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Mutualistic co-evolution of type III effector genes in Sinorhizobium fredii and Bradyrhizobium japonicum.

Kimbrel JA, Thomas WJ, Jiang Y, Creason AL, Thireault CA, Sachs JL, Chang JH - PLoS Pathog. (2013)

Bottom Line: We demonstrate that the rhizobial type III effector genes exhibit a surprisingly high degree of conservation in content and sequence that is in contrast to those of a well characterized plant pathogenic species.This type III effector gene conservation is particularly striking in the context of the relatively high genome-wide diversity of rhizobia.Instead, our results reveal that these loci are relatively static in rhizobial lineages and suggest that fitness conflicts between rhizobia mutualists and their host plants have been largely resolved.

View Article: PubMed Central - PubMed

Affiliation: Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon, United States of America.

ABSTRACT
Two diametric paradigms have been proposed to model the molecular co-evolution of microbial mutualists and their eukaryotic hosts. In one, mutualist and host exhibit an antagonistic arms race and each partner evolves rapidly to maximize their own fitness from the interaction at potential expense of the other. In the opposing model, conflicts between mutualist and host are largely resolved and the interaction is characterized by evolutionary stasis. We tested these opposing frameworks in two lineages of mutualistic rhizobia, Sinorhizobium fredii and Bradyrhizobium japonicum. To examine genes demonstrably important for host-interactions we coupled the mining of genome sequences to a comprehensive functional screen for type III effector genes, which are necessary for many Gram-negative pathogens to infect their hosts. We demonstrate that the rhizobial type III effector genes exhibit a surprisingly high degree of conservation in content and sequence that is in contrast to those of a well characterized plant pathogenic species. This type III effector gene conservation is particularly striking in the context of the relatively high genome-wide diversity of rhizobia. The evolution of rhizobial type III effectors is inconsistent with the molecular arms race paradigm. Instead, our results reveal that these loci are relatively static in rhizobial lineages and suggest that fitness conflicts between rhizobia mutualists and their host plants have been largely resolved.

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PtoDC3000 delivers T3Es of rhizobia in a T3SS-dependent manner.(A) Leaves of Arabidopsis Col-0 (Rps2/Rps2) were infiltrated with PtoDC3000 (top row) and its T3SS-deficient mutant, ΔhrcC (bottom row) carrying no fusion to Δ79avrRpt2 or fusions to P. syringae T3E avrRpm1 or coding sequences from NGR234 candidate T3E genes, nopJ or nopB. (B) Members of the NopB T3E gene family all encode for functional T3Es. Leaves of Arabidopsis Col-0 (Rps2/Rps2) were infiltrated with PtoDC3000 carrying no fusion to Δ79avrRpt2 or fusions to P. syringae T3E avrRpm1 or nopB coding sequences from NGR234, USDA207, or USDA110. Leaves did not respond to infiltrations of ΔhrcC. In all experiments, leaves were scored for the HR ∼20 hpi and the percent of responding leaves are presented (at least 20 leaves infiltrated). Experiments were repeated at least three times.
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ppat-1003204-g002: PtoDC3000 delivers T3Es of rhizobia in a T3SS-dependent manner.(A) Leaves of Arabidopsis Col-0 (Rps2/Rps2) were infiltrated with PtoDC3000 (top row) and its T3SS-deficient mutant, ΔhrcC (bottom row) carrying no fusion to Δ79avrRpt2 or fusions to P. syringae T3E avrRpm1 or coding sequences from NGR234 candidate T3E genes, nopJ or nopB. (B) Members of the NopB T3E gene family all encode for functional T3Es. Leaves of Arabidopsis Col-0 (Rps2/Rps2) were infiltrated with PtoDC3000 carrying no fusion to Δ79avrRpt2 or fusions to P. syringae T3E avrRpm1 or nopB coding sequences from NGR234, USDA207, or USDA110. Leaves did not respond to infiltrations of ΔhrcC. In all experiments, leaves were scored for the HR ∼20 hpi and the percent of responding leaves are presented (at least 20 leaves infiltrated). Experiments were repeated at least three times.

Mentions: PtoDC3000 carrying either the nopB::Δ79avrRpt2 or nopJ::Δ79avrRpt2 fusions elicited HRs within the same time frame (∼20 hours post inoculation; hpi) and to the same degree as the positive control, a fusion between the full-length avrRpm1 P. syringae T3E gene and Δ79avrRpt2 (Figure 2A). Although Arabidopsis ecotype Col-0 can elicit ETI in response to both AvrRpm1 and AvrRpt2, the observed HR is known to be a consequence of perception of the latter by RPS2 [34]. Each of the tested nopB::Δ79avrRpt2 gene fusions were sufficient for PtoDC3000 to trigger an HR at 20hpi, confirming that this family encodes bona fide T3Es (Figure 2B). The NopB family is polymorphic with NopBNGR234 sharing ≥98% amino acid identity with NopBUSDA207, but only 32% with NopBUSDA110. In contrast, PtoDC3000 lacking fusions to Δ79avrRpt2 failed to elicit an HR but eventually showed tissue collapse approximately 28 hpi, indicative of PtoDC3000-caused disease symptoms (data not shown). The T3SS-deficient mutant of PtoDC3000 (ΔhrcC), regardless of the gene it carried, failed to elicit any phenotype throughout the course of the study, thereby demonstrating the T3SS-dependent delivery of T3Es (Figure 2A).


Mutualistic co-evolution of type III effector genes in Sinorhizobium fredii and Bradyrhizobium japonicum.

Kimbrel JA, Thomas WJ, Jiang Y, Creason AL, Thireault CA, Sachs JL, Chang JH - PLoS Pathog. (2013)

PtoDC3000 delivers T3Es of rhizobia in a T3SS-dependent manner.(A) Leaves of Arabidopsis Col-0 (Rps2/Rps2) were infiltrated with PtoDC3000 (top row) and its T3SS-deficient mutant, ΔhrcC (bottom row) carrying no fusion to Δ79avrRpt2 or fusions to P. syringae T3E avrRpm1 or coding sequences from NGR234 candidate T3E genes, nopJ or nopB. (B) Members of the NopB T3E gene family all encode for functional T3Es. Leaves of Arabidopsis Col-0 (Rps2/Rps2) were infiltrated with PtoDC3000 carrying no fusion to Δ79avrRpt2 or fusions to P. syringae T3E avrRpm1 or nopB coding sequences from NGR234, USDA207, or USDA110. Leaves did not respond to infiltrations of ΔhrcC. In all experiments, leaves were scored for the HR ∼20 hpi and the percent of responding leaves are presented (at least 20 leaves infiltrated). Experiments were repeated at least three times.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC3585131&req=5

ppat-1003204-g002: PtoDC3000 delivers T3Es of rhizobia in a T3SS-dependent manner.(A) Leaves of Arabidopsis Col-0 (Rps2/Rps2) were infiltrated with PtoDC3000 (top row) and its T3SS-deficient mutant, ΔhrcC (bottom row) carrying no fusion to Δ79avrRpt2 or fusions to P. syringae T3E avrRpm1 or coding sequences from NGR234 candidate T3E genes, nopJ or nopB. (B) Members of the NopB T3E gene family all encode for functional T3Es. Leaves of Arabidopsis Col-0 (Rps2/Rps2) were infiltrated with PtoDC3000 carrying no fusion to Δ79avrRpt2 or fusions to P. syringae T3E avrRpm1 or nopB coding sequences from NGR234, USDA207, or USDA110. Leaves did not respond to infiltrations of ΔhrcC. In all experiments, leaves were scored for the HR ∼20 hpi and the percent of responding leaves are presented (at least 20 leaves infiltrated). Experiments were repeated at least three times.
Mentions: PtoDC3000 carrying either the nopB::Δ79avrRpt2 or nopJ::Δ79avrRpt2 fusions elicited HRs within the same time frame (∼20 hours post inoculation; hpi) and to the same degree as the positive control, a fusion between the full-length avrRpm1 P. syringae T3E gene and Δ79avrRpt2 (Figure 2A). Although Arabidopsis ecotype Col-0 can elicit ETI in response to both AvrRpm1 and AvrRpt2, the observed HR is known to be a consequence of perception of the latter by RPS2 [34]. Each of the tested nopB::Δ79avrRpt2 gene fusions were sufficient for PtoDC3000 to trigger an HR at 20hpi, confirming that this family encodes bona fide T3Es (Figure 2B). The NopB family is polymorphic with NopBNGR234 sharing ≥98% amino acid identity with NopBUSDA207, but only 32% with NopBUSDA110. In contrast, PtoDC3000 lacking fusions to Δ79avrRpt2 failed to elicit an HR but eventually showed tissue collapse approximately 28 hpi, indicative of PtoDC3000-caused disease symptoms (data not shown). The T3SS-deficient mutant of PtoDC3000 (ΔhrcC), regardless of the gene it carried, failed to elicit any phenotype throughout the course of the study, thereby demonstrating the T3SS-dependent delivery of T3Es (Figure 2A).

Bottom Line: We demonstrate that the rhizobial type III effector genes exhibit a surprisingly high degree of conservation in content and sequence that is in contrast to those of a well characterized plant pathogenic species.This type III effector gene conservation is particularly striking in the context of the relatively high genome-wide diversity of rhizobia.Instead, our results reveal that these loci are relatively static in rhizobial lineages and suggest that fitness conflicts between rhizobia mutualists and their host plants have been largely resolved.

View Article: PubMed Central - PubMed

Affiliation: Department of Botany and Plant Pathology, Oregon State University, Corvallis, Oregon, United States of America.

ABSTRACT
Two diametric paradigms have been proposed to model the molecular co-evolution of microbial mutualists and their eukaryotic hosts. In one, mutualist and host exhibit an antagonistic arms race and each partner evolves rapidly to maximize their own fitness from the interaction at potential expense of the other. In the opposing model, conflicts between mutualist and host are largely resolved and the interaction is characterized by evolutionary stasis. We tested these opposing frameworks in two lineages of mutualistic rhizobia, Sinorhizobium fredii and Bradyrhizobium japonicum. To examine genes demonstrably important for host-interactions we coupled the mining of genome sequences to a comprehensive functional screen for type III effector genes, which are necessary for many Gram-negative pathogens to infect their hosts. We demonstrate that the rhizobial type III effector genes exhibit a surprisingly high degree of conservation in content and sequence that is in contrast to those of a well characterized plant pathogenic species. This type III effector gene conservation is particularly striking in the context of the relatively high genome-wide diversity of rhizobia. The evolution of rhizobial type III effectors is inconsistent with the molecular arms race paradigm. Instead, our results reveal that these loci are relatively static in rhizobial lineages and suggest that fitness conflicts between rhizobia mutualists and their host plants have been largely resolved.

Show MeSH
Related in: MedlinePlus