Rhizosphere Interactions: The Role of Root Exudates

The rhizosphere is a dense and complex environment, in which plants roots negotiate a shifting sea of stimuli, including pathogenic and non-pathogenic microbes, competing plant roots, various invertebrates, and a wide variety of soil conditions. Plant roots play an active role in this environment, exuding chemicals that change the soil’s pH, encourage symbioses, ward off potential pathogens, and regulate the germination and growth of competing plants, including their own offspring. They in turn are acted upon by various rhizosphere inhabitants. The complexity of these multitrophic interactions is almost impossible to replicate in the laboratory, as studies show that the communication between two organisms within the rhizosphere can be “overheard” and influence the behavior of other organisms. For a full discussion of the state of rhizosphere biology, as well as the techniques and limitations for its study, see Bais et al. 2006.

Our laboratory concerns itself with all of the above aspects of rhizosphere biology. Our work on the relationship between plants and pathogenic microbes has shown that the first factor which determines whether a soil microbe will be pathogenic or non-pathogenic is the cocktail of continuously-secreted secondary metabolites which the plant’s roots produce; pathogenic microbes not only tolerate these chemicals, but often are able to inhibit the secretion of certain antimicrobial compounds, thus affecting the plant’s relationship with other organisms as well (Bais et al. 2005).

Likewise, we have shown that soil nematodes can facilitate the symbiotic relationship between legume plants and nitrogen-fixing bacteria (Horiuchi et al. 2005). Some alfalfa plants are able to bring symbiotic bacteria into areas of soil that previously lacked this bacteria; one of the ways they apparently accomplish this is by attracting certain larger organisms, such as soil-dwelling nematodes, which carry the beneficial bacteria on their bodies and in their feces.

We have found that plants are in continual communication with each other as well. Some plant root exudates can regulate the growth and germination of competing individuals of rival species (Bais et al. 2003) and their own species (Perry et al. 2005); such allelopathic interactions have been shown to have a decisive affect on the development of invasiveness in certain exotic weeds. Likewise, allelopathy can be countered and/ or moderated by some species: we found that the North American native species Gaillardia grandiflora and Lupinus sericeus were able to inhibit the production of Centaurea maculosa’s allelochemical catechin through exudates of their own, and that such inhibition had a positive affect on other native species (Weir et al. 2005).

Our work on plant underground communication (for a comprehensive review, see Bais et al. 2004) has tied in with a new interest in the way supposedly passive plants respond, communicate, and even solve problems—a nascent field termed plant neurobiology.