Mitochondria are the power generators of cells that play a diversity of physiological and metabolic roles under conditions of abiotic or biotic stress. They are embedded in responses to an astonishing variety of stresses, from drought, salinity, light, toxins, herbivorous attacks, and pathogen attacks. However, involvement in such a wide range of responses implies mitochondrial multi-stress signaling related to energy and redox balance, which could function as integrators of multiple climate change signals and be involved in acclimation to climate change. When a plant is exposed to environmental stress, this must be sensed either by a receptor on the cell wall/plasma membrane or directly inside the cell. This is then likely signaled to the nucleus and the mitochondria.

Environmental, biotic, abiotic, and chemical stresses applied to plants are well known to induce oxidative stress in plant cells. These stresses alter plant metabolism, growth and development and, at their extremes, can lead to death. Recently, several studies have begun to examine the changes that occur within plant mitochondria following the induction of oxidative stress. Mitochondria contain two terminal oxidases that reduce oxygen to water and the entire ETC. The ETC and OXPHOS sustain the major mitochondrial function of ATP generation in relation to the metabolic dynamics of tricarboxylic acids (TCAs), acetyl-CoA, ADP, oxidized (NAD+) or reduced (NADH) β-nicotinamide adenine dinucleotide, oxidized (FAD) or reduced (FADH2) flavin adenine dinucleotide. The NAD+/NADH and FAD/FADH2 equilibriums are maintained through the TCA cycle, the ETC, and OXPHOS. Photosynthetic carbon capture is a coordinated process that requires cooperation between organelles, and mitochondrial functions are particularly important in sustaining photosynthesis under high light. The fastest photosynthetic rates and the avoidance of photoinhibition are dependent upon mitochondrial function being configured to rapidly transfer electrons from NADH into the water through the non-phosphorylating bypasses of the ETC.

In response to alterations in cellular metabolism and energy demands, mitochondria often undergo changes in their morphology and respiratory capacity by regulating the composition and abundance of the protein machinery. These differences, or heterogeneity of mitochondria, have been observed through reports of tissue-selective phenotypes of mutants. In many recent reverse-genetics studies, mutations of nuclear genes encoding mitochondrial proteins have been reported to yield organ-specific plant phenotypes.

In this way, mitochondria are dynamically tuned to meet the specific needs for energy in different tissue types or in response to the environment that mediates tolerance mechanisms in stress-treated plants, such as altered rates of protein turnover, rebalancing of cellular metabolite pools, altered abundance of ROS species, and changes in the redox ratios within pools of reducing equivalents. Each of these phenomena can be linked to mitochondrial processes through alterations to central metabolic pathways or by their demand for fast rates of ADP: ATP cycling mediated by respiratory oxidative phosphorylation. The assembly of mitochondrial machinery, the signaling by mitochondrial of oxidative stress, and the regulation of respiratory rate are still needed in order to maximize respiration for plant protection in severe environments and to minimize respiratory losses to enhance plant yields.

Mohsina Afreen
Associate Scientist, ASRBC.