A dirigent protein complex directs lignin polymerization and assembly of the root diffusion barrier

In Latin, dirigere means “to align or guide”, so in 1997 when a protein was identified that guides the stereoselectivity of another enzyme it was named as a dirigent protein. Subsequent studies found that the Arabidopsis genome encodes 25 dirigent proteins (DPs). In this new work, Gao et al. characterized the expression and function of several of these DPs. The authors first noted that five DPs are expressed preferentially in the root endodermal cell layer. Fluorescent protein fusions showed that they are localized at the Casparian strip, a lignified barrier that controls movement into the central part of the root. Various single- and double-mutant knock outs showed defects in Casparian strip lignification and function and mislocalization of the CASP-1 protein. Characteristically, at the Casparian strip the plasma membrane is tightly adhered to the cell wall, and this is also disrupted in DP mutants. The original dirigent protein was identified by its influence on lignin biosynthesis, and here the authors also found that the endodermal DPs are required for proper polymerization of monolignols and deposition of lignin in the early stages of Casparian strip formation. (Summary by Mary Williams @PlantTeaching) Science 10.1126/science.adi503