Enantiomeric-Dependent Phytotoxic and Antimicrobial Activity of (±)-Catechin. A Rhizosecreted Racemic Mixture from Spotted Knapweed

In this communication, we unravel part of the mystery surrounding the allelopathic capability of the noxious weed spotted knapweed (Centaurea maculosa). We have found that (−)-catechin is a root-secreted phytotoxin that undoubtedly contributes to spotted knapweed's invasive behavior in the rhizosphere. Although spotted knapweed roots exude (±)-catechin, only the (−)-catechin enantiomer was phytotoxic. (+)-Catechin had antibacterial activity against root-infesting pathogens, which (−)-catechin did not show. This suggests the biological significance for the exudation of racemic catechin, with each enantiomer contributing separate properties for plant aggression and defense. The soil immediately surrounding a plant root constitutes a unique physical, biochemical, and ecological environment. The rhizosphere is to a large extent controlled by the root system itself through chemicals exuded/secreted into the surrounding soil. Root exudates include low-M r compounds such as amino acids, organic acids, sugars, phenolics, and various secondary metabolites, and high-M r compounds like mucilage and proteins (Roshina and Roshina, 1993; Schultze et al., 1994). Through the exudation of a wide variety of compounds, roots may regulate the soil microbial community in their immediate vicinity, cope with herbivores, encourage beneficial symbioses, change the chemical and physical properties of the soil, and inhibit the growth of competing plant species (Nardi et al., 2000). Countering a challenge, roots may respond by secreting certain chemicals such as secondary metabolites, proteins, and even volatiles (Shulaev et al., 1997; De Moraes et al., 2001). Root secretions may play symbiotic or defensive roles as a plant ultimately develops a positive or negative communication, depending on the other elements of its rhizosphere. An example of a negative communication is provided by the Asian native spotted knapweed. This noxious weed, one of the most economically destructive exotic invaders of western North America, displaces other weeds and crops by mounting a chemical warfare mediated by root exudates (Callaway et al., 1999; Callaway and Aschehoug, 2000). Although allelopathy was suggested as the displacing mechanism as early as 1832 (DeCondolle, 1832), there has been minimal success in characterizing any responsible allelochemical(s) from knapweed. Considering the complexity of isolating and characterizing metabolites exuded from roots into the soil, our laboratory addressed this issue by developing a system where knapweed roots, grown in vitro, can secrete or be induced to secrete the allelochemical from its roots into sterile media in a way comparable with secretion into the rhizosphere (Bais et al., 2001).