NO GAMETOPHORES 2is a novel regulator of the 2D to 3D growth transition in the mossPhyscomitrium patens

The colonization of land by plants was one of the most transformative events in the history of life on Earth. The transition from water, which coincided with and was likely facilitated by the evolution of 3-dimensional (3D) growth, enabled the generation of morphological diversity on land. In many plants, the transition from 2-dimensional (2D) to 3D growth occurs during embryo development. However, in the early divergent mossPhyscomitrium patens(formerlyPhyscomitrella patens), 3D growth is preceded by an extended filamentous phase that can be maintained indefinitely. Here, we describe the identification of the cytokinin-responsiveNO GAMETOPHORES 2(PpNOG2) gene, which encodes a shikimate o- hydroxycinnamoyltransferase. In mutants lackingPpNOG2function, transcript levels ofCLAVATAandSCARECROWgenes are significantly reduced, excessive gametophore initial cells are produced, and buds undergo premature developmental arrest. Our results suggest that PpNOG2 functions in the ascorbic acid pathway leading to cuticle formation, and that NOG2-related genes were co-opted into the lignin biosynthesis pathway after the divergence of bryophytes and vascular plants. We present a revised model of 3D growth in which PpNOG2 comprises part of a feedback mechanism that is required for the modulation of gametophore initial cell frequency. We also propose that the 2D to 3D growth transition inP. patensis underpinned by complex auxin-cytokinin crosstalk that is regulated, at least in part, by changes in flavonoid metabolism.