Allelochemicals in Plant/Plant Interactions.

Plants produce phytotoxins, and sometimes one the roles or the only role of such a compound is to give a plant an advantage in plant/plant interactions.  Recent work indicates that, at least in annual plants, these compounds (allelochemicals) are commonly produced and exuded by roots.  As are most soil-applied synthetic herbicides, allelochemicals compounds are usually highly lipophilic.  Their biosynthesis is sometimes induced by the presence of the target plant.  These compounds are sometimes highly active molecules present in low concentrations, and in other cases they are only moderately phytotoxic compounds found at high levels.  Allelochemicals from all biochemical pathways have been reported.  Genetic manipulation of these pathways is proving useful in proving that these compounds function as allelochemicals. Avoidance of autoxicity is poorly understood, but sequestration and rapid exudation into soil are two established mechanisms to avoid self poisoning.  The modes of action of most putative allelochemicals are poorly understood, but for others (e.g., the triketones and sogoleone) molecular target sites have been identified.