Daria Chrobok, Simon R. Law, Bastiaan Brouwer, Pernilla Lindén, Agnieszka Ziolkowska, Daniela Liebsch, Reena Narsai, Bozena Szal, Thomas Moritz, Nicolas Rouhier, James Whelan, Per Gardeström, Olivier Keech

The functions of mitochondria during leaf senescence, a type of programmed cell death aiming 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 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 is also maintained during this time period. Further, after establishing a curated list of genes coding for products targeted to mitochondria, we analysed in isolation their transcript profiles, focusing on several key mitochondrial functions such as the tricarboxylic acid cycle, mitochondrial electron transfer chain, iron-sulphur cluster biosynthesis, transporters as well as catabolic pathways. In tandem with a metabolomic approach, our data indicated that mitochondrial metabolism was reorganised to support the selective catabolism of both amino- and fatty acids. Such adjustments would ensure the replenishment of α-ketoglutarate and glutamate, which provide the carbon backbones for nitrogen remobilisation. 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 appeared 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.