Submitted to: Planta
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 29, 2014
Publication Date: N/A
Interpretive Summary: Many plants accumulate hydroxycinnamoyl esters that likely act as defenses against stresses such as insects, pathogens, ultraviolet light, and ozone. Many of these compounds can also act as potent antioxidents in human and animal health. Caffeic acid esters, in particular, can be oxidized by polyphenol oxidases (PPOs) present in plant tissues. While this oxidation can cause undesirable browning of fresh produce, it also has been shown to prevent protein loss in preserved forage crops. Recently, we showed perennial peanut leaves contain PPO, and we identified at least one PPO substrate, caftaric acid, a hydroxycinnamoyl ester formed from caffeic acid (a hydroxycinnamic acid) and tartaric acid. Perennial peanut leaves also contained compounds presumed to be esters of other hydroxycinnamic acids with tartaric acids. The main purpose of this study was to understand how hydroxycinnamoyl-tartaric acid esters are made in perennial peanut. We found that perennial peanut leaves contain a previously undescribed enzyme capable of making tartaric acid esters with several hydroxycinnamic acids. Our findings will be useful to scientists who study this class of biosynthetic enzymes in order to understand their basic biochemistry. It may also help in efforts to produce useful levels of specific hydroxycinnamoyl-esters in target plant species (for example, to help preserve protein in forages, to protect plants against ultraviolet or ozone exposure, or as a source of nutriceuticals).
Technical Abstract: Many plants accumulate hydroxycinnamoyl esters to protect against abiotic and biotic stresses. Caffeoyl esters, in particular, can be substrates for endogenous polyphenol oxidases (PPOs). Recently, we showed that perennial peanut (Arachis glabrata Benth.) leaves contain PPO and identified one PPO substrate, caftaric acid (trans-caffeoyl-tartaric acid). Additional compounds were believed to be cis- and trans-p-coumaroyl tartaric acid and cis- and trans-feruloyl-tartaric acid, but lack of standards prevented definitive identifications. We have now characterized enzymatic activities in peanut leaves to begin understanding how caftaric acid and related hydroxycinnamoyl esters are made in this species. We showed that peanut leaves contain a hydroxycinnamoyl-CoA:tartaric acid hydroxycinnamoyl transferase (HTT) activity capable of transferring p-coumaroyl, caffeoyl, and feruloyl moieties from CoA to tartaric acid (specific activities of 11±2.8, 8±1.8, 4±0.8 pkat mg-1 crude protein, respectively). The HTT activity was used to make cis- and trans-p-coumaroyl- and -feruloyl-tartaric acid in vitro. These products allowed definitive identification of the corresponding cis- and trans-hydroxycinnamoyl esters extracted from leaves. We tentatively identified sinapoyl-tartaric acid as another major phenolic compound in peanut leaves that likely participates in secondary reactions with PPO-generated quinones. These results suggested hydroxycinnamoyl-tartaric acid esters are made by an acyltransferase, possibly a BAHD family member, in perennial peanut. Identification of a gene encoding HTT and further characterization of the enzyme will aid in identifying determinants of donor and acceptor substrate specificity for this important class of biosynthetic enzymes. An HTT gene could also provide a means via genetic engineering to produce caffeoyl- and other hydroxycinnamoyl-tartaric acid esters in forage crops that lack them.