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PLANT BIOLOGY
The Casparian strip in plant roots is a diffusion barrier that directs water and solutes from the soil to the water-conducting tissues. Proteins involved in making the strip have at long last been identified.
The roots of land plants are usually in direct contact with the soil, which provides water, nutrients and other solutes but may also contain toxic substances. To control the uptake of molecules from the soil, vascular plants are thought to have evolved the protective cell layer that surrounds their water-conducting system. This layer, called the endodermis, acts as an inner skin1. A distinguishing mark of endodermal cells is the local thickening of their transverse cell walls, which was first described in 1865 by Robert Caspary2 and later named the Casparian strip. On page 380 of this issue, Roppolo and colleagues3 now describe a family of proteins that are precisely located in the plasma membrane adjacent to the Casparian strip and are needed for its formation.
The Casparian strip contains the polymeric molecules lignin and suberin, which locally impregnate the transverse cell walls of endodermal cells (Fig. 1a)4. This impregnation hinders diffusion of water and solutes through the cell-wall space1,5, thus forcing them to pass through the outer membrane (plasma membrane) and the interior (cytoplasm) of the endodermal cells that act as a selective barrier (Fig. 1b). Although regulators of asymmetric cell division and cell-fate determination during endodermis formation have been identified6,7, proteins specifically needed to make the Casparian strip have remained unknown.
Roppolo et al.3 reasoned that messenger RNAs encoding proteins specifically involved in Casparian strip formation should be expressed in the endodermis and should include transcripts for membrane proteins. They therefore analysed mRNA transcripts strongly expressed in the root endodermis of the model plant Arabidopsis thaliana and identified five mRNAs encoding...