Class C <scp>ARF</scp>s evolved before the origin of land plants and antagonize differentiation and developmental transitions in <i>Marchantia polymorpha</i>

<jats:title>Summary</jats:title><jats:p> <jats:list list-type="bullet"> <jats:list-item><jats:p>A plethora of developmental and physiological processes in land plants is influenced by auxin, to a large extent via alterations in gene expression by <jats:styled-content style="fixed-case">AUXIN RESPONSE FACTOR</jats:styled-content>s (<jats:styled-content style="fixed-case">ARF</jats:styled-content>s). The canonical auxin transcriptional response system is a land plant innovation, however, charophycean algae possess orthologues of at least some classes of <jats:styled-content style="fixed-case">ARF</jats:styled-content> and AUXIN/INDOLE‐3‐ACETIC ACID (<jats:styled-content style="fixed-case">AUX</jats:styled-content>/<jats:styled-content style="fixed-case">IAA</jats:styled-content>) genes, suggesting that elements of the canonical land plant system existed in an ancestral alga.</jats:p></jats:list-item> <jats:list-item><jats:p>We reconstructed the phylogenetic relationships between streptophyte <jats:styled-content style="fixed-case">ARF</jats:styled-content> and <jats:styled-content style="fixed-case">AUX</jats:styled-content>/<jats:styled-content style="fixed-case">IAA</jats:styled-content> genes and functionally characterized the solitary class C <jats:styled-content style="fixed-case">ARF</jats:styled-content>, Mp<jats:italic><jats:styled-content style="fixed-case">ARF</jats:styled-content>3</jats:italic>, in <jats:italic>Marchantia polymorpha</jats:italic>.</jats:p></jats:list-item> <jats:list-item><jats:p>Phylogenetic analyses indicate that multiple <jats:styled-content style="fixed-case">ARF</jats:styled-content> classes, including class C <jats:styled-content style="fixed-case">ARF</jats:styled-content>s, existed in an ancestral alga. Loss‐ and gain‐of‐function Mp<jats:italic><jats:styled-content style="fixed-case">ARF</jats:styled-content>3</jats:italic> alleles result in pleiotropic effects in the gametophyte, with Mp<jats:italic><jats:styled-content style="fixed-case">ARF</jats:styled-content>3</jats:italic> inhibiting differentiation and developmental transitions in multiple stages of the life cycle. Although loss‐of‐function Mp<jats:italic>arf3</jats:italic> and Mp<jats:italic>mir160</jats:italic> alleles respond to exogenous auxin treatments, strong miR‐resistant Mp<jats:italic><jats:styled-content style="fixed-case">ARF</jats:styled-content>3</jats:italic> alleles are auxin‐insensitive, suggesting that class C <jats:styled-content style="fixed-case">ARF</jats:styled-content>s act in a context‐dependent fashion.</jats:p></jats:list-item> <jats:list-item><jats:p>We conclude that two modules independently evolved to regulate a pre‐existing <jats:styled-content style="fixed-case">ARF</jats:styled-content> transcriptional network. Whereas the auxin‐<jats:styled-content style="fixed-case">TIR</jats:styled-content>1‐<jats:styled-content style="fixed-case">AUX</jats:styled-content>/<jats:styled-content style="fixed-case">IAA</jats:styled-content> pathway evolved to repress class A/B <jats:styled-content style="fixed-case">ARF</jats:styled-content> activity, miR160 evolved to repress class C <jats:styled-content style="fixed-case">ARF</jats:styled-content>s in a dynamic fashion.</jats:p></jats:list-item> </jats:list> </jats:p>