Review Tannins in plant–herbivore interactions Raymond V. Barbehenn a,b , C. Peter Constabel c,⇑ a Department of Molecular, Cellular and Developmental Biology, University of Michigan, Ann Arbor, MI 48109-1048, USA b Ecology and Evolutionary Biology, University of Michigan, Ann Arbor, MI 48109-1048, USA c Centre for Forest Biology and Department of Biology, University of Victoria, P.O. Box 3020, Stn CSC, Victoria, BC, Canada V8W 3N5 article info Article history: Available online 26 February 2011 Keywords: Proanthocyanidin Polyphenol Phenolics Oxidative stress Plant defense abstract Tannins are the most abundant secondary metabolites made by plants, commonly ranging from 5% to 10% dry weight of tree leaves. Tannins can defend leaves against insect herbivores by deterrence and/or tox- icity. Contrary to early theories, tannins have no effect on protein digestion in insect herbivores. By con- trast, in vertebrate herbivores tannins can decrease protein digestion. Tannins are especially prone to oxidize in insects with high pH guts, forming semiquinone radicals and quinones, as well as other reactive oxygen species. Tannin toxicity in insects is thought to result from the production of high levels of reac- tive oxygen species. Tannin structure has an important effect on biochemical activity. Ellagitannins oxi- dize much more readily than do gallotannins, which are more oxidatively active than most condensed tannins. The ability of insects to tolerate ingested tannins comes from a variety of biochemical and phys- ical defenses in their guts, including surfactants, high pH, antioxidants, and a protective peritrophic enve- lope that lines the midgut. Most work on the ecological roles of tannins has been correlative, e.g., searching for negative associations between tannins and insect performance. A greater emphasis on manipulative experiments that control tannin levels is required to make further progress on the defen- sive functions of tannins. Recent advances in the use of molecular methods has permitted the production of tannin-overproducing transgenic plants and a better understanding of tannin biosynthetic pathways. Many research areas remain in need of further work, including the effects of different tannin types on different types of insects (e.g., caterpillars, grasshoppers, sap-sucking insects). Ó 2011 Elsevier Ltd. All rights reserved. Contents 1. Introduction ........................................................................................................ 1551 2. Tannin structure and distribution ....................................................................................... 1552 3. Tannins as protein-binding agents ...................................................................................... 1554 4. Tannins as antioxidants, prooxidants, or toxins ............................................................................ 1555 4.1. Tannins as prooxidants .......................................................................................... 1555 4.2. Toxic effects of tannins .......................................................................................... 1556 5. Effects of tannins on herbivore food choice and performance ................................................................ 1557 6. Defenses against tannins .............................................................................................. 1557 7. Induction of tannins by herbivory....................................................................................... 1558 8. Ecological effects and field studies ...................................................................................... 1559 9. Future research...................................................................................................... 1560 Acknowledgements .................................................................................................. 1561 References ......................................................................................................... 1561 1. Introduction The study of tannin–insect interactions spans a period of at least 40 years, with the bulk of the work in this area done in the first 20 years. This area of research played a dominant role in the field of chemical ecology following the seminal publications of Feeny 0031-9422/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.phytochem.2011.01.040 ⇑ Corresponding author. Tel.: +1 250 472 5140; fax: +1 250 721 6611. E-mail address: cpc@uvic.ca (C. Peter Constabel). Phytochemistry 72 (2011) 1551–1565 Contents lists available at ScienceDirect Phytochemistry journal homepage: www.elsevier.com/locate/phytochem