Root Exudation and Rhizosphere Biology: Allelochemicals and Cell Death
 
Laura G. Perry*, Mark W. Paschke, and Jorge M. Vivanco
Center for Rhizosphere Biology, Colorado State University, Fort Collins CO 80523, USA
*email: lperry@lamar.colostate.edu
 

Plant roots release an enormous variety of secondary metabolites into the soil. These root exudates can play important roles in mediating positive and antagonistic root-root and root-microbe interactions. We will focus our discussion on the role of root exudates in negative and positive communication between roots of Centaurea maculosa and competing plant species.
Centaurea maculosa , a native species in western Europe, is an invasive weed in North America . Our work on C. maculosa root exudates indicates that C. maculosa invasions in North America are partly mediated by root-secretion of a potent phytotoxin, (±)-catechin. Roots of susceptible plants such as Arabidopsis thaliana exposed to (±)-catechin exhibit waves of ROS and Ca 2+ signals that begin at the meristematic and elongation zones, and lead to genome-wide changes in expression and cell death. We investigated the genes involved in early signal transduction triggered by (±)-catechin using T-DNA mutants. Our studies have shown that some T-DNA mutants exhibit resistance to (±)-catechin phytotoxicity and at least one gene mutation increases (±)-catechin-sensitivity in Arabidopsis. These results indicate a complex network of genes working simultaneously to cause cell death and to counteract cell death, possibly by affecting levels of ROS.
Soil (±)-catechin concentrations in C. maculosa populations can be very high (mean = 1.55 mg g -1), and tend to be higher in North American populations than in European populations. Further, North American grassland species tend to be more sensitive to (±)-catechin than the European species with which C. maculosa naturally coexists, suggesting that (1) European species may have co-evolved (±)-catechin-resistance and (2) the (±)-catechin-naïvité of North American species may account for C. maculosa invasiveness in North America. Our research also suggests that high concentrations of (±)-catechin are autotoxic to C. maculosa seedlings and inhibit C. maculosa recruitment in well-established stands, indicating chemical control of population density. In addition, we have found that low catechin concentrations can induce growth and defense responses in (±)-catechin-susceptible species, indicating the potential for chemical facilitation by C. maculosa under some conditions. Finally, our work on mechanisms of (±)-catechin-resistance has shown that at least two North American species (Lupinus sericeus and Gaillardia grandiflora) also rely on root exudates to resist (±)-catechin toxicity. Lupinus and Gaillardia root-secrete oxalic acid in response to exposure to (±)-catechin. Oxalic acid, by working as an antioxidant, protects the resistant species and their (±)-catechin-susceptible neighbors from (±)-catechin toxicity under in vitro conditions and in the field. The presence of the resistant species or exogenous oxalic acid reduces C. maculosa (±)-catechin production, revealing the potential for chemical cross-talk between allelopathic and allelochemical-resistant species, and the coordination of defense responses between plants.

 
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