Directional Growth

The germ tubes of some fungi (for example, those of urediospores of the rust fungi) always enter the host plant through stomata. Rather than the chance encounter of stomata, the growing germ tube is directed toward stomata by the surface properties of the host surface. The presence of hydrophobins (Chapter 1) in the hyphal wall allows close contact of the germ tube with the host surface and senses the surface configurations. The cuticle surface is ridged (Figure 4.4) due to oriented deposition of cuticle wax and the germ tube extends perpendicularly to the ridges. The homing of the germ tube on the stomata opening is guided by the orientation of the ridges around the guard cells (Maheshwari and Hildebrandt, 1967a). To test this hypothesis, Hoch et al. (1987) microfabricated 0.5 ^m ridges on silicon wafers by electron-beam lithography and placed urediospores on it. The hypha grew perpendicularly to the ridges, implying that hypha could sense minute differences in the leaf surface and orient its growth toward stomata. A sharp change in the elevation around the stomata pore is the topographical signal for the differentiation of the appres-sorium, a bulbous swelling of the tip of the germ tube (Figure 4.5), over the stomata. It had long been assumed that foliar penetration of germ tubes results from their chance encounter with stomata opening. However, the observations of germinating urediospores suggests that fungi have evolved mechanisms to perceive surface topography and orient the growing hyphal tip towards stomata. One feature of germ tube growth is a close adherence to the surface. Directional growth was also observed on isolated epidermis or on rubber replicas of leaf surface, showing that the germ tube perceives minute features of a host surface to orient their growth toward natural openings in the host (Maheshwari and Hildebrandt, 1967a; Maheshwari et al., 1967b).

Figure 4.4 Directional growth of germ tubes of rust fungi on a leaf surface. (a) Surface view of leaf epidermis showing cuticular ridges. (b) Urediospore germ tubes oriented perpendicularly to ridges. (From Maheshwari and Hildebrandt (1967).)
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