Cotreatment of Suc and SMX promotes auxin signaling in the hypocotyl. The DII-VENUS reporter system exhibited high levels of fluorescence in the vascular region of the hypocotyl. Treatment with SMX and Suc caused a decrease in VENUS fluorescence, yet this effect was not observed when seedlings were treated with SMX or Suc alone. White numbering indicates the ratio of fluorescence intensity in vasculature relative to the ground tissues (mean ± sd for at least 10 individuals). A, Fluorescence decreased preferentially in the vasculature compared with the ground tissues. Bar = 100 µm. B, DR5::GUS expression was observed in the hypocotyl vascular region during growth in the presence of both Suc and SMX, suggesting auxin signaling is enhanced by the combined action of these two treatments. Bar = 50 µm. Images were taken along the center of the hypocotyl, midway between the shoot apical meristem and the root-hypocotyl junction. C, A proposed model depicting changes in auxin distribution that may underpin the Suc-SMX interaction. In the absence of SMX, auxin is evenly distributed at low concentrations throughout the hypocotyl. SMX causes auxin accumulation in the vasculature, resulting in hypocotyl inhibition via IAA19/ARF7 signaling.