Targeting of Cameleon to different subcellular compartments reveals a strictcytoplasmic/mitochondrial Ca2+ handling relationship in plant cells.

In the present work we describe the unprecedented use of a mitochondria-targeted Cameleon to produce stably transformed Arabidopsis plants that enable analyses of mitochondrial Ca2+ dynamics in planta and allow to monitor the intra mitochondrial Ca2+ 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 the mitochondrial Ca2+ accumulation is strictly related to the intensity of the cytoplasmic Ca2+ raise demonstrating a tight association between mitochondrial and cytoplasmic Ca2+ dynamics. However in all experimental conditions mitochondrial Ca2+ dynamics are substantially different from those monitored in the cytoplasm, demonstrating that mitochondria do not passively sense cytosolic Ca2+, but actively modulate the intramitochondrial level of the cation. In particular, our analyses allow to appreciate that the kinetics of Ca2+ release from mitochondria are much slower than in the cytoplasm and nucleus. The functional implication and mechanisms of these differences are discussed