Elongation growth of the cells in lower hypocotyl and transition zone, and the occurrence of strong HRS1 expression in lower hypocotyl cells, during Arabidopsis seed germination.

Lower hypocotyl (LH) cells are marked by asterisks, whereas the cells in transition zone (TZ) are labeled by filled dots. (A) Comparison of LH and TZ cells in the radicle samples collected at 24 or 48 HOG. Both LH and TZ cells were elongated from 24 to 48 HOG, with the magnitude of the elongation being substantially larger for LH cells. At 48 HOG (immediately post germination), an elongation zone (EZ), differentiated mainly from elongated LH cells, was found in the young seedling root. In addition, four newly developed TZ cells (indicated by diamonds) were observed on top of the root meristematic region. (B) The occurrence of strong HRS1 expression in the LH cells (of the promoter::GUS reporter line RL3-11) as indicated by the blue signals produced by histochemical staining of GUS activity at 36 HOG. The expression pattern shown was typical of four separate staining experiments, and was found for all three independent promoter::GUS reporter lines of HRS1. EZ, elongation zone; H, hypocotyl; LH, lower hypocotyl; RM, root meristem; TZ, transition zone. Bars = 50 μm.