A mechanism for localised lignin deposition in the endodermis

The precise localisation of extracellular matrix and cell wall components is of critical importance for multicellular organisms. Lignin is a major cell wall modification that often forms intricate subcellular patterns that are central to cellular function. Yet, the mechanisms of lignin polymerisation and the subcellular precision of its formation remain enigmatic. Here we show that the Casparian strip, a lignin‐based, paracellular diffusion barrier in plants, forms as a precise, median ring by the concerted action of a specific, localised NADPH oxidase, brought into proximity of localised peroxidases through the action of CASPARIAN STRIP DOMAIN PROTEINS (CASPs). Our findings in Arabidopsis provide a simple mechanistic model of how plant cells regulate lignin formation with subcellular precision. We speculate that scaffolding of NADPH oxidases to the downstream targets of the reactive oxygen species (ROS) they produce might be a widespread mechanism to ensure specificity and subcellular precision of ROS action within the extracellular matrix.