Abstract
BackgroundIn fungi, environmental pH is an important signal for development, and successful host colonization depends on homeostasis. Surprisingly, little is known regarding the role of pH in fungal-fungal interactions. Species of Trichoderma grow as soil saprobes but many are primarily mycotrophic, using other fungi as hosts. Therefore, Trichoderma spp. are studied for their potential in biocontrol of plant diseases. Particularly in alkaline soil, pH is a critical limiting factor for these biofungicides, whose optimal growth pH is 4–6. Gaining an understanding of pH adaptability is an important step in broadening the activity spectrum of these economically important fungi.ResultsWe studied the pH-responsive transcription factor PacC by gene knockout and by introduction of a constitutively active allele (pacCc). ΔpacC mutants exhibited reduced growth at alkaline pH, while pacCc strains grew poorly at acidic pH. In plate confrontation assays ΔpacC mutants showed decreased ability to compete with the plant pathogens Rhizoctonia solani and Sclerotium rolfsii. The pacCc strain exhibited an overgrowth of R. solani that was comparable to the wild type, but was unable to overgrow S. rolfsii. To identify genes whose expression is dependent on pH and pacC, we designed oligonucleotide microarrays from the transcript models of the T. virens genome, and compared the transcriptomes of wild type and mutant cultures exposed to high or low pH. Transcript levels from several functional classes were dependent on pacC, on pH, or on both. Furthermore, the expression of a set of pacC-dependent genes was increased in the constitutively-active pacCc strain, and was pH-independent in some, but not all cases.ConclusionsPacC is important for biocontrol-related antagonism of other fungi by T. virens. As much as 5% of the transcriptome is pH-dependent, and of these genes, some 25% depend on pacC. Secondary metabolite biosynthesis and ion transport are among the relevant gene classes. We suggest that ΔpacC mutants may have lost their full biocontrol potential due to their inability to adapt to alkaline pH, to perceive ambient pH, or both. The results raise the novel possibility of genetically manipulating Trichoderma in order to improve adaptability and biocontrol at alkaline pH.
Highlights
In fungi, environmental pH is an important signal for development, and successful host colonization depends on homeostasis
Degenerate primers for nested amplification (Additional file 1) were designed from the amino acid sequences of the regions most conserved in several pacC orthologs (Aspergillus nidulans, Aspergillus oryzae and Fusarium oxysporum)
The predicted protein product has a molecular mass of 67 kDa and contains 617 amino acid residues. When this sequence was compared to GenBank-deposited sequences from several organisms, the best BLAST similarity scores were obtained with the PacC proteins of the Ascomycete fungi Fusarium oxysporum, Acremonium chrysogenum and Gibberella moniliformis (62%, 64% and 67% similarity, respectively)
Summary
Environmental pH is an important signal for development, and successful host colonization depends on homeostasis. It is known that fungi respond to ambient pH levels via activation of a dedicated transcription factor, PacC [1,2,3,4,5]. This response is initiated by a Fungal pathogens of animals and plants need to sense and respond to local pH. This contrasting regulation suggests that in different species, PacC may play different roles in determining virulence Another interesting example is Candida albicans, a commensal that can become pathogenic in susceptible hosts. A wide variety of genes and activities are controlled by PacC, including xylanases [26], permeases [27], siderophores [28], antibiotic and toxin production [29], and involvement in pathogenic processes [4,21,24]
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