Fig. 1: DAY promotes cotyledon development in the light and BR signaling in the dark.

a Cotyledon opening responses of dexamethasone (DEX)-inducible DAY RNAi (DAYi) transgenic lines #1 and #2. Representative images of cotyledons from seedlings grown in the dark for 5 days in the presence (+) or absence (−) of 1 µM DEX and then exposed to light (D → L) for either 6 or 24 h. Scale bar, 0.1 cm. b, DAY mRNA and protein levels are significantly decreased in DAYi plants upon DEX treatment. Seedlings were grown in the light for 7 days in 1/2 MS medium containing ethanol (EtOH) or 1 µM DEX. RT-qPCR was used to determine DAY transcript levels in the knockdown plants (upper). Transcript levels of DAY were normalized relative to PP2A A3 mRNA. Error bars indicate SD from three technical replicates. Statistical significance was determined by unpaired, two-tailed Student’s t-test. **P ≤ 0.01. Immunoblotting with anti-DAY antibody showed reduced DAY protein levels after DEX treatment (lower). The asterisk indicates a minor isoform of DAY that is consistently detected. DAY iso 1 and DAY iso 2 refer to major isoforms of DAY. c DAY promotes the accumulation of POR in the dark and CHLOROPHYLL A/B BINDING (CAB) proteins during the dark-to-light transition. Immunoblot analyses of seedlings (+) or (−) 1 µM DEX were exposed to light (D → L) for either 6 or 24 h. POR protein levels are very high in the dark in etioplasts, but significantly reduced upon illumination by protein degradation. Anti-POR, anti-CAB, and anti-cpHSP70 antibodies were used to analyze lysates from the two independent transgenic lines indicated on the top. d Reduction of DAY dosage phenocopies a BR signaling mutant. Morphology of 5-day-old dark-grown seedlings of wild-type, bri1-301, DAYi #1 and #2 lines grown on medium containing 1 µM DEX in the absence or presence of 250 nM propiconazole (PPZ), 50 nM brassinolide (BL), or 30 µM bikinin is shown. Scale bars for all pictures, 1 cm. e, f Hypocotyl length of 5-day-old dark-grown DAYi seedlings in the absence (EtOH) or presence of 1 µM DEX at increasing PPZ concentrations (e). Length of 5-day-old dark-grown seedlings on medium containing 1 µM DEX in the absence or presence of 50 nM brassinolide (BL) or 30 µM bikinin (f). Hypocotyl lengths are expressed relative to those of mock-treated samples. Values are means ± SE (n = 10 plants for each treatment). Statistical significance was determined by unpaired, two-tailed Student’s t-test. **P ≤ 0.01. g A model for DAY interplay with BRI1 and POR in the dark and light, respectively.