| Summary: | Evolution of developmental genes is an important mechanism for plant morphological evolution. The LRL genes are an ancient group of bHLH transcription factors that positively regulate root hair development in angiosperms. Here I show that, in the moss <em>Physcomitrella patens</em>, two LRL genes are present and they positively regulate rhizoid and caulonema (a rhizoid-like cell type) development. GUS-transcriptional reporter plants show that both <em>Pp</em>LRL1 and <em>Pp</em>LRL2 are expressed in tissues giving rise to caulonemata. Loss-of-function mutants in either <em>Pp</em>LRL1 or <em>Pp</em>LRL2 led to defective rhizoid and caulonema development, and the <em>Pplrl1</em> <em>Pplrl2</em> double loss-of-function mutants completely lack rhizoids and caulonemata. Consistent with this, gain-of-function mutants show enhanced rhizoid and caulonema development. In addition, I show that the stimulatory effects of auxin and low phosphate on the development of rhizoids and/or caulonemata required <em>Pp</em>LRL gene function. Together, these results show that LRL genes are conserved, positive regulators in tip-growing rooting cell development in land plants. To elucidate whether LRL genes belong to part of a conserved gene network, I use qRT-PCR to determine the transcriptional interaction between LRL genes and the Class I RSL genes, which is another group of conserved regulators of rhizoids and root hairs. Comparing the LRL-RSL network between <em>P. patens</em> and <em>A. thaliana</em> reveals that LRL and Class I RSL genes are transcriptionally independent of each other in <em>P. patens</em> but one LRL gene is transcriptionally downstream of Class I RSL genes in <em>A. thaliana</em>. This suggests that the gene network controlling tip-growing rooting cell development has changed since mosses and angiosperms last shared a common ancestor.
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