First contact with greater gravity: Moss plants adapted via enhanced photosynthesis mediated by AP2/ERF transcription factors.

The emergence of land plants required adaptations to altered water availability and increased effective gravity. Bryophytes underwent major changes in physiology, anatomy, and growth during their emergence onto land. However, the link between gravity, photosynthesis, and genetic control remains unclear. Here, we show that leaf carbon dioxide diffusion enhanced photosynthesis in the model moss Physcomitrium patens under increased gravity (6 and 10 times Earth's gravity), driven by increases in plant (gametophore) number and chloroplast size. RNA sequencing analysis showed that 10 gravity up-regulated several species-specific APETALA2/ethylene-responsive factor (AP2/ERF) transcription factors. Overexpression of one such AP2/ERF, ISSUNBOSHI1 (IBSH1; gene ID = Pp3c1_32440), in P. patens phenocopied plants grown at 10 gravity, and the dominant repressor form of IBSH1 suppressed 10 gravity responses. These results provide evidence that the proliferation of AP2/ERF transcription factors and the establishment of a notable gene network may have been important in adaptation to the terrestrial environment during land plant evolution.