Minimal auxin sensing levels in vegetative moss stem cells revealed by a ratiometric reporter

Efforts to reveal ancestral functions of auxin, a key regulator of plant growth and development, and its importance for evolution have been hampered by a fragmented picture of auxin response domains in early-diverging land plants. We report the mapping of auxin sensing and responses during vegetative moss development using novel reporters. We established a moss-specific ratiometric reporter (PpR2D2) for Auxin Response Element- and AUXIN RESPONSE FACTOR-independent auxin sensing in Physcomitrella patens, and its readout during vegetative development was compared with new promoter-based GmGH3::GFPGUS and DR5revV2::GFPGUS auxin response reporters. The ratiometric reporter responds rapidly to auxin in a time-, dose- and TRANSPORT INHIBITOR RESISTANT1/AUXIN F-BOX-dependent manner and marks known, anticipated and novel auxin sensing domains. It reveals proximal auxin sensing maxima in filamentous tissues and sensing minima in all five vegetative gametophytic stem cell types as well as dividing cells. PpR2D2 readout is compliant with an ancestral function of auxin as a positive regulator of differentiation vs proliferation in stem cell regions. The PpR2D2 reporter is a sensitive tool for high-resolution mapping of auxin sensing, which can increase our knowledge of auxin function in early-diverging land plants substantially, thereby advancing our understanding of its importance for plant evolution.