Arabidopsis DXO1- and RNMT1-Mediated m⁷G Capping of mRNA Modulates ABA Response

Eukaryotic mRNA carries an N7-methylguanosine (m⁷G) 5' cap that is essential for mRNA stability and function. Although traditionally viewed as a constitutive process, m⁷G capping is increasingly recognized as dynamically regulated to control gene expression in response to environmental stimuli. The recent discovery of NAD-capped RNAs adds further complexity to gene regulation through RNA capping. In Arabidopsis, DXO1, an enzyme with NAD cap decapping and exonuclease activities, was recently shown to also promote m⁷G capping through its non-catalytic N-terminal domain, which activates the methyltransferase RNMT1. Here, we show that DXO1's role in abscisic acid (ABA)response and stress tolerance depends primarily on the non-catalytic domain's function in m⁷G capping, with minimal contribution from its enzymatic functions. RNMT1 mutants similarly exhibit reduced ABA sensitivity, mirroring dxo1 phenotypes. Transcriptome analysis reveals that DXO1's and RNMT1's functions in m⁷G capping predominantly regulate genes involved in abiotic stress, defense, light signaling, photosynthesis, and transcription. The enzymatic domain of DXO1 modestly influences the expression of a subset of these m⁷G capping-regulated genes, suggesting coordinated regulation between its m⁷G capping and enzymatic functions. Overall, our study uncovers an important role for m⁷G capping in gene regulation in response to ABA and environmental stimuli.