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

Abstract

SUMMARYHost-specificity and recognition by root-infecting pathogens are reviewed, with emphasis on fungi but drawing parallels with other root parasites, synnbionts 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 w^here host-specificity is paralleled by host-recognition in the pre-infection stages, including hostspecific triggering of propagule germination. The pre-infection sequence of zoosporic pathogens (Pythium, Phyiophthora, Aphanomyces spp.) is reviewed. These fungi seem to have both general (low-affinity) and hostspecific (high-affinity) receptor-based recognition systems for chemotaxis to root difFusates and for induction of encystment by host surface coniponents, Zoospores orientate (dock) during encystment, and the cysts germinate from a fixed point, whereas the non-motiie spores of at least some root parasites (e.g. Idriella bolleyi) can germinate from any point, which is infiuenced 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 mechanisna.

The host guides morphogenesis and stomatal targeting in the grapevine pathogen Plasmopara viticola

“…In this context it would be interesting to search for factors responsible for the so-called docking known from other phytopathogenic oomycetes. These host factors seem to reorient the polarity of the zoospore during the attachment to the host cell such that the prospective germ-tube pole is aligned correctly (Deacon 1996).…”

“…This so called adelphotaxis has been reported for the oomycete Achlya (Thomas and Peterson 1990). In most oomycetes, the polarity of the zoospores seems to be present, but during the attachment to the host cell (so-called docking) it is reoriented such that the prospective germ-tube pole is aligned correctly (Deacon 1996).…”

Evidence for Host Mediation of Preinfection Stages of Phytophthora infestans on the Leaf Surface of Solanum phureja

“…Numerous studies have demonstrated that preinfection processes of root‐infecting oomycetes are driven by host stimuli. This work has been described in several reviews (Carlile, 1987; Deacon, 1997; Tyler, 2002). In most cases, these stimuli appear to be general (e.g.…”

Biocontrol of Aspergillus root rot by tripartite symbiosis in groundnut

“…Very little is known of the factors influencing soil borne fungal pathogens in colonization of root tissues, although root-infecting fungi are important as diseasecausing agents (Deacon 1996). This lack of knowledge is in part a result of the difficulty of studying such processes below ground, and also because of the genetic intractability of many root-infecting organisms.…”