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

Area of Science:

• Plant biology
• Cell biology
• Molecular genetics

Background:

• The root endodermis forms a vital diffusion barrier using Casparian strips.
• The receptor-like kinase SCHENGEN3/GASSHO1 (SGN3/GSO1) is known to be essential for Casparian strip formation and integrity.
• The specific molecular mechanism and ligands regulating SGN3 function remained largely unknown.

Purpose of the Study:

• To identify the molecular factors and ligands that regulate the function of the SCHENGEN3 (SGN3) receptor-like kinase.
• To elucidate the mechanism by which SGN3 establishes and maintains the Casparian strip diffusion barrier in the root endodermis.
• To investigate the role of peptide sulfation in Casparian strip development.

Main Methods:

• Genetic analysis of the schengen2 (sgn2) mutant, which affects peptide sulfation.
• Identification and characterization of stele-expressed peptides, termed CASPARIAN STRIP INTEGRITY FACTORS (CIF1/2).
• Biochemical assays to demonstrate direct peptide binding to recombinant SGN3.
• Phenotypic analysis of Casparian strip formation and integrity in wild-type and mutant plants.

Main Results:

• The schengen2 (sgn2) mutant is defective in an enzyme responsible for sulfating peptide ligands.
• Two stele-expressed peptides, CIF1 and CIF2, were identified as functional ligands for SGN3, complementing the sgn2 mutant at nanomolar concentrations.
• CIF1/2 peptides, when applied exogenously, induce Casparian strip mislocalization and overlignification in a SGN3-dependent manner.
• Direct binding assays confirmed CIF1/2 as ligands for SGN3, suggesting a barrier surveillance system.

Conclusions:

• The CIF1/2 peptides act as ligands for the SGN3 receptor-like kinase, forming a signaling module essential for Casparian strip integrity.
• The CIF1/2-SGN3 pathway is proposed to be a critical component of a surveillance system ensuring the proper sealing of the supracellular Casparian strip network.
• This discovery sheds light on the molecular mechanisms regulating the plant root's primary diffusion barrier.