The PLETHORA Homolog In Marchantia polymorpha is Essential To Meristem Maintenance, Developmental Progression, And Redox Homeostasis.

To adapt to a terrestrial habitat, the ancestors of land plants must make several morphological and physiological modifications, such as a meristem allowing for three-dimensional growth, rhizoids for water and nutrient uptake, air pore complexes or stomata that permit air exchange, and a defense system to cope with oxidative stress that occurs frequently in a terrestrial habitat. To understand how meristem is determined during land plant evolution, we characterized the function of the closest PLETHORA homolog in the liverwort Marchantia polymorpha, which we named MpPLT. Through transgenic approach, we showed that MpPLT is expressed not only in the stem cells at the apical notch but also in the proliferation zone of the meristem, as well as cells that form the air-pore complex and rhizoids. Using the CRISPR method we then created mutants for MpPLT and found that the mutants are not only defective in meristem maintenance but also compromised in air-pore complex and rhizoid development. Strikingly, at later developmental stages, numerous gemma-like structures were formed in Mpplt mutants, suggesting developmental arrest. Further experiments indicate that MpPLT promotes plant growth by regulating MpWOX, which shared a similar expression pattern as MpPLT, and genes involved in auxin and cytokinin signaling pathways. Through transcriptome analyses, we found that MpPLT also has a role in redox homeostasis and that this role is essential to plant growth. Together, these results suggest that MpPLT has a crucial role in liverwort growth and development and hence may have played a crucial role in early land plant evolution.