m⁶A Demethylation in Chloroplasts Enhances Stress Tolerance by Modulating the Stability of Photosynthesis-Associated mRNAs in Arabidopsis.

N⁶-methyladenosine (m⁶A) is the most prevalent epitranscriptomic modification found in eukaryotic mRNA. The levels of m⁶A are determined by the coordinated actions of methyltransferases ('writers') and demethylases ('erasers') that introduce and remove m⁶A marks, respectively. Recent studies have demonstrated that chloroplast mRNAs are highly rich in m⁶A; however, the significance of m⁶A methylation in chloroplasts remains unknown. As no mRNA m⁶A writers and erasers have been identified in chloroplasts, in this study, we artificially imported ALKBH10B, a well-characterised mRNA m⁶A eraser localised in the nucleus and cytoplasm in Arabidopsis (Arabidopsis thaliana), into chloroplasts to uncover the potential impact of m⁶A modification in chloroplasts on photosynthesis and abiotic stress response. We found that the chloroplast-targeted ALKBH10B successfully removes m⁶A marks from numerous mRNAs in chloroplasts. Notably, the ALKBH10B-mediated demethylation f m⁶A marks in chloroplast mRNAs enhanced salt and drought tolerance in Arabidopsis by modulating the stability of m⁶A-modified photosynthesis-associated mRNAs. Overall, our findings reveal that ALKBH10B-mediated m⁶A demethylation in chloroplast mRNAs enhances photosynthesis and stress tolerance, highlighting that modulation of RNA methylation in chloroplasts can be a potential means for breeding stress-tolerant plants.