Plant Mitochondria

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Plant Mitochondria

Abi S Ghifari, The University of Western Australia, Crawley, Western Australia, Australia
Monika W Murcha, The University of Western Australia, Crawley, Western Australia, Australia

Published online: October 2020

DOI: 10.1002/9780470015902.a0029217

Full Article on Wiley Online Library

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Abstract

Plant mitochondria, like the mitochondria of other eukaryotes, originated from an endosymbiotic event and have retained their own genome. Over a billion years of evolution, most mitochondrial genes are laterally transferred to the nucleus. Consequently, most proteins required for mitochondrial biogenesis and activity are synthesised in the cytosol and imported via specific protein import machineries. Mitochondria are energy‐generating organelles that via electron transfer chain (ETC), create a proton gradient and generate adenosine triphosphate (ATP), the energy chemical used by the cell for numerous biochemical reactions. Energy production can come at a cost as reactive oxygen species (ROS) are generated as by‐product. Plant mitochondria have alternative ETC pathways to regulate mitochondrial ROS (mtROS) through modulating the rate of redox reactions, proton gradient and ATP production. Although excessive ROS can damage cellular components and lead to programmed cell death, mtROS also play an important role as signalling molecules. mtROS are involved in nucleus–mitochondria communications (anterograde–retrograde signalling), mitochondrial autophagy (mitophagy) and play a role in the plant response to various environmental stimuli. Considering the pivotal roles of mitochondria in energy generation and cellular signalling, perturbation of mitochondrial biogenesis and activity renders plants to be more susceptible to various stresses and ultimately compromises plant growth, development and reproduction.

Key Concepts

Plant mitochondria are small (1–2 μm long) rod‐shaped organelle that undergo frequent fusion and fission events.
The plant mitochondrial inner membrane houses protein complexes involved in the electron transfer chain (ETC) that creates a proton‐motive gradient across the intermembrane space to drive the production of adenosine triphosphate (ATP) through ATP synthase.
The majority of plant mitochondrial proteins (approximately 2000) are encoded in the nuclear genome, whereas the plant mitochondrial genome contains around 50 genes encoding for ETC subunits, mitoribosomal translation machinery and transfer ribonucleic acids (tRNAs).
The abundance of mtROS produced by the ETC is proportionally linked with the efficiency and rate of electron transfer and ATP production.
Plant mitochondria have alternative ETC pathways to modulate ROS homeostasis, proton gradient and ATP production.
mtROS can damage mitochondrial macromolecules and ultimately organelle function but are also important signalling molecules in various pathways, including stress response and anterograde–retrograde signalling.
The nucleus controls mitochondrial gene expression through anterograde signalling, yet the mitochondria can also influence nuclear gene expression through retrograde signalling.
Disrupted plant mitochondrial function often causes enhanced sensitivity to a number of stresses, disrupted pollen development in cytoplasmic male sterility and affects normal plant growth and development.

Keywords: anterograde and retrograde signalling; cytoplasmic male sterility; electron transfer chain; mitochondrial biogenesis; mitochondrial genome; mitochondrial homeostasis; mitochondrial import; mitophagy; plant mitochondria; reactive oxygen species; stress response

	Figure 1. Structural organisation of mitochondria that mainly divided into six compartments: outer membrane, inner membrane, matrix, cristae, intermembrane space and intercristal space.
	Figure 2. Plant mitochondrial electron transfer chain (ETC) in the inner membrane (IM). Complexes actively involved in the redox reactions and electron transfer are indicated in blue (Complex I–IV). ATP synthase (Complex V) consists of Fo and F1 units are shown in cyan. Plant‐specific alternative ETC pathways are shown in grey, which comprise of alternative oxidase (AOX) and NAD(P)H dehydrogenases (NDs). Electron (e⁻) transfer is depicted as dashed lines and protons are pumped from the matrix (M) to the intermembrane (IMS) and intercristal spaces. Cyt c, cytochrome c; PUMP, plant uncoupling mitochondrial protein; TCA, tricarboxylic acid; UQ, ubiquinone.
	Figure 3. Protein import pathways into plant mitochondria. Four major protein import pathways (dashed lines) for different types of protein from the cytosol (C) that all cross the translocase of outer membrane (TOM) complex. The general pathway (blue) describes the import of protein containing N‐terminal targeting peptide that uses the translocase of inner membrane (TIM17:23) complex and associated presequence‐assisted motor (PAM) complex into the matrix (M). The carrier import pathway (red) depicts the import of protein with internal targeting signal, mostly carrier proteins to the inner membrane via TIM22 gate. The sorting and assembly machinery (SAM) pathway is specifically for β‐barrel proteins targeted to the outer membrane (OM). Both carrier pathway and SAM pathway use small TIMs in assisting the translocation. The mitochondrial intermembrane space (IMS) assembly (MIA) pathway is used for translocating twin‐cysteine proteins into the IMS.
	Figure 4. Intracellular communications within plant cell. The nucleus controls nuclear‐encoded mitochondrial gene expression (anterograde signalling), but mitochondria and chloroplasts influence nuclear gene expression through retrograde signalling, primarily by reactive oxygen species (ROS) and calcium ion (Ca²⁺). These signalling molecules may directly alter nuclear gene expression (red solid lines) or indirectly (blue dashed lines) through activation of redox sensors and post‐translational modifications by kinases (K) and phosphatases (P).

Article Title:	Plant Mitochondria
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