From COP1 Cofactors to Candidate Signaling Hubs: Emerging Roles of SPA Proteins in Plant Growth and Environmental Responses

SUPPRESSOR OF PHYA-105 (SPA) proteins bind the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC 1 (COP1) and, within the COP1/SPA complex, repress light signaling in plants. For two decades, SPA proteins were regarded mainly as COP1 cofactors. Since 2019, Huq and colleagues have reported that recombinant SPA1 phosphorylates PHYTOCHROME INTERACTING FACTOR 1 (PIF1), PIF4, ELONGATED HYPOCOTYL 5 (HY5) and eukaryotic translation initiation factor 2α (eIF2α) in vitro. Here, we review the SPA family, from its evolution and domain architecture to its regulation by phytochromes, cryptochromes and UVR8, and we grade the evidence for each proposed function as established, reported, inferred or hypothetical. We separate in vitro phosphorylation from in vivo SPA dependence and catalytic validation, and SPA-specific genetic evidence from inferences based on COP1 or HY5. We then relate these mechanisms to de-etiolation, shade avoidance, thermomorphogenesis, photoperiodic flowering, circadian timing and anthocyanin accumulation, and we assess soybean SPA-like alleles that shorten internodes and have been associated with higher yield under dense planting in field trials. We propose, as a working hypothesis, that the SPA subunit is the signal-integrating module of the COP1/SPA complex, and we identify the experiments needed to test this model.