Molecular mechanisms of biosynthesis and exudation of strigolactones and their manipulation for regulation of parasitism and symbiosis

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Strigolactones exuded from plant roots elicit seed germination of root parasitic plants and induce hyphal branching of symbiotic arbuscular mycorrhizal (AM) fungi. Therefore, strigolactones function as host recognition signals for both parasitism and symbiosis. Studies were conducted to understand molecular mechanisms of strigolactone biosynthesis and exudation. Through the studies, 1) several novel strigolactones were isolated and their structures determined, 2) effects of nutrients on strigolactone production and exudation were clarified, 3) structure-activity relationships of strigolactones in germination stimulation of root parasitic weed seeds and in induction of hyphal branching of AM fungi were elucidated, and 4) interactions between root parasitic plants with their compatible and incompatible host plants were examined. In addition, excessive branching mutants of rice plants were found to be the deficit either in biosynthesis or signal transduction of strigolactones, demonstrating that strigolactones are a novel class of plant hormones inhibiting shoot branching.