As powerhouses of eukaryotic cells, mitochondria are of crucial importance to fuel germination and early seedling establishment. We have shown earlier that seed mitochondria were endowed with protective mechanisms (LEA, Hsp proteins) to counteract the effects of desiccation and abiotic stress likely to occur during germination. More recently, we showed that the mitochondrial HSP22 protein which is strongly expressed in seeds forms large dispersed oligomers which dissociate into monomers under heat shock conditions, then co-precipitating with unfolded aggregating client proteins (Avelange-Macherel et al., 2020). These "hardened" seed mitochondria can thus provide cellular ATP as soon as tissues are hydrated during imbibition, and thus contribute to the rapid resumption of metabolism needed for germination, followed by the reconfiguration of the chondriome structure via fusion-fission dynamics during germination (Paszkiewicz et al., 2017).  Despite their fragile appearance, Arabidopsis seedlings were found to withstand extreme mineral nutrient starvation for weeks, entering a developmental and metabolic steady state in which mitochondria ensure dissipation of energy through alternative pathways and photorespiration (Rethoré et al., 2019).

To investigate the role of energy-transducing organelles in acquired thermo-tolerance (AcclimHOT project), we take advantage of our seedling liquid culture system in which Arabidopsis seedlings are maintained in a developmental steady-state. It allows us to study how a short priming treatment (38°C, 2h) protects the seedlings from an otherwise lethal heat shock (43°C, 2h) applied 24h later. We have developed an integrative approach (physiology, organelle dynamics, omics) which revealed that, under heat shock, the preservation of primary energy metabolism triggered by priming was playing the role of a switch towards subsequent life or death (manuscript in preparation).

We are also interested by the role of mitochondria in cold tolerance, and indeed, we showed much earlier that isolated pea seed mitochondria could efficiently perform oxidative phosphorylation at negative temperature, a capacity likely allowing peas to germinate on ice in less than a fortnight (Stupnikova et al., 2006). Recently, the suggestion that human mitochondria could operate at a temperature close to 50°C because of their intense activity as energy transducers has led to hot debate because it cannot be explained yet by the laws of thermal physics (Macherel et al., 2021). This led us to propose that self-warming of plant mitochondria could contribute to the efficiency of the organelle at low temperature, and therefore develop a genetic fluorescent thermosensor approach (THERMIT project). We have built a series of Arabidopsis lines expressing genetic fluorescent nanothermometers in several sub-cellular compartments to establish whether (or not) mitochondrial temperature is higher than its immediate surroundings, which would be of utmost interest in terms of plant cold and freezing tolerance.