Scientists discover the genetics inside legumes that
control the production of an oxygen-carrying molecule, crucial to
the plant�??s close relationships with nitrogen-fixing bacteria.
The finding offers the potential to give other plants
the ability to produce ammonia from bacteria �?? reducing the need
for the fossil fuel-dependent and polluting practice of applying
synthetic fertiliser to crops.
The roots of legume plants are home to symbiotic
bacteria. These bacteria can fix nitrogen from the air, turning it
into ammonia, a key nutrient for plants.
In return, the plants house the bacteria in root
nodules, providing sugars and oxygen. The amount of oxygen needs
to be just right to support the symbiosis, the bacteria need
oxygen to fuel their chemical reactions, but too much inhibits a
key enzyme that turns nitrogen in the air into the ammonia that
can be used by the plant.
The plant�??s solution to this �??oxygen paradox of
biological nitrogen fixation�?? is a molecule called leghemoglobin.
Like hemoglobin that carries oxygen in our blood, leghemoglobin
binds to oxygen and is red; it gives legume nodules their pink
colour. Until now it�??s been unclear how plants control how much of
this molecule is produced.
The research team have identified two transcription
factors that control how much leghemoglobin is made in legume
nodules.
�??This gives a key insight into how legume plants create
the microaerobic environment needed for nitrogen-fixation. This
knowledge could be useful for improving nitrogen-fixation in
legumes and would be essential for transfer of nodulation to
non-legume crops�?�, explains corresponding author Dr Jeremy Murray,
CEPAMS Group Leader.
Dr Jeremy Murray continues, �??While many genes involved
in other nodulation processes have been identified, this is the
first breakthrough on the gene regulatory network involved
directly in control of nitrogen fixation.�?�
The research was carried out by a collaborative team,
led by Dr Suyu Jiang in Dr Jeremy Murray�??s group at the CAS-JIC
Centre of Excellence for Plant and Microbial Science (CEPAMS),
Centre for Excellence in Molecular Plant Sciences (CEMPS), Chinese
Academy of Sciences, Shanghai, China, with collaboration from Dr
Pascal Gamas and Dr Marie-Fran?�oise Jardinaud at LIPME (Universit??
de Toulouse, France).
Using the model legume, Medicago
truncatula, the research team looked at a family of
proteins in plants which has several members with roles in
nodulation.
They looked at which proteins in this class are produced
in symbiosis-housing nodules and found that there was two �?? NIN
and NLP2, and that when these are inactive, nitrogen fixation is
reduced. This suggested that they are involved in nitrogen
fixation.
To investigate further, they grew plants in an aeroponic
system, without soil, to be able to look at the nodules, and found
the plants lacking NIN and NLP2 were smaller in size and had
smaller and less-pink nodules. On closer inspection, they had
lower levels of leghemoglobin. Further experiments found that NIN
and NLP2 directly activate the expression of leghemoglobin genes.
�??This research project was purely curiosity-driven, all
we knew at the outset was that the transcription factor we were
studying was highly and specifically expressed in nitrogen-fixing
cells, we were initially not aware of any connection to
leghemoglobins�?�, reflects Dr Murray.
The research has also given insights into the evolution
of this important symbiosis. They found that other members of the
transcription factors family regulate the production of
non-symbiotic hemoglobins found in plants, which are involved in
plant�??s response to low oxygen levels.
Jeremy explains further, �??This was exciting because it
suggests that these transcription factors and their hemoglobin
targets were recruited to nodulation as modules to help improve
energetics in nitrogen-fixing cells, giving a rare glimpse into
how this symbiosis evolved.�?�
How
legumes give oxygen to symbiotic bacteria in their roots
(jic.ac.uk)