Key message We mutated all seven Physcomitrium (Physcomitrella) patens phytochrome genes using highly-efficient CRISPR-Cas9 procedures. We thereby identified phy5a as the phytochrome primarily responsible for inhibiting gravitropism, proving the utility of the mutant library. Abstract The CRISPR-Cas9 system is a powerful tool for genome editing. Here we report highly-efficient multiplex CRISPR-Cas9 editing of the seven-member phytochrome gene family in the model bryophyte Physcomitrium (Physcomitrella) patens. Based on the co-delivery of an improved Cas9 plasmid with multiple sgRNA plasmids and an efficient screening procedure to identify high-order multiple mutants prior to sequencing, we demonstrate successful targeting of all seven PHY genes in a single transfection. We investigated further aspects of the CRISPR methodology in Physcomitrella, including the significance of spacing between paired sgRNA targets and the efficacy of NHEJ and HDR in repairing the chromosome when excising a complete locus. As proof-of-principle, we show that the septuple phy− mutant remains gravitropic in light, in line with expectations, and on the basis of data from lower order multiplex knockouts conclude that phy5a is the principal phytochrome responsible for inhibiting gravitropism in light. We expect, therefore, that this mutant collection will be valuable for further studies of phytochrome function and that the methods we describe will allow similar approaches to revealing specific functions in other gene families.

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Phytochromes are the principle photoreceptors in light-regulated plant development, primarily acting via translocation of the light-activated photoreceptor into the nucleus and subsequent gene regulation. However, several independent lines of evidence indicate unambiguously that an additional cytoplasmic signaling mechanism must exist. Directional responses in filament tip cells of the moss Physcomitrella patens are steered by phy4 which has been shown to interact physically with the blue light receptor phototropin at the plasma membrane. This complex might perceive and transduce vectorial information leading to cytoskeleton reorganization and finally a directional growth response. We developed yeast two-hybrid procedures using photochemically functional, full-length phy4 as bait in Physcomitrella cDNA library screens and growth assays under different light conditions, revealing Pfr-dependent interactions possibly associated with phytochrome cytoplasmic signaling. Candidate proteins were then expressed in planta with fluorescent protein tags to determine their intracellular localization in darkness and red light. Of 14 candidates, 12 were confirmed to interact with phy4 in planta using bimolecular fluorescence complementation. We also used database information to study their expression patterns relative to those of phy4. We discuss the likely functional characteristics of these holophytochrome-interacting proteins (HIP’s) and their possible roles in signaling.

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