Rice Genetics IV - IRRI books - International Rice Research Institute

elements. Two of them were located in the ORFs of the members D and E, suggestingthat these elements may play a role in the diversification of the Xa21 family members.Based on these results, we propose that duplication, recombination, and transpositioncontribute to the amplification and diversification of the Xa21 gene family. Aproposed evolution model of the Xa21 locus is presented in Figure 1.Spectrum of resistance to 32 diverse bacterial blight isolatesThe high level of resistance observed in IRBB21 to multiple Xoo isolates may be dueto the single gene Xa21 or a cluster of tightly linked genes, each of which recognizesa unique isolate-specific determinant. To test these hypotheses, 32 isolates from eightcountries were used to inoculate Xa21 transgenic plants. The transgenic plants expressingthe cloned Xa21 gene conferred resistance to 29 out of 32 diverse isolates,but were susceptible to three isolates from Korea. The resistance spectrum of theengineered line was identical to that of the donor line, indicating that the single clonedgene is sufficient to confer multi-isolate resistance (Wang et al 1996). It is worthnoting that some of the Xa21 transgenic lines showed increased resistance to Xoocompared with the donor lines. This increased resistance may be due to the high copynumber of the transgene in these plants and this has great application for engineeringhigh-level resistance to Xoo in rice cultivars.Xa21 classDup and MutRec and TransXa21D98.0%Dup and MutFRecDup and MutA1A1 classDup and MutRec and TransA2C95.2%EFig. 1. Proposed model for the evolution of Xa21 family members. Boxes represent the openreading frames of Xa21 family members; horizontal bars represent the flanking regions andintrons. The vertical bars indicate gene-coding regions. The same color indicates a high level ofsequence homology. The highly conserved region is shown in yellow. Evolutionary events areindicated as follows: Dup = duplication, Mut = mutation, Rec = recombination, Trans = transposition.The percentage shows the sequence identity among the members in the same class.Structure, function, and evolution of disease resistance . . . 339