Endodermal lignification coordinates with root calcium levels to govern lateral root emergence

Calcium (Ca), an essential macronutrient, is crucial for the growth of young plant tissues, including the formation of lateral roots (LRs) that originate from pericycle cells in the inner root. The Casparian strip (CS) serves as an apoplastic barrier in the root endodermis, optimizing nutrient transport and facilitating plant adaptation to various environments. This study demonstrates that the integrity of the CS regulates local Ca-dependent LR emergence by controlling the lignification of the overlying endodermal cells in Arabidopsis. In mutants with disrupted CS integrity and compensatory lignin deposition, plants require sufficient Ca to maintain auxin gradients and the morphology of LR primordia, thereby promoting LR primordia through the endodermis. Both piperonylic acid treatment and genetic evidence indicate that excessive lignification of the overlying endodermal cells significantly delays LR emergence under low-Ca conditions compared with sufficient Ca. The receptor-like kinase SGN3/GSO1 mediates this excessive lignification. Importantly, the function of the CS in LR emergence is independent of Ca translocation from roots to shoots and, consequently, shoot growth dynamics. Overall, our study highlights the direct involvement of CS integrity in modulating Ca-dependent LR emergence by controlling cell wall stiffness through the lignification of overlying endodermal cells.