CaMK II Inhibitor Gene ID Gonidin and leucodelphinidin (colourless flavan-3,4-cis-diols), respectively. Subsequently, LDOX catalyses the
Gonidin and leucodelphinidin (colourless flavan-3,4-cis-diols), respectively. Subsequently, LDOX catalyses the oxidation of leucocyanidin, leucopelargonidin and leucodelphinidin to cyanidin (red-magenta anthocyanidin), pelargonidin (orange anthocyanidin) and delphinidin (purple-mauve anthocyanidin), respectively. Each of the colours above mentioned refer to a particular environmental condition, i.e., when the anthocyanidins are in an acidic compartment. The final popular step for the production of coloured and steady compounds (anthocyanins) entails the glycosylation of cyanidin, pelargonidin and delphinidin by the enzyme UDP-glucose:flavonoid 3-O-glucosyl transferase (UFGT). Lastly, only cyanidin-3-glucoside and delphinidin-3-glucoside may be further methylated by methyltransferases (MTs), to become converted to peonidin-3-glucoside and petunidin- or malvidin-3-glucoside, respectively. The synthesis of PAs branches off the anthocyanin pathway immediately after the reduction of leucocyanidin (or cyanidin) to catechin (or epicatechin) by the enzymatic activity of a leucoanthocyanidin reductase (LAR), or anthocyanidin reductase (ANR) [30]. The subsequent measures take place in the vacuolar compartments, where the formation of PA polymers occurs by the addition of leucocyanidin molecules to the terminal unit of catechin or epicatechin, possibly catalysed by laccase-like polyphenol oxidases. Having said that, the localization of these enzymes and their actual substrates are still controversial [31,32].Int. J. Mol. Sci. 2013,Figure 1. (A) Scheme in the flavonoid biosynthetic pathway in plant cells. Anthocyanins are synthesized by a multienzyme complicated loosely linked to the endoplasmic reticulum (CHS, chalcone synthase; CHI, chalcone isomerase; F3H, flavanone 3-hydroxylase; F3’H, flavonoid 3′-hydroxylase; F3’5’H, flavonoid 3′,5′-hydroxylase; DFR, dihydroflavonol reductase; LDOX, leucoanthocyanidin oxidase; UFGT, UDP-glucose flavonoid 3-O-glucosyl transferase; MT, methyltransferase). Proanthocyanidins (PAs) synthesis branches off the anthocyanin pathway (LAR, leucoanthocyanidin reductase; ANR, anthocyanidin reductase; STS, stilbene synthase); the black arrows refer to biosynthetic steps missing in grapevine. Numbers next for the flavonoid groups are associated to the chemical structures shown in (B). (B) Chemical structures on the key flavonoid groups.(A)(B)Int. J. Mol. Sci. 2013, 14 3. Mechanisms of Flavonoid Transport in Plant CellsIn the following section, current advances on the models of flavonoid transport into vacuole/cell wall of various plant species, ascribed to a general membrane transporter-mediated transport (MTT), will probably be examined, such as a novel membrane transporter initially found in carnation petals. The establishment of a proton gradient between the cytosol plus the vacuole (or the cell wall) by + H -ATPases (and H+-PPases inside the tonoplast) has been proposed as the major driving force for the transport of some flavonoids and, in unique, anthocyanins into vacuole [33]. As soon as these compounds are inside the vacuoles, the acidic pH inside the vacuolar compartment and also the acylation of flavonoids are each necessary for the induction of a conformational modification, responsible for the appropriate trapping and retention of the metabolites [2,34]. Besides the well-known function in secondary metabolism and xenobiotic detoxification, ATP-binding cassette (ABC) IL-2 Modulator Source transporters have also been claimed to play a role in sequestration of flavonoids in to the vacuole [10,357].