An mA-YTH Module Controls Developmental Timing and Morphogenesis in Arabidopsis.

Methylation of -adenosine (mA) in mRNA is an important posttranscriptional gene regulatory mechanism in eukaryotes. mA provides a binding site for effector proteins ("readers") that influence pre-mRNA splicing, mRNA degradation, or translational efficiency. YT521-B homology (YTH) domain proteins are important mA readers with established functions in animals. Plants contain more YTH domain proteins than other eukaryotes, but their biological importance remains unknown. Here, we show that the cytoplasmic YTH domain proteins EVOLUTIONARILY CONSERVED C-TERMINAL REGION2/3 (ECT2/3) are required for the correct timing of leaf formation and for normal leaf morphology. These functions depend fully on intact mA binding sites of ECT2 and ECT3, indicating that they function as mA readers. Mutation of the close homolog, , enhances the delayed leaf emergence and leaf morphology defects of mutants, and all three ECT proteins are expressed at leaf formation sites in the shoot apex of young seedlings and in the division zone of developing leaves. ECT2 and ECT3 are also highly expressed at early stages of trichome development and are required for trichome morphology, as previously reported for mA itself. Overall, our study establishes the relevance of a cytoplasmic mA-YTH regulatory module in the timing and execution of plant organogenesis.