Invading plant species, such as the Centaurea, establish monocultures in novel habitats by displacing the indigenous plant communities. It is thought that the absence of "natural enemies" and/or the release of phytotoxins from the invading plants by allelopathy promote this process. Previous work has shown that the European spotted knapweed (Centaurea maculosa) releases racemic catechin; the phytotoxin (-)-catechin, and the antimicrobial (+)-catechin. Invasive (-)-catechin levels in North American soil were found to be more than double those found in the natural habitats of C. maculosa. In the September 5 Science, Harsh Bias and colleagues at Colorado State University show that (-)-catechin inhibits growth of a number of nature North American plants by altering gene expression, resulting in extensive root death (Science, 301:1377-1380, September 5, 2003).

Bias et al. used an integrated approach to establish the characteristics of (-)-catechin-mediated inhibition of plant growth. They observed that when field soil was supplemented with "invasive" levels of (-)-catechin, the germination and growth of two native grasses were dramatically reduced compared with more resistant European grasses given the same treatment. Examination of Arabidopsis thaliana and C. diffusa—a close relative of C. maculosa susceptible to (-)-catechin—at the cellular level revealed a wave of cell death, starting at the root tip that proceeded through the central elongation zone toward the stele. (-)-Catechin treatment initiated a rapid reactive oxygen species wave, similar to that observed for root cell death, but proceeding cell death by 5 to 10 minutes. This increase in reactive oxygen species induced Ca²⁺-dependent triggering of cell death. Gene expression analysis of A. thaliana showed that 10 genes were upregulated 10 minutes after treatment, with 956 genes being upregulated 50 minutes later. Many of the 956 gene products are linked to oxidative stress and the phenylpropanoid and terpenoid pathways. At 12 hours, many of these genes were repressed, possibly as a result of the onset of cell death.

"The case we have presented here for allelopathy in C. maculosa challenges the conventional ecological perspective that a species' invasiveness is mainly due to enhanced resource competition after escape from natural enemies and highlights the role for the biochemical potential of the plant as an important determinant of invasive success," conclude the authors.