Everyone has heard of the
nutrient, nitrogen, but why
is it important to plants?
It's a key ingredient in proteins, both in the
grains (seeds) we eat and in leaves. Nitrogen-starved
leaves may turn yellow and have lower rates of [1] photosynthesis,
which turns carbon dioxide from the atmosphere into new plant growth.
To
keep food affordable and protect the environment, crop plants need just the
right amount of nitrogen, but this is a challenge for both farmers and
scientists. Too much nitrogen fertilizer
on crops pollutes rivers and well water and its cost makes food
more expensive.
With too little nitrogen, yields decrease and the imbalance between supply and
demand makes food more expensive. Higher grain
prices can also make it more profitable to clear tropical forests for
agriculture.
Biological
nitrogen fixation is a potential solution to these problems. Legume crops like beans, peanuts, soybeans,
and alfalfa get much of their nitrogen from symbiotic bacteria, known as
rhizobia, which take nitrogen gas from the air and convert it into forms plants
can use, like ammonia. The carbon
compounds the rhizobia need come from their plant hosts. These resources could otherwise have been
used to make more seeds, so legumes have evolved sophisticated mechanisms to
support no more nitrogen fixation than they need. When less-expensive nitrogen sources like
ammonia or nitrate are available in the soil, legumes make fewer of the root
nodules that house the bacteria and may even shut down existing nodules.
Biological nitrogen fixation: Environmental benefits and lower cost?
Could non-legumes
also use nitrogen fixation somehow, with similar environmental benefits? If so, would farmers save enough money on
fertilizer costs to balance the yield decrease from diverting resources from
seed production to nitrogen fixation? Or
might a more-reliable nitrogen supply actually increase yields, despite the
energy cost?
Corn, wheat, and
rice do not make root nodules, but some bacteria that live on or near their
roots can fix small amounts of nitrogen.
In Nature Microbiology, Ryu et al. discuss genetic improvement of
these bacteria. They normally fix little
or no nitrogen if exposed to oxygen, which destroys the key enzyme,
nitrogenase, or if ammonia is available.
Ryu et al. were able to change these traits, but practical applications
of their work are far from certain, even in the long term.
Credit: Ryu et al.
Knocking out
repression of nitrogen fixation by ammonia may seem like a step backwards, from
an environmental point of view. After
all, the ability of legume crops to fix no more nitrogen than they need is key
to limiting nitrate pollution of our water supplies. But we want nitrogen fixation rate to depend
on overall plant needs, not just on ammonia levels around the bacteria. So Ryu et al. engineered bacteria to adjust
nitrogen-fixation rates in response to signals from plants. Energy would still have to come from the
plants, however. Would the benefits of
nitrogen fixation justify this cost?
Let's be optimistic and assume they would, initially.
But bacteria
evolve. Even if they provided nitrogen
at a reasonable cost at first, benefits could decrease in only a few days. The paper mentions the "fitness
burden" of nitrogen fixation, causing rhizobia that fix more to die
out. A “cheating” mutant that fixed only
as much nitrogen as it needed itself, with nothing extra for the plant, would
free up resources for its own reproduction and take over.
Yet, nitrogen
fixation by rhizobia in legume root nodules has persisted for millions of
years. What has limited the spread of
rhizobial “cheaters”? Legumes shut off
the oxygen supply to nodules that fail to fix enough nitrogen, saving resources
for the plant and limiting reproduction by rhizobial “cheaters.” If we want corn, wheat, or rice to get much
of their nitrogen from bacteria, they will need some such mechanism to shut
down nodules that cost more than they are worth.
It may be more
feasible to add nitrogen-fixation genes to new crops themselves, rather than
relying on bacteria. The crops would
still have to pay nitrogen-fixation's high energy cost, but it might be worth
it in at least some cases.
Oxygen would still
be a problem. Even moderate
concentrations of oxygen can destroy nitrogenase, yet oxygen-based respiration
is needed to meet nitrogen-fixation's energy needs. In legumes, regulation of oxygen supply to
the nodule interior helps solve this problem.
But some of the bacteria described in the Nature Microbiology paper
fixed nitrogen in oxygen concentrations above 1%.
If this work points the way to
oxygen-tolerant nitrogen fixation by plants, we might someday be able to grow
corn, wheat, and rice without nitrogen fertilizer.
[1]It is in
chlorophyll, but much more of it is in rubisco.
When plants don’t have enough N to make all the rubisco they need,
there’s no point to make the full amount of chlorophyll, so they don’t. Hence, yellow leaves.
