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Keeping roots water-tight
The Casparian strip provides a waterproofing function to plant roots, protecting them against unregulated influxes of water and minerals. The integrity of the Casparian strip depends on a receptor-like kinase. Doblas et al. and Nakayama et al. now identify the peptide ligands in the core of the root (the stele) that help regulate Casparian strip formation. The receptor is expressed on the outward-facing surface of the root endodermal cells that surround the stele. When the endodermal layer is sealed by the Casparian strip, the peptide ligands cannot reach their receptors. When the endodermal layer is breached, whether by damage or during development, the peptides reach their receptors and activate signaling that encourages lignin deposition, shoring up the strips.
Plants achieve mineral ion homeostasis by means of a hydrophobic barrier on endodermal cells called the Casparian strip, which restricts lateral diffusion of ions between the root vascular bundles and the soil. We identified a family of sulfated peptides required for contiguous Casparian strip formation in Arabidopsis roots. These peptide hormones, which we named Casparian strip integrity factor 1 (CIF1) and CIF2, are expressed in the root stele and specifically bind the endodermis-expressed leucine-rich repeat receptor kinase GASSHO1 (GSO1)/SCHENGEN3 and its homolog, GSO2. A mutant devoid of CIF peptides is defective in ion homeostasis in the xylem. CIF genes are environmentally responsive. Casparian strip regulation is not merely a passive process driven by root developmental cues; it also serves as an active strategy to cope with adverse soil conditions.