Flavonoid biosynthesis - an overview | ScienceDirect Topics
An Arabidopsis group of mutants, collectively named () mutants, are characterized by the reduction or the absence of pigments in the testa (seed coat), showing yellow or pale-brown seeds. mutants with impaired expression of two key enzymes of the flavonoid synthesis (chalcone synthase, CHS, and chalcone isomerase, CHI) are hypersensitive to UV-B radiation although, after UV-B treatment, they manifested only a modest growth retardation compared to wild-type. The unexpected tolerance of these mutants to UV-B may reflect the increased accumulation of other flavonoids . Conversely, an Arabidopsis mutant () with a constitutive elevated accumulation of anthocyanins due to high expression level of CHS gene are tolerant to UV-B radiation .
Mutants affecting flavonoid synthesis have been ..
Anthocyanins are the molecules that confer flower pigmentation . They are responsible, therefore, of pollinator attraction and consequently they play a role in plant reproduction. In fact, colour of flowers provides visual cues that lead pollinators to nectar-filled flowers and attract seed dispersers to ripened fruits . In maize, at least two independent flavonoid biosynthetic pathways have been characterized. A first pathway leads to 3-hydroxy flavonoids such as anthocyanins purple pigment, whereas the other one produces 3-deoxy flavonoids such as the phlobaphene red pigment accumulated in kernel pericarp, silks and cob. Phlobaphenes are synthesized by the oxidation of colorless flavan-4-ol monomers or its polymers as shown in figure .
Flavonoids are secondary metabolites synthesized via the phenylpropanoid pathway. They possess 15 carbon atoms and share a common C6-C3-C6 skeleton, with two benzene rings linked through a heterocyclic pyrane or a pyrone ring (Figure ). According to the C ring substitution pattern, six major subgroups of flavonoids can be identified: chalcones, flavones, flavonols, flavanones, anthocyanins and isoflavonoids. These compounds are bioactive across the plant kingdom with over 9,000 structural variants known . Flavonoids act as scavengers of free radicals (e.g. ROS). Their antioxidant nature is defined mainly by the presence of a B-ring catechol group (dihydroxylated B-ring) capable of readily donating hydrogen (electron) to stabilize a radical species, in according to the following equation:
Flavonoid diversity and biosynthesis in seed …
The endosymbiontic theory identifies two specific groups of bacteria, α-proteobacteria and cyanobacteria, as the closest relatives of mitochondria and chloroplasts, respectively . Many genes were either lost from plastids or transferred to the nucleus during the course of plant evolution . It can be argued that melanins and flavonoids could have evolved from an ancient pigment produced in bacteria and after endosymbiosis some genes involved in pigment biosynthetic pathways have been integrated into animal and plant genomes.
Flavonoid diversity and biosynthesis in seed of ..
The scientific information reported above suggests that cyanobacteria possess a precursor of the UV protecting mechanisms found in plants and animals. It can be hypothesized that before the land colonization, when all organisms lived in aquatic environment, an HGT took place. Bacteria, in particular cyanobacteria, transferred to plants and animals the genes able to confer UV protection. With the emersion from water and the exposure to a higher UV radiation level the living organisms needed to produce more effective antioxidant molecules. Therefore, starting from the genes and pathways controlling MAAs biosynthesis, they evolved new protective compounds such as betalains, melanins and flavonoids. The betalains, present in higher fungi (Basidiomiceteae) and, can be considered the first type of complex antioxidant compound evolved after water emersion. Along with the evolution, plant species became characterized by flavonoids, while melanins were evolved in animals. Fungi, closer to animals than plants, accumulate only melanins, (with the exception of ) and, at least at biochemical level, betalains can also be considered the direct precursor of melanins.
Expression analysis of flavonoid biosynthesis genes …
Ultraviolet (UV) radiations alter a number of metabolic functions in vivant. They produce damages to lipids, nucleic acids and proteins, generating reactive oxygen species such as singlet oxygen (O2), hydroxyl radical (HO) and superoxide anion (O2-). Plants and animals, after their water emersion, have developed biochemical mechanisms to protect themselves from that environmental threat through a common strategy. Melanins in animals and flavonoids in plants are antioxidant pigments acting as free radical scavenging mechanisms. Both are phenol compounds constitutively synthesized and enhanced after exposure to UV rays, often conferring a red-brown-dark tissue pigmentation.