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

对寄主植物分泌的植物根寄生杂草Striga和Orobanche的萌发促进剂脱氧雄酮是单子叶植物玉米、谷子和高粱的主要内酯之一，被认为是丛枝菌根真菌的寄主识别信号。另一方面，在双子叶植物中，主要的植物内酯之一被鉴定为orobancol。除了这些已知的链内酯外，在所研究的植物中至少发现了10种新的链内酯。非菌根植物，如菠菜和白羽扇豆，以及日本红松树，都能分泌出木立内酯。通过对红三叶植物根分泌物中orobandol的LC/MS/MS分析，首次证明在红三叶植物中，磷的供应减少，而培养基中其他矿质营养(N、K、Mg、Ca)的供应没有显著的促进植物根中orobandol的释放。此外，类胡萝卜素生物合成的抑制剂10 NM氟烷酮显著减少了甲壳素的分泌，支持了类胡萝卜素在植物体内合成的假说。这种类胡萝卜素生物合成抑制剂本身就能诱导亚洲千里光种子萌发，也能诱导萌发种子中吸器状的形态发生。基因芯片分析表明，包括两个P450基因和ABC转运蛋白基因在内的多个基因可能参与了植物内酯的生物合成和代谢。对内酯的生物合成和代谢的进一步研究将有助于揭示根际共生和寄生的相互作用。

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.