| SPA1(suppressor of phytochrome A-1051) is one of the three major photomorphogenesis repressors in Arabidopsis. Its over-expression (SPA1-OE) results in hyper-etiolation phenotypes in far-red, red and blue lights. phytochrome B (phyB) is the main photorecepter of red light, and controls primarily shade avoidance responses under low R:FR. Negative interference of phyB with phyA mediating far-red light signaling has been reported, but the mechanism lays largely unknown. Repression of photomorphogenesis in far-red light and promotion of shade avoidance responses under low R:FR by phyB have been investigated in this report using genetics, biochemical and molecular biological methods.In far-red light, over-expression of phyB leads to hyper-etiolation, while phyB-9mutant showing enhanced photomorphogenesis. phyB functions are covered up by phyA and spa1mutants, indicating that Arabidopsis phytochrome B represses phyA signaling depending on both of SPA1and phyA. The expression and protein levels of SPA1gene are facilitated by phyB in dark and far-red with down-regulation of phyA. Therefore, phyB up-regulates SPA1exrpression to erase phyA accumulation in far-red pathway.SPA1-OE displys more stronger shade avoidance responses in phyA-211, phyB-9and cry1-304mutant backgrounds. Interactions between SPA1with phyA, phyB, or cry1are respectively verified using yeast two hybrid assays. The phyB accumulates more protein in both of spal and cop1mutant backgrounds, indicating that SPA1/COP1E3complex may mediate the degradation of phyB protein through26S proteasome pathway. SPA1was conformed a crucial role in regulation of nuclear translocation of COP1via comparison of GUS-COP1activities in wild type, spa1-3and SPA1-OE backgrounds. Taken together, SPA1regulates the nuclear accumulation of COP1to promotes the degradation of phyA, phyB and cry1in low R:FR. Based on microarray data, SPA1may promote shade avoidance responses involving up-regulation of numbers of F-box and auxin signaling-related genes.
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