T6SS: A Key to Pseudomonas's Success in Biocontrol?

Abstract

Bacteria from the genus Pseudomonas have been extensively studied for their capacity to act as biological control agents of disease and pests and for their ability to enhance and promote crop production in agricultural systems. While initial research primarily focused on the human pathogenic bacteria Pseudomonas aeruginosa, recent studies indicate the significance of type VI secretion (T6SS) in other Pseudomonas strains for biocontrol purposes. This system possibly plays a pivotal role in restricting the biological activity of target microorganisms and may also contribute to the bolstering of the survival capabilities of the bacteria within their applied environment. The type VI secretion system is a phage-like structure used to translocate effectors into both prokaryotic and eukaryotic target cells. T6SSs are involved in a myriad of interactions, some of which have direct implications in the success of Pseudomonas as biocontrol agents. The prevalence of T6SSs in the genomes of Pseudomonas species is notably greater than the estimated 25% occurrence rate found in Gram-negative bacteria. This observation implies that T6SS likely plays a pivotal role in the survival and fitness of Pseudomonas. This review provides a brief overview of T6SS, its role in Pseudomonas with biocontrol applications, and future avenues of research within this subject matter.

Keywords: Pseudomonas; biocontrol; type VI secretion system.

Figure 1

(A) T6SS structure, core components, and functions. (B) Schematic representation of T6SS effector delivery, adapted from Coulthurst [25]. A fully assembled T6SS contracts its TSSB/C sheath to facilitate the transfer of TssD-TssI-PAAR spike components out of the cell and into the target cell for effector delivery. T6SS effectors can be cargo or specialized effectors, as described in the main text. After contraction, TssH/ClpV-mediated disassembly of the T6SS prepares the system for subsequent firing events. Created with BioRender.com (accessed on 8 May 2023).

Figure 2

Phylogenetic tree of the Pseudomonas spp. Phylogenetic groups, known BCA potential, isolation source, and number of T6SS loci are depicted. Two Cellvibrio spp. were used as an outgroup. Two Azotobacter spp. were also included in this analysis as these have been shown to fall into the Pseudomonas genus. The tree was constructed with concatenated alignments of four housekeeping genes, 16S, gyrB, rpoB, and rpoD, using the program MrBayes. The number of T6SSs was determined using McSyfinder’s TXXScan. Tree visualization was performed using ggtree 3.4.4.

Figure 3

Relevance of T6SS activity in biocontrol applications. Created using BioRender.com (accessed on 10 September 2023).