Regulation and function of a polarly localized lignin barrier in the exodermis

The endodermal Casparian strip (CS) is a conserved lignin-based barrier in plant roots that seals the endodermal apoplastic space. Most angiosperms possess an additional root cell type, the exodermis, which also forms a barrier. However, its regulation remains poorly understood as this cell type is absent in Arabidopsis thaliana. In tomato (Solanum lycopersicum), Manzano et al. identified a distinct exodermal structure, the polar lignin cap (PLC), which mirrors the function of the CS but is regulated by distinct genetic mechanisms. Through histochemical staining and transmission electron microscopy, the authors confirmed the PLC’s role as an apoplastic barrier. They further demonstrated its functional equivalence to the CS using propidium iodide as a tracer to assess barrier integrity. However, tomato mutant analyses of orthologues of Arabidopsis CS regulators revealed that genetic pathways governing exodermal and endodermal barriers are distinct. By mining gene expression data and analyzing mutants, the researchers identified transcription factors SlSCZ and SlEXO1 as key repressors of PLC formation in inner cortical layers, orchestrating its polar deposition and regulating downstream genes. Despite shared roles as mineral ion checkpoints, the PLC cannot fully substitute the CS’s function in ion regulation, underscoring the specialized and non-redundant roles of these barriers in maintaining root ion homeostasis. This study provides insights into the genetic regulation of the exodermis but also highlights its critical role in water and nutrient transport, broadening our understanding of plant root-environment interactions. (Summary by Elisa De Meo) Nature Plants 10.1038/s41477-024-01864-z