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

Plants colonized the land approximately 470 million years ago, coinciding with the development of apical cells that divide in three planes. The molecular mechanisms that underly the development 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 (m⁶ 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, m⁶ 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 m⁶ A methyltransferase complex (MTC), MTA, MTB and FIP37, in P. patens and demonstrate that their inactivation leads to the loss of m⁶ 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-PpAPB4 transcripts, encoding central factors orchestrating the transition from 2D to 3D growth in P. patens, are modified by m⁶ A, whereas in the Ppmta mutant the lack of the m⁶ A marker is associated with a corresponding decrease in transcript accumulation. Overall, we suggest that m⁶ 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.