MCA1 and MCA2 That Mediate Ca²⁺ Uptake Have Distinct and Overlapping Roles in Arabidopsis

Ca²⁺ is important for plant growth and development as a nutrient and a second messenger. However, the molecular nature and roles of Ca²⁺-permeable channels or transporters involved in Ca²⁺ uptake in roots are largely unknown. We recently identified a candidate for the Ca²⁺-permeable mechanosensitive channel in Arabidopsis (Arabidopsis thaliana), named MCA1. Here, we investigated the only paralog of MCA1 in Arabidopsis, MCA2. cDNA of MCA2 complemented a Ca²⁺ uptake deficiency in yeast cells lacking a Ca²⁺ channel composed of Mid1 and Cch1. Reverse transcription polymerase chain reaction analysis indicated that MCA2 was expressed in leaves, flowers, roots, siliques, and stems, and histochemical observation showed that an MCA2 promoter::GUS fusion reporter gene was universally expressed in 10-d-old seedlings with some exceptions: it was relatively highly expressed in vascular tissues and undetectable in the cap and the elongation zone of the primary root. mca2-null plants were normal in growth and morphology. In addition, the primary root of mca2-null seedlings was able to normally sense the hardness of agar medium, unlike that of mca1-null or mca1-null mca2-null seedlings, as revealed by the two-phase agar method. Ca²⁺ uptake activity was lower in the roots of mca2-null plants than those of wild-type plants. Finally, growth of mca1-null mca2-null plants was more retarded at a high concentration of Mg²⁺ added to medium compared with that of mca1-null and mca2-null single mutants and wild-type plants. These results suggest that the MCA2 protein has a distinct role in Ca²⁺ uptake in roots and an overlapping role with MCA1 in plant growth.