Plant-Pathogenic Agrobacterium tumefaciens Strains Have Diverse Type VI Effector-Immunity Pairs and Vary in In-Planta Competitiveness

Abstract

The type VI secretion system (T6SS) is used by gram-negative bacteria to translocate effectors that can antagonize other bacterial cells. Models predict the variation in collections of effector and cognate immunity genes determine competitiveness and can affect the dynamics of populations and communities of bacteria. However, the outcomes of competition cannot be entirely explained by compatibility of effector-immunity (EI) pairs. Here, we characterized the diversity of T6SS loci of plant-pathogenic Agrobacterium tumefaciens and showed that factors other than EI pairs can impact interbacterial competition. All examined strains encode T6SS active in secretion and antagonism against Escherichia coli. The spectra of EI pairs as well as compositions of gene neighborhoods are diverse. Almost 30 in-planta competitions were tested between different genotypes of A. tumefaciens. Fifteen competitions between members of different species-level groups resulted in T6SS-dependent suppression in in-planta growth of prey genotypes. In contrast, ten competitions between members within species-level groups resulted in no significant effect on the growth of prey genotypes. One strain was an exceptional case and, despite encoding a functional T6SS and toxic effector protein, could not compromise the growth of the four tested prey genotypes. The data suggest T6SS-associated EI pairs can influence the competitiveness of strains of A. tumefaciens, but genetic features have a significant role on the efficacy of interbacterial antagonism.

Effector loading onto the VgrG carrier activates type VI secretion system assembly

“…Sheath-like structures were frequently detected in fractions from C58, and only very few were observed in A The vgrG genetic modules of the tested A. tumefaciens strains. Genes are color-coded according to the predicted function or results of functional assays [24]. The vgrG-associated genes are v1-v6 for 12D1, v1-v9 for 1D1108, and v1-v7 for 15955.…”

“…We therefore tested whether loading of cargo effector onto VgrG to activate T6SS is also used by other A. tumefaciens genomospecies/strains. From these, A. tumefaciens strains 1D1108, 15955, and 12D1, which belong to different genomospecies with only~80% gene content similarity to C58, were selected to test whether effector loading onto VgrG to regulate T6SS occurs in genetically diverse A. tumefaciens [24]. From these, A. tumefaciens strains 1D1108, 15955, and 12D1, which belong to different genomospecies with only~80% gene content similarity to C58, were selected to test whether effector loading onto VgrG to regulate T6SS occurs in genetically diverse A. tumefaciens [24].…”

“…Several strains of A. tumefaciens encode functional T6SSs that are necessary for interbacterial competition [24]. These three strains each carry a T6SS gene cluster similar to C58 but each has only a single vgrG module that circumscribe several downstream genes (v1-6 for 12D1, v1-9 for 1D1108, v1-7 for 15955; Fig 4A) [24]. These three strains each carry a T6SS gene cluster similar to C58 but each has only a single vgrG module that circumscribe several downstream genes (v1-6 for 12D1, v1-9 for 1D1108, v1-7 for 15955; Fig 4A) [24].…”

“…However, in the presence of Tae or Tae-Tai pairs, which are highly conserved across A. tumefaciens genomospecies [24], the Tae effector ensures that the T6SS is Model I: the VgrG-PAAR-effector-adaptor complex forms first before binding to the membrane-associated baseplate complex. In C58, when A. tumefaciens is grown in rich medium, T6SS is only assembled when the cargo effector is loaded onto its cognate VgrG regardless of the presence or absence of Tae effector.…”

Identification ofEscherichia coliClpAP in regulating susceptibility to type VI secretion system-mediated attack byAgrobacterium tumefaciens

“…For examples, A. tumefaciens was able to antagonize E. coli but not its susceptible siblings when co-cultured at an acidic condition where T6SS secretion is active (12,14). In contrast, such intra-species antagonism can occur in planta (14,15). These results suggest that the failure of the intra-species antagonism was not due to insusceptible siblings and that the recipient genetic features might also play a significant role in T6SS killing outcome.…”