Abstract
β–Cyclocitral (β–cc), a β–carotene oxidation product, has been proposed as an early retrograde signal that primes protection against photooxidative stress. Its temporal relationship with photosynthetic acclimation, however, remains unresolved. Exogenous β–cc elicited cytosolic calcium changes within minutes and induced the β–cc–responsive pAER::LUC detoxification reporter within 40 min. We exposed Arabidopsis thaliana plants to 1500 μmol photons m
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under ventilation creating a high-irradiance regime without concomitant heat stress and with a relatively mild photooxidative outcome (RT–EL). Non-photochemical quenching increased, whereas PSII maximum quantum yield progressively decreased during RT-EL, with both responses approaching a plateau by 16 h. Volatile β–cc pretreatment did not improve photosynthetic performance or NPQ dynamics, whereas endogenous β–cc and β-cyclocitric acid accumulated only after prolonged RT–EL exposure (16 h). By contrast, hormone profiling and RNA-seq revealed extensive β–cc–dependent reprogramming that was largely opposite to the RT-EL transcriptional response. Together, these results reveal a kinetic separation between rapid β–cc signaling and its endogenous production. Under RT–EL major photosynthetic adjustments precede detectable β-cc accumulation, whereas β–cc triggers cellular responses within minutes once present. We therefore propose that the timing of β–cc action during excess light is determined primarily by the onset of substantial β–carotene oxidation.