Targeting of Cameleons to various subcellular compartments reveals a strict cytoplasmic/mitochondrial Ca²⁺ handling relationship in plant cells

Here we describe use of a mitochondrial targeted Cameleon to produce stably transformed Arabidopsis plants that enable analyses of mitochondrial Ca 2+ dynamics in planta and allow monitoring of the intra-mitochondrial Ca 2+ concentration in response to physiological or environmental stimuli. Transgenic plants co-expressing nuclear and mitochondrial targeted Cameleons were also generated and analyzed. Here we show that mitochondrial Ca 2+ accumulation is strictly related to the intensity of the cytoplasmic Ca 2+ increase, demonstrating a tight association between mitochondrial and cytoplasmic Ca 2+ dynamics. However, under all experimental conditions, mitochondrial Ca 2+ dynamics were substantially different from those monitored in the cytoplasm, demonstrating that mitochondria do not passively sense cytosolic Ca 2+, but actively modulate the intra-mitochondrial level of the cation. In particular, our analyses show that the kinetics of Ca 2+ release from mitochondria are much slower than in the cytoplasm and nucleus. The mechanisms and functional implications of these differences are discussed.