Opinion on the possible role of flavonoids as energy escape valves: Novel tools for nature's Swiss army knife?

Under different suboptimal growth conditions (e.g. low temperatures, drought and nitrogen deficiency), the ability of plants to process the light energy income can be overwhelmed. Plants respond to this energetic imbalance by adjusting their sucrose biosynthesis and export rates, tuning starch metabolism, down-regulating Calvin cycle enzymes and activating energy dissipation mechanisms. We here hypothesize that a complementary response that is mostly underestimated is increased biosynthesis and subsequent accumulation of flavonoids. These are secondary metabolites involved in a wide array of processes such as flower pigmentation, nodulation and photoprotection. Like other phenylpropanoids, they are synthesized by the shikimate pathway, which has been suggested to act as a sink for reduced carbon and, hence, can act as energy escape valve by consuming trioses phosphate, ATP and NADPH. Compared to the biosynthesis of other phenylpropanoids, that of flavonoids specifically requires the incorporation of malonate, thereby facilitating the diversion of photoassimilate and energy. As flavonoids do not bear nitrogen in their chemical structure, their accumulation does not put an additional pressure on nitrogen resources, which are essential for other processes. In our opinion, this putative protective role of flavonoids as energy escape valve during abiotic stresses has been largely underrated in comparison to their photoprotective and possibly antioxidative functions. (C) 2010 Elsevier Ireland Ltd. All rights reserved.