N6-Methyladenosine modification of mRNA contributes to the transition from 2D to 3D growth in the moss Physcomitrium patens.

Plants colonized the land ca. 470 million years ago, coinciding with the development of apical cells that divide in three planes. The molecular mechanisms that underly the regulation of the 3D growth pattern are poorly understood, mainly because 3D growth in seed plants starts during embryo development. In contrast, the transition from 2D to 3D growth in the moss Physcomitrium patens has been widely studied and it involves a large turnover of the transcriptome to allow the establishment of stage-specific transcripts that facilitate this developmental transition. N6-methyladenosine (m 6 A) is the most abundant, dynamic, and conserved internal nucleotide modification present on eukaryotic mRNA and serves as a layer of post-transcriptional regulation directly affecting several cellular processes and developmental pathways in many organisms. In Arabidopsis, m 6 A has been reported to be essential for organ growth and determination, embryo development and responses to environmental signals. In this study, we identified the main genes of the m 6 A methyltransferase complex (MTC) MTA, MTB and FIP37, in P. patens and demonstrate that their inactivation leads to loss of m 6 A in mRNA, a delay in the formation of gametophore buds, and defects in spore development. Genome-wide analysis revealed several transcripts affected in the Ppmta background. We demonstrate that the PpAPB1-4 transcripts, encoding for central factors regulating the 2D to 3D transition in P. patens, are m 6 A-modified, while in the Ppmta mutant the lack of the m 6 A mark is associated with a corresponding decrease in transcript accumulation. Overall, we suggest that m 6 A is essential to enable the proper accumulation of these and other bud-specific transcripts directing the turnover of stage-specific transcriptomes, and thus promoting the transition from protonema to gametophore buds in P. patens.