phenolics with more than 9000 compounds identified till date [51]. Their basic
scaffold moiety is 2-phenyl-benzyl-γ-pyrone derivative. This group of phenolics is
synthesized in all the developmental stages, tissues, and stress conditions in the
plant. Flavonoids perform various functions on the basis of their localization,
abundance, and temporal synthesis. Its substantial significance lies in defense against
abiotic or biotic stress, pigmentation, fragrance, UV protection, etc. Anthocyanins,
aurones, chalcones, flavonols, and flavanones are generally responsible for floral
pigmentation [52]. Pigmentation and fragrance serve as visual and chemical cues for
insects to identify and locate host plants. Changes in floral pigmentation due to
various secondary metabolites can result in differential host selection. For instance,
Petunia axillaris (white flowers) are pollinated by bees whereas, Petunia integrifolia
(violet flowers) are pollinated by moths [52].
The genesis and diversification of various plant secondary metabolites are in
consonance with their evolution and interaction with varied abiotic or biotic
components. From the fossil records, it has been observed that molecules similar
to flavonoid class might have established 500 Mya [48]. Plant-specific distribution
of the molecules belonging to this class like chalcones, flavanones, and flavonols
were reported in the Silurian period of Paleozoic era (540–250 Mya), the time
when the progression and diversification of bryophytes are observed [53, 52].
There are numerous speculations about functions of these ancestral flavonoids but
most argued it to be in cytoprotection from either UV radiations or signaling
molecules. These flavonoids further evolved to aid plant defense [53]. Proanthocyanins, which portray an intermediate link between early nonpigment
flavonoids and pigmented anthocyanins, are documented to come into existence
in parallel with vascular plants [53]. Following this, anthocyanins, involved in
pollination, were reported to have organized along with flower-producing plants –
gymnosperms and angiosperms [54].
As per fossil records, lignins are reported to be synthesized after flavonoids
and they came into existence along with the tracheophytes (ferns, gymnosperms,
and angiosperms) and remain from the Silurian period [5]. These molecules
build the woody mesh-like tissue through lignification and provide
mechanical strength to the tracheophyte structure. Besides increasing the rigidity
of cell walls, lignins are also reported to have antinutritive effects on herbivorous
insects [55].
Furthermore, there is major plant phenolic compound tannin, which came into
existence post-Carboniferous period [48]. Gymnosperms that appeared in this period
used metabolites like tannins for seed protection against various pathogens and
pests. Tannins are majorly found in leaves of gymnosperms and angiosperms,
specifically the class of woody plants – Fabaceae, Fagaceae, Myrtaceae, and Polygonaceae [56]. As a mode of action, tannins act as pest deterrents and also inhibit
insect digestive enzymes, reducing their digestibility [55]. These primitive classes of
phenolic secondary metabolites further get diversified in corroboration with plant
evolution and serve distinct functions from seed protection to plant-plant communication. In course of evolution, plants also produced various metabolites from other
classes to employ more specialized functions.
28
S. S. Zunjarrao et al.
scaffold moiety is 2-phenyl-benzyl-γ-pyrone derivative. This group of phenolics is
synthesized in all the developmental stages, tissues, and stress conditions in the
plant. Flavonoids perform various functions on the basis of their localization,
abundance, and temporal synthesis. Its substantial significance lies in defense against
abiotic or biotic stress, pigmentation, fragrance, UV protection, etc. Anthocyanins,
aurones, chalcones, flavonols, and flavanones are generally responsible for floral
pigmentation [52]. Pigmentation and fragrance serve as visual and chemical cues for
insects to identify and locate host plants. Changes in floral pigmentation due to
various secondary metabolites can result in differential host selection. For instance,
Petunia axillaris (white flowers) are pollinated by bees whereas, Petunia integrifolia
(violet flowers) are pollinated by moths [52].
The genesis and diversification of various plant secondary metabolites are in
consonance with their evolution and interaction with varied abiotic or biotic
components. From the fossil records, it has been observed that molecules similar
to flavonoid class might have established 500 Mya [48]. Plant-specific distribution
of the molecules belonging to this class like chalcones, flavanones, and flavonols
were reported in the Silurian period of Paleozoic era (540–250 Mya), the time
when the progression and diversification of bryophytes are observed [53, 52].
There are numerous speculations about functions of these ancestral flavonoids but
most argued it to be in cytoprotection from either UV radiations or signaling
molecules. These flavonoids further evolved to aid plant defense [53]. Proanthocyanins, which portray an intermediate link between early nonpigment
flavonoids and pigmented anthocyanins, are documented to come into existence
in parallel with vascular plants [53]. Following this, anthocyanins, involved in
pollination, were reported to have organized along with flower-producing plants –
gymnosperms and angiosperms [54].
As per fossil records, lignins are reported to be synthesized after flavonoids
and they came into existence along with the tracheophytes (ferns, gymnosperms,
and angiosperms) and remain from the Silurian period [5]. These molecules
build the woody mesh-like tissue through lignification and provide
mechanical strength to the tracheophyte structure. Besides increasing the rigidity
of cell walls, lignins are also reported to have antinutritive effects on herbivorous
insects [55].
Furthermore, there is major plant phenolic compound tannin, which came into
existence post-Carboniferous period [48]. Gymnosperms that appeared in this period
used metabolites like tannins for seed protection against various pathogens and
pests. Tannins are majorly found in leaves of gymnosperms and angiosperms,
specifically the class of woody plants – Fabaceae, Fagaceae, Myrtaceae, and Polygonaceae [56]. As a mode of action, tannins act as pest deterrents and also inhibit
insect digestive enzymes, reducing their digestibility [55]. These primitive classes of
phenolic secondary metabolites further get diversified in corroboration with plant
evolution and serve distinct functions from seed protection to plant-plant communication. In course of evolution, plants also produced various metabolites from other
classes to employ more specialized functions.
28
S. S. Zunjarrao et al.