Actualité scientifique
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N° 471
December 2014
When plants and bacteria go hand in hand
Aeschynomene sensitiva (© IRD / L. Markiw).
How can we improve crops without having recourse to nitrogen-based chemical fertilisers?
To answer this question researchers are investigating certain plants, the legumes (groundnut,
soya etc.), which can grow in soils with poor fertility. They are exploring these plants’ apparently
symbiotic relationships with bacteria, known as rhizobium bacteria, which draw the nitrogen
needed by the plant from the air.
An international study, headed by biologists from the IRD and the University of Naples, has
recently revealed the mechanism, which allows these micro-organisms to survive in the soil and
particularly on the host plant. This discovery, published in Nature Communications, will in the
long term allow us to prolong the symbiotic interaction between the plant and the bacteria.
Worth knowing
The ability to form a symbiotic association with the bacteria, known as rhizobia, has made the legumes distinct from
other plants. This has meant that these plants represent more than a quarter of global agricultural production. They
provide an important food resource both for human beings and for animals, but due to this interaction, they are also able
to enrich the nitrogen content of the soil for future crops or for the restoration of vegetation in impoverished ecosystems.
CONTACTS
Two organisms in symbiosis
Once the rhizobium bacteria have made
their way into the host plant, they cause
new organs, the nodules, to form on
the plant’s roots and sometimes on the
stem. At the centre of these growths
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ammonium, a derivative of nitrogen,
which the plant is able to assimilate. This
close relationship, known as symbiosis,
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bacteria. The bacteria thus provide up to
96% of the nitrogen needed by the legume.
In return, the plant releases carbon-based
nutrients produced by photosynthesis,
which are essential for the development of
the bacteria.
Two stressful ways of life
A new study, published in Nature
Communications, lifts the veil on an
important question: how does the
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then in the plant? In fact, it is subjected
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environment of the soil, which may be
saline or water-saturated, then in the centre
of the plant cell of the host plant, an acid,
oxidizing, antimicrobial environment. The
international study, headed by biologists
from the IRD and the University of Naples,
has just shed light on the mechanism,
which enables the bacteria to protect itself
from this external aggression.
A shield membrane which protects the
bacterium
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the role of molecules called hopanoids
about which little was previously known;
these make the membrane of the bacterium
more rigid. These molecules are thus the
equivalent in unicellular organisms to
our cholesterol. The researchers have
revealed that, in order to reinforce the
protection barrier formed by the external
membrane, its main components, the
lipopolysaccharides, combine in almost
indissociably with the hopanoid molecules.
Coordination
Gaëlle COURCOUX
Information and Culture
The study shows that when these
molecules are absent, the bacterium is
much more sensitive to the stresses it
endures in the soil. On the host plant
it dies very rapidly, causing premature
ageing of the nodules and hindering the
proper development of the plant.
Promoting the synthesis of these
molecules by rhizobium bacteria will allow
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that they can enter into symbiosis with a
plant. This will also increase the lifetime of
the nodules on cultivated plants, a lifetime
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This could prolong the symbiotic activity of
the plant-bacterium pair.
Department
T. +33 (0)4 91 99 94 90
'"1$*-"&,2#3/453
www.ird.fr/la-mediatheque
Media Contact
Cristelle DUOS
Partners
University of Naples and CNR–Istituto per i
Polimeri, Compositi e Biomateriali (IPCB) in Italy,
CIRAD, University of Texas and California Institute
of Technology and Howards Hughes Medical
Institute in the United States, Leibniz-Center
for Medicine and Biosciences in Germany and
Chonbuk National University in the Republic of
Korea..
References
A. SILIPO, G. VITIELLO, D. GULLY, L. STURIALE, C.
CHAINTREUIL, J. FARDOUX, D. GARGANI, H.-I. LEE, G.
KULKARNI, N. BUSSET, R. MARCHETTI, A. PALMIGIANO,
H. MOLL, R. ENGEL, R. LANZETTA, L. PADUANO, M.
PARRILLI, W.-S. CHANG, O. HOLST, D. K. NEWMAN,
D. GAROZZO, G. D’ERRICO, E. GIRAUD, A. MOLINARO.
Covalently linked hopanoid-lipid A improves
outer-membrane resistance of a Bradyrhizobium
symbiont of légumes. Nature Communications,
2014, 30;5:5106. doi:10.1038/ncomms6106
Contact
Eric Giraud, IRD researcher
T. +33 (0)4 67 59 37 83
$3#"47#3-,/2#3/453(
Laboratoire des symbioses tropicales et
méditerranéennes – LSTM (IRD / CIRAD /
Montpellier SurAgro / université Montpellier 2)
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© IRD/DIC, Juillet 2014 - Conception et réalisation graphique : L. CORSINI
Tropical soils are poor in nitrogen, which is
essential for plant growth. Certain plants,
such as the legumes, manage to grow in
these infertile soils. In order to do so, they
form a partnership with bacteria in the
soil, called rhizobia. Although it is of great
interest to the agronomic industry, very
little is yet known about this mechanism.
The researchers are therefore keen to
describe it in detail.