Calcium Signaling as an Emerging Integrator of Manganese Homeostasis in Arabidopsis: From Molecular Mechanisms to Adaptive Strategies

Manganese (Mn) is essential for plants, but its fluctuating soil availability—deficiency in alkaline soils and toxicity in acidic soils—challenges crop productivity. Breakthroughs in Arabidopsis have uncovered Ca2+ signaling as a key integrator of Mn status. This review synthesizes these discoveries into an emerging Arabidopsis-centered framework. Under Mn deficiency, sustained Ca2+ oscillations activate CPK21/23, which phosphorylate the importer NRAMP1 at Thr498 to enhance Mn uptake. Under Mn excess, a rapid Ca2+ transient triggers a multi-layered cascade: CPK4/5/6/11 activates MTP8 (Ser31/32) for vacuolar sequestration, while CBL2/3–CIPK3/9/26 sequentially suppresses MTP8 (Ser35, peak 24 h) and MTP11 (Ser194/201, peak 36 h)—a multi-tiered “brake” system. Concurrently, CBL1/9–CIPK23 induces NRAMP1 endocytosis (Ser20/22) to limit Mn uptake. The IRT1 transporter directly binds cytoplasmic Mn2+ and triggers its own degradation via CIPK23, thereby converging with Ca2+ signaling. The BRI1–CNGC12 module generates Mn-induced Ca2+ signals. By organizing current knowledge into a hierarchical framework, this review provides a working model for future research and outlines translational opportunities for engineering Mn-resilient crops.