Metabolism and detoxification of phytoalexins and analogs by phytopathogenic fungi

Abstract

To date, the many examples reporting that fungal pathogens can efficiently detoxify phytoalexins provide strong evidence that the pathogenicity and/or virulence of some fungi is linked to their ability to detoxify their hosts’ phytoalexins. The pathways used by plant pathogenic fungi to metabolize and detoxify phytoalexins are reviewed. Prospects for application of recent findings are discussed.

Introduction

Many defense mechanisms of plants against pathogenic microorganisms involve the production of secondary metabolites, which may be constitutive, phytoanticipins (VanEtten et al., 1994a, VanEtten et al., 1994b, Morrissey and Osbourn, 1999), or inducible, phytoalexins (Smith, 1996, Brooks and Watson, 1985, Bailey and Mansfield, 1982). By definition, phytoalexins are low molecular weight antimicrobial compounds biosynthesized de novo by plants in response to diverse forms of stress, including microbial attack. (Bailey and Mansfield, 1982). Phytoalexins were first described by Müller and Börger (1940) during studies on Phytophthora infestans–Solanum tuberosum (potato) interactions. Since then, the great variety of phytoalexins isolated from very diverse plants indicate that their chemical structures are usually related within a plant family (Brooks and Watson, 1985). For example, many of the phytoalexins from leguminous plants have an isoflavonoid skeleton, crucifers produce indole alkaloids, cereals produce mostly cyclic hydroxamic acids and diterpenoids, whereas plants of the Solanaceae family produce sesquiterpenoids and polyacetylenes. However, stilbenoid phytoalexins have been isolated from different plant families (Harborne, 1999). Initially, research on phytoalexins dealt mainly with the identification of induced antifungal compounds and correlation with disease resistance. Hence, a large number of studies support the idea that phytoalexins have important roles in the defense of plants against pathogens such as bacteria and fungi. More recently, approaches that use in situ localization and quantification have provided evidence that phytoalexins can accumulate at the right time, concentration, and location to be effective in resistance (Hammerschmidt and Dann, 1999).

The first phytoalexin to be isolated and chemically characterized was (+)-pisatin (1) (Cruickshank and Perrin, 1960) from Pisum sativum (pea). (+)-Pisatin (1) was observed to be less toxic to the pea pathogen Ascochyta pisi than to Monilinia fructicola, a pathogen that does not attack pea. The toxicity of (+)-pisatin (1) to 50 fungal strains representing 45 species showed that only five of these fungi were tolerant of pisatin 1 (less than 50% inhibited by 100 μg/mL), and all five were pathogens of pea (Cruickshank, 1962). Only one of the 45 sensitive strains was a pea pathogen. Although subsequent surveys of the sensitivity of fungi to other phytoalexins and even to (+)-pisatin (1) revealed many exceptions to the correlation between tolerance and host range (Smith, 1982, VanEtten et al., 1982), Cruickshank’s initial observation established the concept that tolerance to a phytoalexin might be important in pathogenicity. More recent studies on phytoalexin tolerance in pathogenic fungi have shown a clear relationship between virulence and the ability of fungi to detoxify phytoalexins. Examples illustrating tolerance mechanisms and their role as virulence traits in phytopathogenic fungi were recently reviewed (VanEtten et al., 2001).

The importance of phytoalexins as general defense compounds was demonstrated by transforming tobacco (Nicotiana tabacum), tomato (Lycopersicon esculentum Mill.), and alfalfa (Medicago sativa) with a stilbene synthase that enabled the transformants to synthesize the grapevine (Vitis vinifera) phytoalexin resveratrol (2). The transformants showed an increase in the resistance to Botrytis cinerea, a fungal pathogen of tobacco (Hain et al., 1993), to Phytophthora infestans (Thomzik et al., 1997), and to Phoma medicaginis (Hipskind and Paiva, 2000), respectively. On the other hand, a recent example of transgenic plant tissue with a reduced ability to produce pisatin (1) indicated that such tissue was less resistant to fungal infection (Wu and VanEtten, 2004). Transgenic roots that produced the lowest levels of pisatin were more susceptible to isolates of Nectria haematococca than the control hairy roots. Nonetheless, since phytoalexins are also toxic to plant cells, they can accumulate in whole plants or cell cultures only transiently, some plant enzymes such as extracellular peroxidases can degrade oxidatively phytoalexins (VanEtten et al., 1982).

Currently, enzymatic detoxification of phytoalexins by phytopathogenic fungi is of great interest due to the potential application of results to understand and control plant pathogens (VanEtten et al., 2001). Additional prospects for increasing plant defenses using phytoalexin related pathways are viable alternatives recently reviewed (Essenberg, 2001). In this article the biochemical reactions used in the metabolism and detoxification of phytoalexins and their analogs by plant pathogenic fungi is reviewed.