Figure 5.

Schematic depiction of the GLK-BCM pathway that regulates Chl and LHCP levels in response to environmental factors in Arabidopsis. BCM is required for full activation of GUN4/5 (Chl synthesis) and destabilization of the SGR protein (repression of Chl degradation; Wang et al., 2020). GLKs control the amount of LHCP (LHCI/LHCII) via direct transcriptional regulation of (i) LHCP apoprotein genes and (ii) BCM genes, regulating levels of Chl and stabilizing LHCP. During dark-induced leaf senescence, the reduction in BCM expression caused by the rapid decrease in GLK expression in response to dark treatment is also thought to play a role in the onset of leaf yellowing. Meanwhile, expression of BCM2 is upregulated during the late stage of leaf senescence, thereby fine-tuning the Chl content.