Search

N-terminal (Nt) acetylation (NTA) is an ample and irreversible cotranslational protein modification catalyzed by ribosome-associated Nt-acetyltransferases. NTA on specific proteins can act as a degradation signal (called an Ac/N-degron) for proteolysis in yeast and mammals. However, in plants, the biological relevance of NTA remains largely unexplored. In this study, we reveal that Arabidopsis (Arabidopsis thaliana) SIGMA FACTOR-BINDING PROTEIN1 (SIB1), a transcription coregulator and a positive regulator of salicylic acid-primed cell death, undergoes an absolute NTA on the initiator Met; Nt-acetyltransferase B (NatB) partly contributes to this modification. While NTA results in destabilization of certain target proteins, our genetic and biochemical analyses revealed that plant NatB-involved NTA instead renders SIB1 more stable. Given that the ubiquitin/proteasome system stimulates SIB1 degradation, it seems that the NTA-conferred stability ensures the timely expression of SIB1-dependent genes, mostly related to immune responses. Taking our findings together, here we report a noncanonical NTA-driven protein stabilization in land plants.

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'.

When plant cells are under environmental stress, several chemically distinct reactive oxygen species (ROS) are generated simultaneously in various intracellular compartments and these can cause oxidative damage or act as signals. The conditional flu mutant of Arabidopsis, which generates singlet oxygen in plastids during a dark-to-light transition, has allowed the biological activity of singlet oxygen to be determined, and the criteria to distinguish between cytotoxicity and signalling of this particular ROS to be defined. The genetic basis of singlet-oxygen-mediated signalling has been revealed by the mutation of two nuclear genes encoding the plastid proteins EXECUTER (EX)1 and EX2, which are sufficient to abrogate singlet-oxygen-dependent stress responses. Conversely, responses due to higher cytotoxic levels of singlet oxygen are not suppressed in the ex1/ex2 background. Whether singlet oxygen levels lower than those that trigger genetically controlled cell death activate acclimation is now under investigation.