Although plants have evolved defences against microbial pathogens, it is still not clear why most plants are resistant to most pathogens. A paper published in Nature reports that Arabidopsis thaliana oozes antimicrobials from its roots to fend off potential bacterial pathogens in the rhizosphere.

Bais et al. investigated why only one pathovar, Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), from a collection of eight different P. syringae strains, caused root disease in A. thaliana.

Using a root infection assay, Bais et al. showed that the non-pathogenic strains were poor root colonizers, unlike Pst DC3000. However, two of the same non-pathogenic strains caused disease in A. thaliana leaves. Members of the same research group, led by Jorge Vivanco, had previously shown that A. thaliana roots exude antimicrobial compounds. Here, they showed that the non-pathogenic strains caused disease in A. thaliana roots that had been treated with activated charcoal to soak up antimicrobial exudates. They also showed that the concentrations of antimicrobials were far higher during infection with non-pathogenic strains than in uninfected plants, but that plants infected with Pst DC3000 produced even less antimicrobials than uninfected plants.

Purified antimicrobial compounds were bacteriostatic against the non-pathogenic strains (but not Pst DC3000) in vitro at the same concentrations as those exuded by the roots. Mixed infections of A. thaliana with Pst DC3000 and non-pathogenic strains resulted in disease and failure to produce antimicrobials. As the growth rate of non-pathogenic strains was reduced in vitro by the antimicrobials, even when Pst DC3000 was present, Pst DC3000 did not degrade the antimicrobials.

Finally, mutants of Pst DC3000 in the type III secretion system (TTSS) genes hrcC (hypersensitive response) and hrpL (hypersensitive response and pathogenesis) elicited the same concentrations of antimicrobials from roots as the non-pathogenic strains. Therefore, the TTSS seems to be involved in suppression of this basic plant defence mechanism, presumably by translocating type III effectors into plant root cells. TTSS mutants were more susceptible to antimicrobials, so the TTSS might also be involved in resisting their effects.

Understanding mechanisms of plant resistance could be exploited to generate resistant crop plants and also to understand how bacteria and plants happily cohabit in the rhizosphere.