CNGC2 Is a Ca2+ Influx Channel That Prevents Accumulation of Apoplastic Ca2+ in the Leaf

Ca2+ is absorbed by roots and transported upward through the xylem to the apoplastic space of the leaf, after which it is deposited into the leaf cell. In Arabidopsis (Arabidopsis thaliana), the tonoplast-localized Ca2+/H+ transporters CATION EXCHANGER1 (CAX1) and CAX3 sequester Ca2+ from the cytosol into the vacuole, but it is not known what transporter mediates the initial Ca2+ influx from the apoplast to the cytosol. Here, we report that Arabidopsis CYCLIC NUCLEOTIDEGATED CHANNEL2 (CNGC2) encodes a protein with Ca2+ influx channel activity and is expressed in the leaf areas surrounding the free endings of minor veins, which is the primary site for Ca2+ unloading from the vasculature and influx into leaf cells. Under hydroponic growth conditions, with 0.1 mM Ca2+, both Arabidopsis cngc2 and cax1cax3 loss-of-function mutants grew normally. Increasing the Ca2+ concentration to 10 mM induced H2O2 accumulation, cell death, and leaf senescence and partially suppressed the hypersensitive response to avirulent pathogens in the mutants but not in the wild type. In vivo apoplastic Ca2+ overaccumulation was found in the leaves of cngc2 and cax1cax3 but not the wild type under the 10 mM Ca2+ condition, as monitored by Oregon Green BAPTA 488 5N, a low-affinity and membrane-impermeable Ca2+ probe. Our results indicate that CNGC2 likely has no direct roles in leaf development or the hypersensitive response but, instead, that CNGC2 could mediate Ca2+ influx into leaf cells. Finally, the in vivo extracellular Ca2+ imaging method developed in this study provides a new tool for investigating Ca2+ dynamics in plant cells.