Summary

Many fungal pathogens use contact sensing of the host surface to achieve successful infection. Contact-mediated responses can be induced and experimentally examined on artificial substrata which mimic physical characteristics of the host surface. The necessary physical and spatial characteristics of surfaces which induce contact-mediated responses are described and classified as either topographical or non-topographical. Topographical signals may be in the form of steps, ridges or furrows and these sometimes require very precise dimensions and/or spacings to induce a fungal response. Non-topographical signals lack this specificity because the fungus recognises only that it is in contact with a surface possessing suitable physical features. Examples of contact-mediated responses include the asymmetric organisation of a cell relative to its substratum, adhesion of a cell to the contact surface, changes in spore surface morphology and accompanying release of enzymes, directional growth of hyphae, and induction of appressorium differentiation. Evidence of these processes being contact-mediated is assessed and possible mechanisms by which fungal contact sensing may operate are discussed. Finally, strategies for controlling plant diseases through an understanding of contact sensing are defined.

Introduction

A large number of eukaryotic cells are sensitive to contact (touch) stimuli which affect their behaviour, growth and morphogenesis in a variety of ways. Numerous examples of contact sensing by animal and plant cells have been reported (see e.g. Braam & Davis, 1990; Curtis & Clark, 1990; and references cited therein). Contact sensing also plays an important role in the infection of plants by many fungal pathogens.