Synergistic effects of bacterial endophytes promoting plant invasions
Invasive plants profoundly alter ecosystem processes, and tremendous economic costs are associated with these disturbances. Attributes like higher growth rates, increased biomass, and enhanced chemical defenses have been documented in many invasive plants, often allowing them to outcompete in native communities. Current theories suggest these characteristics are plant-regulated; however, our work shows that bacterial symbionts can regulate these traits. Using culture and molecular approaches, we found that a highly invasive grass (Johnsongrass) harbors several bacterial organisms inside below-ground tissues (rhizomes). We confirmed numerous physiological functions of these bacteria: nitrogen fixation, iron chelation, phosphate solubilization, and plant-growth hormone production. In field studies, we documented alterations to several soil biogeochemical cycles; invaded soils had increased plant-available nitrogen, phosphorus, potassium, and several trace metals. Using a novel antibiotic approach, we manipulated the bacterial symbionts and found they significantly increased plant biomass, and altered resource allocation enhancing rhizome growth. Plants with symbionts significantly inhibited the growth of a native prairie grass (Little Bluestem) frequently displaced by the invader. Restricting bacterial growth completely removed these competitive effects. Plants with bacterial symbionts also had increased production of the herbivore-defense compound, dhurrin, contained in leaves. When leaves from plants with symbionts were fed to a generalist insect herbivore, the insect could not grow and experienced significant mortality. Restricting bacterial growth resulted in <6-fold decrease in dhurrin, and significant increases in insect growth and survival. These results suggest microbial symbionts significantly contribute to Johnsongrass invasions by enhancing the plant traits of biomass, growth rate, competitive effects, and herbivore-defense.