Ralstonia solanacearum requires F-box-like domain-containing type III effectors to promote disease on several host plants

Abstract

The phytopathogenic bacterium Ralstonia solanacearum encodes a family of seven type III secretion system (T3SS) effectors that contain both a leucine-rich repeat and an F-box domain. This structure is reminiscent of a class of typical eukaryotic proteins called F-box proteins. The latter, together with Skp1 and Cullin1 subunits, constitute the SCF-type E3 ubiquitin ligase complex and control specific protein ubiquitinylation. In the eukaryotic cell, depending on the nature of the polyubiquitin chain, the ubiquitintagged proteins either see their properties modified or are doomed for degradation by the 26S proteasome. This pathway is essential to many developmental processes in plants, ranging from hormone signaling and flower development to stress responses. Here, we show that these previously undescribed T3SS effectors are putative bacterial F-box proteins capable of interacting with a subset of the 19 different Arabidopsis Skp1-like proteins like bona fide Arabidopsis F-box proteins. A R. solanacearum strain in which all of the seven GALA effector genes have been deleted or mutated was no longer pathogenic on Arabidopsis and less virulent on tomato. Furthermore, we found that GALA7 is a host-specificity factor, required for disease on Medicago truncatula plants. Our results indicate that the GALA T3SS effectors are essential to R. solanacearum to control disease. Because the F-box domain is essential to the virulence function of GALA7, we hypothesize that these effectors act by hijacking their host SCF-type E3 ubiquitin ligases to interfere with their host ubiquitin͞proteasome pathway to promote disease.plant pathogen ͉ SCF complex ͉ type III secretion system ͉ virulence

Large-Scale, Lineage-Specific Expansion of a Bric-a-Brac/Tramtrack/Broad Complex Ubiquitin-Ligase Gene Family in Rice

“…Alternatively, it has become clear that pathogens have also developed sophisticated methods to exploit or interfere with the host Ub system during infection (Munro et al, 2006;Angot et al, 2007). These methods even include introducing their own E3s, likely to target intracellular host proteins involved in pathogen surveillance for ubiquitination and subsequent turnover (Schrammeijer et al, 2001;Tzfira et al, 2004;Abramovitch et al, 2006;Angot et al, 2006;Janjusevic et al, 2006;Nomura et al, 2006). A role of various E3 types in innate immunity may in turn help explain the preponderance of E3 genes in plants relative to other eukaryotes (Vierstra, 2003;Smalle and Vierstra, 2004).…”

How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model

“…These responses are more extreme and cause more tissue damage than basal defence responses 52 . One example is the root pathogen Ralstonia solanacearum, which requires the GALA7 type 3 secreted protein to cause wilting disease on M. truncatula 130 .…”

Genetic Organization of thehrpGene Cluster inAcidovorax avenaeStrain N1141 and a Novel Effector Protein That Elicits Immune Responses in Rice (Oryza sativaL.)

“…The GALA protein has two motifs, a conserved GAxALA sequence within the LRR region, and an F-box motiflike sequence in the N-terminus. 25,26) This structure is reminiscent of a class of eukaryotic proteins called F-box proteins. 25) F-box proteins, along with the Skp1 and Cullin1 subunits, constitute the SCF-type E3 ubiquitin ligase complex and control specific protein ubiquitinylation.…”

“…25,26) This structure is reminiscent of a class of eukaryotic proteins called F-box proteins. 25) F-box proteins, along with the Skp1 and Cullin1 subunits, constitute the SCF-type E3 ubiquitin ligase complex and control specific protein ubiquitinylation. Although the functions of most GALA proteins are not yet known, the GALA 7 protein of R. solanacearum is required for disease in Medicago truncatula plants, and its F-box domain of GALA 7 is essential for this virulence function.…”