Cytokinins down-regulate photosystem II photochemistry during prolonged darkness in a phytochrome B-dependent manner

Cytokinins (CKs) are well known for their ability to delay dark-induced leaf senescence by inhibiting the degradation of photosynthetic pigments (especially chlorophylls) and proteins, thereby preserving photosynthetic function. However, the molecular mechanisms underlying CK action in photosynthesis remain largely unexplored. When we investigated the effects of exogenous CKs in detached Arabidopsis leaves at different times of their dark incubation, we observed that, in contrast to the positive CK effect on photosystem II (PSII) photochemistry at later stages of dark-induced senescence, CKs unexpectedly downregulated PSII photochemistry at early stages of dark incubation. Transcriptomic analysis revealed that a significant subset of CK-responsive genes are associated with PSII function and red light signalling pathways. Further experiments showed that CK-downregulation of PSII photochemistry required phytochrome B (phyB) and was abolished by far-red light application, indicating that CKs maintain active phyB in darkened leaves. We propose that the CK-phyB-mediated downregulation of PSII photochemistry serves to protect the photosynthetic apparatus from photooxidative damage upon potential sudden illumination of leaves exposed to (prolonged) darkness.