Control of cell fate specification and patterning by an ancestral microRNA

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.