Sheet n°269 - June 2007
A new plant-bacterial
symbiotic mechanism
promising for crop applications
ow can satisfactory
crop yields be obtained
without spreading large quantities of chemical fertilizers
on the fields? To answer this
question, IRD researchers and
their scientific partners (1) are
examining an unusual symbiotic association, between
an aquatic leguminous plant,
Aeschynomene and photosynthetic bacteria of the genus
Bradyrhizobium. These bacteria, able to fix nitrogen taken
from the air, induce nodule
formation on both roots and
stems of the plant. The microorganisms colonize these
nodules which then obtain
nitrogen and provide the plant
with the supply necessary for
its growth.
Genomic studies on bacterial
strains revealed an absence
of nod genes, usually involved in the symbiotic mechanisms of all known rhizobia
bacteria. This remarkable
result, published in the journal
Science, calls into question the
universally accepted theory
envisaging just one exclusive
mode of nodulation initiation.
Understanding of this new
type of molecular communication pathway could, in the
long run, bring improvements
in crop yields, particularly
in the tropical zones where
soil nitrogen deficiency is a
serious handicap.
© IRD/Eric Giraud
H
Transversal section of a nodule on a stem of Aeschynomene sensitiva
The growth of most plants depends on the
presence of sufficient amounts of nitrogen
contained in the soil. However, a family of
plants, the legumes, is partially free of this
constraint thanks to its ability to live in association with soil bacteria of the Rhizobium,
genus, capable of fixing nitrogen from the
air. When these bacteria come into contact
with their host plant, they trigger in the roots
the formation and development of organs,
termed nodules, where they continue to live.
This close relationship is symbiosis, which
benefits both organisms involved: the plant
supplies nutritive elements to the bacteria
which in return pass on the nitrogen they
have stored up.
These interactions improve crop yields of
leguminous plants that are crucial for human
diet (soybean, peas, ground nuts and so
on…) and as animal feed (alfalfa, clover,
sainfoin). In addition, cultivation of legumes
living in symbiotic association with bacteria
can contribute to vegetation regeneration
schemes on soils depleted in nitrogen owing
to overexploitation, erosion or desertification.
The plant cover thus formed can help achieve
ecological restoration, by enriching the soils in
nitrogen. However, the symbiotic processes
studied predominantly concern the leguminous plants of temperate zones, very little
those of the tropics.
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CONTACTS :
ERIC GIRAUD
IRD UR 040 Laboratoire des
Symbioses Tropicales et
Méditerannéennes ( LSTM )
+33(0) 4 67 59 37 83
giraud@mpl.ird.fr
MEDIAS RELATIONS:
+33 (0)1 48 03 75 19 ;
presse@ird.fr
INDIGO, IRD IMAGE LIBRARY :
+33 (0)1 48 03 78 99 ;
indigo@ird.fr
www.ird.fr/indigo
IRD AUDIOVISUEL
+33 (0)1 48 02 56 24 ;
audiovisuel@bondy.ird.fr
www.audiovisuel.ird.fr/
REFERENCE :
ERIC GIRAUD ET AL. Legumes
Symbioses : Absence of Nod
Genes in Photosynthetic
Bradyrhizobia, Science, june 1st
2007.
KEY-WORDS :
SYMBIOSIS, NITROGEN, BRADYRHIZOBIUM,
AESCHYNOMENE, LEGUMINOUS
PLANTS, BACTERIA, NOD GENE,
NODULES
The discovery of the nature of the signal
molecule itself, which remains to be fully
determined, brings a glimpse of future agricultural applications.
Many plants live in symbiosis with bacteria, but
the mechanisms are known for only a small
number of these interactions. The demonstration of alternative pathways capable of triggering the nodulation signal in certain rhizobia
is promising for future techniques for bringing
these bacteria into association with different
leguminous plants. It therefore becomes possible to increase agricultural production of a
greater number of important plants, notably in
tropical countries, while cutting down the use
of fertilizers.
The researchers sequenced (3) the genes
of two bacterial strains of Bradyrhizobium,
ORS278 and BTAi1, in order to find out
their genetic make-up and identify the genes
involved in this rather special form of symbiosis. These bacteria were found to have no
nod genes, usually essential for nodulation.
Bradyrhizobium consequently appeared to
use mechanisms that involved other genes.
This surprising result calls into question the
universally recognized model of molecular
communication that initiates the rhizobia-legume symbiosis. This common model requires
the presence of several nod genes which
allow synthesis of the Nod factor, a compound
elaborated by the bacterium which enables
the plant to recognize it, by molecular recognition, thereby allowing the microorganism
to penetrate inside the plant by the root hairs.
The finding raises the question as to what
signalling pathway Bradyrhizodium might use
to gain entry to the plant and set off nodulation.
The first observation was that the bacteria did
not penetrate the roots of its host-plant by the
hairs. It took advantage of “crack zones” comparable with wound areas. The set of results
obtained from subsequent work, seeking to
identify the genes involved in producing the
unknown signal molecule that plays the role
of Nod factor, prompted the team’s hypothesis
that a molecule similar to a plant hormone (4),
cytokinin, could act in the mechanisms by
triggering nodulation.
(1) This research was conducted in the
‘Laboratoire des Symbioses Tropicales et
Méditerranéennes’, mixed research unit
UMR 113 (IRD, CIRAD, AGRO-M, INRA,
University of Montpellier 2), with the participation of the Genoscope at Evry, the
CEA, the DOE Joint Genome Institute, the
University of Minnesota and the University
of Missouri.
(2) See scientific bulletin n°154 accessible on www.ird.fr/fr/actualites/fiches/2002/
fiche154.htm
(3) sequencing: molecular biology method
that can identify all the DNA sequences of
an organism and therefore its genes.
(4) plant hormones play a role in communication within a plant.
Stéphane Sapolin– IRD
Translation : Nicholas Flay
© IRD/ Eric Giraud
Sheet n°269 - June 2007
For futher information
The team from the IRD’s ‘Laboratoire des
SymbiosesTropicales et Méditerranéennes’
and its partners (1), taking as model a symbiosis between a tropical aquatic legume,
Aeschynomene, and Bradyrhizobium, bacteria of the Rhizobia family, have just revealed
a new mode of communication at molecular
level between these two organisms. The bacteria of this original model have their own
photosynthetic pathway, a unique property in
the rhizobia (2). This special character confers
on it the exceptional, rare ability to form
nodules on the stems of its host-plant. The
plant thus acquires the possibility of fixing
much higher quantities of nitrogen than those
usually measured in leguminous plants which
have nodules only on their roots.
Symbiosis A.Sensitiva-Bradyrhizobium control (left)
stem inoculation and root inoculation (right).
Marie Guillaume-Signoret, coordinatrice
Délégation à l’information et à la communication
Tél. : +33(0)1 48 03 76 07 - fax : +33(0)1 40 36 24 55 - fichesactu@ird.fr