Tannins: chemical characterization and ecological importance

Abstract

Phenolic compounds represent an important class of plant secondary metabolites. Their ecological importance was earlier described as anti-herbivore defenses and attractive to plant pollinators. Recently, it was found that phenolic substances, such as anthocyanins and tannins, are also capable of acting as non-enzymatic antioxidants on plant defense to oxidative burst. According to some authors the evolution of plant defense strategies is nowadays viewed in a phylogenetic perspective: the biosynthetic machinery needed to produce plant defense must be well-conserved and this origin should be monophyletic. This could be supported by the correlations between qualitative plant defenses (alkaloids, iridoids, for example) and the absence of tannins, a quantitative defense, in some Angiosperm groups, meaning that tannins are ubiquitous in woody plants, but almost absent in herbaceous species. However, recently was pointed out the importance of studies concerning the oxidation of tannins and its relationship with herbivore defense. Traditionally, tannins anti-herbivore action was correlated to their capacity of protein precipitation. Condensed tannins and gallotannins have higher protein precipitation capacity than ellagitannins but this property is highly influenced by alkaline gut conditions and at high pH or by the action of polyphenol oxidases and peroxidases at low to neutral pH, ellagitannins are more efficient. The main objective of this study is analyzing tannin composition of one of Brazilian host-parasitic plant model Tapirira guianensis vs. Phoradendron crassifolium. Preliminary results showed a decrease in contents of soluble carbohydrates, starch, lipids, nitrogen, and phenolic compounds in infested branches of T. guianensis, except in the gall region, part where the parasite plant is installed). Non-infested branches from parasitized-plants presented the highest contents of tannins, suggesting that the parasite withdraws nitrogen and soluble carbohydrates from its host and this could result in possible carbon relocation. In addition, accumulation of phenolic compounds in non-infested branches could represent a systemic acquired response (SAR), a defense mechanism well described for plant-pathogen interactions. This mechanism could prevent or even reduce new infestations of a plant already parasitized. Higher amounts of phenolic compounds observed on non-parasitized plants could also point out the importance of those substances to establish patters of resistance/susceptibility to plant parasitism. The second main objective of this proposal is contribute to PolyphenOx project by sampling and analyzing polyphenol composition of plant species representative of Brazilian Atlantic rainforest and Cerrado. A six months of post-doctoral training under supervision of Dr. Juha-Pekka Salminen will be very positive for consolidate a partnership between University of Turku and University of São Paulo, strengthening and improving techniques of polyphenol analysis by Phytochemistry Lab. (IBUSP, Brazil).