Type VI secretion systems in plant-associated bacteria

Abstract

The type VI secretion system (T6SS) is a bacterial nanomachine used to inject effectors into prokaryotic or eukaryotic cells and is thus involved in both host manipulation and interbacterial competition. The T6SS is widespread among Gram-negative bacteria, mostly within the Proteobacterium Phylum. This secretion system is commonly found in commensal and pathogenic plant-associated bacteria. Phylogenetic analysis of phytobacterial T6SS clusters shows that they are distributed in the five main clades previously described (group 1-5). The even distribution of the system among commensal and pathogenic phytobacteria suggests that the T6SS provides fitness and colonization advantages in planta and that the role of the T6SS is not restricted to virulence. This manuscript reviews the phylogeny and biological roles of the T6SS in plant-associated bacteria, highlighting a remarkable diversity both in terms of mechanism and function.

Figure 1

Schematic representation of the T6SS structure. Baseplates components (TssA, TssE, TssF, TssG and TssK are coloured in light orange whereas membrane complex is represented in green (TssJ, TssL and TssM) and the sheath components TssB and TssC in blue.

Figure 2

Number of T6SS clusters in plant associated bacteria. Bacteria are distributed in α‐, β‐ and γ‐proteobacteria. T6SS clusters are represented with different colours according to their phylogenetic groups: group 1 (green), group 2 (red), group 3 (orange), group 4 (blue) and group 5 (purple).

Figure 3

Phylogenetic distribution of T6SS clusters in plant‐associated bacteria. Maximum‐likelihood tree with 1000 bootstrap replicates was built with Mega 6 for the core component protein TssB. T6SS cluster nomenclature (Boyer et al., 2009; Barret et al., 2011) is used to show the major phylogenetic clusters. Five main groups are clearly distinguishable: group 1 (green), group 2 (red), group 3 (orange), group 4 (blue) and group 5 (purple). Subgroups 1.1 and 1.2a and 1.2b are indicated in the tree (green). Subgroup 4A and 4B1 and 4B2 are represented in blue. A grey circle indicates T6SSs involved in interbacterial competition whereas a grey triangle represents systems involved in host manipulation. Characterized T6SSs with specific name in the literature are indicated in the tree next to the strain name.

Figure 4

T6SS clusters in P. putida. Genetic architecture of T6SS clusters belonging to phylogenetic groups 1.1, 1.2a, 1.2b, 2, 3, 4A, 4B1 and 4B2 present in phytobacteria strains. The colour code of the genes correlates with the colour code shown in Fig. 1. PA14 indicates P. aeruginosa PA14; KT2440 indicates P. putida KT2440; Ech586 indicates D. dadantii Ech586; W619 refers to P. putida W619; GMI1000 refers to R. solanacearum GMI1000 and C58 to A. tumefaciens C58. hcp and vgrG genes vary enormously among species, when these genes are found within the clusters in the represented strain, the brackets indicate the positions at which they are located.