Mycosphere Essays 9: Defining biotrophs and hemibiotrophs

Fungi are ubiquitous and exhibit diverse life-styles. Many exhibit a continuum of life-styles ranging from biotrophy, through to necrotrophy and ultimately to saprotrophy. This paper was initiated to establish a set of definitions for fungal life-styles, in an attempt to achieve better documentation in scientific publications of the roles played by taxa. Biotrophism is a life-style where a taxon shows high dependency on a host plant, but causes minimum damage. Biotrophs have total dependency upon living plant cells, whereas hemibiotrophs have an initial biotrophic life-style and a subsequent necrotrophic phase. The necrotrophic life-style involves actively killing host plant cells by secreting cell wall degrading enzymes and phytotoxins. Biotrophic fungi have developed an intimate relationship with the host plant using haustoria for nutrient assimilation. It is believed that biotrophy evolved when fungi developed an ability to modulate plant defense mechanisms. Therefore, biotrophs utilize different strategies to overcome host plant defenses. This paper defines the terms biotrophs and hemibiotrophs in relation to fungi and provides a discussion on its significance, role and life history.