Rice Genetics IV - IRRI books - International Rice Research Institute

trum, IRBB21 conferred a high level of resistance to many diverse Xoo strains fromIndia and the Philippines (Khush et al 1990). Using the restriction fragment lengthpolymorphism (RFLP) mapping technique, the gene was found to be linked to theRFLP marker RG103 on chromosome 11 (Ronald et al 1992). To better understandthe molecular basis of the Xa21 resistance, Song et al (1995) cloned a gene at theXa21 locus using a map-based cloning strategy. Xa21 is a member of a small multigenefamily with at least seven members. Most of these members are linked at a region of270 kb. The deduced amino acid sequence of Xa21 encodes a receptor kinase-likeprotein carrying LRRs in the putative extracellular domain, a single-pass transmembranedomain, and a serine/threonine kinase intracellular domain. Compared withother cloned plant resistance genes, the structure of Xa21 is unique in carrying boththe receptor domain LRR, presumably for recognition, and the kinase domain forsubsequent signal transduction (Ronald 1997).The LRR domain determines race-specific recognitionand is subject to adaptive evolutionGenetic analysis of disease resistance has shown that resistance genes occur at manyloci and genes at each locus are often highly polymorphic for resistance specificities.Recent work on the L and M loci of flax and Cf genes of tomato demonstrated thatalterations in the LRR domain play a significant role in the evolution of resistancegenes and production of new recognition specificities (Anderson et al 1997, Thomaset al 1997, Ellis et al 1999). Within the LRR domain, amino acid differences distinguishingCf4 and Cf9 are confined to their N termini, delimiting a region that determinesthe recognition specificity of ligand binding. The majority of these differencesare in residues interstitial to those of the LRR consensus motif (Thomas et al 1997).Recently, Ellis et al (1999) analyzed the sequences of 13 alleles (L, L1 to L11, andLH) from the flax L locus, which encode Toll/interleukin-1 receptor homology–nucleotide-bindingsite–leucine-rich repeat (TIR-NBS-LRR) rust resistance proteins andfound that specificity differences between alleles can be determined by both the LRRand TIR regions.At least eight RG-103-hybridizing bands are present at the Xa21 locus in IRBB21when digested with restriction enzyme HindIII. Distinct amino acid differences arefound in the LRRs of the Xa21 gene family members, suggesting that the LRR regionis responsible for different specificities. To test this hypothesis, Wang et al (1998)investigated the inheritance of resistance patterns in transgenic rice plants carryingfamily members A1, A2, C, D, E, and F to eight Xoo races of the Philippines. It wasfound that members A1, A2, C, E, and F conferred no observable resistance intransgenic plants whereas Xa21 class member D, designated Xa21D, conferred thesame resistance spectrum as Xa21. However, the resistance level in the Xa21Dtransgenic plants was intermediate compared with that observed for Xa21 (5–7-cmlesion length). Sequence analysis indicated that the presumed open reading frame(ORF) of Xa21D was prematurely truncated because of the presence of a stop codonin the last LRR preceding the transmembrane domain. The truncated ORF encodes aStructure, function, and evolution of disease resistance . . . 337