Prehaustoria of root hemiparasites Rhinanthus minor and Odontites vernus (Orobanchaceae) produce lignin-rich interfacial deposits closely resembling those of attached haustoria

Background and Aims
Lignin and other phenolics are commonly observed at the interfaces between the haustoria of parasitic plants and tissues of their hosts. As known plant defence compounds, their accumulation at haustorial interfaces has been ascribed to mechanical and chemical resistance of host tissues. Although the possibility that the interfacial lignin deposits may have a parasitic origin has not previously been addressed, the fact that certain parasitic plants, including Rhinanthus and Odontites, can form haustoria in the absence of hosts gives us a tool that can be used to help answer this question.

Methods
We compared the interfaces of haustoria of root hemiparasites yellow rattle Rhinanthus minor and red bartsia Odontites vernus (Orobanchaceae) attached to hosts bulbous oat-grass Arrhenatherum elatius ssp. bulbosum and perennial rye grass Lolium perenne (Poaceae) with the contact surfaces of non-infective prehaustoria attached to a pot surface. We performed histochemistry, immunocytochemistry and Raman spectroscopy to characterize the architecture of contact deposits formed by both.

Key Results
Lignolic deposits, which we will term lignin-rich interfacial deposits (LIDs), were found at the interfaces between haustoria and compatible hosts as well as at the pot-appressed facets of prehaustoria. In both cases the deposits were determined as lignin by histology and Raman spectroscopy. Xyloglucan and arabinogalactan protein glycan epitopes were also detected while mixed-linkage glucan, xylans and pectin were not. We demonstrate that prehaustoria can produce lignolic interfacial deposits of high structural similarity to those of haustorium-host interfaces.

Conclusions
LIDs at haustorium–host interfaces may at least partly be attributed to the parasite and benefit the establishment and functioning of the haustorium. A reinterpretation of the origin and role of interfacial lignin in parasitic plant–host interactions may therefore be necessary.