4 Comments

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!)

 

Other Posts

4 Comments
Oldest
Newest Most Voted
john breznak
19 years ago

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

0
Bill Martin
19 years ago

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.

0
Kathryn Jones
18 years ago

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

0
Autumn Cochrane
18 years ago

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

0