Current understanding of the pathways of flavonoid biosynthesis in model and crop plants

Abstract

Flavonoids are a signature class of secondary metabolites formed from a relatively simple collection of scaffolds. They are extensively decorated by chemical reactions including glycosylation, methylation, and acylation. They are present in a wide variety of fruits and vegetables and as such in Western populations it is estimated that 20-50 mg of flavonoids are consumed daily per person. In planta they have demonstrated to contribute to both flower color and UV protection. Their consumption has been suggested to presenta wide range of health benefits. Recent technical advances allowing affordable whole genome sequencing, as well as a better inventory of species-by-species chemical diversity, have greatly advanced our understanding as to how flavonoid biosynthesis pathways vary across species. In parallel, reverse genetics combined with detailed molecular phenotyping is currently allowing us to elucidate the functional importance of individual genes and metabolites and by this means to provide further mechanistic insight into their biological roles. Here we provide an inventory of current knowledge of pathways of flavonoid biosynthesis in both the model plant Arabidopsis thaliana and a range of crop species, including tomato, maize, rice, and bean.

Multi‐tissue integration of transcriptomic and specialized metabolite profiling provides tools for assessing the common bean (Phaseolus vulgaris) metabolome

“…Further glycosyl units can also be added to the sugars directly linked to the aglycone to generate highly decorated flavonoids. Based on this general pathway structure and flavonoid structures previously described in common beans (Tohge et al ., ), we propose the pathway in Figure (c). By comparing the structure of both raw and curated networks, it can be observed that the oxygenation conversions linking two metabolites in Figure (a) actually correspond to a single biochemical step: the basal oxygenation of the kaempferol aglycone giving rise to the parallel quercetin branch, as represented at the top of Figure (c).…”

“…This particular pathway is a great proof of concept as it is well characterized and conserved among higher plants, facilitating its reconstruction and identification of proposed steps. The details are extensively discussed in the literature (Saito et al ., ; Tohge et al ., , ). In summary it starts with the production of naringenin chalcone from p ‐coumaryl‐CoA and malonyl‐CoA via chalcone synthase (CHS).…”

“…Soybean, a species closely related to the common bean (Schmutz et al ., ) is well known to accumulate high levels of isoflavonoids, which in turn exhibit a diverse range of biological activities (Martin et al ., ; Tohge and Fernie, ). While the case of soybean is exceptional and isoflavonoids accumulate at much lower levels in other legumes, this interesting class of specialized metabolite is almost exclusively found in the legume subfamily Papilionoideae (Veitch, ; Tohge et al ., ). The other highly relevant class of metabolites associated with multiple biological activities and often described as being highly abundant in the legumes is the triterpenoid saponins, particularly those derived from β‐amyrin.…”

“…The specialized metabolism of common bean additionally includes other phenolic compounds, mainly flavonoids, with most of this knowledge based on phytochemical analysis of seed coats, sprouts and pods, which are the most commonly consumed parts of the plant (Tohge et al ., ). Importantly, to date, only a few studies on common bean have attempted to investigate the genetic basis underlying their accumulation with most of these studies being related to flavonoids (Tohge et al ., ). A couple of enzymes, namely chalcone synthase (PvCHS) (Ryder et al ., , ) and chalcone isomerase (PvCHI) (Dixon et al ., ; Blyden et al ., ), were characterized in relation to their possible roles in isoflavonoid biosynthesis.…”

“…A couple of enzymes, namely chalcone synthase (PvCHS) (Ryder et al ., , ) and chalcone isomerase (PvCHI) (Dixon et al ., ; Blyden et al ., ), were characterized in relation to their possible roles in isoflavonoid biosynthesis. Through the work of several groups, Mendelian traits linked to seed coat color and flavonoid accumulation were described in different varieties of beans (Tohge et al ., ), and more recently genes involved in anthocyanin biosynthesis in the pods were identified and characterized (Hu et al ., ).…”