Flavonoids as antioxidants and developmental regulators: relative significance in plants and humans

Abstract

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.

Figure 1

Absorbance maxima and ROS-quenching capacity of mono-(apigenin and kaempferol) and dihydroxy B-ring-substituted (quercetin and luteolin) flavonoid glucosides (glc). Absorbance spectra were recorded in phosphate buffer at a metabolite concentration of 50 μM. IC₅₀ denotes the molar concentration required to reduce by 50% the concentrations of superoxide anion (O₂⁻) and the synthetic free radical DPPH (2,2-diphenyl-1-picrylhydrazyl) as estimated following the protocols reported in [10,17].

Figure 2

Inter and intracellular distribution of dihydroxy-B-ring-substituted flavonoid glycosides in leaves of L. vulgare leaves acclimated to full sunlight irradiance. Cross sections were stained with Naturstoff reagent and fluorescence images recorded in a Confocal Laser Scanning Microscope flowing the protocols of [10]. In brief, specimens were excited at 488 nm and fluorescence recorded at 580 ± 10 nm: this excitation-emission set-up has been previously reported to exclusively visualize flavonoids, in particular flavonoids with a catechol group in the B-ring of the flavonoid skeleton [10]. Dihydroxy flavonoids accumulate mostly in the adaxial tissue (up to a distance of 150–170 μm from adaxial surface, as revealed by profiles of their fluorescence acquired at 580 nm, reported in (B). Dotted and solid white arrows in (A) indicate flavonoids in the chloroplast and the vacuole, respectively. Dihydroxy flavonoids are located in the nucleus of mesophyll cells. Nuclei were stained with both DAPI (to visualize the nucleus) and Naturstoff reagent and images were recorded in the blue-channel for detecting DAPI fluorescence (C) or the yellow-channel for detecting flavonoid fluorescence (D).