Grazing of protozoa on rhizosphere bacteria alters growth and reproduction of Arabidopsis thaliana

Plant roots are densely colonized by bacteria which form the basis of the rhizosphere bacterial food web with protozoa as most effective predators. We established a well defined laboratory system with Arabidopsis thaliana as model plant allowing to investigate in detail the effect of rhizosphere interactions on plant performance. We used this system to analyse separate and combined effects of natural rhizobacteria and the protozoa Acanthamoeba castellanii on plants. Protozoa and bacteria increased plant growth with the effect of protozoa markedly exceeding that of bacteria only. Arabidopsis immediately responded to the presence of protozoa by increasing carbon but not nitrogen uptake. Later protozoa enhanced plant uptake of nitrogen from organic material and prolonged vegetative growth of Arabidopsis resulting in strongly increased seed production. It is concluded that the immediate plant response was based on changes in rhizosphere signalling inducing increased plant carbon fixation rather than on protozoa-mediated increase in nitrogen availability. The subsequently increased plant nitrogen uptake presumably originated from nitrogen fixed in bacterial biomass made available by protozoan grazing, i.e. the microbial loop in soil. The results suggest that Arabidopsis prepared for the upcoming mobilization of nitrogen by increasing carbon fixation and root carbon allocation which paid-off later by increased nutrient capture and strongly increased plant reproduction.