Fig. 8

Model for regulation of nitrogen availability response. Our data as well as previous microscopy data suggest that the nitrogen depletion plays a role in chloroplast structure and metabolism [7, 18, 42]. More precisely our data support a two-phase acclimation of the chloroplast during N depletion. A first highlight-like response due to the depletion of stromal protein and supported by the accumulation of LHCII/PSII protein which leads potentially to ROS acclimation, both events could trigger a signal for chloroplast degradation as suggested in Arabidopsis [90]. High phosphorylation levels of CP29 at the early stage of N depletion support those data and could be implicated in the regulation of Fv/Fm [88]. The second step consists in the dismantlement of the chloroplast from 24 h on. Chloroplast dismantlement might involve the BR signaling pathway (see “Results”). Two protein kinases were shown to play a central role, CGK2 and DYRKP. They are connected to central metabolism which shows a clear switch to mitochondrial respiration (a process tightly controlled by protein phosphorylation) and protein translation (tight control by EIF phosphorylation). Data obtained for EIF4B phosphorylation are suggesting a down-regulation of the TOR complex. In this model, we propose that the higher phosphorylation levels observed for NNK1, RPS6, and ATG13 indicate a possible inhibition of the PP2A branch signaling of the TOR pathway by activation of CKIN pathway (CKIN 1, 2, and 3 are the AMPK/SnRK1 orthologs in Chlamydomonas as defined in [6]). This is further suggested by the higher phosphorylation level of CDPKK2, as proposed in yeast [102]. For example, the red-dashed arrow between TOR and RPS6 indicate that TOR indirectly and positively regulates RPS6 by phosphorylation. ROS is reactive oxygen species; PS is photosystem; LHCII is Light-Harvesting complex-II; e represents electron from the electron transfer chain; and LB stands for Lipid Bodies