Extracellular Ca ²⁺ induces desensitized cytosolic Ca ²⁺ rise sensitive to phospholipase C inhibitor which suppresses root growth with Ca ²⁺ dependence

Calcium (Ca) is an essential element for all organisms. In animal cells, the plasma membrane-localized Ca receptor CaSR coupled to a phospholipase C (PLC)-dependent signaling cascade monitors extracellular Ca²⁺ concentrations ([Ca²⁺]_ext) and responds with increases in cytosolic calcium concentrations ([Ca²⁺]_cyt). Plant roots encounter variable soil conditions, but how they sense changes in [Ca²⁺]_ext is largely unknown. In this study, we demonstrate that increasing [Ca²⁺]_ext evokes a transient increase in [Ca²⁺] in the cytosol, mitochondria, and nuclei of Arabidopsis thaliana root cells. These increases were strongly desensitized to repeat applications of [Ca²⁺]_ext, a typical feature of receptor-mediated cellular signaling in animal and plant cells. Treatment with gadolinium (Gd³⁺), a CaSR activator in animal cells, induced concentration-dependent increases in [Ca²⁺]_cyt in roots, which showed self-desensitization and cross-desensitization to [Ca²⁺]_ext-induced increases in [Ca²⁺]_cyt (EICC). EICC was sensitive to extracellular H⁺, K⁺, Na⁺, and Mg²⁺ levels. Treatment with the PLC inhibitor neomycin suppressed EICC and Ca accumulation in roots. The inhibitory effect of neomycin on root elongation was fully rescued by increasing [Ca²⁺]_ext but not [Mg²⁺] or [K⁺] in the growth medium. These results suggest that [Ca²⁺]_ext and the movement of Ca²⁺ into the cytosol of plant roots are regulated by a receptor-mediated signaling pathway involving PLC.

Keywords: Aequorin; Arabidopsis; Calcium; Phospholipase C; Receptor.