Host recognition and infection mechanism by parasitic Orobanchaceae plants

Striga spp. are noxious parasitic weeds that attack important crop species such as rice, maize and sorghum. Estimated yield losses can reach 1 billion US dollars annually. Striga belongs to the Orobanchaceae family, which contains approximately 90 genera of parasitic species. The degree of parasitism varied among species, from facultative parasites that can set seeds without host plants, to obligate holoparasites that cannot photosynthesize and thus completely depend on their hosts. The obligate parasites (e.g., Striga and Orobanche) germinate only when host plants are close by, via recognition of host-secreted strigolactones. The germinated parasitic plants develop an invasive haustorium on their roots. The haustoria penetrate host roots and establish vascular connections between hosts and parasites to absorb water and nutrients. Induction of haustorium is provoked by host-derived small compounds, named haustorium-inducing factors, including quinones and flavonoids. Diversity of germination stimulants and haustorium-inducing factors is likely to contribute to host recognition by parasitic plants. On the other hand, host plants respond to parasitic plants by activating plant immunity. This review focuses on the interaction between the parasites and their hosts.