Messenger RNA 5' NAD+ Capping Is a Dynamic Regulatory Epitranscriptome Mark That Is Required for Proper Response to Abscisic Acid in Arabidopsis.

Although eukaryotic messenger RNAs (mRNAs) normally possess a 5' end N7-methyl guanosine (m7G) cap, a non-canonical 5' nicotinamide adenine dinucleotide (NAD+) cap can tag certain transcripts for degradation mediated by the NAD+ decapping enzyme DXO1. Despite this importance, whether NAD+ capping dynamically responds to specific stimuli to regulate eukaryotic transcriptomes remains unknown. Here, we reveal a link between NAD+ capping and tissue- and hormone response-specific mRNA stability. In the absence of DXO1 function, transcripts displaying a high proportion of NAD+ capping are instead processed into RNA-dependent RNA polymerase 6-dependent small RNAs, resulting in their continued turnover likely to free the NAD+ molecules. Additionally, the NAD+-capped transcriptome is significantly remodeled in response to the essential plant hormone abscisic acid in a mechanism that is primarily independent of DXO1. Overall, our findings reveal a previously uncharacterized and essential role of NAD+ capping in dynamically regulating transcript stability during specific physiological responses.