MCU proteins dominate in vivo mitochondrial Ca2+ uptake in Arabidopsis roots

Ruberti, Cristina; Feitosa-Araujo, Elias; Xu, Zhaolong; Wagner, Stephan; Grenzi, Matteo; Darwish, Essam; Lichtenauer, Sophie; Fuchs, Philippe; Parmagnani, Ambra Selene; Balcerowicz, Daria; Schoenaers, Sebastjen; de la Torre, Carolina; Mekkaoui, Khansa; Nunes-Nesi, Adriano; Wirtz, Markus; Vissenberg, Kris; Van Aken, Olivier; Hause, Bettina; Costa, Alex; Schwarzländer, Markus

Abstract

Ca²⁺ signaling is central to plant development and acclimation. While Ca²⁺-responsive proteins have been investigated intensely in plants, only a few Ca²⁺-permeable channels have been identified, and our understanding of how intracellular Ca²⁺ fluxes is facilitated remains limited. Arabidopsis thaliana homologs of the mammalian channel-forming mitochondrial calcium uniporter (MCU) protein showed Ca²⁺ transport activity in vitro. Yet, the evolutionary complexity of MCU proteins, as well as reports about alternative systems and unperturbed mitochondrial Ca²⁺ 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 Ca²⁺ transport in planta and that this mechanism is the major route for fast Ca²⁺ uptake. Guided by the subcellular localization, expression, and conservation of MCU proteins, we generated an mcu triple knockout line. Using Ca²⁺ imaging in living root tips and the stimulation of Ca²⁺ transients of different amplitudes, we demonstrated that mitochondrial Ca²⁺ uptake became limiting in the triple mutant. The drastic cell physiological phenotype of impaired subcellular Ca²⁺ transport coincided with deregulated jasmonic acid-related signaling and thigmomorphogenesis. Our findings establish MCUs as a major mitochondrial Ca²⁺ entry route in planta and link mitochondrial Ca²⁺ transport with phytohormone signaling.

Keywords
Mitochiondria, Calcium, Transport, Uniporter