N⁶-Methyladenosine is a ubiquitous modification identified in the mRNA of numerous eukaryotes, where it is present within both coding and noncoding regions. However, this base modification does not alter the coding capacity, and its biological significance remains unclear. We show that Arabidopsis thaliana mRNA contains N⁶-methyladenosine at levels similar to those previously reported for animal cells. We further show that inactivation of the Arabidopsis ortholog of the yeast and human mRNA adenosine methylase (MTA) results in failure of the developing embryo to progress past the globular stage. We also demonstrate that the arrested seeds are deficient in mRNAs containing N⁶-methyladenosine. Expression of MTA is strongly associated with dividing tissues, particularly reproductive organs, shoot meristems, and emerging lateral roots. Finally, we show that MTA interacts in vitro and in vivo with At FIP37, a homolog of the Drosophila protein FEMALE LETHAL2D and of human WILMS9 TUMOUR1-ASSOCIATING PROTEIN. The results reported here provide direct evidence for an essential function for N⁶-methyladenosine in a multicellular eukaryote, and the interaction with At FIP37 suggests possible RNA processing events that might be regulated or altered by this base modification.

Summary: N6‐adenosine methylation (m⁶A) of mRNA is an essential process in most eukaryotes, but its role and the status of factors accompanying this modification are still poorly understood. Using combined methods of genetics, proteomics and RNA biochemistry, we identified a core set of mRNA m⁶A writer proteins in Arabidopsis thaliana. The components required for m⁶A in Arabidopsis included MTA, MTB, FIP37, VIRILIZER and the E3 ubiquitin ligase HAKAI. Downregulation of these proteins led to reduced relative m⁶A levels and shared pleiotropic phenotypes, which included aberrant vascular formation in the root, indicating that correct m⁶A methylation plays a role in developmental decisions during pattern formation. The conservation of these proteins amongst eukaryotes and the demonstration of a role in writing m⁶A for the E3 ubiquitin ligase HAKAI is likely to be of considerable relevance beyond the plant sciences.

We previously showed that the N6-methyladenosine (m⁶A) mRNA methylase is essential during Arabidopsis thaliana embryonic development. We also demonstrated that this modification is present at varying levels in all mature tissues. However, the requirement for the m⁶A in the mature plant was not tested. Here we show that a 90% reduction in m⁶A levels during later growth stages gives rise to plants with altered growth patterns and reduced apical dominance. The flowers of these plants commonly show defects in their floral organ number, size, and identity. The global analysis of gene expression from reduced m⁶A plants show that a significant number of down-regulated genes are involved in transport, or targeted transport, and most of the up-regulated genes are involved in stress and stimulus response processes. An analysis of m⁶A distribution in fragmented mRNA suggests that the m⁶A is predominantly positioned toward the 3′ end of transcripts in a region 100–150 bp before the poly(A) tail. In addition to the analysis of the phenotypic changes in the low methylation Arabidopsis plants we will review the latest advances in the field of mRNA internal methylation