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INRA
24, chemin de Borde Rouge –Auzeville – CS52627
31326 Castanet Tolosan CEDEX - France

Dernière mise à jour : Mai 2018

Menu Logo Principal Agrocampus Ouest Angers University   IRHS

IRHS

Mitchondrial Biology

Our team has developed research in mitochondrial biology and bioenergetics for many years, with a special interest for the adaptations and role of mitochondria in the context of seed biology and extreme stress tolerance.
  • Reactivation of mitochondria and energy metabolism in germinating seeds

We are investigating the mechanisms of reactivation of mitochondrial function, dynamics and biogenesis during germination.

Mitochondria are inactive in the dry seed, but must reactivate rapidly upon imbibition to provide ATP for germination. This reactivation requires not only a reactivation of mitochondrial metabolism but also of mitochondrial dynamics. Mitochondria are dynamic organelles in shoots and roots. Mitochondria move on F-actin using myosin motors, but movement can also result from remodelling of the actin cytoskeleton (e.g. El Zawily et al., Plant Physiology 2014). This mitochondrial dynamism underpins their function and maintenance. We were recently able to visualize GFP-tagged mitochondria in the dry seed, and thus provide unambiguous evidence that mitochondria in dry Arabidopsis seeds are ready to function since their membrane potential is regained as soon as the embryo imbibes (Paszkiewicz et al., 2017). Quantitative analyses of mitochondrial populations revealed that mitochondria which were distributed essentially as discrete units in the dry seeds remained static until late germination, when dramatic morphological changes in their organization and dynamics occurred, including massive fusion events allowing redistribution of mitochondrial DNA (Paszkiewicz et al., 2017). Thus, during germination and transition to autotrophic growth, extensive biogenesis and remodeling of mitochondria restore morphological and functional properties of the organelle which are well-known in vegetative tissues. 

Dry seeds being metabolically inactive, this raises intriguing questions about how energy metabolism is reactivated upon imbibition. We have used a biochemical approach to investigate adenylate (ATP, ADP, AMP) metabolism during seed imbibition and drying using intact pea seeds or fragmented tissues (Raveneau et al., 2017). AMP was confirmed as the major adenylate stored in dry seeds, and normal adenylate balance was rapidly restored upon rehydration of the tissues. Conversely, re-drying of fully imbibed seeds reversed the balance toward AMP accumulation. The overall analysis, supported by in vitro enzyme mimicking experiments, shows that during tissue dehydration, when oxidative phosphorylation is no longer efficient because of decreasing water content, the ATP metabolic demand is met by adenylate kinase, resulting in accumulation of AMP. During seed imbibition, adenylate balance is rapidly restored from the AMP stock by the concerted action of adenylate kinase and mitochondria. The adenylate balance in orthodox seeds, and likely in other desiccation-tolerant organisms, appears to be simply driven by water content throughout the interplay between ATP metabolic demand, adenylate kinase and oxidative phosphorylation, which requires mitochondria to be energetically efficient from the onset of imbibition.

  • Functional analysis of LEA and sHSP mitochondrial stress proteins

This project focuses on the role of stress proteins expressed in seed mitochondria.

We have previously shown that pea seed mitochondria accumulate a small HSP (HSP22) and LEAM, a late embryogenesis abundant protein. LEAM was shown to be an intrinsically disordered protein that folds into a class A helix upon drying to provide protection to the inner membrane in the dry state (Tolleter et al., 2007). The genetic variability of LEAM and HSP22 in pea was studied by analyzing the protein expression profile in the seeds of a collection of 91 genotypes (coll. Judith Burstin, UMR Agroecology, Dijon). While HSP22 did not exhibit variation, three isoforms of LEAM were uncovered. Cloning of the respective cDNAs revealed the presence of a variable InDel, which did not alter the class A helix motif which is essential for LEAM function (Avelange-Macherel et al., 2015).

We further explored the role of mitochondrial LEA proteins and small HSPs in Arabidopsis to take advantage of reverse genetics tools. This required the unambiguous identification of mitochondrial LEA proteins, and so we performed an extensive characterization of the subcellular localization of the more than 50 LEA proteins encoded in the Arabidopsis genome (Candat et al. 2014). Thirty-six LEA proteins localized to the cytosol, with most being able to diffuse into the nucleus. Three proteins were exclusively localized in plastids or mitochondria, while two others were found dually targeted to these organelles. Targeting cleavage sites could be determined for five of these proteins. Three proteins were found to be endoplasmic reticulum (ER) residents, two were vacuolar, and two were secreted. A single protein was identified in pexophagosomes. The broad subcellular distribution of LEA proteins highlights the requirement for each cellular compartment to be provided with protective mechanisms to cope with desiccation or cold stress. The general importance of LEA proteins for anhydrobiosis led us to develop bioinformatics approach with the building of an interactive database (LEAPdb) dedicated to the identification and in silico analysis of LEA proteins (Jaspard et al., 2012).

The physiological role and molecular function of the five mitochondrial LEA proteins and three sHSP were investigated within the frame of the ANR MITOZEN project (2012-16), in collaboration with the team of Annie Marion-Poll (INRA, IJPB Versailles). A range of single and multiple mutants, as well as overexpressing lines have been constructed, and are currently under characterization using molecular and physiological approaches.

See also

Seedling Vigour Lire >>>

Nitrate perception & Signalling Lire >>>