Root exudation: The role of secondary metabolites, their localisation in roots and transport into the rhizosphere

Root exudates contain numerous biologically active low molecular weight secondary metabolites that are produced by plants. Due to their physical and biochemical properties, they are able to interact with many diverse targets in subcellular locations to elicit various activities in microbes, plants and animals. We present specific examples of two families of bioactive secondary plant products which have been well documented as allelochemcals and discuss their production and transport in the plants which produce them and their respective roles in the rhizosphere. We focus on flavonoids which play important roles in transport of auxin, root and shoot development, pollination, modulation of reactive oxygen species, and signalling of symbiotic bacteria in the legume Rhizobium symbiosis. In addition, they possess antibacterial, antifungal, antiviral and anticancer activities. Flavonoids are transported within and between plant tissues and cells by specific transport proteins or transporters, and are released into the rhizosphere by roots where they are involved in numerous interactions including allelopathy. Released by root exudation or tissue degradation over time, both aglycones and glycosides of flavonoids and other bioactive secondary metabolites are found in soil solutions and the rhizosphere.We describe their activity and fate in the soil rhizosphere in selected examples involving legumes. Long chain hydroquinones, in contrast, are lipophilic molecules that are released by passive exudation from living Sorghum spp. root hairs, and are involved in allelopathic interactions, and in chemical signalling causing stimulation of germination by Striga spp. They are also thought to be involved in nematicidal activity associated with Sorghum haplotypes and are antibacterial to certain soil bacteria. Hydroquinones including sorgoleone are released continuously through pores in the tips of root hairs where they bind to soil particles and organic matter. Once exuded, sorgoleone and its metabolites remain bioactive in the rhizosphere for several days after introduction to living soils. We also discuss the potential for future research to further elucidate the role of secondary metabolites and their fate in the soil rhizosphere.