Haustoria of two African parasitic Scrophulariaceae: ontogeny and interface.
The haustoria of Buchnera hispida and Rhamphicarpa fistulosa, growing on pearl millet (Pennisetum americanum [P. glaucum]), were studied with regard to their ontogeny, anatomy and ultrastructure. Immunocytological observations were carried out at light and electron microscopic levels using JIM 5, JIM 7 and LM 1 monoclonal antibodies. Special attention was paid to the interface of the 2 species in order to elucidate whether penetration occurs by mechanical or enzymatic mechanisms. Buchnera hispida and Rhamphicarpa fistulosa were found to penetrate the roots of the host exclusively by means of secondary haustoria, formed as lateral organs on parasite roots. The haustoria in both species originate from the rhizodermis and/or the hypodermis. Mature haustoria have an axis of differentiated xylem elements which connect the xylem elements of the host and parasite. Penetration of the host cortex seems to take place quite easily. The presence of several layers of crushed and collapsed host cell walls at the host-parasite interface indicates mainly mechanical penetration. These layers are strongly labelled by LM 1 antibodies reacting with hydroxyproline-rich glycoproteins. As there is also evidence for the presence of lignins and other phenolic substances at the interface, the host cells are believed to show a strong defence reaction. As cell walls of monocotyledonous plants are generally poor in pectin, the JIM 5 and JIM 7 antibodies recognizing different types of pectins are useful for distinguishing between host and parasite cell walls at the interface of the two.