Phtheirospermum japonicum, like most hemiparasitic species in the Orobanchaceae, forms multiple haustoria along its roots when it detects a nearby host. In this PNAS Brief Report, we present the discovery of an endogenous peptide, PjCLE1, which stimulates protohaustorium formation when applied externally. This makes the 12-amino acid-long PjCLE1 peptide the first identified peptidic haustorium-inducing factor (HIF). Initially, the discovery of an endogenous peptidic HIF in P. japonicum surprised us because haustorium formation in P. japonicum is strongly tied to the presence of a host or host-derived HIFs such as DMBQ. However, the HIF activity of PjCLE1 is less surprising when considering that PjCLE1 is expressed in mature haustoria. Additionally, we have identified a protease, PjSBT1.2.3, that activates proPjCLE1 through proteolytic cleavage. PjSBT1.2.3 was previously found to be a marker for mature haustoria. With PjCLE1 and PjSBT1.2.3 accumulating in mature haustoria, we propose a model in which fully-formed haustoria promote further haustoria formation after the successful initial infection of a host. The haustoriogenesis-promoting effect of PjCLE1 was strongest in the presence of the host-derived HIF, DMBQ, and partially dependent on sucrose availability. Interestingly, PjCLE1 shares sequence similarity with CLEs auto-regulating nodulation in legumes, despite the independent evolution of parasitism in the Orobanchaceae and nodulation in legumes.
We hope to initiate with this finding further research into how haustoriogenesis is regulated and integrated into the physiology of parasitic Orobanchaceae.