Casbene: An anti-fungal diterpene produced in cell-free extracts of Ricinus communis seedlings

The capacity of cell-free extracts of 2·5-day-old castor bean seedlings for synthesis of casbene from mevalonic acid were compared for seedings which had been germinated under sterile conditions and seedlings which were intentionally exposed to fungal cultures. Extracts from seedlings exposed to cultures of Rhizopus stolonifer, Aspergillus niger or Fusarium moniliforme produced much higher levels of casbene than extracts from sterile controls; the initial rates of casbene synthesis were 20–40 times higher in the extracts of seedlings that had been exposed to fungus. Although some variation in the capacity for synthesis of other diterpene hydrocarbons from mevalonic acid was seen in the two types of extracts, no consistent or striking stimulation in the synthesis of any of these was noted under these conditions of exposure of the seedlings to fungi. The potato-dectrose agar used as a fungal growth medium did not itself evoke the increase in casbene synthesis. Intact mycelia and cell-free extracts of mycelia of Rhizopus stolonifer gave no indication of diterpene biosynthesis from mevalonic acid. Purified casbene at concentrations of 10 μg ml⁻¹ or greater retarded the development of A. niger on potato-dextrose medium. Casbene was also found to inhibit the endogenous and gibberellic acid-stimulated growth of leaf sheaths of the dwarf-5 mutant of Zea mays and of the growth of the K-12 strain of Escherichia coli on glucose-minerals medium. It is suggested that casbene may serve the castor bean plant as a phytoalexin.