Metabolic engineering and applications of flavonoids.

A rapid and substantial progress to a broad understanding of flavonoid biosynthesis marked the previous reviews of this series of books (Hahlbrock and Grisebach, 1975; Ebel and Hahlbrock, 1982; Heller and Forkmann, 1988), and several other reviews have also appeared (Harborne, 1988; Zaprometov, 1989; Stafford, 1990). Advances achieved in the past few years are restricted to specific steps, filling some of the remaining gaps in this metabolic pathway. Among these reactions are the formation of 3-deoxyanthocyanins and 5-deoxyflavonoids, and the synthesis of the structurally derived pterocarpans, which represent the most prominent group of flavonoid phytoalexins. Among the flavonoid modifications, enzymatic formation of flavonoid sulfate esters and aromatic acylation of anthocyanins with hydroxycinnamic acid glucose esters as donors can be mentioned. Yet the key reaction to one of the main flavonoid classes, the anthocyanins, as well as the formation of epicatechin, the proanthocyanidins and some minor flavonoid-related compounds, such as aurones and dihydrochalcones, still remains a matter of debate.

Biosynthesis of flavonoids

Details of Contributors. Preface. 1. Plant secondary metabolism R. Verpoorte. 2. General strategies R. Verpoorte, et al. 3. Agrobacterium, a natural metabolic engineer of plants P.J.J. Hooykaas. 4. Particle gun methodology as a tool in metabolic engineering M.J. Leech, et al. 5. Modulation of plant function and plant pathogens by antibody expression R. Fischer, et al. 6. Transcriptional regulators to modify secondary metabolism J. Memelink, et al. 7. Plant colour and fragrance K.M. Davies. 8. Metabolic engineering of condensed tannins and other phenolic pathways in forage and fodder crops M.P. Robbins, P. Morris. 9. Metabolic engineering of crops with the tryptophan decarboxylase of Catharanthus roseus V. De Luca. 10. Metabolic engineering of enzymes diverting amino acids into secondary metabolism J. Berlin, L. Fecker. 11. Modification of plant secondary metabolism by genetic engineering R. Hain, B. Grimmig. 12. Expression of the bacterial UBIC gene opens a new biosynthetic pathway in plants L. Heide. 13. Regulation of tropane alkaloid metabolism in plants an plant cell cultures K.M. Oksman-Caldentey, R. Arroo.

Metabolic engineering of plant secondary metabolism

Cytochromes P450 play important roles in biosynthesis of flavonoids and their coloured class of compounds, anthocyanins, both of which are major floral pigments. The number of hydroxyl groups on the B-ring of anthocyanidins (the chromophores and precursors of anthocyanins) impact the anthocyanin colour, the more the bluer. The hydroxylation pattern is determined by two cytochromes P450, flavonoid 3′-hydroxylase (F3′H) and flavonoid 3′,5′-hydroxylase (F3′5′H) and thus they play a crucial role in the determination of flower colour. F3′H and F3′5′H mostly belong to CYP75B and CYP75A, respectively, except for the F3′5′Hs in Compositae that were derived from gene duplication of CYP75B and neofunctionalization. Roses and carnations lack blue/violet flower colours owing to the deficiency of F3′5′H and therefore lack the B-ring-trihydroxylated anthocyanins based upon delphinidin. Successful redirection of the anthocyanin biosynthesis pathway to delphinidin was achieved by expressing F3′5′H coding regions resulting in carnations and roses with novel blue hues that have been commercialized. Suppression of F3′5′H and F3′H in delphinidin-producing plants reduced the number of hydroxyl groups on the anthocyanidin B-ring resulting in the production of monohydroxylated anthocyanins based on pelargonidin with a shift in flower colour to orange/red. Pelargonidin biosynthesis is enhanced by additional expression of a dihydroflavonol 4-reductase that can use the monohydroxylated dihydrokaempferol (the pelargonidin precursor). Flavone synthase II (FNSII)-catalysing flavone biosynthesis from flavanones is also a P450 (CYP93B) and contributes to flower colour, because flavones act as co-pigments to anthocyanins and can cause blueing and darkening of colour. However, transgenic plants expression of a FNSII gene yielded paler flowers owing to a reduction of anthocyanins because flavanones are precursors of anthocyanins and flavones.

https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.2012.0432

Nigel K. Giveno

Papers

Floral flavonoids and the potential for pelargonidin biosynthesis in commercial chrysanthemum cultivars