Plant phenolic compounds and oxidative stress: integrated signals in fungal–plant interactions
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Upon invasion of a host, fungal pathogens are exposed to a variety of stresses. Plants release reactive oxygen species, and mount a variety of preformed and induced chemical defenses. Phenolic compounds are one example: they are ubiquitous in plants, and an invading pathogen encounters them already at the leaf surface, or for soil-borne pathogens, in the rhizosphere. Phenolic and related aromatic compounds show varying degrees of toxicity to cells. Some compounds are quite readily metabolized, and others less so. It was known already from classical studies that phenolic substrates induce the expression of the enzymes for their degradation. Recently, the ability to degrade phenolics was shown to be a virulence factor. Conversely, phenolic compounds can increase the effectiveness of antifungals. Phenolics are known antioxidants, yet they have been shown to elicit cellular responses that would usually be triggered to counter oxidant stress. Here, we review the evidence for a connection between the fungal response to phenolics as small-molecule signals, and the response to oxidants. The connections proposed here should enable genetic screens to identify specific fungal receptors for plant phenolics. Furthermore, understanding how the pathogen detects plant phenolic compounds as a stress signal may facilitate new antifungal strategies.
KeywordsFungal Phenolic acids Plant pathogen β-Ketoadipate pathway Oxidative stress
We are grateful to Drauzio E. N. Rangel, and to the reviewers of the first version of this paper, for their comments and suggestions which added to the depth and scope of our review. We are pleased to contribute to this special issue which was made possible by the São Paulo Research Foundation (FAPESP) grant #2014/01229-4. Research in the authors’ lab was funded in part by Israel Science Foundation grant 332/13.
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