Fig. 2. Decreased auxin levels correlate with increased cell volume and SAUR expression during hypocotyl elongation.

(A and B) Decreased auxin levels correlate with increased cell volume of bottom (A) and top (B) cells of R2D2 hypocotyls for the indicated time. Auxin levels were proxied by the inverse n3 × Venus/ntdTomato signal ratio (mDII/DII). Cells were assumed to have a cylindrical shape, and cell volume was estimated by the formula for the volume of a cylinder [π * (width/2)² * length]. Five cells each from the bottom and the top of the hypocotyl were used for analysis. Values represent sample means ± SEM from three replicates. (C) Quantification of hypocotyl volumes at indicated times. (D) Quantification of endogenous IAA in the elongating hypocotyls at indicated times. Auxin concentrations were expressed in picograms of IAA per volume of hypocotyl (mm³). Different letters indicate statistical differences between groups with P < 0.01 using analysis of variance (ANOVA) test. (E) Expression of auxin-responsive genes in etiolated hypocotyls at the indicated times. (F) SAUR22::GUS expression in etiolated hypocotyls at the indicated times. Scale bars, 1 mm. (G) Expression of auxin-responsive genes in etiolated hypocotyls grown on different media. Germinated seeds were transferred to ½ MS medium supplemented with 1% sucrose and with DMSO (mock) or 1 μM IAA (IAA) for growth. Expression analysis was performed at 36HPG. (E and G) Expression levels of indicated genes were normalized against ACTIN7 expression and presented as the fold change to the expression at 12HPG (E) or mock (G). Data are means ± SEM from three biological replicates. *P < 0.01.