Cytokinins induce prehaustoria coordinately with quinone and phenolic signals in the parasitic plant Striga hermonthica

Abstract

Orobanchaceae parasitic plants are major threats to global food security, causing severe agricultural damage worldwide. Parasitic plants derive water and nutrients from their host plants through multicellular organs called haustoria. The formation of a prehaustorium, a primitive haustorial structure, is provoked by host-derived haustorium-inducing factors (HIFs). Quinones, including 2,6-dimethoxy-p-benzoquinone (DMBQ), and phenolics, including syringic acid, are of most potent HIFs for various species in Orobanchaceae, but except non-photosynthetic holoparasites, Phelipanche and Orobanche spp. On the other hand, cytokinin phytohormones was reported to induce prehaustoria in Phelipanche ramosa. However, little is known about whether cytokinins act as HIFs in the other parasitic species. Moreover, the signaling pathways for quinones, phenolics and cytokinins in prehaustorium induction are not well understood. This study showed that cytokinins act as HIFs in Striga hermonthica but not in Phtheirospermum japonicum. Using chemical inhibitors for each type of HIF, we demonstrated that cytokinins activate prehaustorium formation through a signaling pathway that overlaps with the quinone and phenolic HIF pathways in S. hermonthica. Host root exudates activated S. hermonthica cytokinin biosynthesis and signaling genes, and inhibitors blocking any of three types of HIFs perturbed the prehaustorium-inducing activity of exudates, indicating that host root exudates include a mixture of HIFs. Our study reveals the importance of cytokinins for prehaustorium formation in obligate parasitic plants.