Antioxidant properties of dihydroxy B-ring flavonoids modulate circadian amplitude in <i>Arabidopsis</i>

Flavonoids are abundant specialized metabolites produced by plants for a range of functions, including pigmentation, hormonal signaling, UV protection, and drought tolerance. We previously showed that flavonoids also influence the circadian clock in <i>Arabidopsis</i> Here, we report that the antioxidant properties of dihydroxy B-ring flavonoids are responsible for influencing the amplitude of the core clock gene luciferase reporter, <i>TOC1:LUC</i> We found the elevated amplitude of <i>TOC1:LUC</i> rhythms correlates with the elevated cellular H₂O₂ content of flavonoid-deficient seedlings. Moreover, two different chemical approaches to reduce the accumulation of reactive oxygen species rescued the elevated <i>TOC1:LUC</i> amplitude in flavonoid-deficient seedlings, whereas chemically reducing auxin transport, a known function of flavonoids, had no impact. Interestingly, Ca²⁺ levels in the chloroplast, but not the cytosol, were also altered in flavonoid-deficient seedlings, hinting at retrograde signaling as a possible target of flavonoid action. This study advances our understanding of the relationship between flavonoids and the circadian clock, as well as the mechanisms underlying this interaction.