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 gene-knockout-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.
Highlights
Bacteria can interact closely with eukaryotic hosts, as recently illustrated in studies on the core microbiomes that are associated with plants and supported by the proposal that plants and their associated microorganisms may be considered as “superorganisms” denoted holobionts (Vandenkoornhuyse et al, 2015)
T3SS Mediated Bacterial–Fungal Interactions holobiont are thought to be tuned by molecular communication, especially that relying on systems such as the type III secretion system (Lemanceau et al, 2016)
We critically examine the literature on this topic, with the following specific objectives: (1) Consideration of the evolution of the T3SS and its connection with interactions with particular hosts, (2) Evaluation of the role of the T3SS in bacterial–fungal interactions (BFIs) in soil and soil-related habitats, (3) Outlook and identification of future research directions
Summary
Bacteria can interact closely with eukaryotic hosts, as recently illustrated in studies on the core microbiomes that are associated with plants and supported by the proposal that plants and their associated microorganisms may be considered as “superorganisms” denoted holobionts (Vandenkoornhuyse et al, 2015). These belonged mostly to Oxalobacteraceae and differed from the bacteria attached to non-hyphal plant roots or glass wool substrate These and other studies suggested that fungal hyphae tend to drive associations with specific bacterial groups. Bacterial type three secretion systems T3SS, which for a long time were only considered to constitute virulence determinants of Gram-negative bacteria, may play roles in the modulation of bacterial-fungal interactions (BFI) in soil and soil functioning. Troisfontaines and Cornelis, 2005) These different types of T3SSs are grossly related to the ecological roles their hosts play when interacting with eukaryotic hosts (Abby et al, 2014), as discussed in the following. The ecological roles of the different T3SSs are further explored (s)
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