<p>The spatial expression pattern of <i>CRA2</i> was analyzed using a promoter (1,8 kb)-GUS fusion (<b>A–D</b> and <b>I–N</b>) or by <i>in situ</i> hybridization (<b>E–H</b>). <b>A</b>, A root apex observed in bright field with dichroic illumination (Nomarski). The dotted lines indicate the position of the “cone-shaped” transition zone (TZ) between the cell proliferation zone (CPZ) and the cell elongation zone (CEZ). <b>B</b> and <b>D</b>, Root transversal sections in the CEZ (<b>B</b>) and the CPZ (<b>D</b>). <b>C</b>, Detail of the root meristem transition zone using a z-stack projection in confocal sections. The cell walls are visualized with a Propidium Iodide counterstaining, and GUS staining appears in reflectance as blue dots. <b>E–F</b>, <i>In situ</i> hybridization of the <i>CRA2</i> transcripts in root longitudinal sections. (<b>F</b>) Detail of the purple signal associated with vasculature strands. <b>G–H</b>, <i>In situ</i> hybridization of the <i>CRA2</i> transcripts in stem transversal sections (<b>G</b>, purple signal) or with a sense probe used as a negative control (<b>H</b>). Brackets indicate vascular bundles. <b>I–J</b>, Detail of the stele in the root differentiated region in bright field (<b>I</b>) to visualize the GUS staining (in dark blue) and phloem vascular bundles poles (Phl, in turquoise blue) or under UV illumination (<b>J</b>, same section as I) to visualize the blue autofluorescence of the xylem vascular bundle poles (Xy) and endodermis (End). <b>K–L</b>, Lateral root primordium initiation (<b>K</b>, arrow) and emergence (<b>L</b>) observed in bright field. <b>K</b> is a root transversal section. <b>M–N</b>, Nodule primordium (<b>M</b>, three days post-inoculation [dpi] with <i>S. meliloti</i> 1021) and mature nitrogen-fixing nodule (<b>N</b>, 14 dpi) observed in bright field. Bars = 100 µm.</p
