Mitochondrial Electron Transport and Plant Stress

Abstract

Due to the sessile nature of plants, it is crucial for their survival and growth that they can handle a constantly changing, and thus stressful, ambient environment by modifying their structure and metabolism. The central metabolism of plants is characterized by many alternative options for metabolic pathways, which allow a wide range of adjustments of metabolic processes in response to environmental variations. Many of the metabolic pathways in plants involve the processing of redox compounds and the use of adenylates. They converge at the mitochondrial electron transport chain (ETC) where redox compounds from carbon degradation are used for powering ATP synthesis. The standard ETC contains three sites of energy conservation in complexes I, III, and IV, which are in common with most other eukaryotes. However, the complexity of the plant metabolic system is mirrored in the ETC. In addition to the standard enzymes, plants have a large set of supplementary electron transport enzymes. Many of these, such as the external and internal NAD(P)H dehydrogenases, proline dehydrogenase, and glycerol-3-phosphate dehydrogenase, feed into the ubiquinone pool and they therefore bypass the first site of energy conservation in the ETC. The alternative oxidase provides a non-energy-conserving alternative to electron transport through complexes III and IV. There also appears to be a special coupling between specific NAD(P)H dehydrogenases and specific members of the alternative oxidase family. These additional enzymes therefore give a great flexibility in the type and origin of the substrate, the electron transport route(s) used, and the energy yield. At the same time special reactions, such as ascorbate biosynthesis, can take place. In this way, the mitochondrial ETC can mediate major adjustments in cellular metabolism that is important for cellular function under a great variety of stress conditions such as low temperature and drought.

Glossary

Acclimation:

A plant stress response, physiologic, and/or morphologic, that increases the ability of the plant to tolerate stress.

Alternative oxidase:

A peripheral membrane protein that oxidizes ubiquinol to ubiquinone and reduces O₂ to H₂O, and which constitutes an energy-bypass.

Anoxia:

The special case of hypoxia where O₂ is virtually absent.

Ascorbate:

A major cellular antioxidant (vitamin c), synthesized by the electron transport chain in plants.

Compatible solute:

A molecule that is synthesized in response to osmotic stress in so large amounts that the osmotic potential is lowered significantly without harming cellular constituents,

Electrochemical proton gradient:

The combined effect of the difference in electrical charge and in proton concentration across a membrane, determining the energy change for protons transported across the membrane.

Electron transport chain:

A set of redox enzymes linked by associated smaller molecules that allow electrons to pass from reduced substrates (e.g., NADH) to acceptors (e.g., O₂).

Energy-bypass:

A protein that forms a parallel path for electron or proton transport, bypassing one or several of the sites for energy coupling between electron transport and ATP synthesis.

Glycine oxidation:

The conversion of two glycines to serine, CO₂, and NH ⁺₄ . This reaction is the major provider of NADH in the mitochondrial matrix in leaves of C3-plants in the light.

Hypoxia:

The condition where the O₂ concentration is below a threshold level imposing a deficiency effect on the plant.

Male sterility:

A defect in production of functional pollen often associated with changes in mitochondria.

NAD(P)H dehydrogenases:

Membrane proteins that oxidize NADH to NAD⁺, and/or NADPH to NADP⁺ and reduce ubiquinone to ubiquinol. Plants contain type I (complex I) and type II NAD(P)H dehydrogenases, the latter being energy-bypasses to the former.

NO:

Nitric oxide; a signaling molecule and inhibitor of complex IV.

Oxidative stress:

A condition where reactive oxygen species accumulate to such high levels that they negatively affect cellular function.

Reactive oxygen species:

ROS; oxygen-containing molecules that can damage cellular constituents by chemical reaction with them. Many ROS are radicals.

Reductant:

A molecule (e.g., NADH, NADPH, malate, and reduced ferredoxin) that carries low redox potential electrons, which can be donated to another molecule via a redox reaction.

Retrograde regulation:

The process where the functional status in an organelle is signaled to the nucleus and affects gene expression.

Ubiquinone:

An electron and proton carrier located in the mitochondrial inner membrane, where it mediates electron and proton transport.

Uncoupling protein:

A proton transporter and energy-bypass that allows protons exported by the electron transport chain to flow back into the matrix without passing the ATP synthase.