Early differentiation of Casparian strip mediated by nitric oxide is required for efficient K transport under low K conditions in Arabidopsis.

The Casparian strip (CS) is a cell wall modification made of lignin that functions as an apoplastic barrier in the root endodermis to restrict nutrient and water transport between the soil and stele. CS formation is affected by nutritional conditions and its physiological roles have been discussed. In this study, we found that low K condition affects CS permeability, lignin deposition and MYB36 mRNA accumulation. To understand the mechanism underling these findings, we focused on nitric oxide (NO). NO is known to act as a signaling molecule and participates in cell wall synthesis, especially for lignin composition. However, the mechanism by which NO affects lignin deposition and corrects CS formation in the plant roots remains unclear. Through the combining fluorescent observation with histological stains, we demonstrated that the root endodermal cell lignification response to low-potassium (K) conditions is mediated by NO through the MYB36-associated lignin-polymerizing pathway. Furthermore, we discovered the noteworthy ability of NO to maintain nutrient homeostasis for adaptation to low-K conditions by affecting the correct apoplastic barrier formation of CS. Collectively, our results suggest that NO is required for the lignification and apoplastic barrier formation in the root endodermis during adaptation to low-K conditions, which revealing the novel physiological roles of CS under low nutrient conditions and making a significant contribution to CS biology.