Evolution of Plant Resistance Genes: their Structure and Function.

Home page of Dr. Tony Pryor:
http://www.pi.csiro.au/Research/GenEng/SubPrograms/YB.htm

In the August, 2000 issue of Current Opinion in Plant Biology (Evolution of Plant Resistance Genes: their Structure and Function, (3(4):278-84.), Dr. Tony Prior and his associates at CSIRO, Australia have described the various factors contributing to the evolution of plant resistance genes classed under  R locus. In spite of their differences in DNA base pairs, all R genes are characterized by the same kind of proteins they encode. These proteins contain a nucleotide binding site (NBS) and a leucine repeat region (LRR). This class of genes are abundant in plant species and in Arabidopsis their number has been estimated  to be 200, approxi- mately 1 percent of its total genome size. R genes are classed into two groups: one containing, an additional region, called TIR  Toll/interleukin-1-receptor and the other in which TIR is absent. The R genes in Arabidopsis fall into the first category.

Two basic processes, namely perception of pathogen attack and responses by the host to contain  disease are involved in gene-for-gene plant disease resistance.  Receptors perceive responses with high degrees of specificity and counteract  invading pathogens with specific proteins to defend against their attack. A number of distantly related R genes are involved in this process. In order to interact with the rapidly evolving pathogens, R genes have evolved giving rise to R-gene polymorphism through gene duplication, change of base pairs through point mutations, deletion and duplication of intragenic DNA repeats . Furthermore, meiotic recombination in which reassortment of sequence diversity occurs led to rapid evolution of R locus.   Some of these diversified R genes encoding LRR of different composition are better suited to combat new strains of pathogens after being subjected to diversifying selection pressure.

One  interesting observation has come to light through recent studies on R locus. Some R genes such as RPP1 (resistance to Peronospora parasitica), RPP5 and RPP8 are found in complex loci with 2-9 paralogues (originating from the same locus by duplication followed by divergence), each locus containing 2 or more resistance specificities. On the other hand, some, such as RPS2 (resistance to Pseudomonas syringe) are simple loci conferring resistance to only one specific pathogen, in this case, against Pseudomonas syringe 2.

R genes and pathogen avirulence (Avr)  genes evolve simultaneously and the capacity of an R-gene to combat a disease is lost as a result of mutation of the corresponding pathogen. This imposes selection pressure on the host for new resistance specificities. Results of a recent study show that resistance conferred by RPM1 locus is associated with a single NBS-LRR gene whereas a large deletion covering the RPM1 locus is responsible for susceptibility. Both alleles are widely distributed in natural populations.

The authors conclude that although considerable progress has been made recently to understand the specificity of R genes and how they have evolved, there are a still a number of important questions which need to be addressed to. Among others these questions are: nature of interactions  between R-gene products and their corresponding avirulence proteins, Dr. Tony Prior and his associates of CSIRO, Australia have described the various factors contributing to the evolution of plant resistance genes classed under  R locus. In spite of  their  differences in DNA base pairs, all R genes are characterized by the same kind of proteins they encode. These proteins contain a nucleotide binding site (NBS) and a leucine repeat region (LRR). This class of genes are abundant in plant species and in Arabidopsis their number has been estimated  to be 200, approxi- mately 1 percent of its total genome size. R genes are classed into two groups: one containing, an additional region, called TIR  Toll/interleukin-1-receptor and the other in which TIR is absent. The R genes in Arabidopsis fall into the first category.

Toll/interleukin-1-receptor and the other in which TIR is absent. The R genes in Arabidopsis fall into the first category.

Two basic processes, namely perception of pathogen attack and responses by the host to contain  disease are involved in gene-for-gene plant disease resistance.  Receptors perceive responses with high degrees of specificity and counteract  invading pathogens with specific proteins to defend against their attack. A number of distantly related R genes are involved in this process. In order to interact with the rapidly evolving pathogens, R genes have evolved giving rise to R-gene polymorphism through gene duplication, change of base pairs through point mutations, deletion and duplication of intragenic DNA repeats . Furthermore, meiotic recombination in which reassortment of sequence diversity occurs led to rapid evolution of R locus.   Some of these diversified R genes encoding LRR of different composition are better suited to combat new strains of pathogens after being subjected to diversifying selection pressure.

One  interesting observation has come to light through recent studies on R locus. Some R genes such as RPP1 (resistance to Peronospora parasitica), RPP5 and RPP8 are found in complex loci with 2-9 paralogues (originating from the same locus by duplication followed by divergence), each locus containing 2 or more resistance specificities. On the other hand, some, such as RPS2 (resistance to Pseudomonas syringe) are simple loci conferring resistance to only one specific pathogen, in this case, against Pseudomonas syringe 2.

R genes and pathogen avirulence (Avr)  genes evolve simultaneously and the capacity of an R-gene to combat a disease is lost as a result of mutation of the corresponding pathogen. This imposes selection pressure on the host for new resistance specificities. Results of a recent study show that resistance conferred by RPM1 locus is associated with a single NBS-LRR gene whereas a large deletion covering the RPM1 locus is responsible for susceptibility. Both alleles are widely distributed in natural populations.

The authors conclude that although considerable progress has been made recently to understand the specificity of R genes and how they have evolved, there are a still a number of important questions which need to be addressed to. Among others these questions are: nature of interactions  between R-gene products and their corresponding avirulence proteins

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