The Type VI Secretion Systems in Plant-Beneficial Bacteria Modulate Prokaryotic and Eukaryotic Interactions in the Rhizosphere

Abstract

Rhizosphere colonizing plant growth promoting bacteria (PGPB) increase their competitiveness by producing diffusible toxic secondary metabolites, which inhibit competitors and deter predators. Many PGPB also have one or more Type VI Secretion System (T6SS), for the delivery of weapons directly into prokaryotic and eukaryotic cells. Studied predominantly in human and plant pathogens as a virulence mechanism for the delivery of effector proteins, the function of T6SS for PGPB in the rhizosphere niche is poorly understood. We utilized a collection of Pseudomonas chlororaphis 30–84 mutants deficient in one or both of its two T6SS and/or secondary metabolite production to examine the relative importance of each T6SS in rhizosphere competence, bacterial competition, and protection from bacterivores. A mutant deficient in both T6SS was less persistent than wild type in the rhizosphere. Both T6SS contributed to competitiveness against other PGPB or plant pathogenic strains not affected by secondary metabolite production, but only T6SS-2 was effective against strains lacking their own T6SS. Having at least one T6SS was also essential for protection from predation by several eukaryotic bacterivores. In contrast to diffusible weapons that may not be produced at low cell density, T6SS afford rhizobacteria an additional, more immediate line of defense against competitors and predators.

Shared and host-specific transcriptomic response of Pseudomonas donghuensis P482 to the exudates of tomato (Dicot) and maize (Monocot) shed light on the host-adaptive traits in the promiscuous root colonizing bacteria

“…In P. aeruginosa , TagT, TagQ and TagF are proteins involved in the regulatory cascade that controls the activation of the type VI secretion system (T6SS). This secretion system enables bacteria to inject proteins into other bacterial cells and is best known for its role in interbacterial competition (50, 51). In P. aeruginosa, protein secretion is regulated post-translationally by phosphorylation of threonine, with PpkA kinase required for the assembly of T6SS.…”

“…It therefore seems that rhizosphere-associated pseudomonas generally do not suffer from iron starvation when supplied with root exudates. However, a greater effort in terms of iron acquisition could be required in the rhizosphere in the presence of microbial competitors (51). Production of siderophores was shown to be an important mechanism of biocontrol, that is the ability of root-associated microbiota to suppress the activity of plant pathogens (111).…”

“…T6SS is a nanomachine used by bacteria to inject effectors to other procaryotic or eucaryotic cells (112). It is known for its role in interbacterial competition and was shown to protect PGPR from eukaryotic bacterivores (51).…”

Copiotrophs dominate rhizosphere microbiomes and growth rate potential is a major factor explaining the rhizosphere effect

“…We observed that in PhyloGLM results, flagellar proteins were associated to rhizosphere and copiotrophs in Alphaproteobacteria, Acidobacteria, and Gammaproteobacteria, to copiotrophs in Actinobacteria, and never associated to soil or oligotroph, confirming the importance of active motility in rhizosphere colonization in the different taxa (Supplementary Table 4). Linked to this cluster we found an inter-membrane structural component of the type VI protein secretion system (T6SS, COG3521), which has been associated with modulating the microbial interactions and promoting rhizosphere competence of plant-beneficial bacteria [49, 50]. When we searched for the other COGs which compose the T6SS, we found that, when significantly enriched, in all cases they were associated either with rhizosphere or with copiotrophs, especially in Gammaproteobacteria (Supplementary Table 4).…”

Aquitalea palustris sp. nov., isolated from a wild cranberry bog in the Cape Cod National Seashore

“…RASTtk annotation indicates the presence of two catalase genes, katG and katE, supporting the positive test for catalase activity. Annotation also indicates that MWU14-2270 and MWU14-2238 each contain 15 putative genes for antibacterial type VI secretion systems [38], but MWU14-2217 T does not. Each strain contains three to four putative genes for VgrG, and 23 predicted genes for VgrG-related proteins, which are generally part of type VI secretion systems or bacteriophage injection mechanisms, and which are associated with microbiome competition strategies such as the establishment and defence of a microniche, or antibacterial activity [39,40].…”