Dissecting the Metabolic Role of Mitochondria during Developmental Leaf Senescence

Abstract

The functions of mitochondria during leaf senescence, a type of programmed cell death aimed at the massive retrieval of nutrients from the senescing organ to the rest of the plant, remain elusive. Here, combining experimental and analytical approaches, we showed that mitochondrial integrity in Arabidopsis (Arabidopsis thaliana) is conserved until the latest stages of leaf senescence, while their number drops by 30%. Adenylate phosphorylation state assays and mitochondrial respiratory measurements indicated that the leaf energy status also is maintained during this time period. Furthermore, after establishing a curated list of genes coding for products targeted to mitochondria, we analyzed in isolation their transcript profiles, focusing on several key mitochondrial functions, such as the tricarboxylic acid cycle, mitochondrial electron transfer chain, iron-sulfur cluster biosynthesis, transporters, as well as catabolic pathways. In tandem with a metabolomic approach, our data indicated that mitochondrial metabolism was reorganized to support the selective catabolism of both amino acids and fatty acids. Such adjustments would ensure the replenishment of α-ketoglutarate and glutamate, which provide the carbon backbones for nitrogen remobilization. Glutamate, being the substrate of the strongly up-regulated cytosolic glutamine synthase, is likely to become a metabolically limiting factor in the latest stages of developmental leaf senescence. Finally, an evolutionary age analysis revealed that, while branched-chain amino acid and proline catabolism are very old mitochondrial functions particularly enriched at the latest stages of leaf senescence, auxin metabolism appears to be rather newly acquired. In summation, our work shows that, during developmental leaf senescence, mitochondria orchestrate catabolic processes by becoming increasingly central energy and metabolic hubs.

Metabolite Profiling for Leaf Senescence in Barley Reveals Decreases in Amino Acids and Glycolysis Intermediates

“…Metabolic changes during leaf senescence have been mainly investigated in Arabidopsis [11,13,32]. They showed many similar features to what we found here in barley, such as the accumulation of minor CHO and the decrease in most of the amino acids like glutamine, glutamate, aspartate, serine, glycine and arginine.…”

“…While pipecolate and α-aminoadipate decreased in primary leaves with senescence, they sharply accumulated in flag leaves with ageing, suggesting that lysine was used for mitochondria respiration in seedlings but not so much in the flag leaves. The anapleurotic respiration of amino acids has been mainly observed in Arabidopsis in response to dark stress [32,34,37] and Chrobok et al [32] showed that the whole metabolic pathway of lysine degradation was up-regulated in Arabidopsis individual darken leaves. It could then be possible that the differences of light environment between primary and flag leaves are the reason of these discrepancies and that lysine level could be related to both glutamate content and TCA replenishment.…”

“…It seems thus that the mitochondrial respiration is not affected in a similar manner by ageing at reproductive and vegetative stages. The maintenance of mitochondrial respiration during flag leaf senescence could explain that TCA compounds are poorly affected by ageing [32]. However, pipecolate and α-aminoadipate, which are both markers of the lysine anapleurotic respiratory pathway, increased during senescence in the flag leaf, suggesting a specific role of this pathway in flag leaf compared to seedling leaves [33,34].…”

Fission and Fusion of Plant Mitochondria, and Genome Maintenance

“…During leaf senescence in individually darkened leaves in Arabidopsis, the number of mitochondria decreases (Keech et al, 2007), probably by autophagy . The remaining mitochondria retain their motility and respiration activity until the last step of leaf senescence (Keech et al, 2007;Keech, 2011;Ruberti et al, 2014;Chrobok et al, 2016). These mitochondria are thought to provide energy and metabolites for degrading the cell components and relocating them to other younger parts of the plant (Keech et al, 2007;Keech, 2011;Ruberti et al, 2014;Chrobok et al, 2016).…”

Triphosphate Tunnel Metalloenzyme Function in Senescence Highlights a Biological Diversification of This Protein Superfamily

“…Other organelles, such as mitochondria and peroxisomes, remain active much longer and provide energy for the cell as well as take over a number of metabolic tasks during this process (Keech et al, 2007;Chrobok et al, 2016). While yeast and animals possess mitochondrial PPases (Lundin et al, 1991;Curbo et al, 2006), so far no mitochondrial PPase has been identified in plants.…”