The phytochrome A-specific signaling intermediate SPA1 interacts directly with COP1, a constitutive repressor of light signaling in Arabidopsis

SPA1 is a phytochrome A (phyA)-specific signaling intermediate that acts as a light-dependent repressor of photomorphogenesis in Arabidopsis seedlings . It contains a WD-repeat domain that shows high sequence similarity to the WD-repeat region of the constitutive repressor of light signaling, COP1. H ere, using yeast two-hybrid and in vitro interaction assays, we show that S PA1 strongly and selectively binds to COP1. Domain mapping studies indicate that the putative coiled-coil domain of SPA1 is necessary and sufficient f or binding to COP1. Conversely, similar deletion analyses of the COP1 prote in suggest that SPA1 interacts with the presumed coiled-coil domain of COP1 . To further investigate SPA1 function in the phyA signaling pathway, we te sted whether SPA1, like COP1, mediates changes in gene expression in respon se to light. We show that spa1 mutations increase the photoresponsiveness o f certain light-regulated genes within 2 h of light treatment. Taken togeth er, the results suggest that SPA1 may function to link the phytochrome A-sp ecific branch of the light signaling pathway to COP1. Hence, our data provi de molecular support for the hypothesis that COP1 is a convergence point fo r upstream signaling pathways dedicated to individual photoreceptors.