primarily by altering plant hormonal balance, while their
impact on plant growth may also involve trophic microbial
effects. For example, the potential for symbiotic nitrogen
fixation is typically in the order of 60–200 kg N/ha/year, for
which the plant will invest 3–25 % of net photosynthates,
versus only 5–25 kg N/ha/year for free nitrogen fixation.
Nevertheless, the symbiotic nitrogen fixation potential is
often poorly exploited in modern agriculture, which relies
on massive use of chemical nitrogen fertilizers. Mycorrhization improves plant nutrition in phosphorus and nitrogen, and to a lesser extent in potassium and iron, especially
in low-fertility soils (Brundrett 2009). Phosphorus can be
recovered by mycorrhizal hyphae at more than 7 cm from the
root. Mycorrhizal fungi also improve plant resistance to
biotic stress (disease, herbivory; Bennett and Bever 2007)
and abiotic stress (drought, metal pollution; Auge 2004).
Regarding plant health, the role of plant-protecting bacteria
and fungi is important in the rhizosphere. In soils that are
suppressive to disease, their interactions are sufficient to limit
disease severity despite the presence of the pathogen, plant
susceptibility to disease, and environmental conditions favorable
for infection (Alabouvette et al. 1996; Garbeva et al. 2004).
These disease-suppressive soils are documented for root diseases
caused by fungi and to a lesser extent by bacteria or nematodes.
In some pathosystems, suppressiveness gradually develops during crop monoculture in soils that were initially conducive to
the disease. This is the case of wheat take-all caused by
Gaeumannomyces tritici; take-all severity increases over the
early years of wheat monoculture, reaches a peak (usually in
years 4–8), and decreases thereafter to minor levels in the
following years (Lucas and Sarniguet 1998; Raaijmakers and
Weller 1998). In other pathosystems, disease suppressiveness is
a natural property of the soil and does not require monoculture,
as for soils suppressive to Fusarium wilt (caused by F.
oxysporum) (Alabouvette et al. 1996) or to Thielaviopsis black
root rot (caused by Thielaviopsis basicola) (Kyselkova ´ et al.
2009). Soil suppressiveness is often attributed to Pseudomonas
PGPR subpopulations producing 2,4-diacetylphloroglucinol,
although other phytobeneficial microorganisms might also play
an important role (Kyselkova ´ et al. 2009).
Plant–microbe interactions have a significant impact on
natural plant communities (Cardon and Whitbeck 2007).
Nitrogen-fixing and mycorrhizal symbioses may be important to promote the establishment of pioneer plant groups.
This is the case for plant recolonization of ground surfaces
following deglaciation (at the end of ice ages or with current
global warming), volcanic eruptions, or forest fires (van der
Maarel 2005). Pioneer plant species will modify soil microbial community composition/functioning which, in turn,
may then influence plant succession and the composition of
intermediate groups, both by favoring certain plant species
and counterselecting others. Finally, when the climax stage
is reached, pathogens adapted to the main plant species may
decrease their dominance or eliminate enough individuals to
allow re-installation of pioneer species (within forest
clearings) and reinitiate plant succession (van der Maarel
2005). In the particular case of invasive plant species, the
soil microbial community may facilitate their establishment
at the expense of native plants. Plant–microbe interactions
are also important for the functioning of plant communities,
particularly in terms of recycling of plant litter by microbial
decomposers and nitrogen fixation (van der Maarel 2005).
Organic matter transformations contribute to pedogenesis
and to soil functioning, and as such they impact both on
plant growing conditions and plant community successions.
11.3.7 Biotechnological Uses of Plant–Microbe
Interactions
Certain microorganisms are used as inoculum in agriculture
(Fig. 11.11b), for biofertilization, phytostimulation, or biocontrol purposes (Dobbelaere et al. 2001; Alabouvette et al.
2006). The goal is to improve crop yield (productivity) and/
or reduce chemical inputs (environmental quality).
Biofertilizers aim at improving plant mineral nutrition, e.
g., symbiotic nitrogen-fixing bacteria (Rhizobium and other
genera) and to a lesser extent mycorrhizal fungi. For PGPR
having mainly hormonal effects on the plant, the term
phytostimulation is used instead of biofertilization. The
main phytostimulators used belong to the genus
Azospirillum, especially for cereals (0.5–1 million ha of
corn inoculated each year). Azospirillum inoculation resulted
in increased cereal yield (by 10–30 %) in two-thirds of the
cases (Dobbelaere et al. 2001). Biological control is most
often implemented in commercial greenhouse, using bacteria or fungal inoculants. The objective is to protect crops
against phytopathogens (often fungi or oomycetes, sometimes bacteria), parasitic nematodes or plants, or weeds.
The microorganisms used correspond to Firmicutes (Bacillus), Proteobacteria (Pseudomonas), and Actinobacteria
(Streptomyces) for bacteria as well as Deuteromycetes
(Trichoderma, Gliocladium, Coniothyrium minitans) for
fungi (Gilbert et al. 1993; Harman et al. 2004; Rezzonico
et al. 2005). Inoculum formulation is a key issue determining
inoculant performance, and spore-forming microorganisms
have better storage capacities. The inoculum is often applied
to the seed before sowing or in the furrow, more rarely on
the soil surface or on the plant during growth. Current
regulations on the use of inoculants in agriculture vary
greatly from one country to the next (regarding the need for
toxicological assessment, proof of effectiveness, guarantee
for inoculum level in the product), which can make it expensive in certain countries to develop new microbial products,
especially for biological control purposes.
11 Microorganisms and Biotic Interactions
419
impact on plant growth may also involve trophic microbial
effects. For example, the potential for symbiotic nitrogen
fixation is typically in the order of 60–200 kg N/ha/year, for
which the plant will invest 3–25 % of net photosynthates,
versus only 5–25 kg N/ha/year for free nitrogen fixation.
Nevertheless, the symbiotic nitrogen fixation potential is
often poorly exploited in modern agriculture, which relies
on massive use of chemical nitrogen fertilizers. Mycorrhization improves plant nutrition in phosphorus and nitrogen, and to a lesser extent in potassium and iron, especially
in low-fertility soils (Brundrett 2009). Phosphorus can be
recovered by mycorrhizal hyphae at more than 7 cm from the
root. Mycorrhizal fungi also improve plant resistance to
biotic stress (disease, herbivory; Bennett and Bever 2007)
and abiotic stress (drought, metal pollution; Auge 2004).
Regarding plant health, the role of plant-protecting bacteria
and fungi is important in the rhizosphere. In soils that are
suppressive to disease, their interactions are sufficient to limit
disease severity despite the presence of the pathogen, plant
susceptibility to disease, and environmental conditions favorable
for infection (Alabouvette et al. 1996; Garbeva et al. 2004).
These disease-suppressive soils are documented for root diseases
caused by fungi and to a lesser extent by bacteria or nematodes.
In some pathosystems, suppressiveness gradually develops during crop monoculture in soils that were initially conducive to
the disease. This is the case of wheat take-all caused by
Gaeumannomyces tritici; take-all severity increases over the
early years of wheat monoculture, reaches a peak (usually in
years 4–8), and decreases thereafter to minor levels in the
following years (Lucas and Sarniguet 1998; Raaijmakers and
Weller 1998). In other pathosystems, disease suppressiveness is
a natural property of the soil and does not require monoculture,
as for soils suppressive to Fusarium wilt (caused by F.
oxysporum) (Alabouvette et al. 1996) or to Thielaviopsis black
root rot (caused by Thielaviopsis basicola) (Kyselkova ´ et al.
2009). Soil suppressiveness is often attributed to Pseudomonas
PGPR subpopulations producing 2,4-diacetylphloroglucinol,
although other phytobeneficial microorganisms might also play
an important role (Kyselkova ´ et al. 2009).
Plant–microbe interactions have a significant impact on
natural plant communities (Cardon and Whitbeck 2007).
Nitrogen-fixing and mycorrhizal symbioses may be important to promote the establishment of pioneer plant groups.
This is the case for plant recolonization of ground surfaces
following deglaciation (at the end of ice ages or with current
global warming), volcanic eruptions, or forest fires (van der
Maarel 2005). Pioneer plant species will modify soil microbial community composition/functioning which, in turn,
may then influence plant succession and the composition of
intermediate groups, both by favoring certain plant species
and counterselecting others. Finally, when the climax stage
is reached, pathogens adapted to the main plant species may
decrease their dominance or eliminate enough individuals to
allow re-installation of pioneer species (within forest
clearings) and reinitiate plant succession (van der Maarel
2005). In the particular case of invasive plant species, the
soil microbial community may facilitate their establishment
at the expense of native plants. Plant–microbe interactions
are also important for the functioning of plant communities,
particularly in terms of recycling of plant litter by microbial
decomposers and nitrogen fixation (van der Maarel 2005).
Organic matter transformations contribute to pedogenesis
and to soil functioning, and as such they impact both on
plant growing conditions and plant community successions.
11.3.7 Biotechnological Uses of Plant–Microbe
Interactions
Certain microorganisms are used as inoculum in agriculture
(Fig. 11.11b), for biofertilization, phytostimulation, or biocontrol purposes (Dobbelaere et al. 2001; Alabouvette et al.
2006). The goal is to improve crop yield (productivity) and/
or reduce chemical inputs (environmental quality).
Biofertilizers aim at improving plant mineral nutrition, e.
g., symbiotic nitrogen-fixing bacteria (Rhizobium and other
genera) and to a lesser extent mycorrhizal fungi. For PGPR
having mainly hormonal effects on the plant, the term
phytostimulation is used instead of biofertilization. The
main phytostimulators used belong to the genus
Azospirillum, especially for cereals (0.5–1 million ha of
corn inoculated each year). Azospirillum inoculation resulted
in increased cereal yield (by 10–30 %) in two-thirds of the
cases (Dobbelaere et al. 2001). Biological control is most
often implemented in commercial greenhouse, using bacteria or fungal inoculants. The objective is to protect crops
against phytopathogens (often fungi or oomycetes, sometimes bacteria), parasitic nematodes or plants, or weeds.
The microorganisms used correspond to Firmicutes (Bacillus), Proteobacteria (Pseudomonas), and Actinobacteria
(Streptomyces) for bacteria as well as Deuteromycetes
(Trichoderma, Gliocladium, Coniothyrium minitans) for
fungi (Gilbert et al. 1993; Harman et al. 2004; Rezzonico
et al. 2005). Inoculum formulation is a key issue determining
inoculant performance, and spore-forming microorganisms
have better storage capacities. The inoculum is often applied
to the seed before sowing or in the furrow, more rarely on
the soil surface or on the plant during growth. Current
regulations on the use of inoculants in agriculture vary
greatly from one country to the next (regarding the need for
toxicological assessment, proof of effectiveness, guarantee
for inoculum level in the product), which can make it expensive in certain countries to develop new microbial products,
especially for biological control purposes.
11 Microorganisms and Biotic Interactions
419