Cell wall diversity in the haustoria of hemiparasitic plants Rhinanthus minor and Odontites vernus (Orobanchaceae).

Haustoria are structures produced by parasitic plants to attach to host plant organs and to infiltrate them for nutrients. While their primary functions are to access and abstract host solutes, additional roles in their processing have also been suggested.
Haustorial cell walls of two hemiparasitic herbs were investigated using (immuno)histochemistry at light and electron microscopy level as well as Raman spectroscopy. Cell wall modifications were observed in specific regions of haustoria and included cell wall thickening in the interfacial parenchyma and parenchyma abutting haustorial xylem as well as paramural deposits in the central, metabolically active parenchymateous tissue know as the hyaline body. Immunocytochemistry showed that the distribution of arabinogalactan protein (AGPs) epitopes coincided with four strategic areas of haustoria: 1) protoplasts of the differentiating xylem bridges, 2) cell walls, paramural deposits and ergastic globular inclusions of the hyaline bodies 3) interfacial parenchyma walls and 4) an extramural secretion complex of lignin-like substances at the interface with the host. Examination of metahaustoria, non-infecting haustoria attached to pot surface, allowed the determination of this phenolic-rich layer as a product of the parasite rather than the host.
The demonstrated diversity of haustorial cell walls is likely to reflect a wide range of functions fulfilled by these specialised organs. The striking localisation of AGPs in parts of haustoria crucial for their functioning implies important roles in their development and functioning while the discovery of interfacial lignin production by haustoria suggests that it contributes to the parasite’s virulence rather than host defence.