Isolation and Characterization of a Novel Arabidopsis thaliana Mutant Unable to Develop Wilt Symptoms After Inoculation with a Virulent Strain of Ralstonia solanacearum.

ABSTRACT To characterize host genes required for a compatible interaction, we identified a novel recessive Arabidopsis thaliana mutant, nws1 (no wilt symptoms), that failed to develop wilt symptoms in response to virulent strains of the phytopathogenic bacterium, Ralstonia solanacearum. The absence of wilting in nws1 plants was not correlated with a cell death phenotype or a constitutive expression of salicylic acid-, jasmonic acid- or ethylene-associated genes. In addition, this mutation, which conferred a symptomless phenotype in response to all the R. solanacearum strains tested, was highly specific to this pathogen, because nws1 responses to other plant pathogens, including oomycetes, nematodes, viruses, and other bacteria, were identical to those of wild-type Col-5 plants. Finally, the lack of disease development was shown to be different than RRS1-R-mediated resistance. The identification of mutants such as nws1, that are unable to develop disease, should lead to the isolation of target host factors required for pathogen growth or fitness, or of factors modified by the invading microorganism to avoid or inactivate plant defense mechanisms, and should bring a better understanding of bacterial wilt diseases.

Physical interaction between RRS1-R, a protein conferring resistance to bacterial wilt, and PopP2, a type III effector targeted to the plant nucleus.

RRS1-R confers broad-spectrum resistance to several strains of the causal agent of bacterial wilt, Ralstonia solanacearum. Although genetically defined as recessive, this R gene encodes a protein whose structure combines the TIR-NBS-LRR domains found in several R proteins and a WRKY motif characteristic of some plant transcriptional factors and behaves as a dominant gene in transgenic susceptible plants. Here we show that PopP2, a R. solanacearum type III effector, which belongs to the YopJ/AvrRxv protein family, is the avirulence protein recognized by RRS1-R. Furthermore, an interaction between PopP2 and both RRS1-R and RRS1-S, present in the resistant Nd-1 and susceptible Col-5 Arabidopsis thaliana ecotypes, respectively, was detected by using the yeast split-ubiquitin two-hybrid system. This interaction, which required the full-length R protein, was not observed between the RRS1 proteins and PopP1, another member of the YopJ/AvrRxv family present in strain GMI1000 and that confers avirulence in Petunia. We further demonstrate that both the Avr protein and the RRS1 proteins colocalize in the nucleus and that the nuclear localization of the RRS1 proteins are dependent on the presence of PopP2.

Resistance to Ralstonia solanacearum in Arabidopsis thaliana is conferred by the recessive RRS1-R gene, a member of a novel family of resistance genes.

The identification of two Arabidopsis thaliana genes involved in determining recessive resistance to several strains of the causal agent of bacterial wilt, Ralstonia solanacearum, is reported. Dominant (RRS1-S) and recessive (RRS1-R) alleles from susceptible and resistant accessions encode highly similar predicted proteins differing in length and which present a novel structure combining domains found in plant Toll-IL-1 receptor-nucleotide binding site-leucin-rich repeat resistance proteins and a WRKY motif characteristic of some plant transcriptional factors. Although genetically defined as a recessive allele, RRS1-R behaves as a dominant resistance gene in transgenic plants. Sequence analysis of the RRS1 genes present in two homozygous intragenic recombinant lines indicates that several domains of RRS1-R are essential for its resistance function. Additionally, RRS1-R-mediated resistance is partially salicylic acid- and NDR1-dependent, suggesting the existence of similar signaling pathways to those controlled by resistance genes in specific resistance.