xanthomonas infects a wide variety of plants including both monocotyledonous and dicotyledonous species. In order to test the effect of Xa21 in dicots, we transformed tobacco with the Xa21 genomic clone. RT PCR performed on the primary transformants showed that Xa21 is transcribed in tobacco plants and that the intron present in the kinase is spliced. These plants display a rapid HR (hypersensitive response) when inoculated with Xanthomonas. The rapid HR phenotype cosegregates with the presence of the transgene in the progeny of the primary transformant. Finally, in order to study the role of Xa21 homologs in tomato, we designed degenerate primers in regions of Xa21 so that the amplified product would span the transmembrane domain. PCR amplification in tomato led to the identification of sequences with high homology to Xa21. The map position of these clones will be presented.