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Edited by May R. Berenbaum, University of Illinois at Urbana-Champaign, Urbana, IL (received for review August 11, 2005)
Abstract
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 ≈100 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.
Footnotes
↵‡ To whom correspondence should be addressed. E-mail: konno{at}affrc.go.jp.
↵† K. Konno and H.O. contributed equally to this work.
↵∥ Present address: National Institute of Vegetable and Tea Science, Ano, Mie 514-2392, Japan.
Author contributions: K. Konno and H.O. designed research; K. Konno and H.O. performed research; K. Konno, H.O., M.N., K.T., C.H., Y.T., M.H., A.K., and K. Kohno contributed new reagents/analytic tools; K. Konno and H.O. analyzed data; and K. Konno wrote the paper.
Conflict of interest statement: No conflicts declared.
This paper was submitted directly (Track II) to the PNAS office.
Abbreviations: d-AB1, 1,4-dideoxy-1,4-imino-d-arabinitol; DNJ, 1-deoxynojirimycin; HMWF, high-molecular-weight fraction.
- Received August 11, 2005.
- Accepted December 6, 2005.
- Copyright © 2006, The National Academy of Sciences
Mulberry latex rich in antidiabetic sugar-mimic alkaloids forces dieting on caterpillars
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