Fig. 1. 3D reconstruction of PX in different regions of soybean hypocotyls. (A-E) Apical region of the hypocotyl. (A) PX with ring-shaped, lignified SW (red fluorescence) and GRPCWM (green fluorescence) interconnecting the lignified SW. (B) PX with a helical, lignified SW. The attachments of the GRPCWM are clearly visible and are often somewhat broadened along the lignified SW thickening (arrow). (C) Two PX. The younger one, with closely spaced SW, does not yet contain GRPCWM. The older one is characterized by more widely spaced SW and the typical strands of the GRPCWM. The wall between the two is also clearly fluorescent (arrows). (D) The ends of two PX joined together end-by-end. The arrows indicate the GRP-containing wall connecting these two cells with a third PX (not retained in the preparation). The same type of wall, seen here in front view, is seen in (C) in side view. The ends of the two PX are outlined with a hatched line. (E) Rabbit serum control: no green fluorescence is observed. Ethidium bromide stains the SW and (very faintly) the GRPCWM. Notice the small protuberances on the SW in the direction of the longitudinal axis of the hypocotyl (arrows). (F) Middle region of the hypocotyl: PX with strongly elongated helical SW. Notice the stress-fibre-like appearance of the GRPCWM and the attachments between the GRPCWM and the SW (arrows). In contrast to the apical region of the hypocotyl, where the GRPCWM spans only the distance between adjacent SW, long, continuous filaments or sheets are typically observed in the mature regions of the hypocotyl (arrowheads). (G-I) Basal region of the hypocotyl. New MX are differentiating but the structure of the PX remains much the same as in the middle region of the hypocotyl. (G) Three stretched helical SW are visible to the left and a newly formed PX with a flat helical SW to the right. Notice the GRPCWM along the helical SW of this PX. Arrows indicate a faintly stained cell corner. (H) Overview, showing (from left to right) a MX (m) and a series of younger to older PX. The older elements are characterized by passively elongated helical SW and intense staining with GRP1.8 antibodies. Little staining is observed in the MX, with the exception of a cell corner adjacent to the youngest PX (arrow). (I) Rabbit serum control: no green fluorescence is observed. Ethidium bromide stains the SW and, faintly, the GRPCWM (arrows). Bars, 10 µm.