Do soil protozoa enhance plant growth by hormonal effects?

We investigated changes in root morphology of watercress seedlings (Lepidium sativum L.) and effects on the composition of the rhizosphere bacterial community to test the hypothesis that rhizosphere protozoa affect plant growth by a grazing-induced stimulation of plant growth-promoting rhizobacteria. The presence of Acanthamoebae (Protozoa: Amoebida) induced changes in root morphology of watercress seedlings as soon as the root protruded from the seed. The root system was greater and more branched. These changes resembled hormonal effects and were accompanied by an increase in the proportion of auxin, indolyl-3-acetic acid (IAA) producing rhizosphere bacteria. IAA did not originate from amoebal metabolism, but resulted from changes in the composition and activity of the microbial community. Therefore, amoebae affected both the functioning and turnover of rhizosphere microoganisms. We propose a new mechanism based on hormonal effects of protozoa on root growth. Protozoa function as bacteria-mediated mutualists promoting plant growth by hormonal feed-back mechanisms and nutrient effects based on nutrient release from grazed bacterial biomass, i.e. the microbial-loop.