Mechanisms for cellular transport and release of allelochemicals from plant roots into the rhizosphere

Allelochemicals and other metabolites released by plant roots play important roles in rhizosphere signalling, plantdefence and responses to abiotic stresses. Plants use a variety of sequestration and transport mechanisms to moveand export bioactive products safely into the rhizosphere. The use of mutants and molecular tools to study geneexpression has revealed new information regarding the diverse group of transport proteins and conjugationprocesses employed by higher plants. Transport systems used for moving secondary products into and out of rootcells are similar to those used elsewhere in the plant but are closely linked to soil environmental conditions and localroot health. Root cells can rapidly generate and release large quantities of allelochemicals in response to stress orlocal rhizosphere conditions, so the production and transport of these compounds in cells are often closely linked.Plants need to manage the potentially toxic allelochemicals and metabolites they produce by sequestering them tothe vacuole or other membrane-bound vesicles. These compartments provide secure storage areas and systems forsafely moving bioactive chemicals throughout the cytosol. Release into the apoplast occurs either by exocytosis orthrough membrane-bound transport proteins. This review discusses the possible transport mechanisms involved inreleasing specific root-produced allelochemicals by combining microscopic observations of the specialized rootcells with the physical and chemical properties of the exudates.