Talmudic Question #2

Why have nitrogen-fixing bacterial endosymbionts of plants not evolved into organelles (“azoplasts”)? (Nitrogenase’ sensitivity to oxygen is not a satisfying answer. Rhizobia have solved that problem!)

Comments

[Autumn Cochrane]

In order to become an organelle, wouldn't the endosymbiont first have to "give up" most of its genome to the plant cell? Because, organelles might have one little plasmid of DNA, but they are not free-living inside their cells. Also, how would it be possible for the circular bacterial chromosome to be taken up into the linear chromosomes of a plant cell? I know that theoretically it can occur, especially in a lab. But in nature, after plants have been evolved to deal with linear chromosomes and have been doing so for ages, isn't it a little late? I mean, when mitochondria and chloroplasts became organelles, it was obviously very early in evolutionary history, possibly during a transition stage of cells when they actually could have had circular chromosomes. This would have made the process infinitely easier to occur naturally. I just can't see that happening in nature during this time period. May be, if the world exploded in nuclear war, the radiation would revert genomes back into circular DNA, and a whole slew of new organelles could arise...

Autumn,

Would that every student in their first microbiology class were as enthusiastic and questioning as you! Indeed, the endosymbiont has to give up much to become an organelle. Some of its genes are transferred to the nuclear chromosomes of the host, others are completely lost. Circular chromosomes insert into linear ones readily. Plasmids and phage, for example, do this all the time. You can see how by drawing this out on paper. With the circular chromosome next to the linear one, make a break in both chromosomes and rejoin each end of the circle to one of the ends of the linear chromosome. Voila! There are some other related posts on our blog that you might find interesting: Play It Again, Cyan, Some Like It Linear, and (for a mitochondrial digression), From Free-Living Bacterium to Cellular Citizen.

Merry

[Kathryn Jones]

The endosymbiotic events that produced mitochondria and chloroplasts occurred in a unicellular host.
Rhizobial bacteria that invade plant hosts infect root cells and are not introduced into the plant germ line. (So far, no one has succeeded in culturing and propagating root cells infected with rhizobia.) So, these endosymbiotic events are terminal and aren’t passed to the next generation.

But why hasn’t a rhizobium formed an endosymbiosis with a unicellular eukaryote such as an algae to produce an azoplast? Maybe we just haven’t found it yet.

Why hasn’t a nitrogen-fixing cyanobacterium formed an endosymbiosis with a protist to produce a combination chloroplast/azoplast?
Evidence suggests that it has--perhaps more than once.
In Martin et al., (2002)*, the authors point out that “the cyanobacterial ancestor of the plastids was…closer to [Nostoc] punctiforme than to [Prochlorococcus] marinus or Synechocystis sp. [PCC 6803]”. Since N. punctiforme is a nitrogen-fixing cyanobacterium, Martin et al. entertain the possibility that early plastids could fix nitrogen. In fact, N. punctiforme is able to form an endosymbiosis with the plant Gunnera.
Also, in the article on page 186 of this issue, Prof. Capone cites Foster et al., (2006)** who found a marine dinoflagellate, Histioneis sp. host 1 carrying an endosymbiotic cyanobacteria that was able to produce significant amounts of the nitrogen-fixing enzyme nitrogenase.

One would think that the advantage of carrying a nitrogen-fixing symbiont would be great enough that the eukaryotic cell would find a way to trap it, unless the costs outweigh the benefits.

*Martin W, Rujan T, Richly E, Hansen A, Cornelsen S, Lins T, Leister D, Stoebe B, Hasegawa M, Penny D: Evolutionary analysis of Arabidopsis, cyanobacterial, and chloroplast genomes reveals plastid phylogeny and thousands of cyanobacterial genes in the nucleus. Proc Natl Acad Sci USA 2002, 99:12246-12251.

**Foster, R. A., E. J. Carpenter, and B. Bergman. 2006. Unicellular symbionts in open ocean dinoflagellates, radiolarians and tintinnids: Ultrastructural characterization and immuno-localization of phycoerythrin and nitrogenase. Journal of Phycology 42:453

[Bill Martin]

Well, some of us would probably say that the limiting step in the transition from a prokaryotic endosymbiont (of which there are oodles and oodles in nature) to a prokaryote-derived organelle (a mitochondrion or a chloroplast) is the origin of the protein import apparatus that organelles use to import proteins from the cytosol. Endosymbionts encode in their own genome all of the proteins that they require, organelles don't. Maybe that's "why" endosymbionts are extremely common in nature (and probably always have been since phagocytosis arose) but mitochondria and chloroplasts each arose only one single time (each) in all of cell history. It is apparently very difficult to become an organelle, otherwise it would happen all the time (which it doesn't). It is easy to become an endosymbiont, and it does indeed happen all the time.

[john breznak]

We're probably seeing rhizobia in the middle of that very evolution into plant "azoplasts" or
"nitrogenosomes" as we speak. Let's check back on them in another
million years or so and see what they look like!

John Breznak