The peptide hormone Pj CLE1 stimulates haustorium formation in the parasitic plant Phtheirospermum japonicum

Phtheirospermum japonicum is a hemiparasitic plant of the Orobanchaceae, the largest family of parasitic plants. It extracts water and nutrients from other plants through haustoria along its roots. Haustoriogenesis, the formation of haustoria, is initiated by host-derived haustorium-inducing factors (HIFs). The first step in haustoriogenesis is the development of parasitically inactive protohaustoria. Here, we report that an endogenous peptide hormone, CLAVATA3/Embryo Surrounding Region 1 ( Pj CLE1), is sufficient to induce protohaustorium formation. Pj CLE1 hyperactivated HIF-responses and caused prolific protohaustoria formation. PjCLE1 expression and activation by the subtilisin-type protease Pj SBT1.2.3 occur in fully developed, mature haustoria, suggesting that Pj CLE1 acts as an internal signal produced by mature haustoria to stimulate additional protohaustorium formation for effective extraction of resources from hosts. Pj CLE1 is similar in sequence to CLEs regulating nodulation in legumes and part of a regulatory system for haustoria formation in parasitic plants.