Latex is a sticky emulsion that exudes upon damage from specialized canals in about 10% of flowering plant species. Latex has no known primary metabolic function and has been strongly implicated in defense against herbivorous insects. Here we review historical hypotheses about the function of latex, evidence that it serves as a potent defense, and the chemistry and mode of action of the major constituent defense chemicals and proteins across a diversity of plant species. We further attempt to synthesize the characteristics of latex as a coordinated plant defense system. Herbivores that feed on latex-bearing plants typically evade contact with latex by severing the laticifers or feeding intercellularly, or may possess physiological adaptations. Convergent evolution appears to be rampant both in plants with latex and insects that exploit latex-bearing plants. Because latex shows phenotypic plasticity, heritability, and macoevolutionary lability, it is an ideal system to study plant-herbivore interactions using evolutionary approaches.

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SummaryMany plants contain latex that exudes when leaves are damaged, and a number of proteins and enzymes have been found in it. The roles of those latex proteins and enzymes are as yet poorly understood. We found that papain, a cysteine protease in latex of the Papaya tree (Carica papaya, Caricaceae), is a crucial factor in the defense of the papaya tree against lepidopteran larvae such as oligophagous Samia ricini (Saturniidae) and two notorious polyphagous pests, Mamestra brassicae (Noctuidae) and Spodoptera litura (Noctuidae). Leaves of a number of laticiferous plants, including papaya and a wild ®g, Ficus virgata (Moraceae), showed strong toxicity and growth inhibition against lepidopteran larvae, though no apparent toxic factors from these species have been reported. When the latex was washed off, the leaves of these lactiferous plants lost toxicity. Latexes of both papaya and the wild ®g were rich in cysteine-protease activity. E-64, a cysteine protease-speci®c inhibitor, completely deprived the leaves of toxicity when painted on the surface of papaya and ®g leaves. Cysteine proteases, such as papain, ®cin, and bromelain, all showed toxicity. The results suggest that plant latex and the proteins in it, cysteine proteases in particular, provide plants with a general defense mechanism against herbivorous insects.

Leaves of the privet tree, Ligustrum obtusifolium, contain a large amount of oleuropein, a phenolic secoiridoid glycoside, which is stably kept in a compartment separate from activating enzymes. When the leaf tissue is destroyed by herbivores, enzymes localized in organelles start to activate oleuropein into a very strong protein denaturant that has protein-crosslinking and lysine-decreasing activities. These activities are stronger than ever reported from plant systems and have adverse effects against herbivores by decreasing the nutritive value of dietary protein completely. We report here that strong oleuropein-specific ␤-glucosidase in organelles activates oleuropein by converting the secoiridoid glucoside moiety of oleuropein into a glutaraldehyde-like structure, which is also an ␣,␤-unsaturated aldehyde. Oleuropein activated by ␤-glucosidase had very strong proteindenaturing, protein-crosslinking, and lysine-alkylating activities that are very similar to, but stronger than, those of glutaraldehyde. Aucubin, another iridoid glycoside, had similar activities after ␤-glucosidase treatment. We also detected polyphenol oxidase activity in organelles that activate the dihydroxyphenolic moiety to have protein-crosslinking activities. These data suggest that the privet tree has developed an effective defense mechanism with oleuropein, a unique multivalent alkylator ideal as a protein-crosslinker. Our results that iridoid glycosides are precursors of alkylators may elucidate the chemical bases that underlie various bioactivities and ecological roles of iridoid glycosides.

Since ancient times, mulberry leaves (Morus spp.) have been used to rear the silkworm Bombyx mori. Because the silkworm grows well on mulberry leaves, the toxicities and defensive activities of these leaves against herbivorous insects have been overlooked. Here we show that mulberry leaves are highly toxic to caterpillars other than the silkworm B. mori, because of the ingredients of the latex, a milky sap exuded from mulberry leaf veins. The toxicity of mulberry leaves was lost when the latex was eliminated from the leaves, and artificial diets containing latex showed toxicity. Mulberry latex contained very high concentrations of alkaloidal sugar-mimic glycosidase inhibitors reported to have antidiabetic activities, such as 1,4-dideoxy-1,4-imino-D-arabinitol, 1-deoxynojirimycin, and 1,4-dideoxy-1,4-imino-D-ribitol. The overall concentrations of these inhibitors in latex reached 1.5-2.5% (8 -18% dry weight) in several mulberry varieties, which were Ϸ100 times the concentrations previously reported from whole mulberry leaves. These sugar-mimic alkaloids were toxic to caterpillars but not to the silkworm B. mori, indicating that the silkworm can circumvent the mulberry tree's defense. Our results suggest that latex ingredients play key roles in defense of this tree and of other plants against insect herbivory, and they imply that plant latexes are treasuries of bioactive substances useful as medicines and pesticides.plant defense ͉ plant-insect interactions ͉ Morus spp. ͉ Eri silkworm ͉ Bombyx mori