The emergence of grass root chemical ecology.

A llelopathy is defined by most scientists as the adverse effect of one plant species on another through production of phytotoxins (allelochemicals), although more expansive definitions have been formulated. Allelopathy is but one component of plant/plant interference, the other being competition for resources such as nutrients, light, and water. Allelopathy has been a recognized phenomenon for many years (1), but prominent ecologists have argued that allelopathy is seldom a significant component of interference (e.g., ref. 2). This point of view was bolstered by the lack of scientific rigor of much of the allelopathy research that attempted to explain allelopathy through the effects of known, weakly phytotoxic, easy-to-quantify phytochemicals such as ferulic acid. More recent studies using bioassay-guided isolation and subsequent structure determination of potent, root-exuded phytotoxins built strong evidence for allelopathy, especially in grass species (reviewed in refs. 3–5, and see Table 1). The article by Bertin et al. (6) in this issue of PNAS adds significantly to this growing body of supportive literature. The work provides clear evidence of a novel, root-exuded allelochemical produced by an allelopathic grass, a variety of a Festuca rubra subspecies. It establishes that m-tyrosine is a highly active allelochemical causing most, if not all, of the effects of the root exudate of this allelopathic fescue variety described in that article and an earlier one (7). Although some nonprotein amino acids have functions in plant primary metabolism (e.g., -aminolevulinic acid), others are thought to be involved in protection of plants from a variety of biotic threats, particularly herbivores. The modes of action of these compounds range from direct neurotoxicity, such as produced by -N-methylaminoL-alanine (8), to incorporation into proteins to produce aberrant molecules, leading to multiple physiological problems (9). Nonprotein amino acids have previously been implicated in allelopathy. For example, mimosine has been associated with allelopathy of the legume tree Leucaena leucocephala (10). L-DOPA, a compound structurally related to m-tyrosine, has been implicated in allelopathy of Mucuna pruriens (11). Roots of pea (Pisum sativa) exude -(3isoxazolin-5-on-2yl)-alanine, which inhibits root growth on nonlegume plant species (12), although this nonprotein amino acid is much less phytotoxic than m-tyrosine. Finding that m-tyrosine is a potent phytotoxin leads to many interesting questions deserving further inquiry. First, how does the producing species protect itself from autotoxicity? It seems that m-tyrosine is broadly phytotoxic with some differences in plant species susceptibility, so what mechanism does the producing plant use to avoid the effects seen on other species? Does the plant avoid accumulation of the com-