PHYTOCHROME INTERACTING FACTORs from Physcomitrella patens are active in Arabidopsis and complement the pif quadruple mutant.

Phytochromes are red/far-red light receptors in plants involved in the regulation of growth and development in response to changes in the ambient environment. An important mode of action of plant phytochromes depends on their light-regulated relocation from the cytosol into the nucleus and control of gene expression; in addition, there is also evidence for a cytosolic or plasma membrane associated function of phytochromes in different species. The PHYTOCHROME INTERACTING FACTORs (PIFs) form a subgroup of the bHLH transcription factors and it is well established that PIFs are key components of phytochrome downstream signalling in the nucleus of seed plants. Recent studies identified members of the PIF family also in the liverwort Marchantia polymorpha and the moss Physcomitrella patens. Here, we show that all four potential PIF homologs from Physcomitrella have PIF function when expressed in the Arabidopsis pifQ mutant, which is deficient in multiple PIFs. We propose that PIFs are ancient components of nuclear phytochrome signalling that have emerged in the last common ancestor of today's land plants.