Control of cell fate specification and patterning by an ancestral microRNA

ABSTRACT The formation of an organized body requires the establishment and maintenance of cells with structural and functional distinctive characteristics. A central question in developmental biology is how changes in the regulation of genes drive cell specification and patterning 1 . microRNAs (miRNAs) are small non-coding RNAs that regulate development through mRNA cleavage and/or translational repression 2 . In plants, miRNAs regulate key aspects including growth, development, stem cell maintenance, vegetative phase change, leaf morphogenesis, floral organ formation and flowering time 3 . Biogenesis of plant miRNAs depends on the activity of DICER-LIKE 1 (DCL1), an RNase type III endonuclease that processes double stranded RNA to give rise to mature miRNAs 4 . The genomes of today’s flora contain at least one bona fide copy of DCL1 5,6 . Using Marchantia polymorpha -a model bryophyte that allows comparative approaches to infer characteristics of the ancestral land plant-, we demonstrate that Mp DCL1a is required for the biogenesis of miRNAs and uncovered a central role for miR166/Homeodomain Zipper Class III-regulated auxin synthesis in the specification of cell identity, patterning, meristem function, laminar expansion and the development of the body in the last common ancestor of extant land plants.