A Clade-D Auxin Response Factor is a Major Regulator of Auxin Signaling in Physcomitrium patens.

Auxin Response Factors (ARFs) are a family of transcription factors that are responsible for regulating gene expression in response to changes in auxin level. The analysis of ARF sequence and activity indicates that there are two major groups-activators and repressors. One clade of ARFs, clade-D, is sister to clade-A activating ARFs, but are unique in that they lack a DNA binding domain. Clade-D ARFs are present in lycophytes and bryophytes but absent in pteridophytes and spermatophytes. The transcriptional activity of clade-D ARFs, as well as how they regulate gene expression, is not well understood. Here, we report that clade-D ARFs are transcriptional activators in the model bryophyte P. patens and have a major role in the development of this species. {Delta}arfd1,d2 protonemata exhibit a delay in filament branching, as well as a delay in a key cell differentiation event. Additionally, leafy gametophore development in {Delta}arfd1,d2 lines lag behind wild-type. We present evidence that ARFd1 interacts with activating, but not repressing, ARFs. An ARFd1 hypomorph, arfd1T653L, cannot multimerize. Therefore, we propose a model by which clade-D ARFs enhance gene expression by oligomerizing to DNA-bound archetypal ARFs.