Evolutionary insights into β-cyclocitral signaling in Physcomitrium patens

Summary How did a product of photooxidative damage become a signal for stress tolerance? β-cyclocitral (β-cc) promotes acclimation in several angiosperms, yet has detrimental effects in other photosynthetic organisms. We investigated this contrast in the moss Physcomitrium patens to explore the evolution of β-cc signaling. We combined apocarotenoid treatments and excess light exposure with photosynthetic measurements, biochemical analyses and RNA sequencing. Comparative phylogenomics traced the conservation of regulatory and detoxification components associated with β-cc responses. Physcomitrium accumulated β-cc under excess light and mounted a strong transcriptional response to exogenous β-cc, substantially overlapping with its light-stress response. Nevertheless, β-cc impaired photosynthetic performance and provided no protection. At a phylogenetic level, while MBS1 was deeply conserved, the downstream detoxification module showed marked divergence. Notably, ANAC081/ANAC102-like proteins were recovered in terrestrial monocots but not in Physcomitrium or Lemna turionifera , both showing detrimental responses to β-cc. Our findings suggest that the evolution of protective β-cc signaling involved the recruitment of downstream detoxification regulators, with the SCL14/TGAII- and ANAC081/ANAC102-related lineage emerging as a candidate link between an ancient oxidative cue and beneficial acclimation.