Lignin as a contributor to virulence rather than defence ? — novel insights from parasitic plant haustoria

Parasitic plants produce organs called haustoria to attach to host plants, infiltrate their vasculature and absorb nutrients. Haustoria of several parasite species attached to resistant hosts are often encapsulated by a lignified interfacial layer assumed to be the main mechanism of host defence. While the precise structure, composition and assembly of encapsulation layers are poorly characterised, interfacial lignins have frequently been suggested as a crucial component.

The interfaces of Rhinanthus minor (a hemiparasitic herb of species-rich meadows) with a grass host, Arrhenatherum elatius, and a eudicot non-host, Plantago lanceolata, were investigated using histology, immunocytochemistry and Raman spectroscopy. Lignin-like substances consistently localised to the cell walls of haustorial interfacial parenchyma and an interfacial extramural layer, where they co-localised with xyloglucans and arabinogalactan proteins, and were even found at the contact surface of non-infective haustoria appressed to pots (lab-grown plants). Lignification was not observed in the cell walls of hosts or non-hosts.

These findings suggest that at least some of the interfacial polyphenolic substances previously assumed to contribute to host defence are synthesised by and beneficial to the parasite. Consequently, reinterpretation of interfacial lignin’s role in parasitic plant-host interactions is needed.