Phytochrome higher order mutants reveal a complex set of light responses in the moss Physcomitrium patens

O_LIPhytochromes are photoreceptors enabling plants to respond to various light conditions. Independent gene duplication events resulted in small phytochrome gene families in mosses, ferns, and seed plants. This phytochrome diversity is hypothesised to be critical for sensing and adapting to different light conditions, but experimental evidence for this idea is lacking for mosses and ferns. C_LIO_LIThe model moss species Physcomitrium patens contains seven phytochromes grouped into three clades, PHY1/3, PHY2/4, and PHY5. Here, we used CRISPR/Cas9 generated single and higher order mutants to investigate their role in light-regulation of protonema and gametophore growth, protonema branching, and induction of gametophores. C_LIO_LIWe found both specific and partially overlapping roles for the three clades of moss phytochromes in regulating these responses in different light conditions, and we identified a mechanism for sensing simulated canopy shade different from the mechanism in seed plants. PHY1/3 clade phytochromes act as primary far-red light receptors, while PHY5 clade phytochromes are the primary red light receptors. PHY2/4 clade phytochromes have functions in both red and far-red light. C_LIO_LISimilar to seed plants, gene duplication events in the phytochrome lineage in mosses were followed by functional diversification into red and far-red light sensing phytochromes. C_LI