FMT, a protein that affects mitochondrial distribution, interacts with translation-related proteins in Arabidopsis thaliana

Collective mitochondrial dynamics resolve conflicting cellular tensions: From plants to general principles

Mitochondria play diverse and essential roles in eukaryotic cells, and plants are no exception. Plant mitochondria have several differences from their metazoan and fungal cousins: they often exist in a fragmented state, move rapidly on actin rather than microtubules, have many plant-specific metabolic features and roles, and usually contain only a subset of the complete mtDNA genome, which itself undergoes frequent recombination. This arrangement means that exchange and complementation is essential for plant mitochondria, and recent work has begun to reveal how their collective dynamics and resultant "social networks" of encounters support this exchange, connecting plant mitochondria in time rather than in space. This review will argue that this social network perspective can be extended to a "societal network", where mitochondrial dynamics are an essential part of the interacting cellular society of organelles and biomolecules. Evidence is emerging that mitochondrial dynamics allow optimal resolutions to competing cellular priorities; we will survey this evidence and review potential future research directions, highlighting that plant mitochondria can help reveal and test principles that apply across other kingdoms of life. In parallel with this fundamental cell biology, we also highlight the translational "One Health" importance of plant mitochondrial behaviour - which is exploited in the production of a vast amount of crops consumed worldwide - and the potential for multi-objective optimisation to understand and rationally re-engineer the evolved resolutions to these tensions.

FRIENDLY is required for efficient dark-induced mitophagy and controlled senescence in Arabidopsis

Mitochondria play essential roles in plant metabolism, supporting both development and stress responses. To maintain a healthy pool of mitochondria, several quality control systems are in place. Selected degradation of mitochondria by autophagy -mitophagy- has been extensively studied in yeast and animals, but information on mitophagy components in plants is limited. The 'Friendly Mitochondria' (FRIENDLY; FMT) protein, homologous to 'clustered mitochondria protein homolog' CLU in animals, was recently suggested to mediate mitophagy of depolarized mitochondria. In this study, we evaluated the role of FMT in carbon starvation- and dark senescence-induced mitophagy in Arabidopsis. Using mitophagy flux assays, we show that loss of FMT results in decreased mitophagy during dark-induced senescence. Mitophagy induced by inhibition of Target of Rapamycin (TOR) signalling is also partially impaired in fmt mutants. The FMT protein is mostly localised in the cytosol, but we show that during darkness FMT is redistributed into speckles, some of which associate with mitochondria. Fmt mutants were initially identified for their abnormal mitochondrial morphology, with mitochondria often forming clusters. We found that dark senescence strongly increases the number and size of mitochondrial clusters in fmt mutants. In agreement with a role for FMT in mitophagy, we show that fmt mutants show accelerated senescence phenotypes comparable to autophagy 11 (atg11) mutants, but less prominent than in atg5 mutants. Furthermore, FMT prevents excessive dark-induced cell death and hydrogen peroxide production, and supports mitophagy and greening in etiolated seedlings. Our findings thus indicate that FMT contributes to mitophagy and provide evidence that mitophagy is required for controlled senescence and prevention of accelerated cell death. We propose that FMT mediates efficient mitophagy by preventing mitochondrial clustering, thereby allowing mitochondria to be captured more effectively by autophagosomes, rather than by acting as a direct mitophagy receptor.

FRIENDLY (FMT) is an RNA binding protein associated with cytosolic ribosomes at the mitochondrial surface

The spatial organization of protein synthesis in the eukaryotic cell is essential for maintaining the integrity of the proteome and the functioning of the cell. Translation on free polysomes or on ribosomes associated with the endoplasmic reticulum has been studied for a long time. More recent data have revealed selective translation of mRNAs in other compartments, in particular at the surface of mitochondria. Although these processes have been described in many organisms, particularky in plants, the mRNA targeting and localized translation mechanisms remain poorly understood. Here, the Arabidopsis thaliana Friendly (FMT) protein is shown to be a cytosolic RNA binding protein that associates with cytosolic ribosomes at the surface of mitochondria. FMT knockout delays seedling development and causes mitochondrial clustering. The mutation also disrupts the mitochondrial proteome, as well as the localization of nuclear transcripts encoding mitochondrial proteins at the surface of mitochondria. These data indicate that FMT participates in the localization of mRNAs and their translation at the surface of mitochondria.