Abstract
Metschnikowia pulcherrima synthesizes the red pigment pulcherrimin, from cyclodileucine (cyclo(Leu-Leu)) as a precursor, and exhibits strong antifungal activity against notorious plant pathogenic fungi such as Botrytis and Gibberella (i.e., Fusarium). This yeast therefore has great potential for biocontrol applications against fungal diseases; particularly in the phyllosphere where this species is frequently found. To elucidate the molecular basis of the antifungal activity of M. pulcherrima, we compared a wildtype strain with a spontaneously occurring, pigmentless, weakly antagonistic mutant derivative. Whole genome sequencing of the wildtype and mutant strains identified a point mutation that creates a premature stop codon in the transcriptional regulator SNF2 in the mutant strain. Complementation of the snf2 mutant strain with the wildtype SNF2 gene restored pigmentation and recovered the strong antifungal activity of M. pulcherrima against plant pathogens in vitro and on cherries. Ultra-performance liquid chromatography-high resolution heated electrospray ionization mass spectrometry (UPLC HR HESI-MS) proved the presence and structure of the pulcherrimin precursors cyclo(Leu-Leu) and pulcherriminic acid and also identified new compounds that likely represented an additional precursor and degradation products of pulcherriminic acid and/or pulcherrimin. All of these compounds were identified in the wildtype and complemented strain, but were undetectable in the pigmentless snf2 mutant strain. These results thus identify SNF2 as a regulator of antifungal activity and pulcherriminic acid biosynthesis in M. pulcherrima and provide a starting point for deciphering the molecular functions underlying the antagonistic activity of this yeast.
Significance statement Metschnikowia pulcherrima is a strongly antifungal yeast and a most promising species for the control of notorious plant diseases. This multidisciplinary study on the M. pulcherrima mode of action compared a wildtype isolate with a pigmentless mutant exhibiting reduced antifungal activity. The transcriptional regulator Snf2 was identified as a “biocontrol regulator” controlling antifungal activity of M. pulcherrima via PUL gene transcription, cyclodipeptide synthesis and additional, yet uncharacterized mechanisms. The identification of cyclo(Leu-Leu), pulcherriminic acid, as well as novel precursor and degradation products of pulcherrimin, opens up new avenues for research on the metabolism and functions of pulcherrimin. Overall, this works establishes M. pulcherrima as a genetically tractable model and will benefit the development of biocontrol solutions for important plant diseases.
