14.12 How pathogens attack plants
Pathogens need to invade their hosts in order to cause disease and assure their own survival, so they are equipped with mechanisms for attachment, identification and invasion of suitable hosts. Entering the host tissues might involve finding and entering through an existing aperture, such as a stoma or an injury, or direct mechanical attack. How do they find the right place to invade?
A spore that alights on a leaf surface adheres to that surface by hydrophobic interactions between hydrophobins in the spore wall and the hydrophobic surface of the leaf cuticle (see the section entitled On the far side in Chapter 6; CLICK HERE to view the page). The hypha that emerges from the spore senses an appropriate site and when it finds it, the hyphal tip enlarges (differentiating into an appressorium) and strengthens the adhesion to the leaf surface. A strong hold on the leaf surface is necessary to support the amount of mechanical force used to penetrate into the plant.
Growth of conidial germ tubes of these pathogens on leaves or artificial substrates is usually perpendicular to ridges and furrows on the surface. Ridges or grooves 0.5 µm high by 2.0 µm wide in artificial surfaces induce appressorium formation within four minutes after contact. This is a thigmotropism, a directional growth in response to touch or physical contact stimuli with a solid object; a similar mechanism operates in animal pathogens also (Nikawa et al., 1997).
In plant pathogens, the object with which the fungus is interacting is the plant cuticle, which is external to the cell and on the surface of the cell wall of external, usually epidermal, cells. Components of the cuticle are in layers in a matrix of cutin. The outer surface is mainly, or only, composed of very hydrophobic waxes, with the inner layers, those closest to the epidermal cell wall, being very hydrophilic (cellulose-rich) in composition. This shift from hydrophilic to hydrophobic between inner and outer surface provides the plant with a uniform protective barrier that retards moisture loss from the plant cell surface. The growing hypha is able to sense very minute alterations in the surface texture and the spacing between ridges on the natural leaf surface such as those that occur between epidermal cells and especially around stoma. Extension growth of the germ tube hypha ceases after its apex has grown over a stomatal guard-cell lip (or a depression on an artificial surface in vitro) and the appressorium is formed over the stoma.
Thigmotropism is not the only surface cue involved in plant pathogens; surface hardness and, separately, surface hydrophobicity stimuli are essential for appressorium formation and differentiation in the rice blast fungus Magnaporthe grisea and involve regulators of G-protein signalling cascades and accumulation of cAMP (Liu et al., 2007). Additionally, there are indications that these mycelia also respond chemotropically to the stomata, presumably to the gases emerging from the substomatal space.
Updated December 17, 2016