Red light controls adventitious root regeneration by modulating hormone homeostasis inPicea abiesseedlings

Vegetative propagation relies on the capacity of plants to regenerate de novo adventitious roots (ARs), a quantitative trait controlled by the interaction of endogenous factors such as hormones and environmental cues among which light plays a central role. However, the physiological and molecular components mediating light cues during AR initiation (ARI) remain largely elusive. We explored the effect of light spectral quality on ARI in de-rooted Norway spruce seedlings as well as on hormone metabolism with sensitive mass spectrometry-based methods. We coupled this to gene expression analysis to identify potential signaling pathways and to extensive anatomical characterization to investigate ARI at the cellular level. We showed that in contrast to white light and blue light, red light promoted ARI likely by reducing jasmonate (JA) and JA-isoleucine biosynthesis and repressing the accumulation of isopentyl-adenine-type cytokinins and abscisic acid. We confirmed that exogenously applied JA and/or CK inhibit ARI, and found that they possibly act in the same pathway. The negative effect of JA was confirmed at the histological level. We showed that JA represses the early events of ARI. In conclusion, RL promotes ARI by repressing the accumulation of the wound-induced phytohormones JA and CK. Highlight Blue and red light have an opposite effect on adventitious root initiation in Norway spruce hypocotyl, red light having a promoting effect by modulating hormone homeostasis.