# Origin and Function of Structural Diversity in the Plant Specialized Metabolome
## 4 Decoration and Conjugation in Specialized Metabolism
### Function of Decorations  
An excellent review recently discussed the current knowledge of the biological function of flavonoid hydroxylation, methylation, glycosylation and acylation [134]. Glycosylation and aliphatic acid are generally seen as strategies to detoxy or increase the solubility of specialized metabolites or to change their biological activity. Regulatory functions of acylations have also been demonstrated. For example, malonylation of flavonoid glucosides increases the transport efficiency of flavonoid glucosides into the vacuole, indicating that acylation participates in the regulation of metabolic homeostasis [135; 136]. In phenylpropanoid biosynthesis, \(p\)-counaroyl shikimate is preferred over \(p\)-counaric acid as a substrate for 3-hydroxylation by \(p\)-COUMARATE 3-HYDROXYLASE (C3H), and the product, caffeoyl shikimate, is subsequently hydrolyzed back to the carboxylic acid level (caffeic acid) (Figure 5) [137; 138; 139]. The occurrence of shikimate ester intermediates represents an energy consuming biosynthetic detour, and has been proposed to play a regulating role in the phenylpropanoid biosynthesis pathway [69]. Regarding biological activity, aromatic acylation of glycosylated flavonoids enhances UV absorption, and it has been proposed that this results from an intramolecular co-pigmentation-like effect, due to the interaction between the aromatic A-ring of the flavonoid and the phenolic ring of the acyl [134]. Depending on the glycosylation site and the anthocyanin type, glycosylation can decrease, leave unchanged or increase the antioxidant activity of anthocyanins [140]. The above-described structure-function relationships of undecorated flavonoids and anthocyanins predict a similar antioxidant activity of these two categories, or a slightly stronger effect of flavonoids, in plants. However, it has been repeatedly shown that, in plant extracts, anthocyanins contribute more to antioxidant activity than flavonoids [141; 142]. In berries and small fruits (raspberry, blackberry, sour cherry, strawberry, chokeberry, elderberry and blueberry), antioxidant activity was more strongly correlated with total anthocyanin content (r = 0.99) than with total flavonoids, or total quercetin derivatives (r = 0.93) [141]. In the fern _Pteris vittata_, subterranean parts have stronger antioxidant activity than aerial parts and the main difference in the metabolomics of these two parts resides in the presence of anthocyanins in the subterranean parts that were undetected in aerial parts [142]. These observations emphasize that functional studies of aglycones need to be relativized and that the final activity of the specialized metabolome is determined by the decorations. It should also be noted that studies relating antioxidant activity of plant extracts to their metabolome, published today, are almost always biased by the targeted metabolite profiling methods on which they are based. A plethora of structures is present in the specialized plant metabolome, of which the majority has never been described, is not included in databases and is hard to detect by targeted approaches. Detection and characterization of biochemically related compounds can be aided by untargeted approaches via the construction of molecular networks. In this approach, pairs of features (ionized metabolites detected by LC-MS) are connected based on mass spectral similarity or based on mass differences that potentially represent well-known decorations or bitotransformations [3; 143]. Only a wider coverage of the specialized metabolome, including the modifications and decorations that were not recorded in databases, will allow more reliable studies of its functioning. The decorations are subject to dynamic change during growth and development of the plants or in response to environmental stimuli [73].