Parasitic plants absorb water and nutrients from their host plants via an invasive organ called a haustorium. Following host attachment, a subset of haustorial cells differentiates into tracheary elements that form a xylem bridge that connects the parasite and host vascular systems. Although the plant hormone auxin has been implicated in haustorium formation, its precise role in establishing host-parasite connections is unclear. Now, Ken Shirasu and colleagues report that an auxin transport network underlies the development of xylem bridges between Arabidopsis and the hemi-parasitic plant Phtheirospermum japonicum. They first show that an auxin response, as detected by the DR5 auxin response reporter, is present at sites of tracheary element differentiation. They further show that an auxin efflux inhibitor blocks xylem bridge formation. The researchers also characterize the expression and localization of various auxin efflux and influx transporters during haustorium development. Based on their analyses, they suggest that PjPIN1 and PjPIN9 likely function in xylem bridge formation, while PjLAX1, PjLAX2 and PjLAX5 likely fine-tune the auxin gradient. Indeed, the researchers report that RNAi-mediated knockdown of PjPIN1 and PjPIN9 blocks tracheary element differentiation and reduces xylem connections. Overall, these findings lead the authors to propose a model in which the cooperative action of auxin and auxin transporters controls xylem bridge formation.
