Abstract
BackgroundTwo opposing evolutionary constraints exert pressure on plant pathogens: one to diversify virulence factors in order to evade plant defenses, and the other to retain virulence factors critical for maintaining a compatible interaction with the plant host. To better understand how the diversified arsenals of fungal genes promote interaction with the same compatible wheat line, we performed a comparative genomic analysis of two North American isolates of Puccinia graminis f. sp. tritici (Pgt).ResultsThe patterns of inter-isolate divergence in the secreted candidate effector genes were compared with the levels of conservation and divergence of plant-pathogen gene co-expression networks (GCN) developed for each isolate. Comprative genomic analyses revealed substantial level of interisolate divergence in effector gene complement and sequence divergence. Gene Ontology (GO) analyses of the conserved and unique parts of the isolate-specific GCNs identified a number of conserved host pathways targeted by both isolates. Interestingly, the degree of inter-isolate sub-network conservation varied widely for the different host pathways and was positively associated with the proportion of conserved effector candidates associated with each sub-network. While different Pgt isolates tended to exploit similar wheat pathways for infection, the mode of plant-pathogen interaction varied for different pathways with some pathways being associated with the conserved set of effectors and others being linked with the diverged or isolate-specific effectors.ConclusionsOur data suggest that at the intra-species level pathogen populations likely maintain divergent sets of effectors capable of targeting the same plant host pathways. This functional redundancy may play an important role in the dynamic of the “arms-race” between host and pathogen serving as the basis for diverse virulence strategies and creating conditions where mutations in certain effector groups will not have a major effect on the pathogen’s ability to infect the host.
Highlights
Two opposing evolutionary constraints exert pressure on plant pathogens: one to diversify virulence factors in order to evade plant defenses, and the other to retain virulence factors critical for maintaining a compatible interaction with the plant host
Comparative genomic analysis of Puccinia graminis f. sp. tritici (Pgt) isolates MCCFC and RKQQC Genomic sequence similarity was assessed between the two North American isolates of Pgt 59KS19, 99KS76A-1 [40], and the previously sequenced and annotated isolate 75-36-700-3 [41]
Understanding how specific Pgt effectors are able to cause these types of changes within a host plant is an important factor when designing resistant crop varieties and monitoring the evolution of virulent pathogen populations [62, 73]
Summary
Two opposing evolutionary constraints exert pressure on plant pathogens: one to diversify virulence factors in order to evade plant defenses, and the other to retain virulence factors critical for maintaining a compatible interaction with the plant host. Like other biotrophic plant pathogens, Pgt infects susceptible host plants by engaging in a complex interaction that involves multiple proteins from both the fungus and plant. Effector proteins secreted by the fungus interact directly with host plant factors and function to alter plant cellular defenses, architecture, and metabolism, leading to a compatible plant-pathogen interaction [4,5,6,7,8]. Recognition of effector proteins by plant host resistance genes forms the basis of effector-triggered immunity and drives fast evolution of effector complement in the pathogen populations [9, 10]. Mutations in effector encoding genes are suggested to be one of the major factors rendering resistance genes ineffective against new pathogen populations [11,12,13,14,15,16,17]. The resulting pathogen population shifts were shown to be linked with significant losses in crop production in last decades [2, 18]
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