Clarification of survival strategy and host recognition mechanism of root parasitic weeds

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Characterization of strigolactones, germination stimulants for root parasitic weeds, Striga and Orobanche, exuded by their host plants was conducted. One of the major strigolactones of monocotyledonous plants, maize, millet and sorghum, was found to be deoxystrigol which is known as a host recognition signal for arbuscular mycorrhizal (AM) fungi. On the other hand, one of the major strigolactones of dicotyledonous plants was identified as orobanchol. In addition to these known strigolactones, at least 10 novel strigolactones were found in plant species examined. Non-mycorrhizal plants, such as spinach and white lupin, and Japanese red pine tree were found to exude strigolactones. These results indicate that strigolactones are distributed widely in the plant kingdom and other factors as well as strigolactones appear to be involved in host recognition of AM fungi.Through LC/MS/MS analyses of orobanchol in the root exudates, it was first demonstrated that in red clover plants reduced supply of phosphorus but not of other mineral nutrients examined (N, K, Mg, Ca) in the culture medium significantly promoted the release of orobanchol by the root of the plant. In addition, 10 nM fluridone, an inhibitor of carotenoid biosynthesis, reduced orobanchol exudation markedly, supporting the hypothesis that strigolactones are synthesized via carotenoids in planta. This carotenoid biosynthetic inhibitor alone elicited seed germination of Striga asiatica and also induced haustorium-like morphogenesis in the germinated seeds.DNA microarray analyses of Arabidopsis thaliana treated with strigol suggested that several genes including two P450 genes and ABC transporter gene are potentially involved in the biosynthesis and metabolism of strigolactones.Further study on biosynthesis and metabolism of strigolactones should shed light to both symbiotic and parasitic interactions in the rhizosphere.