• This forum is specifically for the discussion of factual science and technology. When the topic moves to speculation, then it needs to also move to the parent forum, Science Fiction and Fantasy (SF/F).

    If the topic of a discussion becomes political, even remotely so, then it immediately does no longer belong here. Failure to comply with these simple and reasonable guidelines will result in one of the following.
    1. the thread will be moved to the appropriate forum
    2. the thread will be closed to further posts.
    3. the thread will remain, but the posts that deviate from the topic will be relocated or deleted.
    Thank you for understanding.​

Botany: Why don't all plants "fix" nitrogen?

Introversion

Pie aren't squared, pie are round!
Kind Benefactor
Super Member
Registered
Joined
Apr 17, 2013
Messages
10,642
Reaction score
14,865
Location
Massachusetts
Nitrogen fixation seems to involve a painful tradeoff.

Ars Technica said:
Plants, like all living things, need nitrogen to build amino acids and other essential biomolecules. Although nitrogen is the most abundant element in air, the molecular form of nitrogen found there is largely unreactive. To become useful to plants, that nitrogen must first be "fixed," or busted out of its molecular form and linked with hydrogen to make ammonia. The plants can then get at it by catalyzing reactions with ammonia.

But plants can't fix nitrogen. Bacteria can.

Some legumes and a few other plants have a symbiotic relationship with certain bacterial species. The plants build specialized structures on their roots called nodules to house and feed the bacteria, which in turn fix nitrogen for the plants and assure them a steady supply of ammonia. Only 10 families of plants have the ability to do this, and even within these families, most genera opt out. Ever since the symbiosis was discovered in 1888, plant geneticists have wondered: why? If you could ensure a steady supply of nitrogen for use, why wouldn't you?

A global consortium of geneticists sequenced and compared the genomes of 37 plants—some symbiotic, some not; some that build nodules, some not; some agriculturally relevant, some not—to try to find out what was going on. The group's genetic analysis of the conundrum was reported in Science.

The authors considered three possible influences on the presence of the plant-bacterial partnership: (1) an ancestor underwent a predisposition event that allowed the symbiosis to evolve; (2) symbiosis independently evolved multiple times; and (3) symbiosis was also independently lost multiple times.

They figured that any genes required for a predisposing event would be present in all of the nodulating species, and found only in them and their clade—the other plants descended from the same forbears. They found zero genes meeting this criterion. So if a predisposing event did happen, it did so by coopting extant genes instead of via new ones.

Then they looked for the expansion of a gene family in symbiotic plants, since this is one method by which a trait can evolve multiple times. They didn’t find genes like this either.

To see if genes governing symbiosis had been lost—which seems weird, since nitrogen fixation is so vital—they looked for genes found only in nodulating species and plants outside their clade but not in most non-nodulating species within their clade. The existence of such a gene would imply that an ancestor would have had it but the non-modulating species would have lost it along the way.

Lo and behold, a gene called NIN (named for its function, Nodule Inception) fit the bill. The non-nodulating species had the same genes surrounding NIN as their nodulating cousins, but eight different deletion events had erased the NIN gene from different species. One other gene, also required to house the symbiotic bacteria, followed a similar pattern.

"Use it or lose it" applies to genes as much as (or maybe even more than) anything else. Genes involved in biological processes are lost if the trait they confer is unused or unnecessary. For most of the time that plants have lived on this planet, nitrogen has been limiting, so symbiosis should have been favored. But it was jettisoned at least eight different times. Maybe it was too energetically costly for most plants to keep: maybe housing and feeding the bacteria wasn't worth it, or non-nitrogen fixing bacteria took advantage and moved in.

...
 

blacbird

Super Member
Registered
Joined
Mar 21, 2005
Messages
36,987
Reaction score
6,158
Location
The right earlobe of North America
Lightning plays a crucial role in the nitrogen cycle. This, from the lead of an article in Kimball's Biology Pages:

www.bbc.co.uk


The enormous energy of lightning breaks nitrogen molecules and enables their atoms to combine with oxygen in the air forming nitrogen oxides. These dissolve in rain, forming nitrates, that are carried to the earth. Atmospheric nitrogen fixation probably contributes some 5– 8% of the total nitrogen fixed.


caw
 

Introversion

Pie aren't squared, pie are round!
Kind Benefactor
Super Member
Registered
Joined
Apr 17, 2013
Messages
10,642
Reaction score
14,865
Location
Massachusetts
Yeah, I thought I’d read that. Reality is awesome sometimes.