AbstractCasparian strips (CS) are cell wall modifications of vascular plants restricting extracellular free diffusion into and out the vascular system. This barrier plays a critical role in controlling the acquisition of nutrients and water necessary for normal plant development. CS are formed by the precise deposition of a band of lignin approximately 2 μm wide and 150 nm thick spanning the apoplastic space between adjacent endodermal cells. Here, we identified a copper-containing protein, Uclacyanin1 (UCC1) that is sub-compartmentalised within the CS. UCC1 forms a central CS nanodomain in comparison with other CS-located proteins that are found to be mainly accumulated at the periphery of the CS. We found that loss-of-function of two uclacyanins (UCC1andUCC2) reduces lignification specifically in this central CS nanodomain, revealing a nano-compartmentalised machinery for lignin polymerisation. This lack of lignification leads to increased endodermal permeability, and consequently to a loss of mineral nutrient homeostasis.