A chloroplast-localized mitochondrial calcium uniporter transduces osmotic stress in Arabidopsis

Chloroplasts are integral to sensing biotic and abiotic stress in plants, but their role in transducing Ca²⁺-mediated stress signals remains poorly understood^1,2. Here we identify cMCU, a member of the mitochondrial calcium uniporter (MCU) family, as an ion channel mediating Ca²⁺ flux into chloroplasts in vivo. Using a toolkit of aequorin reporters targeted to chloroplast stroma and the cytosol in cMCU wild-type and knockout lines, we provide evidence that stress-stimulus-specific Ca²⁺ dynamics in the chloroplast stroma correlate with expression of the channel. Fast downstream signalling events triggered by osmotic stress, involving activation of the mitogen-activated protein kinases (MAPK) MAPK3 and MAPK6, and the transcription factors MYB60 and ethylene-response factor 6 (ERF6), are influenced by cMCU activity. Relative to wild-type plants, cMCU knockouts display increased resistance to long-term water deficit and improved recovery on rewatering. Modulation of stromal Ca²⁺ in specific processing of stress signals identifies cMCU as a component of plant environmental sensing.