title: Root-secreted malic acid recruits beneficial soil bacteria.
abstract: Beneficial soil bacteria confer immunity against a wide range of foliar diseases by activating plant defenses, thereby reducing a plant's susceptibility to pathogen attack. Although bacterial signals have been identified that activate these plant defenses, plant metabolites that elicit rhizobacterial responses have not been demonstrated. Here, we provide biochemical evidence that the tricarboxylic acid cycle intermediate L-malic acid (MA) secreted from roots of Arabidopsis (Arabidopsis thaliana) selectively signals and recruits the beneficial rhizobacterium Bacillus subtilis FB17 in a dose-dependent manner. Root secretions of L-MA are induced by the foliar pathogen Pseudomonas syringae pv tomato (Pst DC3000) and elevated levels of L-MA promote binding and biofilm formation of FB17 on Arabidopsis roots. The demonstration that roots selectively secrete L-MA and effectively signal beneficial rhizobacteria establishes a regulatory role of root metabolites in recruitment of beneficial microbes, as well as underscores the breadth and sophistication of plant-microbial interactions.
authors.affiliation: Department of Plant and Soil Sciences, University of Delaware, Newark, Delaware 19716, USA.
authors.forename: Thimmaraju
authors.initials: T
authors.lastname: Rudrappa
authors.forename: Kirk J
authors.initials: KJ
authors.lastname: Czymmek
authors.forename: Paul W
authors.initials: PW
authors.lastname: Paré
authors.forename: Harsh P
authors.initials: HP
authors.lastname: Bais
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D001412:Bacillus subtilis / Q000378:metabolism*; D001422:Bacterial Adhesion; D018441:Biofilms; D008293:Malates / Q000378:metabolism*; D018517:Plant Roots / Q000378:metabolism*; D015398:Signal Transduction; D012988:Soil Microbiology
chemical_list: D008293:Malates; C030298:malic acid