Agriculture Reference
In-Depth Information
a) Microorganisms that incorporate nitrogen into the plant-soil system through
biological nitrogen fixation
The most efficient nitrogen fixers are bacteria belonging to the genera
Rhizobium
,
Ensifer
(formerly
Sinorhizobium
),
Mesorhizobium
,
Bradyrhizobium, Azorhizobium
and
Allorhizobium
(Bloemberg & Lugtenberg, 2001).
b) Microorganisms that increase nutrient and water uptake
This category includes the fungi which associate with plants to form the symbiotic
relationship known as mycorrhizae, which play an important role in water, phosphorus, zinc,
sulfur, and copper uptake (Nelson & Spaner, 2010), and bacteria such as
Azospirillum
spp.,
which increases the capacity of water and nutrient uptake of plants by stimulating root growth
through hormonal action. (Hayat et al.
,
2010).
c) Microorganisms that increase the availability of nutrients found in the soil in not
assimilable forms
This category includes phosphate-solubilizing microorganisms capable of producing
phosphatases or organic acids (
e.g.
Bacillus megaterium
or
Pseudomonas fluorescens
), sulfur-
oxidizing bacteria that convert elemental sulfur or any reduced form of this element into
sulfates, which are the usable form by plants, microorganisms producing siderophores, such
as certain species of the genera
Pseudomonas
,
Bacillus
and
Flavobacterium
that increase the
availability of iron to the plants and microorganisms capables of destroy the structures
potassium-bearing mineral (
e.g.
Bacillus, Pseudomonas,
and
Clostridium
and fungi such as
Aspergillus, Penicillium
and
Mucor
; Guevara-Granja, 2010)
d) Microorganisms that possess antagonistic activities against plant pathogens
This mechanism is sustained by the fact that a healthy plant will fed and function better
and, consequently, will be able to tolerate more efficiently the effect of nutritional
deficiencies or adverse environmental conditions. Different species of
Pseudomonas,
Bacillus, Serratia, Flavomonas, Curtobacterium
and
Trichoderma
, are included in this group
(Aguado-Santacruz, 2012).
Additionally to the previous classification, other categories have been proposed for
referencing the mechanisms by which the microorganisms promote the growth of plants:
a) Biopesticides
Include microorganisms that stimulate plant growth by controlling phytopathogenic
agents. For example,
Pseudomonas
aurantiaca
is an orange colored, Gram negative, growth-
promoting bacterium originally isolated from the rhizosphere of potato that produces the
compound di-2, 4 diacetylphloroglucylmethane, which is an effective antibiotic against
various phytopathogenic soil microorganisms (Esipov et al., 1975; Felker et al.
,
2005).
Additionally, some endophytic microorganisms such as
Cladosporium sphaerosperum,
Neotyphodium sp., Phomopsis oblonga, Bacillus subtilis
or
Pseudomonas
fluorescens
have
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