Roles of the UPP during Root Development Figure 1. Effects of different inhibitors on primary root elongation. a–d. Cold-pretreated seeds were cultured in 1/2 MS with 1% Suc for 24 h, and then uniform seedlings of similar size and primary root length were transferred to the same fresh medium containing 0 (a), 20 mM MG132 (b), 40 mM MG132 (c) or 80 mM MG132 (d), respectively, for additional 48 h. Bars = 1000 mm. e–h. Cold-pretreated seeds were cultured in 1/2 MS with 1% Suc for 24 h, and then uniform seedlings of similar size and primary root length were transferred to the same fresh medium containing various concentrations of MG 132 (e), MG115 (f), E-64 (g) and DMSO (h), for additional 48 days. The values are the means 6 SD of about 15 seedlings of each treatment. The experiment was repeated thrice with consistent results. doi:10.1371/journal.pone.0045673.g001 meristem in MG132-treated seedlings were 2864/240615 mm (20 mM), 2563/201631 mm (40 mM), and 1362/144612 (80 mM) (Fig. 2 b–d). All these data indicated that MG132 treatment caused a significantly decrease in meristem size in a dose-dependent manner. Besides, FM staining also revealed that MG132 treatment is concomitant with a more rapid cell differentiation in the elongation zone (Fig. 2). Given that root-meristem size is determined not only by the rate of cell differentiation, but also by the rate of cell division [1]. To assess whether the decline in PLOS ONE | www.plosone.org 3 September 2012 | Volume 7 | Issue 9 | e45673