Soil bacteria as sources of virulence signal providers promoting plant infection by Phytophthora pathogens.

Phytophthora species are known as "plant destroyers" capable of initiating single zoospore infection in the presence of a quorum of chemical signals from the same or closely related species of oomycetes. Since the natural oomycete population is too low to reach a quorum necessary to initiate a disease epidemic, creation of the quorum is reliant on alternate sources. Here, we show that a soil bacterial isolate, Bacillus megaterium Sb5, promotes plant infection by Phytophthora species. In the presence of Sb5 exudates, colonization of rhododendron leaf discs by 12 Phytophthora species/isolates was significantly enhanced, single zoospores of P. nicotianae infected annual vinca and P. sojae race 25 successfully attacked a non-host plant, Nicotiana benthamiana as well as resistant soybean cultivars with RPS1a or RPS3a. Sb5 exudates, most notably the fractions larger than 3 kDa, promoted plant infection by improving zoospore swimming, germination and plant attachment. Sb5 exudates also stimulated infection hypha growth and upregulated effector gene expression. These results suggest that environmental bacteria are important sources of virulence signal providers that promote plant infection by Phytophthora species, advancing our understanding of biotic factors in the environmental component of the Phytophthora disease triangle and of communal infection of plant pathogens.