Investigating how plants recruit antibiotic-producing Streptomyces bacteria to protect themselves against disease

Humans have been using antibiotics to treat disease for less than 100 years but their use in nature stretches back for tens of millions of years. Most of the antibiotics we use are made by a group of common soil bacteria called Streptomyces and plants and insects started using them long before humans. Plants including the model Arabidopsis thaliana and important food crops like wheat, potato and rice, all have Streptomyces living inside their roots. Indeed, several recent high profile papers have shown that Streptomyces bacteria are enriched in the rhizosphere (the soil coating the roots) and even more greatly enriched inside the roots of Arabidopsis compared to the surrounding soil. Research published in Science in August 2015 suggests that Streptomyces bacteria are specifically attracted by salicylic acid (SA) that is produced by the plants as part of their general stress response. The suggestion is that Streptomyces (but not all bacteria) can use SA as a food source and thus the plants can specifically select bacteria that are useful to them out of the trillions of bacteria present in the soil. The antibiotics made by these Streptomyces strains are thought to protect the plant roots against fungal infection. In this project we will use Arabidopsis as a model to test (1) if Streptomyces bacteria in the rhizosphere and roots are really using plant-produced SA as food, (2) if any and all Streptomyces strains can live inside plant roots, (3) if SA metabolism is essential for root colonisation by Streptomyces and (4) if Streptomyces-produced antifungals do indeed protect the plants against fungal infection. Ultimately this work will lay the foundation for engineering improved Streptomyces strains for use in agriculture and thus contribute to global food security.