The Casparian strip functions as a bidirectional gate for calcium homeostasis in Arabidopsis

Calcium (Ca) is an essential mineral nutrient transported in plants predominantly through the apoplastic pathway. While the molecular assembly of the Casparian strip (CS), the critical apoplastic barrier in the root endodermis, has been elucidated significantly over the past decade, the precise mechanisms by which the CS regulates Ca uptake, transport and translocation remain unclear. Building on our previous identification of LOTR1 as a regulator of CS formation, we here establish its genetic interaction with the receptor-like kinase SGN3. We show that LOTR1 promotes CS fusion additively with SGN3 but corrects CS positioning only partially through SGN3-dependent pathways. By analyzing single and double mutants across varying Ca gradients, we uncover a bidirectional gating function of the CS. Specifically, an intact CS is required to retain Ca within the stele under Ca limitation, while simultaneously restricting excessive Ca entry under high-Ca to prevent toxicity in shoots. Moreover, using non-invasive micro-test technology, we reveal that disruption of CS function reduces net Ca 2+ influx in both meristematic and mature root zones. Together, these findings demonstrate that the CS is essential for plant adaptation to fluctuating Ca environments by sustaining efficient root Ca uptake and strictly regulating root-to-shoot Ca translocation.