Fig. 4.

The nitrate upshift enhances the expression of selected SAUR genes. Seedlings transferred from 0.5 mM to 5.0 mM nitrate 1 h after the beginning of the photoperiod of day 4 are compared to the controls that remained on 0.5 mM nitrate. (A) Genes with expression enhanced (Left) or reduced (Right) by the nitrate upshift in the wild type and the associated overrepresented gene ontology (GO) terms. Each datum point is the average expression of one gene in Col-0 or pif4 under control or nitrate upshift conditions (three biological replicates, RNA sequencing, Dataset S1). (B) Nuclear fluorescence driven by the pDR5:GFP transgene measured by confocal microscopy 3 h after the nitrate upshift. (C and D) DII-VENUS measured by confocal microscopy in cotyledon (C) or hypocotyl (D) cells 1.5 h after the nitrate upshift. (E) GUS staining in the hypocotyl driven by the pSAUR21:GUS transgene, 3.5 h after the shift in nitrate. (F) Correlation between normalized SAUR gene expression and normalized hypocotyl growth across genotypes and nitrate conditions. Each datum point corresponds to the expression in one transcriptome experiment and the growth of the hypocotyl under the same condition during the same experiment (circles: Col-0; triangles: pif4). The significance of the correlation is indicated. The ratio between the response to the nitrate upshift in pif4 and Col-0 was calculated for each experiment and the mean ± SE is indicated. (G–I) The nlp7 mutation reduced the effect of the nitrate on SAUR6 (G) and SAUR67 (H) but not SAUR21 (I) expression (measured 3.5 h after the nitrate upshift). Data are means ± SE and individual values (A–E and G–I). Asterisks indicate the significance of the effect of the nitrate upshift in Student’s t tests (B–E), Bonferroni tests following significant interaction (int, G and I) or the main effect of nitrate following nonsignificant interaction (H): *P < 0.05; **P < 0.01; ***P < 0.001; ns, not significant.