Interface between haustoria of parasitic members of the Scrophulariaceae and their hosts: a histochemical and immunocytochemical approach.
The haustorial structures of 3 African parasitic members of the family Scrophulariaceae (Buchnera hispida, Rhamphicarpa fistulosa and Striga hermonthica) were studied at the interface with invaded host roots. Immunocytochemical observations using light and electron microscopy were carried out with monoclonal antibodies against pectin, JIM5, JIM7 and the hydroxyproline-rich glycoprotein (HRGP) LM1. Lignins were visualized by phloroglucinol-hydrochloric acid staining. At the margin of the lateral interface (contact area of host root cortex and parasite cells), JIM5- and JIM7-labelled substances accumulate between parasite papillae and the host root surface, indicating that pectins are implicated in sealing the parasite to the attacked host organ. The lateral interface is characterized by the presence of compressed, necrotic host cells, whereas the central interface (contact area between host stele and parasite cells) is generally devoid of host cell remnants. Phenolic substances and/or lignins can be found at the site of penetration of the haustorium into the host root. These observations and the fact that HRGPs accumulate at the host side of the interface support the view of, at least, a partial defense reaction in the invaded host root tissues. Within haustoria, HRGPs were restricted to differentiating xylem elements, implying a spatio-temporal regulation of HRGPs in developmental processes.