Plant cell biology in the frame!

Image: Andrew Maule, Wikimedia Commons.
Image: Andrew Maule, Wikimedia Commons.

Notwithstanding the centuries we’ve spent peering at, poking, prodding and penetrating the inner workings of plant cells with various types of microscopes and decades undertaking investigations at the sub-cellular level, there are still new discoveries to be made. Here are two, united by the theme of cell–cell transport. First, the recent revelation by Deborah Barton et al. (The Plant Journal 66: 806–817, 2011) that small molecules – up to 10.4 kDa in size – can move between adjacent plant cells via the plasmodesmata. No, that’s not the news, the novelty is the fact that this transport took place within the lumen of the endoplasmic reticulum, which extends between adjacent cells and constitutes the desmotubule, a feature within the plasmodesma itself. Investigating Nicotiana trichomes and Tradescantia epidermides using a fluorescent technique, the group propose that the ER lumen of plant cells is continuous with that of their neighbours, and allows movement of small ER-luminal molecules between cells. All of which adds more intrigue to these curious plant-specific cell–cell portals (http://en.wikipedia.org/wiki/Plasmodesmata). And second, that favourite feature of root anatomists, the Casparian strip, which has just given up some of its secrets. This famous endodermal wall structure acts as a barrier to apoplastic (‘extracellular’) movement of materials within the root and ‘forces’ those solutes across the semi-permeable barrier that is the plasma membrane of the endodermal cells and to follow a symplastic transport pathway. Until now it was unknown why the strip forms where it does, but Daniele Roppolo and co-workers (Nature 473: 380–383, 2011) appear to have solved that puzzle with their identification of CASPs (Casparian strip membrane domain proteins). CASPs specifically mark a membrane domain that predicts the formation of Casparian strips in the endodermal cells and are considered to be the first molecular factors that are shown to establish a plasma membrane and extracellular diffusion barrier in plants. And a timely reminder of the pivotal role that plant cell biology still plays in botanical science is provided by Simon Gilroy’s article entitled ‘Plant cell biology: with grand challenges come great possibilities’ (Frontiers in Plant Science; doi:10.3389/fpls.2011.00003).