Plant Secretes Malic Acid to Attract Beneficial Soil Bacteria

Plants in their natural environment need all the help they can get. They face a multitude of threats: frost, drought, herbivores and pathogens such as fungi, nematodes, bacteria and viruses. Scientists have known that beneficial soil bacteria confer immunity against a wide range of plant diseases by activating plant defenses, thereby reducing a plant's susceptibility to pathogen attack. Plants use an array of metabolites to defend themselves against harmful organisms and to attract others that are beneficial. Although bacterial signals have been identified that activate these plant defenses, plant metabolites that elicit rhizobacterial responses have not been demonstrated.

Scientists at the University of Delaware provided evidence tthat the metabolic intermediate malic acid (MA) secreted from roots of Arabidopsis selectively signals and recruits the beneficial rhizobacterium Bacillus subtilis in a dose-dependent manner. Secretions of malic acid are elicited by the foliar pathogen Pseudomonas. Binding of the rhizobacterium triggers induced systemic resistance, and in turn provides resistance against the foliar pathogen.

The discovery underscores the breadth and sophistication of plant-microbial interactions. The use of microorganisms to control plant pathogens is accepted as a durable and environmentally friendly alternative in plant disease management.

The open-access paper published by Plant Physiology is available at http://dx.doi.org/10.1104/pp.108.127613