A mutation in the Arabidopsis mTERF-related plastid protein SOLDAT10 activates retrograde signaling and suppresses (1)O(2)-induced cell death.

The conditional flu mutant of Arabidopsis thaliana generates singlet oxygen ((1)O(2)) in plastids during a dark-to-light shift. Seedlings of flu bleach and die, whereas mature plants stop growing and develop macroscopic necrotic lesions. Several suppressor mutants, dubbed singlet oxygen-linked death activator (soldat), were identified that abrogate (1)O(2)-mediated cell death of flu seedlings. One of the soldat mutations, soldat10, affects a gene encoding a plastid-localized protein related to the human mitochondrial transcription termination factor mTERF. As a consequence of this mutation, plastid-specific rRNA levels decrease and protein synthesis in plastids of soldat10 is attenuated. This disruption of chloroplast homeostasis in soldat10 seedlings affects communication between chloroplasts and the nucleus and leads to changes in the steady-state concentration of nuclear gene transcripts. The soldat10 seedlings suffer from mild photo-oxidative stress, as indicated by the constitutive up-regulation of stress-related genes. Even though soldat10/flu seedlings overaccumulate the photosensitizer protochlorophyllide in the dark and activate the expression of (1)O(2)-responsive genes after a dark-to-light shift they do not show a (1)O(2)-dependent cell death response. Disturbance of chloroplast homeostasis in emerging soldat10/flu seedlings seems to antagonize a subsequent (1)O(2)-mediated cell death response without suppressing (1)O(2)-dependent retrograde signaling. The results of this work reveal the unexpected complexity of what is commonly referred to as 'plastid signaling'.