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.