ATHB5 protein abundance and EXPA3 promoter activity are spatiotemporally coregulated during Arabidopsis seed germination. Shown is digital single-cell quantification of ATHB5 protein abundance (A–D) and EXPA3 promoter activity (E–H), at BTR (A and E), ETR (B and F), LTR (C and G), and JG (D and H). Reporter quantification data were derived from four embryos each. The mean GUS quantity in each cell was calculated and normalized to GUS activity as determined by quantitative fluorometric assays for each sample. White scale bar for all in D is 50 µm. See also Supplemental Figure S4, Supplemental File S2, and Supplemental Movie S2.