Exudate molecules initiating fungal responses to seeds and roots

Abstract

Plant pathogenic fungi survive in soils in a quiescent state. In order for many root-pathogen interactions to be initiated, dormant propagules must be activated by molecules present in seed and root exudates. Without the release of such stimulatory molecules, the majority of root infections do not occur. Currently, little is known about the specific molecules involved in stimulating propagule germination and initiating root-pathogen interactions. Although certain molecules can be shown to elicit germination responses in vitro, responses of propagules reared on conventional culture media do not always reflect the responses of those formed on plant tissues in soil. Consequently, it is not possible to extend conclusions from laboratory determinations of the role of specific exudate molecules in stimulating fungal propagule germination to soil systems. The interaction of Pythium species with germinating seeds has served as a model system to answer questions about propagule behavior and the role of exudate stimulant molecules in establishing root-fungus interactions. The potential role of both volatile and water-soluble molecules in stimulating propagule germination are discussed.

Microconidia germination of the tomato pathogenFusarium oxysporumin the presence of root exudates

“…Propagules of several plant pathogenic fungi have been shown to germinate in the presence of root exudates (Nelson 1991), but little is known yet on the microconidia germination of F. oxysporum in the presence of root exudates. Microconidia are efficient fungal propagules of F. oxysporum for the infection of tomato plants and have primarily been used in a number of studies (e.g.…”

“…Background information about the activation of quiescent fungal propagules by such exudates is fundamental in understanding the initiation and control of root infections by soil-borne pathogens (Nelson 1991).…”

“…Most substances exuded by plant roots, such as sugars, amino acids, organic acids, phenolic compounds, flavonoids, enzymes, fatty acids, growth regulators, nucleotides, tannins, carbohydrates, steroids, terpenoids, alkaloids, polyacetylenes and vitamins, are involved in plant metabolic processes (Bertin et al 2003), but as reviewed by Nelson (1991) little is known about the specific exudate molecules initiating vegetative growth of fungal propagules in the rhizosphere.…”

“…All stages are present in infected tissues or soil (Nelson 1981). Details of the colonization process of F. oxysporum within and outside the vascular system have been studied by several authors (Brammall & Higgins 1988, Charest et al 1984, Lagopodi et al 2002, Olivain & Alabouvette 1997, Rodriguez-Gálvez & Mendgen 1995 and the mycelial development in the vicinity of plant roots has been investigated by Steinberg et al (1999), but little is known about the germination of F. oxysporum propagules, a key step in plant-pathogen interactions (Nelson 1991). The purpose of our study was to assess the effect of root exudates of host (tomato) and non-host plants on spore germination of Fusarium oxysporum pathogenic on tomato.…”

Relationships between M. enterolobii and F. solani: spatial and temporal dynamics in the occurrence of guava decline

“…This strongly supports the hypothesis that when parasitism by M. enterolobii occurs, quantitative and/or qualitative changes in the root exudates allow the fungus to invade and induce decline in root system. In work by Nelson [16] , root exudates were clearly seen to trigger and modulate fungal response to the roots. According to that author, germination of F. solani f. sp.…”