Metabolites from soil bacteria affect plant water relations

Water-soluble compounds move naturally in soil moisture toward roots of transpiring plants. To test for effects of rhizosphere food-web molecules on plants, low concentrations of common microbial products were supplied to bean (Phaseolus vulgaris L.) roots. Stomatal conductance and transpiration increased significantly (+20 to +30%, P ≤ 0.05) 42 h after 10 nM homoserine lactone (HL) was supplied to roots. Because transpiration helps both a plant and its root-colonizing bacteria obtain diffusion-limited mineral nutrients, such as phosphorus, any increase triggered by a degradation product of N-acyl-homoserine lactone (AHL) regulatory signals commonly used among plant-associated bacteria may represent a mutualistic plant-microbe interaction. Results from these initial physiological tests justify further screening to identify other rhizosphere compounds that control plant functions important for root-colonizing microorganisms.