Bacteria had been around long before multi‐cellular eukaryotes evolved and quickly began to colonize these new life forms as another suitable habitat and potential source of nutrients. Nearly all higher organisms across taxa now have bacterial communities living on and within them—from mutually beneficial symbioses to antagonistic parasitic conflicts. They have co‐evolved with their microbiota and come to depend on it for a range of functions in immunity, metabolism and nutrient absorption. In humans, the microbiota, especially bacteria in the gut, play important roles in all areas, by breaking down indigestible polysaccharides, providing vitamins, mediating nutrient absorption in the gut and helping train the immune system. While research on the human microbiome has stolen the limelight, plant scientists have also been making progress towards elucidating the composition and function of plant microbiomes. Their work has great potential for the future of humanity, as it could help to establish more sustainable agriculture with less dependence on fertilizers, pesticides and herbicides, while increasing yield and nutrient content.
Research on the plant microbiome actually predates the animal and human counterparts because microorganisms play an obvious and crucial role in fixing nutrients for the plant, especially nitrogen and phosphorus. Given their vital importance for protein and DNA synthesis and energy production, many plants have evolved close symbiotic relationships with soil bacteria and fungi that supply them with both elements in convenient forms with minimum energy expenditure. Indeed, the close relationships with two specific families of microorganisms set apart the plant microbiome from its animal counterpart: rhizobia nitrogen‐fixing bacteria and mycorrhizal fungi, which greatly enhance the ability of roots to extract various nutrients from the soil. Neither group can survive without the host plant that in return supplies oxygen and products of photosynthesis, mainly proteins and carbohydrates.
A three‐way relationship
The relationship with mycorrhizal fungi seems to be …