Abstract
Mitochondria increase in number by the fission of existing mitochondria. Mitochondrial fission is needed to provide mitochondria to daughter cells during cell division. In Arabidopsis thaliana, four kinds of genes have been reported to be involved in mitochondrial fission. Two of them, DRP3 (dynamin-related protein3) and FIS1 (FISSION1), are well conserved in eukaryotes. The other two are plant-specific ELM1 (elongated mitochondria1) and PMD (peroxisomal and mitochondrial division). To better understand the commonality and diversity of mitochondrial fission factors in land plants, we examined mitochondrial fission-related genes in a liverwort, Marchantia polymorpha. As a bryophyte, M. polymorpha has features distinct from those of the other land plant lineages. We found that M. polymorpha has single copies of homologues for DRP3, FIS1 and ELM1, but does not appear to have a homologue of PMD. Citrine-fusion proteins with MpDRP3, MpFIS1 and MpELM1 were localized to mitochondria in M. polymorpha. MpDRP3- and MpELM1-defective mutants grew slowly and had networked mitochondria, indicating that mitochondrial fission was blocked in the mutants, as expected. However, knockout of MpFIS1 did not affect growth or mitochondrial morphology. These results suggest that MpDRP3 and MpELM1 but neither MpFIS1 nor PMD are needed for mitochondrial fission in M. polymorpha.
Highlights
Almost all eukaryotes have mitochondria, organelles that are essential for energy production and supply, for the control of diverse metabolic pathways and for regulation of cell death
In wild-type Arabidopsis, ELM1 surrounds mitochondria and interacts with both DRP3A and DRP3B32. These results suggest that ELM1 is required for relocalization of DRP3A from the cytosol to mitochondrial fission sites
The amino acid sequences of MpDRP3s and MpDRP3l are highly similar to their Arabidopsis and Physcomitrella homologues (Fig. S1a)
Summary
The Marchantia genome has single copies of homologues of DRP3, FIS1 and ELM1. For searches of the M. polymorpha genome, we used the JGI M. polymorpha EST and genome databases ver. 3.1 (https://phytozome.jgi.doe.gov/pz/portal.html). In transgenic plants expressing these proteins and stained with MitoTracker, a part of the Citrine-MpDRP3s and Citrine-MpDRP3l signals localized to the constriction sites of mitochondria (Fig. 2a, arrowheads). As with the fluorescence microscopy, transmission electron microscopy showed that most of mitochondria in Mpelm[1] and Mpdrp[3] were much longer than those in the wild-type and Mpfis[1] (Fig. 4c). Enlarged structures of mitochondria included vesicle-like structures (black arrowhead) and vacuolated structures (black arrow) These structures might be similar to the dark areas inside mitochondrial red bodies (shown in the insets of Fig. 4c (yellow arrows)), which were not stained with MitoTracker. Mpelm[1] cells with MpELM1-Citrine have discrete complemented mitochondria (Fig. 5b)
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