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 ofAnthoxanthum odoratum andPanicum sphaerocarpon. We tested whether host-specific populations ofBacillus mycoides affected the growth of theirAnthoxanthum andPanicum hosts and whether host-specific differences inBacillus populations modified the strong detrimental effect of the root fungal pathogen, Pythium macrosporum. Our results showed both positive and negative effects ofBacillus inoculation and thatAnthoxanthum andPanicum plants responded differently toBacillus isolates that originated from different host plants.Anthoxanthum grew relatively better with isolates fromPanicum, whilePanicum grew relatively better with isolates fromAnthoxanthum, consistent with a negative feedback. In both experimentsPythium infection was detrimental to plant growth, andPanicum was more negatively affected by inoculation withPythium. Overall,Bacillus ameliorated the pathogenic effect ofPythium. However, there was no evidence that host-specificBacillus populations had different effects on the interaction between these plant species andPythium. Both host-specific differences in rhizosphere bacteria and host-specific accumulation of a fungal pathogen can generate negative feedback between these two plant species.