Cell wall immunocytochemistry and histology of hemiparasitism in Rhinanthus minor L. and Odontites vernus (Bellardi) Dumort: interactions at haustorial interfaces and implications for grassland biodiversity
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Parasitic plants develop specialised grafting organs called haustoria to attach to and infiltrate host plant organs to access nutrients. The outcome of the parasitic process is largely determined where cell walls of the host and parasite form the initial zone of contact at the haustorial interface. While haustorial cell walls show adaptation to virulence, host walls can be actively modified in response to attack. A combination of field and analytical microscopy techniques were used to investigate haustoria of Rhinanthus minor and Odontites vernus, annual native Irish hemiparasites, and grassland community structure associated with parasitism. The two species were confirmed to be associated with species-rich grassland habitats. In addition to lignified walls of haustorial xylem, required for solute uptake, other specialised wall types included; 1) unlignified flange-like thickenings of parenchyma adjacent to the xylem bridge, 2) thickened, unlignified walls of interfacial parenchyma and 3), primary cell walls of the hyaline body with associated paramural deposits. Arabinogalactan proteins (proteoglycans implicated in plant development) localised to the interfacial parenchyma and hyaline body and co-localised with extensins in partly differentiated xylem protoplasts suggesting roles in haustorial development and functioning. Metahaustoria, attached to the pot surface rather than host roots, provided new insights into the derivation of phenolic substances typically found at the interfaces between hosts and parasitic plants and generally believed to be synthesised by hosts as a defence response. A phenolic-rich interfacial secretion complex was produced by metahaustoria which analytical methods, including histological staining and Raman spectroscopy, indicated are compositionally similar to the interfacial secretions between hosts and non-hosts. This suggests the interfacial lignin-like substances are not related to resistance but instead are produced by haustoria to facilitate the parasitic process. Parasitic plants might be the only organisms known to produce lignin to aid virulence.