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Phosphate solubilizing bacteria and their role in plant growth promotion …
Phosphate solubilizing bacteria and their role in plant growth promotion
Hilda Rodríguez *, Reynaldo Fraga
most powerful phosphate solubilizers
Pseudomonas
Bacillus
Rhizobium
механизмы стимуляции роста растений
прямые
synthesis of phytohormones
N2 fixation
уменьшение мембранного потенциала корней
synthesis of some enzymes (such as
ACC
deaminase
) that modulate the level of plant hormones
solubilization of inorganic phosphate
and
mineralization of organic phosphate
, which makes phosphorous available to the plants
непрямые
decrease or prevention of deleterious effect of pathogenic microorganisms
synthesis of antibiotics
siderophores
Phosphate in soil
400–1200 mg/kg в почве
1 форма - минеральные фосфаты
:star:
Mineral phosphate can be also found associated with the surface of hydrated oxides of
Fe, Al, and Mn
, which are poorly soluble and assimilable (ferralitic soils)
rocks and other deposits
fixation and precipitation of P in soil is generally highly dependent on pH and soil type
acid soils
phosphorus is fixed by
free oxides and hydroxides of aluminum and iron
alkaline soils
is fixed by
calcium
, causing a low efficiency of soluble P fertilizers, such as super calcium
2 форма - органический фосфор (organic matter)
:star:
30–50%
of the total phosphorus in most soils,
although it may range from
as low as 5% to as high as 95%
1 форма
- Organic P in soil is largely in the form of
inositol phosphate (soil phytate)
инозитол-фосфат (почвенный питат)
It is synthesized by microorganisms and plants
the most stable of the organic forms of phosphorus in soil
up to 50% of the total organic P
2 форма
-
phosphomonoesters, phosphodiesters
including phospholipids and nucleic acids, and
phosphotriesters
компоненты гидролиза
cytosine, adenine, guanine, uracil, hypoxanthine, and xanthine
(decomposition products of guanine and adenine)
Of the total organic phosphorus in soil,
only approximately 1%
phospholipids
choline
product of the hydrolysis of lecithin
that only approximately
1–5 ppm
of phospholipids phosphorus occur in soil, although values as high as
34 ppm
have been detected
Many of these P compounds are high molecular-weight material
they must first be bioconverted
to soluble ionic phosphate (Pi, HPO4-2, H2PO4)
assimilation by the cell
or low molecular-weight organic phosphate
assimilation by the cell
Phosphate solubilizing bacteria
Mineral phosphate solubilization :check:
inorganic phosphate compounds
tricalcium phosphate
dicalcium phosphate
hydroxyapatite
rock phosphate
Pseudomonas, Bacillus, Rhizobium, Burkholderia, Achromobacter, Agrobacterium, Microccocus, Aereobacter, Flavobacterium, Erwinia
Visual detection and even semiquantitative estimation of the phosphate solubilization ability of mko
plate screening methods
clearing zones around the microbial colonies in media containing insoluble
mineral phosphates (mostly tricalcium phosphate or hydroxyapatite)
as the single P source
contradictory results between plate halo detection and P solubilization in liquid cultures (In some cases)
метод Gupta et al. [58]
medium containing
bromophenol blue
yellow colored halos
are formed around the colonies in response to the pH drop produced by the release of organic acids, which are responsible for phosphate solubilization.
Link Title
dynamics of phosphate solubilization by bacterial strains
measurement of P release into culture broth
The rate of P solubilization is often estimated by subtracting the final P concentration (minus that of an inoculated control) from the initial theoretical P supplied by the P substrate.
This estimation has the disadvantage
of not taking into account the P utilized by the cells during growth
certain mineral elements play a role in the process of solubilization
A critical
K concentration
is necessary for optimum solubilization rates
Mg и Na
важен для некоторых грибов, но для псевломонад
Instability of the phosphate-solubilizing character of some strains after several cycles of inoculation has been reported :!!:
quantitative comparison
Rhizobium, Pseudomonas, Bacillus
species are among the most powerful solubilizers
tricalcium phosphate and hydroxyapatite
seem to be more degradable substrates than rock phosphate
Organic phosphate solubilization :check:
To make this form of P available for plant nutrition, it must be hydrolyzed to inorganic P
Mineralization of most organic phosphorous compounds -
phosphatase enzymes
- фосфотаза
Rhizobium, Enterobacter,
Serratia, Citrobacter, Proteus, Klebsiella, Pseudomonas,
Bacillus
a significant level of acid phosphatases
Phosphate-solubilizing bacteria as plant growth promoters
количество:
their numbers are not high enough to compete with other bacteria commonly established in the rhizosphere
the amount of P liberated by them is generally not sufficient for a substantial increase in in situ plant growth :!:
production by these strains of other metabolites beneficial to the plant, such as phytohormones, antibiotics, or siderophores, among others, has created confusion about the specific role of phosphate solubilization in plant growth
evidence supporting the role of this mechanism in plant growth enhancement
soil microorganisms, including bacteria, improve the supply of P to plants as a consequence of their capability for inorganic or organic P solubilization
Burkholderia cepacia
indoleacetic acid production
: displaying significant mineral phosphate solubilization and moderate phosphatase activity
препараты
Phylazonit-M
(Hungary)
Bacillus megaterium; Azotobacter chroococcum
KYUSEI EM’
(EM Technologies, Inc.)
a mixed inoculum including lactic acid bacteria, the lactic acid being the agent for mineral phosphate solubilization
Mechanisms of phosphate solubilization
Solubilization of mineral phosphates
:check:
major mechanism
action of organic acids synthesized by soil microorganisms
acidification of the microbial cell and its surroundings
Pi may be released from a mineral phosphate by proton substitution for Ca2+
organic acids
gluconic acid
seems to be the most frequent agent of mineral phosphate solubilization
Pseudomonas sp., Erwinia herbicola, Pseudomonas cepacia and Burkholderia cepacia
2. 2-ketogluconic acid
Rhizobium leguminosarum, Rhizobium meliloti, Bacillus firmus
3. mixtures of lactic, isovaleric, isobutyric, acetic acids
Bacillus liqueniformis, Bacillus amyloliquefaciens
4. Other organic acids: glycolic, oxalic, malonic, and succinic acid
experimental evidence
organic acids Rhizobium leguminosarum solubilized an amount of P nearly equivalent to the amount that was solubilized by the whole culture
treatment of the culture filtrates from several Rhizobium strains with pepsin or removal of proteins by acetone precipitation did not affect phosphate release capacity, showing that this was not an enzymatic process. However, neutralization with NaOH destroyed the solubilization activity
Goldstein [31,116] has proposed that the direct periplasmic oxidation of glucose to gluconic acid, and often 2-ketogluconic acid, forms the metabolic basis of the mineral phosphate solubilization phenotype in some Gram negative bacteria
Alternative possibilities other than organic acids for mineral phosphate solubilization
lack of a linear correlation between pH and the amount of solubilized P
no significant amounts of organic acid production could be detected from a phosphate solubilizer fungus, Penicillium sp
Studies have shown that the
release of H+
to the outer surface in exchange for cation uptake or with the help of H+ translocation ATPase could constitute alternative ways for solubilization of mineral phosphates
Other mechanisms
production of chelating substances
production of inorganic acids ( sulphidric, nitric, and carbonic acid)
However, the effectiveness of these processes has been questioned and their contribution to P release in soil appears to be negligible
Mineralization of organic phosphorus
:check:
decomposition of organic matter
numerous saprophytes
produce the release of radical orthophosphate from the carbon structure of the molecule
microbial mineralization of organic phosphorus is strongly influenced by environmental parameters; in fact,
moderate alkalinity favors the mineralization of organic phosphorus
The degradability of organic phosphorous compounds
depends mainly on the physicochemical and biochemical properties of their molecules
nucleic acids, phospholipids, and sugar phosphates are easily broken down :check:
phytic acid, polyphosphates, and phosphonates are decomposed more slowly :red_cross:
mineralization
phosphatases (also called phosphohydrolases)
dephosphorylating reactions
involve the hydrolysis of phosphoester or phosphoanhydride bonds
phosphohydrolases
1.acid
optimal catalytic activity at acidic to neutral pH values
специфические
3'-nucleotidases and 5'-nucleotidases
hexose phosphatases
phytases
неспецифические
2.alkaline
Some phosphohydrolases are secreted outside the plasma membrane
released in a soluble form
retained as membrane-bound proteins
This localization allows them to act as scavenging enzymes on organic phosphoesters that are components of high molecular weight material (i.e. RNA and DNA) and cannot cross the cytoplasmic membrane
Genetics
Genetics of organic phosphate mineralization
Genetics of phosphate solubilizing bacteria
Genetics of mineral phosphate solubilization
Mps+ phenotype
feature
the selection by classical genetic methods of mutants with increased production of organic acids and/or phosphatase activity, could constitute an effective approach that can not be underestimated