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1.  Flavonoids as Antioxidants and Developmental Regulators: Relative Significance in Plants and Humans 
Phenylpropanoids, particularly flavonoids have been recently suggested as playing primary antioxidant functions in the responses of plants to a wide range of abiotic stresses. Furthermore, flavonoids are effective endogenous regulators of auxin movement, thus behaving as developmental regulators. Flavonoids are capable of controlling the development of individual organs and the whole-plant; and, hence, to contribute to stress-induced morphogenic responses of plants. The significance of flavonoids as scavengers of reactive oxygen species (ROS) in humans has been recently questioned, based on the observation that the flavonoid concentration in plasma and most tissues is too low to effectively reduce ROS. Instead, flavonoids may play key roles as signaling molecules in mammals, through their ability to interact with a wide range of protein kinases, including mitogen-activated protein kinases (MAPK), that supersede key steps of cell growth and differentiation. Here we discuss about the relative significance of flavonoids as reducing agents and signaling molecules in plants and humans. We show that structural features conferring ROS-scavenger ability to flavonoids are also required to effectively control developmental processes in eukaryotic cells.
doi:10.3390/ijms14023540
PMCID: PMC3588057  PMID: 23434657
auxin movement; chloroplast flavonoids; dihydroxy B-ring-substituted flavonoids; MAPK; nuclear flavonoids; reactive oxygen species (ROS); signaling molecules
2.  Flavonols: old compounds for old roles 
Annals of Botany  2011;108(7):1225-1233.
Background
New roles for flavonoids, as developmental regulators and/or signalling molecules, have recently been proposed in eukaryotic cells exposed to a wide range of environmental stimuli. In plants, these functions are actually restricted to flavonols, the ancient and widespread class of flavonoids. In mosses and liverworts, the whole set of genes for flavonol biosynthesis – CHS, CHI, F3H, FLS and F3′H – has been detected. The flavonol branch pathway has remained intact for millions of years, and is almost exclusively involved in the responses of plants to a wide array of stressful agents, despite the fact that evolution of flavonoid metabolism has produced >10 000 structures.
Scope
Here the emerging functional roles of flavonoids in the responses of present-day plants to different stresses are discussed based on early, authoritative views of their primary functions during the colonization of land by plants. Flavonols are not as efficient as other secondary metabolites in absorbing wavelengths in the 290–320 nm spectral region, but display the greatest potential to keep stress-induced changes in cellular reactive oxygen species homeostasis under control, and to regulate the development of individual organs and the whole plant. Very low flavonol concentrations, as probably occurred in early terrestrial plants, may fully accomplish these regulatory functions.
Conclusions
During the last two decades the routine use of genomic, chromatography/mass spectrometry and fluorescence microimaging techniques has provided new insights into the regulation of flavonol metabolism as well as on the inter- and intracellular distribution of stress-responsive flavonols. These findings offer new evidence on how flavonols may have performed a wide array of functional roles during the colonization of land by plants. In our opinion this ancient flavonoid class is still playing the same old and robust roles in present-day plants.
doi:10.1093/aob/mcr234
PMCID: PMC3197460  PMID: 21880658
Auxin transport; early flavonoid genes; evolution of early terrestrial plants; flavonol metabolism; Myb genes; ROS scavengers; stress-responsive flavonoids; sub-cellular flavonoid distribution; UV-B screening
3.  Stress-induced flavonoid biosynthesis and the antioxidant machinery of plants 
Plant Signaling & Behavior  2011;6(5):709-711.
There is a growing body of evidence that flavonoids do not primarily function as UV-B screening pigments in photoprotection. Recent findings support the idea that excess light stress, irrespective of the relative proportions of the solar wavebands reaching the leaf surface, upregulates the biosynthesis of dihydroxy B-ring-substituted flavonoid glycosides, as a consequence of and aimed at countering the generation of ROS. Intriguingly, the very conditions that lead to the inactivation of antioxidant enzymes can also upregulate the biosynthesis of antioxidant flavonoids, which suggests flavonoids constituting a secondary ROS-scavenging system in plants exposed to severe/prolonged stress conditions. H2O2 may diffuse out of the chloroplast at considerable rates and be transported to the vacuole, the storing site for flavonoids, by tonoplast intrinsic proteins, under severe excess light conditions. We suggest that the unanticipated key role of the vacuole in the ROS homeostasis might be mediated by flavonoids.
doi:10.4161/psb.6.5.15069
PMCID: PMC3172844  PMID: 21448007
antioxidant enzymes; excess light stress; flavonol metabolism; hydrogen peroxide (H2O2); mesophyll flavonoids; quercetin glycosides; oxidative stress
4.  Mesophyll distribution of ‘antioxidant’ flavonoid glycosides in Ligustrum vulgare leaves under contrasting sunlight irradiance 
Annals of Botany  2009;104(5):853-861.
Background and Aims
Flavonoids have the potential to serve as antioxidants in addition to their function of UV screening in photoprotective mechanisms. However, flavonoids have long been reported to accumulate mostly in epidermal cells and surface organs in response to high sunlight. Therefore, how leaf flavonoids actually carry out their antioxidant functions is still a matter of debate. Here, the distribution of flavonoids with effective antioxidant properties, i.e. the orthodihydroxy B-ring-substituted quercetin and luteolin glycosides, was investigated in the mesophyll of Ligustrum vulgare leaves acclimated to contrasting sunlight irradiance.
Methods
In the first experiment, plants were grown at 20 % (shade) or 100% (sun) natural sunlight. Plants were exposed to 100 % sunlight irradiance in the presence or absence of UV wavelengths, in a second experiment. Fluorescence microspectroscopy and multispectral fluorescence microimaging were used in both cross sections and intact leaf pieces to visualize orthodihydroxy B-ring-substituted flavonoids at inter- and intracellular levels. Identification and quantification of individual hydroxycinnamates and flavonoid glycosides were performed via HPLC-DAD.
Key Results
Quercetin and luteolin derivatives accumulated to a great extent in both the epidermal and mesophyll cells in response to high sunlight. Tissue fluorescence signatures and leaf flavonoid concentrations were strongly related. Monohydroxyflavone glycosides, namely luteolin 4′-O-glucoside and two apigenin 7-O-glycosides were unresponsive to changes in sunlight irradiance. Quercetin and luteolin derivatives accumulated in the vacuoles of mesophyll cells in leaves growing under 100 % natural sunlight in the absence of UV wavelengths.
Conclusions
The above findings lead to the hypothesis that flavonoids play a key role in countering light-induced oxidative stress, and not only in avoiding the penetration of short solar wavelengths in the leaf.
doi:10.1093/aob/mcp177
PMCID: PMC2749533  PMID: 19633310
Confocal laser scanning microscopy (CLSM); flavonoid glycosides; fluorescence microimaging; fluorescence microspectroscopy; hydroxycinnamates; intra-cellular flavonoid localization; Ligustrum vulgare; photoprotection; UV stress
5.  Responses to Changes in Ca2+ Supply in Two Mediterranean Evergreens, Phillyrea latifolia and Pistacia lentiscus, During Salinity Stress and Subsequent Relief 
Annals of Botany  2008;102(4):609-622.
Background and Aims
Changes in root-zone Ca2+ concentration affect a plant's performance under high salinity, an issue poorly investigated for Mediterranean xerophytes, which may suffer from transient root-zone salinity stress in calcareous soils. It was hypothesized that high-Ca2+ supply may affect differentially the response to salinity stress of species differing in their strategy of Na+ allocation at organ level. Phillyrea latifolia and Pistacia lentiscus, which have been reported to greatly differ for Na+ uptake and transport rates to the leaves, were studied.
Methods
In plants exposed to 0 mm or 200 mm NaCl and supplied with 2·0 mm or 8·0 mm Ca2+, under 100 % solar irradiance, measurements were conducted of (a) gas exchange, PSII photochemistry and plant growth; (b) water and ionic relations; (c) the activity of superoxide dismutase and the lipid peroxidation; and (d) the concentration of individual polyphenols. Gas exchange and plant growth were also estimated during a period of relief from salinity stress.
Key Results
The performance of Pistacia lentiscus decreased to a significantly smaller degree than that of Phillyrea latifolia because of high salinity. Ameliorative effects of high-Ca2+ supply were more evident in Phillyrea latifolia than in Pistacia lentiscus. High-Ca2+ reduced steeply the Na+ transport to the leaves in salt-treated Phillyrea latifolia, and allowed a faster recovery of gas exchange and growth rates as compared with low-Ca2+ plants, during the period of relief from salinity. Salt-induced biochemical adjustments, mostly devoted to counter salt-induced oxidative damage, were greater in Phillyrea latifolia than in Pistacia lentiscus.
Conclusions
An increased Ca2+ : Na+ ratio may be of greater benefit for Phillyrea latifolia than for Pistacia lentiscus, as in the former, adaptive mechanisms to high root-zone salinity are primarily devoted to restrict the accumulation of potentially toxic ions in sensitive shoot organs.
doi:10.1093/aob/mcn134
PMCID: PMC2701781  PMID: 18701601
Calcium–sodium interactions; gas exchange; Na allocation; Na uptake and transport; oxidative damage; Phillyrea latifolia; Pistacia lentiscus; polyphenols; PSII photochemistry; relief from salinity; water relations

Results 1-5 (5)