MCU proteins dominate in vivo mitochondrial Ca(2+) uptake in Arabidopsis roots

Ca(2+) signaling is central to plant development and acclimation. While Ca(2+)-responsive proteins have been investigated intensely in plants, only a few Ca(2+)-permeable channels have been identified, and our understanding of how intracellular Ca(2+) fluxes is facilitated remains limited. Arabidopsis thaliana homologs of the mammalian channel-forming mitochondrial calcium uniporter (MCU) protein showed Ca(2+) transport activity in vitro. Yet, the evolutionary complexity of MCU proteins, as well as reports about alternative systems and unperturbed mitochondrial Ca(2+) uptake in knockout lines of MCU genes, leave critical questions about the in vivo functions of the MCU protein family in plants unanswered. Here, we demonstrate that MCU proteins mediate mitochondrial Ca(2+) transport in planta and that this mechanism is the major route for fast Ca(2+) uptake. Guided by the subcellular localization, expression, and conservation of MCU proteins, we generated an mcu triple knockout line. Using Ca(2+) imaging in living root tips and the stimulation of Ca(2+) transients of different amplitudes, we demonstrated that mitochondrial Ca(2+) uptake became limiting in the triple mutant. The drastic cell physiological phenotype of impaired subcellular Ca(2+) transport coincided with deregulated jasmonic acid-related signaling and thigmomorphogenesis. Our findings establish MCUs as a major mitochondrial Ca(2+) entry route in planta and link mitochondrial Ca(2+) transport with phytohormone signaling.