Caspary’s conductor

Abstract

Cell walls are essential features of plant cells and modifications of this apoplastic space implement a number of structural and physiological functions. The Casparian strip (CS), named after its 1865 discoverer Robert Caspary (1), is a remarkable cell wall modification that all biology students become acquainted with in undergraduate plant anatomy courses. Typically, CSs occur in the endodermis of roots, the organ performing the uptake of water and nutrients. The CS can be envisioned as a belt-like structure, specifically localized in the middle of the anticlinal wall all around an endodermal cell (Fig. 1). CSs, chemically dominated by lignin (2, 3), bridge the cell wall of adjacent endodermal cells and thus seal the extracellular space. Consequently, apoplastic passage of ions is restricted and transporter and channel proteins in stele and soil-directed plasma membranes selectively permit the radial transport of ions from the soil to the stele (4). Conversely, CSs in the endodermis also prevent the apoplastic backflow of ions from the stele to the cortex. The resulting ion build-up in the stele and subsequent osmotic water movements generate the root pressure, enabling vertical water and nutrient transport when transpiration is limited. When roots develop further, suberin, a cell wall modification of aromatic and aliphatic materials, is deposited as lamella all around the endodermal cell (5). This additionally prevents transcellular paths across membranes and thus complicates the evaluation of the CS's impact on water relations and nutrient homeostasis. However, although a consistent root structure and physiologically essential, little is known about the molecular components that regulate CS localization and biogenesis. In PNAS, Kamiya et al. (6) present a great step forward in the understanding of how and what controls CS formation.