Mechanisms of Plant Species Coexistence: Roles of Rhizosphere Bacteria and Root Fungal Pathogens

Two independent experiments were conducted to investigate the influence of rhizosphere bacteria on the growth of Anthoxanthum odoratum and Panicum sphaerocarpon. We tested whether host-specific populations of Bacillus mycoides affected the growth of their Anthoxanthum and Panicum hosts and whether host-specific differences in Bacillus populations modified the strong detrimental effect of the root fungal pathogen, Pythium macrosporum. Our results showed both positive and negative effects of Bacillus inoculation and that Anthoxanthum and Panicum plants responded differently to Bacillus isolates that originated from different host plants. Anthoxanthum grew relatively better with isolates from Panicum, while Panicum grew relatively better with isolates from Anthoxanthum, consistent with a negative feedback. In both experiments Pythium infection was detrimental to plant growth, and Panicum was more negatively affected by inoculation with Pythium. Overall, Bacillus ameliorated the pathogenic effect of Pythium. However, there was no evidence that host-specific Bacillus populations had different effects on the interaction between these plant species and Pythium. Both host-specific differences in rhizosphere bacteria and host-specific accumulation of a fungal pathogen can generate negative feedback between these two plant species.