Fig. 3.

(A–D) Pioneer root comparative anatomy (transverse sections stained with Safranin and Fast Green). Only undifferentiated cells are located at the sites of subsequent xylem (arrows) and tracheary elements (TEs) are lacking in the stele of 1-day-old roots (A). The first TEs located in four xylem poles of 2-day-old roots (B). Well-developed primary xylem in 3- to 4-day-old roots (C, D). Note the large nuclei in the thin-walled cells, adjacent to mature vessels (D; arrowheads). (E–L) Visualization of nuclear DNA fragmentation through detection of TUNEL-positive nuclei (arrows) during xylogenesis on the second (E–H) and third–fourth days (I–L) of growth of pioneer roots. Note that no TUNEL-positive nuclei were seen in the mature, already differentiated vessels (x). E, endodermis; x, primary xylem. Scale bars = 50 μm.