The Arabidopsis COP1/SPA complex is a key repressor of photomorphogenesis that suppresses light signaling in the dark. Both COP1 and SPA proteins are essential components of this complex. Although COP1 also exists in humans, SPA genes are specific to the green lineage.To elucidate the evolution of SPA genes we analyzed SPA functions in the moss Physcomitrella patens by characterizing knockout mutants in the two Physcomitrella SPA genes PpSPAa and PpSPAb.Light-grown PpspaAB double mutants exhibit smaller gametophores than the wild-type. In the dark, PpspaAB mutant gametophores show enhanced continuation of growth but etiolate normally. Gravitropism in the dark is reduced in PpspaAB mutant protonemata. The expression of light-regulated genes is mostly not constitutive in PpspaAB mutants. PpSPA and PpCOP1 interact; PpCOP1 also interacts with the transcription factor PpHY5 and, indeed, PpHY5 is destabilized in dark-grown Physcomitrella. Degradation of PpHY5 in darkness, however, does not require PpSPAa and PpSPAb.The data suggest that COP1/SPA-mediated light signaling is only partially conserved between Arabidopsis and Physcomitrella. Whereas COP1/SPA interaction and HY5 degradation in darkness is conserved, the role of SPA proteins appears to have diverged. PpSPA genes, unlike their Arabidopsis counterparts, are only required to suppress a subset of light responses in darkness.

SUMMARY The Arabidopsis COP1/SPA ubiquitin ligase suppresses photomorphogenesis in darkness. In the light, photoreceptors inactivate COP1/SPA to allow a light response. While SPA genes are specific to the green lineage, COP1 also exists in humans. This raises the question of when in evolution plant COP1 acquired the need for SPA accessory proteins. We addressed this question by generating Physcomitrium Ppcop1 mutants and comparing their visible and molecular phenotypes with those of Physcomitrium Ppspa mutants. The phenotype of Ppcop1 nonuple mutants resembles that of Ppspa mutants. Most importantly, both mutants produce green chloroplasts in complete darkness. They also exhibit dwarfed gametophores, disturbed branching of protonemata and absent gravitropism. RNA‐sequencing analysis indicates that both mutants undergo weak constitutive light signaling in darkness. PpCOP1 and PpSPA proteins form a complex and they interact via their WD repeat domains with the VP motif of the cryptochrome CCE domain in a blue light‐dependent manner. This resembles the interaction of Arabidopsis SPA proteins with Arabidopsis CRY1, and is different from that with Arabidopsis CRY2. Taken together, the data indicate that PpCOP1 and PpSPA act together to regulate growth and development of Physcomitrium. However, in contrast to their Arabidopsis orthologs, PpCOP1 and PpSPA proteins execute only partial suppression of light signaling in darkness. Hence, additional repressors may exist that contribute to the repression of a light response in dark‐exposed Physcomitrium.