Heide-Jørgensen H S, Kuijt J. 1995. The haustorium of the root parasite Triphysaria (Scrophulariaceae), with special reference to xylem bridge ultrastructure. Amer. J. Bot. 82: 782-797.
Abstract/Summary: Haustoria of Triphysaria pusilla and T. versicolor subsp. faucibarbata from a natural habitat were analyzed by light and electron microscopy. Secretory trichomes (root hairs) participate in securing the haustorium to the surface of the host root. The keel-shaped intrusive part of the secondary haustorium penetrates to the depth of the vascular tissue of the host. Some of the epidermal interface cells differentiate into xylem elements. A significant number of haustoria do not differentiate further, but in most haustoria one to five of the epidermal xylem elements terminate a similar number of xylem strands. The strands mostly consist of vessel members and they connect host xylem or occasionally host parenchyma to the plate xylem adjacent to the stele of the parasite root. Each strand of this xylem bridge is accompanied by highly protoplasmic parenchyma cells with supposed transfer cell function. Increased surface area of the plasmalemma occurs in these cells as it does in interface parenchyma cells. Graniferous tracheary elements are restricted to the haustorium and occur most frequently in the plate xylem. The plate xylem is also accompanied by highly protoplasmic parenchyma cells. Hyphae of mycorrhizal fungi of the host root occasionally penetrate into the distal part of the xylem bridge. We combine structural observations and physiological facts into a hypothesis for translocation of water and nutrients between host and parasite. Some evolutionary aspects related to endogeny/exogeny of haustoria are discussed, and it is argued that the Triphysaria haustorium represents a greatly advanced and/or reduced condition within Scrophulariaceae.
Figure legend for the diagram:
Semidiagrammatic sketch of mature haustorium of Triphysaria having a xylem bridge with a single xylem strand. Upper Fig. Three-dimensional diagram of diarch parasite root having only primary structures and pentarch host root. The plate xylem (G) includes parenchyma cells (no internal structures shown) and tracheary elements.
Lower Fig. Transverse section of parasite and host root. The primary xylem is diarch. Opposite the haustorium the cortex consists of three cell layers including exodermis. G is part of the plate xylem.
Abbreviations: E, epidermis. EN, endodermis. G, graniferous tracheary element of plate xylem. X, host xylem element. IW, lignified interface cell wall. R, trichome. ST, interface xylem element. XB, Vessel member of single-stranded xylem bridge.