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

The exodermis does not form a Casparian Strip (CS). Instead, it creates a Polar Lignin Cap (PLC), that functions similarly to the CS. The SlSCZ and SlEXO1 transcription factors regulate the formation of the PLC. The presence of distinct lignin structures acting as apoplastic barriers has intriguing implications for a root's responses.

Plants have specialized root barriers to regulate how they interact with the environment. One of these barriers is the Casparian strip (CS), a ring-like structure made of lignin that seals the root endodermis functioning as an apoplastic barrier. Most flowering plants also have an additional root barrier, the exodermis, which is generally located beneath the epidermis. However, our understanding of exodermis development and function is limited as this cell type is absent in the model plant Arabidopsis thaliana. In this work, we demonstrate that the exodermis forms a different apoplastic barrier, the Polar Lignin Cap (PLC) which functions like the CS. The genetic regulation of the PLC differs from that of the CS. Specifically, the SlSCZ and SlEXO1 transcription factors control the formation of the PLC genetically interacting. Additionally, repression of the exodermal PLC in inner cortical layers is also mediated by the SlSCZ and SlEXO1 genes. Several target genes downstream of SlSCZ and SlEXO1 in the exodermis have been identified. Although both the exodermis and endodermis create apoplastic barriers to regulate mineral uptake, the PLC in the exodermis cannot fully replace CS apoplastic function. The presence of distinct lignin structures acting as apoplastic barriers has exciting implications for a root's response to abiotic and biotic stimuli.

Original Paper: Manzano, C., Morimoto, K.W., Shaar-Moshe, L., Mason, G.A., Cantó-Pastor, A., Gouran, M., De Bellis, D., Ursache, R., Kajala, K., Sinha, N., Bailey-Serres, J., Geldner, N., del Pozo, J.C., Brady, S.M. 2024. Regulation and function of a polarly localized lignin barrier in the exodermis. Nature Plants. DOI: 10.1038/s41477-024-01864-z