Many plants contain latex that exudes when leaves are damaged, and a number of proteins and enzymes were found in it, however, roles of those latex proteins and enzymes are poorly understood. We found that papain, a cysteine protease in latex of Papaya tree (Carica papaya, Caricaceae), is the crucial factor that accounts for the defense of papaya tree against herbivorous lepidopteran larvae. Leaves of a number of laticiferous plants, including Papaya and a wild fig (Ficus virgata, Moraceae), showed strong toxicity and growth inhibition against Eri silkmoth (Samia ricini, Saturniidae) larvae, even though there have been no apparent toxic factors reported from these species. When latex was washed off, the leaves of these lactiferous plants lost toxicity. Latexes of both papaya and the wild fig were rich in cysteine-protease activity. Painting of a cysteine protease-specific inhibitor, on the surface of papaya and fig leaves completely deprived the leaves of toxicity. Cysteine proteases such as papain, ficin and bromelain, all showed toxicity. The results suggest that plant latex and proteins in it, cysteine proteases in particular, are defenses against herbivorous insects.
Species 1: Lepidoptera Saturniidae Samia ricini (Eri silkworm)
Species 2: Violales Caricaceae Carica papaya (papaya tree)
Species 3: Urticales Moraceae Ficus virgata (fig tree)
Keywords: cysteine proteases, plant latex
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