Tropic Orientation Responses of Pathogenic Fungi

Cellular orientation allows growth, differentiation and behaviour to respond to vectorial cues generated in the environment and in relation to cells of the same organisms or different organisms that exist in proximity to one another. In the case of fungal pathogens, the orientation of hyphae may allow the fungus to detect a host and to make strategic penetrations at points of weakness on the host surface. Within a host, tropic orientation may facilitate colonisation, ramification and dispersal within the host tissues. To achieve this, cells have to be able to coordinate their cell cycles, growth and expansion of their margins with directional growth responses. In this chapter, we review the tropic orientation responses of fungi and, with an emphasis on fungal pathogenesis, discuss and speculate on the underlying molecular mechanisms that regulate cellular tropisms. Examples are taken across the fungal kingdom, including from work on saprophytes, plant and animal pathogens, to construct a working model that speculates how a wide range of tropisms may be controlled by a more-or-less common tropic mechanism that regulates the orientation of the hyphal tip.