The soil surrounding a plant root, known as the rhizosphere, is a unique physical, biochemical and ecological environment. The rhizosphere is to a large extent controlled by the root system through chemicals secreted into the surrounding soil. These root secretions (exudates) may play symbiotic or defensive roles as a plant communicates with its environment, ultimately developing a positive or negative response to the other elements in its rhizosphere. An example of negative communication is provided by the Asian native Centaurea maculosa L. (spotted knapweed).
Centaurea maculosa Lam. (spotted knapweed) is one of the most invasive and destructive introduced weeds in North America, particularly in the fields and pastures of the northwest U.S. Although it has long been proposed that spotted knapweed achieves its dominance through secretion of a plant-toxic chemical (allelochemical), no such chemical had been identified until last year, when it was determined that an exudate of spotted knapweed roots is indeed toxic to other plants. The active "ingredient" of this chemical is composed of racemic catechin, a 50:50 mixture of (+)-catechin and (-)-catechin. Although (+)-catechin is widespread throughout the plant kingdom, both (-)-catechin and the racemic form are extremely rare. Further, the anti-plant part of the exudate was entirely due to (-)-catechin, while (+)-catechin was inactive. Conversely, (+)-catechin was active against certain bacteria, while (-)-catechin was not. (-)-Catechin was found to be toxic against Arabidopsis and a variety of crop plant species and was also toxic to C. diffusa (diffuse knapweed), a close relative of spotted knapweed. (-)-Catechin did not affect seedlings or seed germination of C. maculosa itself. In Arabidopsis thaliana, spotted knapweed's toxic chemical triggers a rapid cascade of stress responses which kill cells in the root meristem and elongation zone and ultimately lead to the death of the entire root system. The proposed research will shed light on the molecular and biochemical interactions that occur in conjunction with this negative communication process in the subterranean part of this plant.
Specifically, the proposal will examine the negative control exerted by (-)-catechin in the rhizosphere of C. maculosa using the following aims: 1) to understand the toxic chemical processes initiated by (-)-catechin in Arabidopsis roots, 2) to understand the mechanism by which C. maculosa detoxifies (-)-catechin, and 3) to clone the gene(s) involved in (-)-catechin production. The interaction of (-)-catechin with plants such as Arabidopsis is an event that occurs under natural soil conditions, and thus has biological relevance. In addition, this research will provide the broader benefits of training two postdoctoral scholars, one graduate student and undergraduates in a multidisciplinary research atmosphere. Furthermore, the research outlined has practical implications to both U.S. agriculture and to preserving plant diversity of the Western U.S.
