First and second authors: Department of Food Science and Technology, Cornell University, Geneva, NY 14456; and third author: Department of Organic Chemistry, Cornell University, Ithaca, NY 14853
Cell suspension cultures of the scab-resistant apple (Malus × domestica) cultivar Liberty were challenged with yeast extract to mimic the effect of biological stress such as fungal invasion. The cells responded to the challenge by production of novel compounds. Suspension cultures of the scab-susceptible cultivar McIntosh, when similarly challenged, showed no detectable response. The major compound produced by scab-resistant cells in response to the challenge has been identified as the 2,4-methoxy-3-hydroxy-9-O-β-D-glucosyloxydibenzofuran by UV, mass spectrometry, 1H-nuclear magnetic resonance (NMR), and 13C-NMR spectroscopy. We suggest the trivial name malusfuran for the compound. Malusfuran production was initiated approximately 24 h after being challenged. Malusfuran inhibited spore germination and growth of Venturia inaequalis at millimolar concentrations, indicating its role as a possible phytoalexin. The aglycone of malusfuran, 2,4-methoxy-3,9-hydroxy-dibenzofuran, showed higher toxicity to V. inaequalis than to the parent malusfuran. In vitro cultures of V. inaequalis produced a β-glucosidase that hydrolyzed ortho- and para-substituted nitrophenyl-β-glucosides, suggesting that the aglycone may act as the actual phytoalexin.