The endodermis in roots acts as a selectivity filter for nutrient and water transport essential for growth and development. This selectivity is enabled by the formation of lignin-based Casparian strips. Casparian strip formation is initiated by the localization of the Casparian strip domain proteins (CASPs) in the plasma membrane, at the site where the Casparian strip will form. Localized CASPs recruit Peroxidase 64 (PER64), a Respiratory Burst Oxidase Homolog F, and Enhanced Suberin 1 (ESB1), a dirigent-like protein, to assemble the lignin polymerization machinery. However, the factors that control both expression of the genes encoding this biosynthetic machinery and its localization to the Casparian strip formation site remain unknown. Here, we identify the transcription factor, MYB36, essential for Casparian strip formation. MYB36 directly and positively regulates the expression of the Casparian strip genes CASP1, PER64, and ESB1. Casparian strips are absent in plants lacking a functional MYB36 and are replaced by ectopic lignin-like material in the corners of endodermal cells. The barrier function of Casparian strips in these plants is also disrupted. Significantly, ectopic expression of MYB36 in the cortex is sufficient to reprogram these cells to start expressing CASP1–GFP, correctly localize the CASP1–GFP protein to form a Casparian strip domain, and deposit a Casparian strip-like structure in the cell wall at this location. These results demonstrate that MYB36 is controlling expression of the machinery required to locally polymerize lignin in a fine band in the cell wall for the formation of the Casparian strip. Significance Casparian strips play a critical role in sealing endodermal cells in the root to block uncontrolled extracellular uptake of nutrients and water. Building Casparian strips requires the construction of extracellular lignin structures that encircle cells within the cell wall and that are anchored to the plasma membranes of adjacent cells to form tight seals between them. The transcription factor we have discovered, and the set of genes it regulates, now provides us with the detailed “parts list” necessary to build Casparian strips. This finding has clear implications for better understanding the nature of tight cellular junctions in biology and also has practical implications of agricultural, offering the potential for improved water and nutrient use efficiencies and enhanced resistance to abiotic stresses.

The endodermis represents the main barrier to extracellular diffusion in plant roots, and it is central to current models of plant nutrient uptake. Despite this, little is known about the genes setting up this endodermal barrier. In this study, we report the identification and characterization of a strong barrier mutant, schengen3 (sgn3). We observe a surprising ability of the mutant to maintain nutrient homeostasis, but demonstrate a major defect in maintaining sufficient levels of the macronutrient potassium. We show that SGN3/GASSHO1 is a receptor-like kinase that is necessary for localizing CASPARIAN STRIP DOMAIN PROTEINS (CASPs)—major players of endodermal differentiation—into an uninterrupted, ring-like domain. SGN3 appears to localize into a broader band, embedding growing CASP microdomains. The discovery of SGN3 strongly advances our ability to interrogate mechanisms of plant nutrient homeostasis and provides a novel actor for localized microdomain formation at the endodermal plasma membrane.

CLAVATA3/EMBRYO SURROUNDING REGION (CLE) peptides are secreted endogenous plant ligands that are sensed by receptor kinases (RKs) to convey environmental and developmental inputs. Typically, this involves an RK with narrow ligand specificity that signals together with a more promiscuous co-receptor. For most CLEs, biologically relevant (co-)receptors are unknown. The dimer of the receptor-like protein CLAVATA 2 (CLV2) and the pseudokinase CORYNE (CRN) conditions perception of so-called root-active CLE peptides, the exogenous application of which suppresses root growth by preventing protophloem formation in the meristem. clv2 as well as crn null mutants are resistant to root-active CLE peptides, possibly because CLV2-CRN promotes expression of their cognate receptors. Here, we have identified the CLE-RESISTANT RECEPTOR KINASE (CLERK) gene, which is required for full sensing of root-active CLE peptides in early developing protophloem. CLERK protein can be replaced by its close homologs, SENESCENCE-ASSOCIATED RECEPTOR-LIKE KINASE (SARK) and NSP-INTERACTING KINASE 1 (NIK1). Yet neither CLERK nor NIK1 ectodomains interact biochemically with described CLE receptor ectodomains. Consistently, CLERK also acts genetically independently of CLV2-CRN. We, thus, have discovered a novel hub for redundant CLE sensing in the root.