Plant Genome IV Conference Plant Genome IV Conference
The rice gene Xa21 confers resistance against the bacterial pathogen Xanthomonas oryzae (Xoo) (Ronald et al., 1992). We have previously demonstrated that the Xa21 locus consists of a small multigene family and that transgenic lines (T0) expressing a single member of the Xa21 gene family confer resistance to Xoo race 6 (Song et al., 1995). Unlike all other cloned plant disease resistance genes, the deduced amino acid sequence of Xa-21 encodes a glycosylated leucine rich repeat (LRR) extracellular domain, a single pass transmebrane domain and a serine threonine kinase intracellular domain. The sequence of the predicted protein of Xa21 is strongly suggestive of a role in cell surface recognition of a peptide ligand and subsequent activation of an intracellular defense response.
To determine the resistance spectrum of the cloned resistance gene Xa21, 300 T1 progeny from the transgenic line 106-17 containing Xa21 were tested for resistance to Xoo. Inoculation results indicated that the addition of only a single gene, Xa-21 , is sufficient to confer resistance to all 7 Philippine races of Xoo and 27 strains from China, Columbia, India, Indonesia, Korea, Malaysia, Nepal and Thailand. Only three Korean strains were weakly virulent on the transgenic lines. Resistance to Philippine race 1-6 cosegregated with the PCR profile of the Xa21 transgene in a 3:1 ratio. Southern analysis confirmed the PCR results, indicating that a single member of the Xa21 multi-gene family can confer resistance to diverse strains isolated from eight countries.
1. Ronald PC, Albano B, Tabien R, Abenes L, Wu KS, McCouch S, Tanksley
SD. 1992. Genetic and physical analysis of the rice bacterial blight
disease resistance locus, Xa21. Mol Gen Genet 236:113-120.
2.Song WY, Wang GL, Chen L, Kim KS, Holsten T, Wang B, Zhai Z, Zhu LH,
Fauquet C, Ronald PC. The rice disease resistance gene, Xa21, encoded
a receptor kinase-like protein. Science in press.