Figure 2. Sucrose-producing SuSy activity in crude extracts of (A) leaves and (B) stems of WT and sus1/sus2/sus3/sus4 Arabidopsis plants. SuSy activity was measured following the two-step assay method described in the main text. The SuSy reaction assay mixture of step 1 included 50 mM HEPES (pH 7.5), 3 mM MgCl₂, 1 mM UDPG and 200 mM fructose. After 3 min at 25°C (still under initial velocity conditions), reactions were stopped by boiling the assay mixture for 1 min. Sucrose was then measured by HPLC with pulsed amperometric detection on a DX-500 system (Dionex) fitted to a CarboPac PA10 column. The results are the mean ± SD of three independent experiments.