Histology of control and challenged roots of Phlomis purpurea.
a and b: Cross section of a control root of Phlomis purpurea stained with toluidine blue showing the presence of lignin evidenced by the bright blue colour (a) and the reinforcement (double arrows) over the outer tangential wall of the epidermal cells (b). An exodermis whose radial walls have Casparian strips is evident in b (arrows). c and d: Paraffin embedded sections of control roots of P. purpurea, stained with direct red and observed under UV light. An intense blue light fluorescence evidence the Casparian strips in the exodermis and endodermis (arrows). In d is shown a magnification of Casparian strips (arrow). e and f: Cross section of a P. purpurea root challenged with Phytophthora cinnamomi along 72 h. Although no differences were observed in radial walls of the exodermis between control and challenged plant roots, a statistically significant reinforcement of the outer tangential walls was detected in challenged ones (compare with a and b and see Additional file 1). g and h: Low Temperature Scanning Electron Microscopy (LTSEM) of P. purpurea root epidermis. In g view of the outer tangential wall of the epidermal cells of P. purpurea root; in h
P. cinnamomi hyphae growing over the outer tangential wall (arrows)