Establishing durable disease resistance in agricultural crops, where much of the plant defense is provided through effector-R gene interactions, is complicated by the ability of pathogens to overcome R gene resistance by losing the corresponding effector gene. Many proposed methods to maintain disease resistance in the field depend on the idea that effector gene loss results in a fitness cost to the pathogen. In this article we test for fitness costs of effector gene function loss. We created directed knockouts of up to four effector genes from the bacterial plant pathogen Xanthomonas axonopodis pv. vesicatoria (Xav) and examined the effect of the loss of a functional gene product on several important fitness parameters in the field. These traits included transmission, lesion development, and epiphytic survival. We found that the products of all four effector genes had significant and often additive effects on fitness traits. Additional greenhouse tests revealed costs of effector gene loss on in planta growth and further showed that the effects on lesion development were separable from the effects on growth. Observable fitness effects of the three plasmidborne effector genes were dependent upon the loss of functional avrBs2, indicating that complex functional interactions exist among effector genes with Xav.
SummaryThe phytopathogen Pseudomonas syringae competes with other epiphytic organisms, such as filamentous fungi, for resources. Here we characterize a gene in P. syringae pv. syringae B728a and P. syringae pv. tomato DC3000, termed phcA, that has homology to a filamentous fungal gene called het-c. phcA is conserved in many P. syringae strains, but is absent in one of the major clades, which includes the P. syringae pathovar phaseolicola. In the filamentous fungus Neurospora crassa, HET-C regulates a conserved programmed cell death pathway called heterokaryon incompatibility (HI). Ectopic expression of phcA in N. crassa induced HI and cell death that was dependent on the presence of a functional het-c pin-c haplotype. Further, by co-immunoprecipitation experiments, a heterocomplex between N. crassa HET-C1 and PhcA was associated with phcA-induced HI. P. syringae was able to attach and extensively colonize N. crassa hyphae, while an Escherichia coli control showed no association with the fungus. We further show that the P. syringae is able to use N. crassa as a sole nutrient source. Our results suggest that P. syringae has the potential to utilize phcA to acquire nutrients from fungi in nutrientlimited environments like the phyllosphere by the novel mechanism of HI induction.
The bacterial plant pathogen Xanthomonas axonopodis pv. vesicatoria, also known as Xanthomonas campestris pv. vesicatoria group A, is the causal agent of bacterial spot in pepper and tomato. In order to test different models that may explain the coevolution of avrBs2 with its host plants, we sequenced avrBs2 and six chromosomal loci (total of 5.5 kb per strain) from a global sample of 55 X. axonopodis pv. vesicatoria strains collected from diseased peppers. We found an extreme lack of genetic variation among all X. axonopodis pv. vesicatoria genomic loci (average nucleotide diversity, ؍ 9.1 ؋ 10 ؊5 ), including avrBs2. This lack of diversity is consistent with X. axonopodis pv. vesicatoria having undergone a recent population bottleneck and/or selective sweep followed by population expansion.
Establishing durable disease resistance in agricultural crops, where much of the plant defense is provided through effector-R gene interactions, is complicated by the ability of pathogens to overcome R gene resistance by losing the corresponding effector gene. Many proposed methods to maintain disease resistance in the field depend on the idea that effector gene loss results in a fitness cost to the pathogen. In this article we test for fitness costs of effector gene function loss. We created directed knockouts of up to four effector genes from the bacterial plant pathogen Xanthomonas axonopodis pv. vesicatoria (Xav) and examined the effect of the loss of a functional gene product on several important fitness parameters in the field. These traits included transmission, lesion development, and epiphytic survival. We found that the products of all four effector genes had significant and often additive effects on fitness traits. Additional greenhouse tests revealed costs of effector gene loss on in planta growth and further showed that the effects on lesion development were separable from the effects on growth. Observable fitness effects of the three plasmid-borne effector genes were dependent upon the loss of functional avrBs2, indicating that complex functional interactions exist among effector genes with Xav.
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