The role of cytokinins in the interactions between parasitic plants and host plants

Abstract

Cytokinins are widely recognized as crucial phytohormones regulating plant growth and development by promoting cell division and differentiation. They are also known to be involved in inter-organism interactions, including the interaction between parasitic plants and host plants. Orobanchaceae parasitic plants form prehaustoria, primitive haustorial structures before host invasion, when in close proximity to the hosts. The induction of prehaustoria requires host-derived compounds, namely haustorium-inducing factors (HIFs), including quinones, phenolics and flavonoids, with structural similarity to lignin monomers. Notably, recent reports showed that cytokinins are capable to induce prehaustoria in the obligate parasitic plants in Orobanchaceae. Once prehaustoria invade into host roots, they become mature haustoria, which efficiently extract water and nutrients from the host. Cytokinins are also involved in this stage, as they are synthesized by parasitic plants and transported to host plants, causing hypertrophy and promoting efficient parasitism. In this review, we summarize the roles of cytokinins in the interaction between parasitic plants and host plants, with a specific focus on current understanding of prehaustorium induction, hypertrophy and germination of obligate parasites.