Cell surface interactions between sorghum roots and the parasitic weed Striga hermonthica: cytochemical aspects of cellulose distribution in resistant and susceptible host tissues

Abstract

Cell surface interactions between sorghum roots and the parasitic weed Striga hermonthica were investigated using light and electron microscopy. An exoglucanase–gold complex was used to study the distribution of cellulose in host tissues of resistant and susceptible cultivars. In the susceptible cultivar, intrusive cells of S. hermonthica could reach xylem elements, whereas in the resistant cultivar, development occurred mainly in the outer cortex. Invasion of the susceptible cultivar was accompanied by wall alterations and by the release of cellulose-rich fragments likely detached from host walls through the action of lytic enzymes. The occurrence of wall alterations at a short distance from the point of parasite penetration provided indirect evidence for extracellular diffusion of cellulolytic enzymes. Tylose formation and accumulation of coating material along secondary thickenings of some xylem vessels were late responses observed in the susceptible cultivar. In the resistant cultivar, heavy deposition of successive layers of cellulosic material occurred in outer tissues. Accumulation of this material may constitute a defense reaction against penetration. Electron-dense coating material was noticeable in cortical cells and occasionally in xylem vessels. Defense reactions observed in both sorghum cultivars upon parasite attack differed in their temporal and spatial distribution. Key words: Striga, sorghum, haustorium, root anatomy, gold cytochemistry, cellulose, resistance.