Ecological implications of recognition events in the pre-infection stages of root pathogens

Host-specificity and recognition by root-infecting pathogens are reviewed, with emphasis on fungi but drawing parallels with other root parasites, symbionts and rhizosphere micro-organisms. Two major levels of specificity are identified: family-specific parasitism, exemplified by many parasites that characteristically infect Gramineae, and the possibly recent segregation of pathogen species into crop-specific strains. In both of these cases there are examples where host-specificity is paralleled by host-recognition in the pre-infection stages, including host-specific triggering of propagule germination. The pre-infection sequence of zoosporic pathogens (Pythium, Phytophthora, Aphanomyces spp.) is reviewed. These fungi seem to have both general (low-affinity) and host-specific (high-affinity) receptor-based recognition systems for chemotaxis to root diffusates and for induction of encystment by host surface components. Zoospores orientate (dock) during encystment, and the cysts germinate from a fixed point, whereas the non-motile spores of at least some root parasites (e.g. Idriella bolleyi) can germinate from any point, which is influenced by the host. The possession of both general and host-specific recognition systems by a parasite could enable it to exploit rhizosphere niches or to invade compromised plants in the absence of preferred hosts. Evidence from graminicolous (family-specific) Pythium spp. suggests that host-specific recognition is quantitative rather than absolute but, when compounded over successive cycles of infection, is a powerful selective mechanism.