Root diffusion barrier control by a vasculature-derived peptide binding to the SGN3 receptor.

The root endodermis forms its extracellular diffusion barrier by developing ringlike impregnations called Casparian strips. A factor responsible for their establishment is the SCHENGEN3/GASSHO1 (SGN3/GSO1) receptor-like kinase. Its loss of function causes discontinuous Casparian strips. SGN3 also mediates endodermal overlignification of other Casparian strip mutants. Yet, without ligand, SGN3 function remained elusive. Here we report that schengen2 (sgn2) is defective in an enzyme sulfating peptide ligands. On the basis of this observation, we identified two stele-expressed peptides (CASPARIAN STRIP INTEGRITY FACTORS, CIF1/2) that complement sgn2 at nanomolar concentrations and induce Casparian strip mislocalization as well as overlignification-all of which depend on SGN3. Direct peptide binding to recombinant SGN3 identifies these peptides as SGN3 ligands. We speculate that CIF1/2-SGN3 is part of a barrier surveillance system, evolved to guarantee effective sealing of the supracellular Casparian strip network

Molecular mechanism for the recognition of sequence-divergent CIF peptides by the plant receptor kinases GSO1/SGN3 and GSO2.

Plants use leucine-rich repeat receptor kinases (LRR-RKs) to sense sequence diverse peptide hormones at the cell surface. A 3.0-Å crystal structure of the LRR-RK GSO1/SGN3 regulating Casparian strip formation in the endodermis reveals a large spiral-shaped ectodomain. The domain provides a binding platform for 21 amino acid CIF peptide ligands, which are tyrosine sulfated by the tyrosylprotein sulfotransferase TPST/SGN2. GSO1/SGN3 harbors a binding pocket for sulfotyrosine and makes extended backbone interactions with CIF2. Quantitative biochemical comparisons reveal that GSO1/SGN3-CIF2 represents one of the strongest receptor-ligand pairs known in plants. Multiple missense mutations are required to block CIF2 binding in vitro and GSO1/SGN3 function in vivo. Using structure-guided sequence analysis we uncover previously uncharacterized CIF peptides conserved among higher plants. Quantitative binding assays with known and novel CIFs suggest that the homologous LRR-RKs GSO1/SGN3 and GSO2 have evolved unique peptide binding properties to control different developmental processes. A quantitative biochemical interaction screen, a CIF peptide antagonist and genetic analyses together implicate SERK proteins as essential coreceptor kinases required for GSO1/SGN3 and GSO2 receptor activation. Our work provides a mechanistic framework for the recognition of sequence-divergent peptide hormones in plants

Polarly localized kinase SGN1 is required for Casparian strip integrity and positioning.

Casparian strips are precisely localized and aligned ring-like cell wall modifications in the root of all higher plants. They set up an extracellular diffusion barrier analogous to animal tight junctions, and are crucial for maintaining the homeostatic capacity of plant roots. Casparian strips become localized because of the formation of a highly stable plasma membrane domain, consisting of a family of small transmembrane proteins called Casparian strip membrane domain proteins (CASPs). Here we report a large-scale forward genetic screen directly visualizing endodermal barrier function, which allowed us to identify factors required for the formation and integrity of Casparian strips. We present the identification and characterization of one of the mutants, schengen1 (sgn1), a receptor-like cytoplasmic kinase that we show localizes in a strictly polar fashion to the outer plasma membrane of endodermal cells and is required for the positioning and correct formation of the centrally located CASP domain

SCHENGEN receptor module drives localized ROS production and lignification in plant roots.

Production of reactive oxygen species (ROS) by NADPH oxidases (NOXs) impacts many processes in animals and plants, and many plant receptor pathways involve rapid, NOX-dependent increases of ROS. Yet, their general reactivity has made it challenging to pinpoint the precise role and immediate molecular action of ROS. A well-understood ROS action in plants is to provide the co-substrate for lignin peroxidases in the cell wall. Lignin can be deposited with exquisite spatial control, but the underlying mechanisms have remained elusive. Here, we establish a kinase signaling relay that exerts direct, spatial control over ROS production and lignification within the cell wall. We show that polar localization of a single kinase component is crucial for pathway function. Our data indicate that an intersection of more broadly localized components allows for micrometer-scale precision of lignification and that this system is triggered through initiation of ROS production as a critical peroxidase co-substrate