Retrograde Control of Cytosolic Translation Targets Synthesis of Plastid Proteins and Nuclear Responses for High-Light Acclimation

Canonical retrograde signalling comprises information transmission from organelles to the nucleus and in particular controls gene expression for organellar proteins. The need to re-assess this paradigm was suggested by discrepancies between de novo protein synthesis and transcript abundance in response to excess light. Here we uncover major components of a translation-dependent retrograde signalling pathway that first impacts translation and then transcription. The response realization depends on the kinases Mitogen-activated protein kinase 6 (MPK6) and Sucrose non-fermenting 1-related kinase (SnRK1) subunit, AKIN10. Global ribosome foot-printing revealed differential ribosome association of 951 transcripts within 10 min after transfer from low to high light. Despite predominant translational repression, 15 % of transcripts were increased in translation and enriched for chloroplast-localized photosynthetic proteins. About one third of these transcripts, including Stress associated proteins (SAP) 2 and 3, share regulatory motifs in their 5`-UTR that act as binding sites for glyceraldehyde-3-phosphate dehydrogenase (GAPC) and light responsive RNA binding proteins (RBPs). SAP2 and 3 are both translationally regulated and interact with the calcium sensor Calmodulin-like 49 (CML49), which promotes relocation to the nucleus inducing a translation-dependent nuclear stress response. Thus, translation-dependent retrograde signalling bifurcates to directly regulate a translational circuit of chloroplast proteins and simultaneously initiate a nuclear circuit synchronizing retrograde and anterograde response pathways, serving as a rapid mechanism for functional acclimation of the chloroplast CML49 KO and SAP3 KO 0' and 60' of low light (8µE) to high light (800µE) transfer in comparison to Col-0 As in Oelze et al 2012: sap3, cml49 KO and Col-0 WT plants were grown for 3 weeks under normal light (80µE) before shifting to low light (8µE) for 10 days. Plants were then shifted to high light (800µE) for 60 min and harvested at mid photo period (3 pm) together with low light control (8µE).