PHYTOALEXIN PRODUCTION IN AN APPLE CULTIVAR RESISTANT TO VENTURIA-INAEQUALIS

Cell suspension cultures of the scab-resistant apple (Malus x domestic a) cultivar Liberty were challenged with yeast extract to mimic the ef fect of biological stress such as fungal invasion. The cells responded to the challenge by production of novel compounds. Suspension culture s of the scab-susceptible cultivar McIntosh, when similarly challenged , showed no detectable response. The major compound produced by scab-r esistant cells in response to the challenge has been identified as the 2,4-methoxy -3-hydroxy-9-O-beta-D-glucosyloxydibenzofuran by UV, mass spectrometry, H-1-nuclear magnetic resonance (NMR), and C-13-NMR spec troscopy. We suggest the trivial name malusfuran for the compound. Mal usfuran production was initiated approximately 24 h after being challe nged. Malusfuran inhibited spore germination and growth of Venturia in aequalis at millimolar concentrations, indicating its role as a possib le phytoalexin. The aglycone of malusfuran, 2,4-methoxy-3,9-hydroxy-di benzofuran, showed higher toxicity to V. inaequalis than to the parent malusfuran. In vitro cultures of V. inaequalis produced a beta-glucos idase that hydrolyzed ortho- and para-substituted nitrophenyl-beta-glu cosides, suggesting that the aglycone may act as the actual phytoalexi n.