Strigol induces germination of the facultative parasitic plant <i>Phtheirospermum japonicum</i> in the absence of nitrate ions

Root parasitic plants in the family Orobanchaceae, such as <i>Striga</i> and <i>Orobanche</i> spp., infest major crops worldwide, leading to a multibillion-dollar loss annually. Host-derived strigolactones (SLs), recognized by a group of α/β hydrolase receptors (KAI2d) in these parasites, are important determinants for germinating root parasitic plants near the roots of host plants. <i>Phtheirospermum japonicum</i>, a facultative hemiparasitic Orobanchaceae plant, can germinate and grow in the presence or absence of the host and can also exhibit root chemotropism to host-derived SLs that are perceived via KAI2d. However, the importance of SLs in <i>P. japonicum</i> germination remains unclear. In this study, we found that germination of <i>P. japonicum</i> was suppressed in the absence of nitrate ions and that germination of <i>P. japonicum</i> was promoted by exogenous strigol, an SL, under such conditions. We propose a model in which <i>P. japonicum</i> may select either independent living or parasitism in response to ambient nitrogen conditions and host presence.