Function of NOT9B and the CCR4-NOT complex in phytochrome A dependent light signalling
Final Report Abstract
Phytochromes (phys) are red (R)/far-red (FR) photoreceptors in plants that play a critical role in the adaptation of growth and development to changes in the environment. They are synthesised in the inactive Pr state and convert – upon absorption of light – to the physiologically active Pfr state. In seed plants, mosses, and ferns, phytochromes are usually encoded by small gene families, consisting of PHYA-E in Arabidopsis. PhyA and phyB are the main phytochromes involved in regulation of growth and development, including seed germination, de-etiolation, flowering, and responses to canopy shade. PhyB is the primary receptor for R light, while phyA is required for sensing FR light and light of very low intensity. Phytochromes localise to the cytosol in the inactive state and translocate into the nucleus upon activation by light. Two downstream signalling pathways in the nucleus that link phytochrome-mediated light perception to regulation of gene expression are well described. Members of the PIF subfamily of bHLH transcription factors generally suppress light responses and the COP1/SPA E3 ubiquitin ligase complex targets positive regulators of light responses for degradation in the absence of light. Light-activated phytochromes repress both PIF and COP1/SPA action and thereby promote downstream signalling in light. Over the past years, we identified a number of proteins that bind to phyA and/or phyB in the lightactivated state, suggesting these proteins could play a role in alternative downstream signalling pathway that potentially do not depend on PIFs and COP1/SPA. In the project on which we report here, we describe NOT9B, a component of the CCR4-NOT complex, as novel phyA signalling component. The not9b loss-of-function mutant is slightly hypersensitive to FR light and in part unable to fully suppress light signalling in the dark. In contrast, lines overexpressing NOT9B are less sensitive to FR light, indicating that NOT9B is a negative regulator of phyA signalling. Using Y2H, pull down, and microscopy-based approaches, we could demonstrate that the same binding site in NOT9B is required for interaction with both phyA and NOT1, the scaffold protein of the CCR4-NOT complex. This prompted us to test whether phyA can control the association of NOT9B with the CCR4-NOT complex. Indeed, light-activated phyA competes with NOT1 for binding NOT9B, leading to partial dissociation of NOT9B from CCR4-NOT in light. We propose that the CCR4-NOT complex is a repressor of light signalling and that binding of phyA to NOT9B leads to partial inactivation of the CCR4-NOT complex to promote photomorphogenesis.
Publications
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Uncovering a novel function of the CCR4-NOT complex in phytochrome A-mediated light signalling in plants. eLife, 10.
Schwenk, Philipp; Sheerin, David J.; Ponnu, Jathish; Staudt, Anne-Marie; Lesch, Klara L.; Lichtenberg, Elisabeth; Medzihradszky, Katalin F.; Hoecker, Ute; Klement, Eva; Viczián, András & Hiltbrunner, Andreas
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Phytochrome A Mediates the Disassembly of Processing Bodies in Far-Red Light. Frontiers in Plant Science, 13.
Schwenk, Philipp & Hiltbrunner, Andreas
