The Ecological Role of Type Three Secretion Systems in the Interaction of Bacteria with Fungi in Soil and Related Habitats Is Diverse and Context-Dependent

Abstract

Bacteria and fungi constitute important organisms in many ecosystems, in particular terrestrial ones. Both organismal groups contribute significantly to biogeochemical cycling processes. Ecological theory postulates that bacteria capable of receiving benefits from host fungi are likely to evolve efficient association strategies. The purpose of this review is to examine the mechanisms that underpin the bacterial interactions with fungi in soil and other systems, with special focus on the type III secretion system (T3SS). Starting with a brief description of the versatility of the T3SS as an interaction system with diverse eukaryotic hosts, we subsequently examine the recent advances made in our understanding of its contribution to interactions with soil fungi. The analysis used data sets ranging from circumstantial evidence to geneknockout-based experimental data. The initial finding that the abundance of T3SSs in microbiomes is often enhanced in fungal-affected habitats like the mycosphere and the mycorrhizosphere is now substantiated with in-depth knowledge of the specific systems involved. Different fungal-interactive bacteria, in positive or negative associations with partner fungi, harbor and express T3SSs, with different ecological outcomes. In some particular cases, bacterial T3SSs have been shown to modulate the physiology of its fungal partner, affecting its ecological characteristics and consequently shaping its own habitat. Overall, the analyses of the collective data set revealed that diverse T3SSs have assumed diverse roles in the interactions of bacteria with host fungi, as driven by ecological and evolutionary niche requirements.

Chromobacterium violaceum Pathogenicity: Updates and Insights from Genome Sequencing of Novel Chromobacterium Species

“…Bacterial T3SSs have been demonstrated to be key determinants of virulence for many Gram-negative plant and animal pathogens via delivery of effector proteins into the cytosol of host eukaryotic cells ( Deng et al, 2017 ). Gene clusters encoding T3SSs are also found in genomes of non-pathogenic bacteria, and the roles of these T3SSs are not restricted to pathogenesis, but seem to include other processes during interactions involving bacteria and their hosts in diverse ecological contexts ( Nazir et al, 2017 ). As mentioned above, earlier genome sequencing of the strain type C. violaceum ATCC 12472 revealed the presence of two T3SSs (Cpi-1/1a and Cpi-2), of which, Cpi-1/1a is absolutely required for virulence ( Vasconcelos et al, 2003 ; Miki et al, 2010 ).…”

Stable isotope informed genome-resolved metagenomics uncovers potential trophic interactions in rhizosphere soil

“…Some of the genomes appeared to encode secretion systems associated with bacteriaeukaryotic interactions. Six genomes encode multiple genes from type III secretion systems, which are known to be important in symbiotic colonization and infection of plants, fungi, singlecelled eukaryotes, and animals (49). We do not know the intended targets of the type III secretion systems because of the diversity of possible hosts.…”

Mechanism of interaction of an endofungal bacterium Serratia marcescens D1 with its host and non-host fungi

“…Involvement of type II secretion system (T2SS) was reported in Burkholderia endosymbiont for active invasion of fungal hyphae [23]. Role of type II secretion system (T2SS) in bacterial-fungal interaction has been critically reviewed by Nazir et al, (2017) [24].…”