Marvellously mobile mitochondria…or are they?

Can anybody doubt how marvellously dynamic a plant cell is when one watches videos of such phenomena as cytoplasmic streaming?

And plant cells aren’t limited to mass movements of organelles within the cytoplasm, individual organelles can also undergo various movements, too, e.g. Golgi, mitochondria and peroxisomes. Who said plants were boring or ‘don’t do anything’?

But, these are intracellular movements. What about intercellular movement of organelles? Does that happen? Yes, if one takes at face value the claim that is unambiguously made by Rutgers University (New Jersey, USA)-based Csanad Gurdon et al. in their paper entitled “Cell-to-cell movement of mitochondria in plants”.

The work concerned cellular transfer of materials across a graft junction between two members of the Nicotiana genus, N. sylvestris (wood tobacco) and N. tabacum (smoking tobacco). Although grafting is interesting (e.g. Charles Melnyk and Elliot Meyerowitz), it is not as attention-grabbing as the claim in the article’s title. One had to read more, and one duly did. But, having read the paper I can’t find any clear, unambiguous evidence that it is in fact mitochondria – the organelle – that have been translocated from one partner to the other in the graft union that was studied. I understand the reported evidence that supports the view that mitochondrial genes from one party appear in cells of the other participant, but nowhere (and much as I’d love to see it!) do I see the proof positive that intact mitochondria have made this journey.

Mitochondrion
Image: Kelvinsong / Wikipedia

Without that definitive proof, isn’t the title of the paper a little inaccurate? Which raises the not unimportant matter of ‘hype’ in the titles of science papers. Whilst we are used to news reports frequently over-playing the importance or significance of scientific work, should we expect similar over-statement of the work by the practitioners themselves, in scientific papers? Shouldn’t scientific journals be bastions of self-restraint and objectivity?

Yes, it is important that practitioners communicate the work that scientists do – which is often funded by government ultimately from the members of the public via their taxes – but how much responsibility should they bear for honest reporting of the work? This is not a difference of opinion on semantic lines between cell biologists and molecular biologists; mitochondrial DNA is not a mitochondrion. Similar grafting work a few years previously by Potsdam (Germany)-based Sandra Stegemann et al. concerning horizontal transfer of chloroplast genes was more cautious in announcing “Horizontal transfer of chloroplast genomes between plant species” with no suggestion that this involved intact chloroplasts. However, such was the claim made by Gregory Thyssen et al. (from the same US-university as the Gurnod et al. paper) – “Cell-to-cell movement of plastids in plants”. Although, in the abstract, they were much more cautious in saying, “Here, we present evidence for cell-to-cell movement of the entire 161-kb plastid genome in these plants, most likely in intact plastids.” Is this evidence of a trans-Atlantic divide – Germany (Europe…) versus USA – in the style of presentation of scientific articles?

Anyway – and even if proven – the short hop for a mitochondrion from one angiosperm to another via a graft is as nothing compared to the monumental journey that was made when – as current understanding would have it – an ancestral precursor of that modern-day organelle was assimilated within another entity thereby helping to create the eukaryotic cell*, millions of years ago as a component of the Serial Endosymbiotic Theory (SET).

* In the interests of some sort of balance, for a recent item on chloroplasts and the SET, try this contribution at the Plant Scientist blog site by guest blogger Joram Schimmeyer.

[Ed. – Although P Cuttings’ interest in this story has been from an organelle movement/science veracity point of view, the press has – understandably – concentrated on the ‘accidental’ genetic modification of plants that may have been unwittingly carried out by farmers and others who’ve practised the ancient art of grafting for hundreds of years, which has often resulted in so-called horizontal gene transfer.]

Reference List

Csanad Gurdon, Zora Svab, Yaping Feng, Dibyendu Kumar, Pal Maliga, 2016, 'Cell-to-cell movement of mitochondria in plants', Proceedings of the National Academy of Sciences, vol. 113, no. 12, pp. 3395-3400 http://dx.doi.org/10.1073/pnas.1518644113

Charles W. Melnyk, Elliot M. Meyerowitz, 2015, 'Plant grafting', Current Biology, vol. 25, no. 5, pp. R183-R188 http://dx.doi.org/10.1016/j.cub.2015.01.029

S. Stegemann, M. Keuthe, S. Greiner, R. Bock, 2012, 'Horizontal transfer of chloroplast genomes between plant species', Proceedings of the National Academy of Sciences, vol. 109, no. 7, pp. 2434-2438 http://dx.doi.org/10.1073/pnas.1114076109

G. Thyssen, Z. Svab, P. Maliga, 2012, 'Cell-to-cell movement of plastids in plants', Proceedings of the National Academy of Sciences, vol. 109, no. 7, pp. 2439-2443 http://dx.doi.org/10.1073/pnas.1114297109

Alexandros A. Pittis, Toni Gabaldón, 2016, 'Late acquisition of mitochondria by a host with chimaeric prokaryotic ancestry', Nature http://dx.doi.org/10.1038/nature16941

Patrick J. Keeling, Jeffrey D. Palmer, 2008, 'Horizontal gene transfer in eukaryotic evolution', Nature Reviews Genetics, vol. 9, no. 8, pp. 605-618 http://dx.doi.org/10.1038/nrg2386

Ignacia Fuentes, Sandra Stegemann, Hieronim Golczyk, Daniel Karcher, Ralph Bock, 2014, 'Horizontal genome transfer as an asexual path to the formation of new species', Nature, vol. 511, no. 7508, pp. 232-235 http://dx.doi.org/10.1038/nature13291