Abstract
Phytophthora plant pathogens contain many hundreds of effectors potentially involved in infection of host plants. Comparative genomic analyses have shown that these effectors evolve rapidly and have been subject to recent expansions. We examined the recent sequence evolution of RXLR-class effector gene families in the sudden oak death pathogen, P. ramorum. We found that P. ramorum RXLR effectors have taken multiple evolutionary paths, including loss or gain of repeated domains, recombination or gene conversion among paralogs, and selection on point mutations. Sequencing of homologs from two subfamilies in P. ramorum’s closest known relatives revealed repeated gene duplication and divergence since speciation with P. lateralis. One family showed strong signatures of recombination while the other family has evolved primarily by point mutation. Comparison of a small number of the hundreds of RXLR-class effectors across three clonal lineages of P. ramorum shows striking divergence in alleles among lineages, suggesting the potential for functional differences between lineages. Our results suggest future avenues for examination of rapidly evolving effectors in P. ramorum, including investigation of the functional and coevolutionary significance of the patterns of sequence evolution that we observed.
Highlights
The interactions between plants and their pathogens are complex, encompassing several layers of defense and counterdefense by the plant host and multiple routes to pathogenicity and evasion of detection by the pathogen [1,2,3]
Plants have evolved mechanisms to detect the presence of pathogens via recognition of pathogen-associated molecular patterns (PAMPs), which result in a biochemical signaling cascade and eventually defense gene expression in the host
The specific recognition of pathogens by host plants has been known for many years [4,5], but only relatively recently have researchers recognized the very large numbers of genes in eukaryotic plant pathogens that directly interact with plant host molecules [6,7,8]
Summary
The interactions between plants and their pathogens are complex, encompassing several layers of defense and counterdefense by the plant host and multiple routes to pathogenicity and evasion of detection by the pathogen [1,2,3]. Plants have evolved mechanisms to detect the presence of pathogens via recognition of pathogen-associated molecular patterns (PAMPs), which result in a biochemical signaling cascade and eventually defense gene expression in the host. PAMPtriggered immunity (PTI), a general defense mechanism, restricts pathogen infection and growth. Plant pathogens may secrete hundreds of proteins that act on plant targets, termed effectors, in order to colonize and reproduce in host plant tissues. The specific recognition of pathogens by host plants has been known for many years [4,5], but only relatively recently have researchers recognized the very large numbers of genes in eukaryotic plant pathogens that directly interact with plant host molecules [6,7,8]. Effectors are thought to play a role in determining pathogen host range [9,10]
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