How Striga parasitizes its host: A TEM and SEM study

The cellular contact between Striga hermonthica and Striga asiatica and their hosts, Zea mays and Sorghum bicolor, was investigated by light, transmission electron and scanning electron microscopy. The xylem connections between parasites and hosts involve very specific, clustered intrusions into the host's water conducting elements, predominantly into the large vessel elements. A single haustorial cell can penetrate a host vessel element with more than one intrusion. All intrusions become covered by an additional electron-opaque wall layer. During subsequent differentiation, a dissolution of specific wall parts of the cell intrusions occurs so that open, cup- or trunk-like structures result. The vessel-like host contact can comprise up to five openings within a single intrusion. Concomitantly, the intrusions and the haustorial cells to which they belong lose their protoplasts and transform into elements which rake up water. The walls of the haustorial cells and both wall parts of their appendages become strongly lignified. The water and nutrient absorbing structures inserted into the host vessel are named 'oscula'. Within the whole haustorial complex of both Striga species no phloem elements were detected. Translocation of substances from host to parasite are briefly discussed. (C) 1997 Annals of Botany Company.