Cryptochrome 2 is necessary for promoting a blue light-independent chloroplast signal to control cellular degradation

Abstract

To thrive in dynamic environments, plants can use their photosynthetic plastid (chloroplast) organelles to sense environmental changes and relay this information to the cell. This signaling can be initiated within chloroplasts by the accumulation of reactive oxygen species (ROS) such as singlet oxygen (1O2), which can accumulate under stress and initiate retrograde (chloroplast-to-nucleus) signaling, chloroplast degradation, and programmed cell death (PCD). However, the mechanisms driving these signals, and the relationship between gene expression and cellular degradation, are unknown. To understand this physiology, here we use the Arabidopsis thaliana plastid ferrochelatase 2 (fc2) mutant, which conditionally produces chloroplast 1O2 and initiates signaling. We show that the blue light photoreceptor cryptochrome 2 (CRY2) is necessary to promote 1O2-induced chloroplast degradation and PCD in the fc2 mutant and that cry2 blocks cellular degradation without affecting bulk 1O2 levels, suggesting that signaling, rather than photo-oxidative stress, has been blocked. Furthermore, a global RNA transcript analysis demonstrated that the impact of the cry2 mutation on retrograde signaling is limited and that the effect of 1O2 on nuclear gene expression and cellular degradation can be uncoupled. Under permissive conditions, fc2 cry2 mutants exhibited constitutive expression of photo-oxidative stress genes, suggesting that cry2 may lead to transcriptional priming to photo-oxidative stress. Finally, we demonstrate that this function of CRY2 is independent of blue light-signaling and that CRY2 isoforms lacking canonical blue light signaling capabilities restore 1O2-induced PCD to fc2 cry2. Together, these results reveal a blue light-independent role for CRY2 in chloroplast signaling and stress acclimation pathways.