Trichoderma virens is a biocontrol agent used in agriculture to antagonize pathogens of crop plants. In addition to direct mycoparasitism of soil-borne fungal pathogens, T. virens interacts with roots. This interaction induces systemic resistance (ISR), which reduces disease in aboveground parts of the plant. In the molecular dialog between fungus and plant leading to ISR, proteins secreted by T. virens provide signals. Only a few such proteins have been characterized previously. To study the secretome, proteins were characterized from hydroponic culture systems with T. virens alone or with maize seedlings, and combined with a bioassay for ISR in maize leaves infected by the pathogen Cochliobolus heterostrophus. The secreted protein fraction from coculture of maize roots and T. virens (Tv؉M) was found to have a higher ISR activity than from T. virens grown alone (Tv). A total of 280 fungal proteins were identified, 66 showing significant differences in abundance between the two conditions: 32 were higher in Tv؉M and 34 were higher in Tv. Among the 34 found in higher abundance in Tv and negatively regulated by roots were 13 SSCPs (small, secreted, cysteine rich proteins), known to be important in the molecular dialog between plants and fungi. The role of four SSCPs in ISR was studied by gene knockout. All four knockout lines showed better ISR activity than WT without affecting colonization of maize roots. Furthermore, the secreted protein fraction from each of the mutant lines showed improved ISR activity compared with WT. These SSCPs, apparently, act as negative effectors reducing the defense levels in the plant and may be important for the fine tuning of ISR by Trichoderma. The down-regulation of SSCPs in interaction with plant roots implies a revision of the current model for the Trichoderma-plant symbiosis and its induction of resistance to pathogens. Molecular & Cellular Proteomics

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.

SummaryThe transcription factor ChAP1 of the fungal pathogen of maize, Cochliobolus heterostrophus, responds to oxidative stress by migration to the nucleus and activation of antioxidant genes. Phenolic and related compounds found naturally in the host also trigger nuclear localization of ChAP1, but only slight upregulation of some antioxidant genes. ChAP1 thus senses phenolic compounds without triggering a strong antioxidant response. We therefore searched for genes whose expression is regulated by phenolic compounds and/or ChAP1. The C. heterostrophus genome contains a cluster of genes for metabolism of phenolics. One such gene, catechol dioxygenase CCHD1, was induced at least 10-fold by caffeic and coumaric acids. At high phenolic concentrations (Ն 1.6 mM), ChAP1 is needed for maximum CCHD1 expression. At micromolar levels of phenolics CCHD1 is as strongly induced in chap1 mutants as in the wild type. The pathogen thus detects phenolics by at least two signalling pathways: one causing nuclear retention of ChAP1, and another triggering induction of CCHD1 expression. The low concentrations required for induction of CCHD1 indicate fungal receptors for plant phenolics. Symbiotic and pathogenic bacteria are known to detect phenolics, and our findings generalize this to a eukaryotic pathogen. Phenolics and related compounds thus provide a ubiquitous plant-derived signal.

The necrotrophic maize pathogen Cochliobolus heterostrophus senses plant-derived phenolic compounds, which promote nuclear retention of the redox-sensitive transcription factor ChAP1 and alter gene expression. The intradiol dioxygenase gene CCHD1 is strongly upregulated by coumaric and caffeic acids. Plant phenolics are potential nutrients but some of them are damaging compounds that need to be detoxified. Using coumaric acid as an inducer (16 to 160 μM), we demonstrated the rapid and simultaneous upregulation of most of the β-ketoadipate pathway genes in C. heterostrophus. A cchd1 deletion mutant provided genetic evidence that protocatechuic acid is an intermediate in catabolism of a wide range of aromatic acids. Aromatics catabolism was slowed for compounds showing toxicity, and this was strongly correlated with nuclear retention of GFP-ChAP1. The activity of a structure series of compounds showed complementary requirements for upregulation of CCHD1 and for ChAP1 nuclear retention. Thus, there is an inverse correlation between the ability to metabolize a compound and the stress response (ChAP1 nuclear retention) that it causes. The ability to metabolize phenolics and to respond to them as signals should be an advantage to plant pathogens and may explain the presence of at least two response pathways detecting these compounds.

The transcription factors ChAP1 and Skn7 of the maize pathogen Cochliobolus heterostrophus are orthologs of Yap1 and Skn7 in yeast, where they are predicted to work together in a complex. Previous work showed that in C. heterostrophus, as in yeast, ChAP1 accumulates in the nucleus in response to reactive oxygen species (ROS). The expression of genes whose products counteract oxidative stress depends on ChAP1, as shown by impaired ability of a Δchap1 mutant to induce these 'antioxidant' genes. In this study, we found that under oxidative stress, antioxidant gene expression is also partially impaired in the Δskn7 mutant but to a milder extent than in the Δchap1 mutant, whereas in the double mutant - Δchap1-Δskn7 - none of the tested genes was induced, with the exception of one catalase gene, CAT2. Both single mutants are capable of infecting the plant, showing similar virulence to the WT. The double mutant, however, showed clearly decreased virulence, pointing to additive contributions of ChAP1 and Skn7. Possible mechanisms are discussed, including additive regulation of gene expression by oxidative stress.