Abstract

Whether flavonoids play significant antioxidant roles in plants challenged by photooxidative stress of different origin has been largely debated over the last few decades. A critical review of the pertinent literature and our experimentation as well, based on a free-of-scale approach, support an important antioxidant function served by flavonoids in plants exposed to a wide range of environmental stressors, the significance of which increases with the severity of stress. On the other side, some questions need conclusive answers when the putative antioxidant functions of plant flavonoids are examined at the level of both the whole-cell and cellular organelles. This partly depends upon a conclusive, robust, and unbiased definition of “a plant antioxidant”, which is still missing, and the need of considering the subcellular re-organization that occurs in plant cells in response to severe stress conditions. This likely makes our deterministic-based approach unsuitable to unveil the relevance of flavonoids as antioxidants in extremely complex biological systems, such as a plant cell exposed to an ever-changing stressful environment. This still poses open questions about how to measure the occurred antioxidant action of flavonoids. Our reasoning also evidences the need of contemporarily evaluating the changes in key primary and secondary components of the antioxidant defense network imposed by stress events of increasing severity to properly estimate the relevance of the antioxidant functions of flavonoids in an in planta situation. In turn, this calls for an in-depth analysis of the sub-cellular distribution of primary and secondary antioxidants to solve this still intricate matter.

Highlights

  • It has been known for decades that UV, UV-B radiation, triggers the biosynthesis of flavonoids, the vast class of phenylpropanoids comprising more than 8000 structures with a huge array of decorations [3]

  • This supports the view the redox and/or reactive oxygen species (ROS) signals are key drivers for the biosynthesis of flavonols. This conforms to the notion that the activity of R2R3MYB proteins, regulating key genes of flavonol biosynthesis, is under strict redox control, even because of posttranslational modifications [70,71,72,73,74]. It has been reasonably inferred, that the biosynthesis of flavonoids is activated by alteration in the redox potential of the cell and, in turn, flavonoids may help to limit oxidative damage primarily acting as antioxidants and not as light screeners in photoprotection [12,30,31]

  • We suggest that the reduction of oxidative stress may be well suited to estimate the antioxidant role of flavonoids in an in planta situation

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Summary

Introduction

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