Structure-Dependent Phytotoxicity of Catechins and Other Flavonoids: Flavonoid Conversions by Cell-free Protein Extracts of Centaurea maculosa (Spotted Knapweed) Roots

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Abstract
Invasive plants are believed to succeed in part by secretion of allelochemicals, thus displacing competing plant species. Centaurea maculosa (spotted knapweed) provides a classic example of this process. We have previously reported that spotted knapweed roots secrete (±)-catechin and that (−)-catechin, but not (+)-catechin, is phytotoxic and hence may be a major contributor to C. maculosa's invasive behavior in the rhizosphere. In this communication, we explore both structure/activity relationships for flavonoid phytotoxicity and possible biosynthetic pathways for root production of (±)-catechin. Kaempferol and dihydroquercetin were shown to be phytotoxic, while quercetin was not. Kaempferol was converted to dihydroquercetin and (±)-catechin when treated with total root protein extracts from C. maculosa, but quercetin was not. This finding suggests an alteration in the standard flavonoid biosynthetic pathway in C. maculosa roots, whereby kaempferol is not a dead-end product but serves as a precursor to dihydroquercetin, which in turn leads to (±)-catechin production. Keywords: Centaurea maculosa; allelopathy; (−)-catechin; (±)-catechin; kaempferol; phytotoxicity; flavonoid biosynthesis; in vitro