How Do Microbes Choose Their Neighbors?
The rhizosphere contains several species of biocontrol bacteria that produce phenazines, toxic molecules which help keep phytopathogenic fungi at bay. However, there are biocontrol strains of fungi which are also susceptible to phenazine assault and must live directly next to phenazine producers.
We have isolated a plant-promoting biocontrol fungus, Aspergillus ustus, that was found with an adherent bacterial partner, Paraburkholderia S0S3. P. SOS3 is able to detoxify and sequester phenazines, including the environmentally relevant phenazine-1-carboxylic acid (PCA). Intriguingly, P. SOS3 only does so in the presence of A. ustus, allowing the fungus to grow even in the presence of phenazines and phenazine-producing bacteria. This appears to be accomplished by a structural rearrangement of P. SOS3.
In the absence of PCA, P. SOS3 is dispersed throughout the fungal colony. However, in the presence of PCA this bacterium forms spheres within the fungal colony that appear to reduce and sequester PCA, permitting A. ustus growth. This behavior appears to be species-specific as other biocontrol bacteria cannot effectively protect A. ustus.
Given that the relative abundance of phenazine producers in the soil rises with temperature, understanding how biocontrol fungi interact with partnered bacteria to withstand these kinds of stresses is important to the future of agriculture in a warmer climate.