In a broader sense, the biotechnological use of
plant–microbe interactions in agriculture also includes the
use of plants benefiting from symbiotic nitrogen fixation
(Moulin et al. 2001). This is particularly the case of
Fabaceae, as forage or cash crops, traditionally used to
improve soil fertility, particularly in crop rotations. Some
of the Fabaceae are used as green manure, i.e., once grown
they are incorporated into soil rather than being harvested. In
agroforestry, Fabaceae shrub species and actinorhizal plants
(e.g., Casuarina) can be used for alley cropping with herbaceous companion crops. Two other types of nitrogen-fixing
symbioses, which do not lead to the formation of nodules,
are also important from a biotechnological point of view.
One involves aquatic ferns and cyanobacteria, such as the
Azolla–Anabaena symbiosis, which can provide in the order
of 30–60 kg N/ha/year for submerged rice fertilization
(Roger et al. 1993). The other is formed by sugarcane and
endophytic Proteobacteria such as Gluconacetobacter
diazotrophicus. These nitrogen-fixing symbioses may provide up to 100–150 kg N/ha/year, making sugarcane particularly attractive for biofuel production. This crop contributes
to atmospheric flux of the greenhouse gas N 2 O, but much
less than other crops (Crutzen et al. 2007). Finally, crop
plants are often involved in cooperative interactions with
PGPR and/or endophytic fungi. This capacity is of agronomic interest, but it varies depending on plant genotype
(Picard and Bosco 2006), and so far it has been largely
ignored in crop breeding schemes.
In nonagricultural soils, plant–microbe interactions are of
interest for the establishment of windbreaks and landscaping of
roadside and other anthropogenic sites. Nitrogen-fixing
symbioses based on Fabaceae or actinorhizal shrubs are often
used in this context, all the more as these species also benefit
from mycorrhizal symbioses. These symbioses are particularly
useful for revegetation of land low in organic matter (abandoned quarries or eroded slopes; Fig. 11.11c), degraded, highly
acidic, and/or rich in heavy metals (mining areas, etc.) (Roy
et al. 2007). In the latter case, revegetation (cf. Sect. 16.2.1) can
stabilize the contaminated soil (phytostabilization), and interaction with PGPR displaying ACC deaminase activity and/or
with mycorrhizal fungi is important for improving plant tolerance to the metals (Belimov et al. 2005).
In the case of contaminated soils, interactions between
plants and microorganisms are also of interest for two
phytoremediation techniques, i.e., phytoextraction and
rhizoremediation (Cardon and Whitbeck 2007; Roy et al.
2007). Regarding metal pollution, some of the rhizosphere
bacteria and fungi can solubilize metals such as chromium,
lead, and arsenic (Khan 2005), while promoting their
absorption by the roots (phytoextraction). For some organic
pollutants, rhizodeposit availability can promote their biodegradation by the rhizosphere microbial community, whose
action may be supplemented by enzymes released by the
root. Organic pollutants that can be treated through such
rhizoremediation include pesticides, simple hydrocarbons,
polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) (Rentz et al. 2005). Microbial
biodegradation by rhizospheric microorganisms also helps
protect the plant from phytotoxic effects of organic
pollutants, at the same time promoting site revegetation.
Plant–microbe interactions also have an interest for purification of effluents. This is the case of rhizoremediation
systems, including those based on reeds or bamboo, used
to decompose organics in certain water treatment operations
(cf. Chap. 16). However, the role of plant–microbe
interactions in these systems is poorly documented. A
simplified system based on unicellular microalgae (Chlorella) and bacteria (Azospirillum) co-immobilized in alginate beads was also developed for tertiary treatment of
wastewater, allowing removal of nitrogen (ammonium
mainly) and phosphorus (de-Bashan et al. 2004).
Finally, the ability of Agrobacterium to transfer genes
into higher plants is one of the means used to obtain transgenic plants (Tzfira and Citovsky 2006). This ability relies
on the pTi plasmid, which allows the transfer of a particular
DNA segment (the T-DNA) located between 25-bp flanking
repeats and that includes genes involved in the synthesis of
phytohormones (auxins and cytokinins) and opines. The TDNA is transferred in single-stranded form, combined with
VirD2 and VirE2 proteins, and is randomly integrated into
the nuclear genome of dicotyledonous plants. To obtain
transgenic plants, the genes of interest replace genes
between the flanking sequences. This technology has been
enhanced to allow genetic modification of monocots, a transfer that does not occur under natural conditions.
11.3.8 Epilogue
Interactions between microorganisms and plants have played
a fundamental role in the evolution of plants, allowing the
emergence of the first plant cells (thanks to cyanobacteria)
and, later, land colonization by plants thanks noticeably to the
endomycorrhizal symbiosis (Simon et al. 1993).
Plant–microbe interactions are very diverse, ranging from
facultative relationships with minor consequences to obligatory partnership with major evolutionary consequences. The
microbial community contributes significantly to the functioning and ecology of the plant partner and to soil quality.
Indeed, microorganisms modulate the growth, nutrition, and
health of plants. Conversely, plants grow in biotopes where
thousands of microbial taxa lead an existence whose focal
point is the plant as a source of exudates or litter or as a
partner to attack or collaborate with. It is therefore inevitable
that plants have evolved and will continue to do so according
to the microorganisms interacting with them. Mankind must
420
Y. Moe ¨nne-Loccoz et al.
plant–microbe interactions in agriculture also includes the
use of plants benefiting from symbiotic nitrogen fixation
(Moulin et al. 2001). This is particularly the case of
Fabaceae, as forage or cash crops, traditionally used to
improve soil fertility, particularly in crop rotations. Some
of the Fabaceae are used as green manure, i.e., once grown
they are incorporated into soil rather than being harvested. In
agroforestry, Fabaceae shrub species and actinorhizal plants
(e.g., Casuarina) can be used for alley cropping with herbaceous companion crops. Two other types of nitrogen-fixing
symbioses, which do not lead to the formation of nodules,
are also important from a biotechnological point of view.
One involves aquatic ferns and cyanobacteria, such as the
Azolla–Anabaena symbiosis, which can provide in the order
of 30–60 kg N/ha/year for submerged rice fertilization
(Roger et al. 1993). The other is formed by sugarcane and
endophytic Proteobacteria such as Gluconacetobacter
diazotrophicus. These nitrogen-fixing symbioses may provide up to 100–150 kg N/ha/year, making sugarcane particularly attractive for biofuel production. This crop contributes
to atmospheric flux of the greenhouse gas N 2 O, but much
less than other crops (Crutzen et al. 2007). Finally, crop
plants are often involved in cooperative interactions with
PGPR and/or endophytic fungi. This capacity is of agronomic interest, but it varies depending on plant genotype
(Picard and Bosco 2006), and so far it has been largely
ignored in crop breeding schemes.
In nonagricultural soils, plant–microbe interactions are of
interest for the establishment of windbreaks and landscaping of
roadside and other anthropogenic sites. Nitrogen-fixing
symbioses based on Fabaceae or actinorhizal shrubs are often
used in this context, all the more as these species also benefit
from mycorrhizal symbioses. These symbioses are particularly
useful for revegetation of land low in organic matter (abandoned quarries or eroded slopes; Fig. 11.11c), degraded, highly
acidic, and/or rich in heavy metals (mining areas, etc.) (Roy
et al. 2007). In the latter case, revegetation (cf. Sect. 16.2.1) can
stabilize the contaminated soil (phytostabilization), and interaction with PGPR displaying ACC deaminase activity and/or
with mycorrhizal fungi is important for improving plant tolerance to the metals (Belimov et al. 2005).
In the case of contaminated soils, interactions between
plants and microorganisms are also of interest for two
phytoremediation techniques, i.e., phytoextraction and
rhizoremediation (Cardon and Whitbeck 2007; Roy et al.
2007). Regarding metal pollution, some of the rhizosphere
bacteria and fungi can solubilize metals such as chromium,
lead, and arsenic (Khan 2005), while promoting their
absorption by the roots (phytoextraction). For some organic
pollutants, rhizodeposit availability can promote their biodegradation by the rhizosphere microbial community, whose
action may be supplemented by enzymes released by the
root. Organic pollutants that can be treated through such
rhizoremediation include pesticides, simple hydrocarbons,
polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs) (Rentz et al. 2005). Microbial
biodegradation by rhizospheric microorganisms also helps
protect the plant from phytotoxic effects of organic
pollutants, at the same time promoting site revegetation.
Plant–microbe interactions also have an interest for purification of effluents. This is the case of rhizoremediation
systems, including those based on reeds or bamboo, used
to decompose organics in certain water treatment operations
(cf. Chap. 16). However, the role of plant–microbe
interactions in these systems is poorly documented. A
simplified system based on unicellular microalgae (Chlorella) and bacteria (Azospirillum) co-immobilized in alginate beads was also developed for tertiary treatment of
wastewater, allowing removal of nitrogen (ammonium
mainly) and phosphorus (de-Bashan et al. 2004).
Finally, the ability of Agrobacterium to transfer genes
into higher plants is one of the means used to obtain transgenic plants (Tzfira and Citovsky 2006). This ability relies
on the pTi plasmid, which allows the transfer of a particular
DNA segment (the T-DNA) located between 25-bp flanking
repeats and that includes genes involved in the synthesis of
phytohormones (auxins and cytokinins) and opines. The TDNA is transferred in single-stranded form, combined with
VirD2 and VirE2 proteins, and is randomly integrated into
the nuclear genome of dicotyledonous plants. To obtain
transgenic plants, the genes of interest replace genes
between the flanking sequences. This technology has been
enhanced to allow genetic modification of monocots, a transfer that does not occur under natural conditions.
11.3.8 Epilogue
Interactions between microorganisms and plants have played
a fundamental role in the evolution of plants, allowing the
emergence of the first plant cells (thanks to cyanobacteria)
and, later, land colonization by plants thanks noticeably to the
endomycorrhizal symbiosis (Simon et al. 1993).
Plant–microbe interactions are very diverse, ranging from
facultative relationships with minor consequences to obligatory partnership with major evolutionary consequences. The
microbial community contributes significantly to the functioning and ecology of the plant partner and to soil quality.
Indeed, microorganisms modulate the growth, nutrition, and
health of plants. Conversely, plants grow in biotopes where
thousands of microbial taxa lead an existence whose focal
point is the plant as a source of exudates or litter or as a
partner to attack or collaborate with. It is therefore inevitable
that plants have evolved and will continue to do so according
to the microorganisms interacting with them. Mankind must
420
Y. Moe ¨nne-Loccoz et al.