The epitranscriptomic mark -methyladenosine (mA) can be written, read, and erased via the action of a complex network of proteins. mA binding proteins read mA marks and transduce their downstream regulatory effects by altering RNA metabolic processes. The characterization of mA readers is an essential prerequisite for understanding the roles of mA in plants, but the identities of mA readers have been unclear. Here, we characterized the YTH-domain family protein ECT2 as an mA reader whose mA binding function is required for normal trichome morphology. We developed the formaldehyde cross-linking and immunoprecipitation method to identify ECT2-RNA interaction sites at the transcriptome-wide level. This analysis demonstrated that ECT2 binding sites are strongly enriched in the 3' untranslated regions (3' UTRs) of target genes and led to the identification of a plant-specific mA motif. Sequencing analysis suggested that ECT2 plays dual roles in regulating 3' UTR processing in the nucleus and facilitating mRNA stability in the cytoplasm. Disruption of accelerated the degradation of three ECT2 binding transcripts related to trichome morphogenesis, thereby affecting trichome branching. The results shed light on the underlying mechanisms of the roles of mA in RNA metabolism, as well as plant development and physiology.

The biological functions of the epitranscriptomic modification N 6 -methyladenosine (m 6 A) in plants are not fully understood. CPSF30-L is a predominant isoform of the polyadenylation factor CPSF30 and consists of CPSF30-S and an m 6 A-binding YTH domain. Little is known about the biological roles of CPSF30-L and the molecular mechanism underlying its m 6 A-binding function in alternative polyadenylation. Here, we characterized CPSF30-L as an Arabidopsis m 6 A reader whose m 6 A-binding function is required for the floral transition and abscisic acid (ABA) response. We found that the m 6 A-binding activity of CPSF30-L enhances the formation of liquid-like nuclear bodies, where CPSF30-L mainly recognizes m 6 A-modified far-upstream elements to control polyadenylation site choice. Deficiency of CPSF30-L lengthens the 3 0 untranslated region of three phenotypes-related transcripts, thereby accelerating their mRNA degradation and leading to late flowering and ABA hypersensitivity. Collectively, this study uncovers a new molecular mechanism for m 6 A-driven phase separation and polyadenylation in plants.

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