Induction of microbial genes for pathogenesis and symbiosis by chemicals from root border cells

Reporter strains of soil-borne bacteria were used to test the hypothesis that chemicals released by root border cells can influence the expression of bacterial genes required for the establishment of plant-microbe associations. Promoters from genes known to be activated by plant factors included virE, required for Agrobacterium tumefaciens pathogenesis, and common nod genes from Rhizobium leguminosarum bv viciae and Rhizobium meliloti, required for nodulation of pea (Pisum sativum) and alfalfa (Medicago sativum), respectively. Also included was phzB, an autoinducible gene encoding the biosynthesis of antibiotics by Pseudomonas aureofaciens. The virE and nod genes were activated to different degrees, depending on the source of border cells, whereas phzB activity remained unaffected. The homologous interaction between R. leguminosarum bv viciae and its host, pea, was examined in detail. Nod gene induction by border cells was dosage dependent and responsive to environmental signals. The highest levels of gene induction by pea (but not alfalfa) border cells occurred at low temperatures, when little or no bacterial growth was detected. Detached border cells cultured in distilled water exhibited increased nod gene induction (ini) in response to signals from R. leguminosarum bv viciae.