Ionomics reveals as role for dirigent proteins in lignin deposition

New research into the function of ESB1 (Enhanced Suberin 1) has revealed that this dirigent protein plays an essential role in building lignin-based Casparian strips in cell walls of endodermal cells. These discoveries recently published in PNAS start to dissect the molecular mechanisms involved in endodermal control of solute entry into plants. Further, they provide in vivo evidence for proteins in the dirigent family functioning as part of the machinery that builds extracellular lignin-based structures, opening a new avenue to determine the elusive role of this protein family in planta.

Hosmani PS, Kamiya T, Danku J, Naseer S, Geldner N, Guerinot ML, Salt DE. Dirigent domain-containing protein is part of the machinery required for formation of the lignin-based Casparian strip in the root. Proc Natl Acad Sci U S A. 2013 Aug 27;110(35):14498-503. doi: 10.1073/pnas.1308412110. Epub 2013 Aug 12. PubMed PMID: 23940370.

Abstract: The endodermis acts as a “second skin” in plant roots by providing the cellular control necessary for the selective entry of water and solutes into the vascular system. To enable such control, Casparian
strips span the cell wall of adjacent endodermal cells to form a tight junction that blocks extracellular diffusion across the endodermis. This junction is composed of lignin that is polymerized by oxidative coupling of monolignols through the action of a NADPH oxidase and peroxidases. Casparian strip domain proteins
(CASPs) correctly position this biosynthetic machinery by forming a protein scaffold in the plasma membrane at the site where the Casparian strip forms. Here, we show that the dirigent-domain containing protein, enhanced suberin1 (ESB1), is part of thismachinery, playing an essential role in the correct formation of Casparian strips. ESB1 is localized to Casparian strips in a CASP-dependent manner, and in the absence of ESB1, disordered and defective Casparian strips are formed. In addition, loss of ESB1 disrupts the localization of the CASP1 protein at the casparian strip domain, suggesting a reciprocal requirement for both ESB1 and CASPs in forming the Casparian Strip Domain.