Figure 8.

The Plasma Membrane in Polarized Cells

In addition to simply forming the limiting membrane of the plant cell, the PM has recently been found to have multiple kinds of specialized sub-domains in cell that are polarized or that are responding to particular stresses or developmental signals. For example, in studies reported by Assaad et al (2004), an epidermal cell responds to infection by powdery mildew (Erysiphe cichoracearum; red) by establishment of a particular PM structure called papillae. Two PM-SNAREs, Qa-PEN1 (A) and Qa-SYP122 (B), visualized by confocal microscopy in cell expressing GFP- or CFP-fusions (respectively), become enriched in these domains and are essential for resisting the fungal infection. In a similar way, the Qa-SNARE KNOLLE is also important for establishment of the cell plate in dividing cells (C),here visualized in the root tips of plants expressing KNOLLE::CFP-KNOLLE. The best studied examples of polarization in plant cells has been shown by the auxin efflux (PINs) and influx (AUX1s) transporters. For example, in work reported in Kleine-Vehn et al. (2006) the authors performed whole-mount immunofluorescent microscopy to show how the protophloem cells of the root have independent apical and basal domains enriched in AUX1 and PIN1 (respectively (D-F)). Images (A) and (B) are from Assaad et al., 2004; images (D-F) are from Kleine-Vehn et al., 2006.