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INTERACTIONS
BETWEEN WHITE LUPIN (LUPINUS ALBUS) AND THE BACTERIA ASSOCIATED
TO ITS PROTEOID ROOTS
Communication
between plant and bacteria, analysis of the microflora
and root excretion physiology.
Keywords
: Lupinus albus, proteoid roots, rhizosphere,
phosphate, citrate, rhizospheric competence, PGPR.
The
interest of working with White Lupine
 In
contrast to most herbaceous plants, Lupinus albus does not form
any mycorrhizal association. However, it is resistant to phosphate
starvation. This resistance is based on several mechanisms :
* Formation of dense cluster-roots called " proteoid
roots "
* Exudation by these roots of large amounts of organic acids
(especially citrate)
* Secretion of acid phosphatases
* ... Maybe a particular helpful microflora...
The organic acid exudation and consequently the pH of the root
environment undergo important changes during the development
of the proteoid roots : The rhizosphere of White Lupine and
especially the proteoid roots, which can be considered as hot
spots of activity, represent an environment where the nutrients
as well as the physical and chemical conditions vary in an important
and rapid way.
There
is no doubt that these changes have an important influence on
the bacterial communities and this is the reason for studying
the bacteria associated to the different types of roots.
Moreover,
in contrast to the physiology
of these roots, which is intensively studied (http://www.unine.ch/biol//pv.htm),
the impact of the bacteria on the plant (proteoid roots formation,
phosphate solubilization,...) has not yet been investigated.
Objectives
We have two main objectives in this study : on the one hand,
a description of the bacterial communities associated with the
roots of Lupinus albus and on the other hand, an attempt to
better understand the function of the bacterial microflora in
the " plant-soil-bacteria "system.
Methods
We
will use three different approaches in order to study the bacterial
communities associated with L. albus roots :
* Direct observation
The
bacteria will be stained with a fluorescent dye and then observed
with a microscope.
* Cultural methods
We
will use several culture media and try to simulate the natural
situation as closely as possible, with regard to the carbon source
(citrate, malate or glucose) and to the pH (from 5 to 7). We will
then isolate and characterize the bacteria growing in each medium
and subject them to physiological tests (plant hormone production,
phosphate solubilization,...)
* Molecular methods
The
SSCP analysis (Singe Strand Conformation Polymorphism), will enable
us to separate the bacterial populations in function of the sequence
of the 16S gene, and this will give us an idea of the bacterial
biodiversity associated to the different types of roots.
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