Calcium Flux across Plant Mitochondrial Membranes: Possible Molecular Players.

Luca Carraretto,Ildikó Szabó,Sara De Bortoli,Enrico Teardo,Elide Formentin,Vanessa Checchetto,Alex Costa

doi:10.3389/fpls.2016.00354

Abstract

Plants, being sessile organisms, have evolved the ability to integrate external stimuli into metabolic and developmental signals. A wide variety of signals, including abiotic, biotic, and developmental stimuli, were observed to evoke specific spatio-temporal Ca2+ transients which are further transduced by Ca2+ sensor proteins into a transcriptional and metabolic response. Most of the research on Ca2+ signaling in plants has been focused on the transport mechanisms for Ca2+ across the plasma- and the vacuolar membranes as well as on the components involved in decoding of cytoplasmic Ca2+ signals, but how intracellular organelles such as mitochondria are involved in the process of Ca2+ signaling is just emerging. The combination of the molecular players and the elicitors of Ca2+ signaling in mitochondria together with newly generated detection systems for measuring organellar Ca2+ concentrations in plants has started to provide fruitful grounds for further discoveries. In the present review we give an updated overview of the currently identified/hypothesized pathways, such as voltage-dependent anion channels, homologs of the mammalian mitochondrial uniporter (MCU), LETM1, a plant glutamate receptor family member, adenine nucleotide/phosphate carriers and the permeability transition pore (PTP), that may contribute to the transport of Ca2+ across the outer and inner mitochondrial membranes in plants. We briefly discuss the relevance of the mitochondrial Ca2+ homeostasis for ensuring optimal bioenergetic performance of this organelle.

Highlights

Molecular identification and pharmacological characterization of mitochondria-located ion channels allowed a deep understanding of the crucial importance of these proteins for organelle function and even for determining cell fate in animals (Leanza et al, 2014; Szabo and Zoratti, 2014)
In plant mitochondria the current knowledge is more limited than in the animal system and only few electrophysiological studies deal with plant mitochondrial ion channels
permeability transition pore (PTP) could still be opened in the absence of spastic paraplegia 7 (SPG7), at higher matrix calcium concentrations, suggesting that SPG7, to cyclophilin D (CypD), acts a regulator rather than a crucial pore-forming moiety of the PTP (Bernardi and Forte, 2015)

Summary

Mitochondria and Calcium Homeostasis

Molecular identification and pharmacological characterization of mitochondria-located ion channels allowed a deep understanding of the crucial importance of these proteins for organelle function and even for determining cell fate in animals (Leanza et al, 2014; Szabo and Zoratti, 2014). In plant mitochondria the current knowledge is more limited than in the animal system and only few electrophysiological studies deal with plant mitochondrial ion channels These include the voltage-dependent anion channel (VDAC), e.g., PTP could still be opened in the absence of SPG7, at higher matrix calcium concentrations, suggesting that SPG7, to CypD, acts a regulator rather than a crucial pore-forming moiety of the PTP (Bernardi and Forte, 2015) Apart from their proteolytic roles, the m-AAA proteases mediate ATPdependent membrane dislocation of the heme-binding reactive oxygen scavenger protein Ccp (Tatsuta et al, 2007), possibly linking PTP activation to oxidative stress

VDAC of the Outer Mitochondrial Membrane

Permeability Transition Pore