# Community interactions among microbes give rise to host-microbiome mutualisms in an aquatic plant

## TL;DR

Microbial communities, not just single microbes, can form mutualistic relationships with plants, boosting both plant growth and microbial productivity.

## Contribution

Demonstrates that host-microbiome mutualisms can emerge from community interactions among microbes.

## Key findings

- 10-strain synthetic communities increased duckweed growth and microbial productivity more than single strains.
- Most single strains were commensals, but multi-strain communities showed mutualistic effects.
- Microbial productivity and host fitness were positively correlated.

## Abstract

Microbiomes often benefit plants, conferring resistance to pathogens,
improving stress tolerance, or promoting plant growth. As potential plant
mutualists, however, microbiomes are not a single organism but a community
of species with complex interactions among microbial taxa and between
microbes and their shared host. The nature of ecological interactions among
microbes in the microbiome can have important consequences for the net
effects of microbiomes on hosts. Here, we compared the effects of individual
microbial strains and 10-strain synthetic communities on microbial
productivity and host growth using the common duckweed Lemna
minor and a synthetic, simplified version of its native
microbiome. Except for Pseudomonas protegens, which was a
mutualist when tested alone, all of the single strains we tested were
commensals on hosts, benefiting from plant presence but not increasing host
growth relative to uninoculated controls. However, 10-strain synthetic
microbial communities increased both microbial productivity and duckweed
growth more than the average single-strain inoculation and uninoculated
controls, meaning that host-microbiome mutualisms can emerge from community
interactions among microbes on hosts. The effects of community inoculation
were sub-additive, suggesting at least some competition among microbes in
the duckweed microbiome. We also investigated the relationship between
L. minor fitness and that of its microbes, providing
some of the first empirical estimates of broad fitness alignment between
plants and members of their microbiomes; hosts grew faster with more
productive microbes or microbiomes.

There is currently substantial interest in engineering synthetic microbiomes
for health or agricultural applications. One key question is how
multi-strain microbial communities differ from single microbial strains in
their productivity and effects on hosts. We tested 20 single bacterial
strains and 2 distinct 10-strain synthetic communities on plant hosts and
found that 10-strain communities led to faster host growth and greater
microbial productivity than the average, but not the best, single strain.
Furthermore, the microbial strains or communities that achieved the greatest
cell densities were also the most beneficial to their hosts, showing that
both specific single strains and multi-strain synthetic communities can
engage in high-quality mutualisms with their hosts. Our results suggest that
~5% of single strains, as well as multi-strain synthetic communities
comprised largely of commensal microbes, can benefit hosts and result in
effective host-microbe mutualisms.