Uptake of non-pathogenic E. coli by Arabidopsis induces down-regulation of heat shock proteins.

We recently demonstrated that non-pathogenic and non-symbiotic microbes E. coli and yeast are taken up by roots and used as a source of nutrients by the plant. Although this process appears to be beneficial for the plant, the nutritional gain of microbe incorporation has to exceed the energy expense of microbe uptake and digestion, and the question remains whether the presence of microbes triggers pathogen- and other stress-induced responses. Here, we present evidence that digesting microbes is accompanied by strong down-regulation of genes linked to stress response in Arabidopsis. Genome-wide transcription analysis shows that uptake of E. coli by Arabidopsis roots is accompanied by a pronounced down-regulation of heat shock proteins. Plants up-regulate heat shock proteins in response to environmental stresses including temperature, salt, light and disease agents including microbial pathogens. The pronounced down-regulation of heat shock proteins in the presence of E. coli indicates that uptake and subsequent digestion of microbes does not induce stress. Additionally it suggests that resources devoted to stress resistance in control plants may be re-allocated to the process of microbe uptake and digestion. This observation adds evidences to the notion that uptake of microbes is an active, purposeful and intentional behavior of the plant.