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Alcohol Oxidase Is a Novel Pathogenicity Factor for Cladosporium fulvum, but Aldehyde Dehydrogenase Is Dispensable

¹Department of Physiology, Carlsberg Laboratory, Gamle Carlsberg Vej 10, DK-2500 Copenhagen Valby, Denmark; ²Center for Agricultural. Biotechnology, 5115 Plant Sciences Building, University of Maryland, College Park 20742, U.S.A.; ³University of East Anglia, School of Biological Sciences, Norwich NR4 7TJ, U.K.; ⁴Institute of Food Research, Norwich Research Park, Colney, Norwich NR4 7UA, U.K.

Cladosporium fulvum is a mitosporic ascomycete pathogen of tomato. A study of fungal genes expressed during carbon starvation in vitro identified several genes that were up regulated during growth in planta. These included genes predicted to encode acetaldehyde dehydrogenase (Aldh1) and alcohol oxidase (Aox1). An Aldh1 deletion mutant was constructed. This mutant lacked all detectable ALDH activity, had lost the ability to grow with ethanol as a carbon source, but was unaffected in pathogenicity. Aox1 expression was induced by carbon starvation and during the later stages of infection. The alcohol oxidase enzyme activity has broadly similar properties (K_m values, substrate specificity, pH, and heat stability) to yeast enzymes. Antibodies raised to Hansenula polymorpha alcohol oxi-dase (AOX) detected antigens in Western blots of starved C. fulvum mycelium and infected plant material. Antigen reacting with the antibodies was localized to organelles resembling peroxisomes in starved mycelium and infected plants. Disruption mutants of Aox1 lacked detectable AOX activity and had markedly reduced pathogenicity as assayed by two different measures of fungal growth. These results identify alcohol oxidase as a novel pathogenicity factor and are discussed in relation to peroxisomal metabolism of fungal pathogens during growth in planta.