Agriculture Reference
In-Depth Information
convert TCP to di- and monobasic phosphates with the net result of an enhanced
availability of the element to plants.
2.2.1 Mineralization: Enzymatic Degradation of Complex
Organic P Compounds
Organic P compounds undergo mineralization, and the resulting P is taken up as
nutrient by plants. In this regard, numerous soil microbes or rhizosphere microflora
possess the ability to transform organic P into soluble forms of P (Tarafdar and
Claassen
1988
; Rodriguez et al.
2006
). This mineralization process is mediated by
the enzymes especially phosphatases (Tarafdar et al.
1988
; Yadav and Tarafdar
2003
; Aseri et al.
2009
) and phytases (Maougal et al.
2014
), released by the soil
microbes. The enzyme phosphatases (e.g. acid and alkaline phosphatases) released
exterior to the cell (exo-enzymes) are non-specific in nature and use organic P as a
substrate to convert it into inorganic form (Beech et al.
2001
). Of the two phospha-
tases, acid phosphatases (To-O et al.
2000
), a widely distributed enzyme and
commonly found in fungi (To-O et al.
1997
; Abd-Alla and Omar
2001
),
for example,
Colletotrichum graminicola
(Schadeck et al.
1998a
,
b
), are considered
as the principal mechanism for mineralization of soil organic P (Hilda and Fraga
1999
) where it catalyses the release of inorganic P from organic P compounds
such as inositol hexaphosphate (Nozawa et al.
1998
; Tarafdar and Gharu
2006
;
Yadav and Tarafdar
2007
,
2011
). However, the degradation of organic P mediated
by phosphatases varies greatly among different fungi (Guimar˜es et al.
2006
).
Another attractive application of P-dissolving enzymes is the mineralization of
soil organic P through phytate degradation mediated by the enzyme phytase, which
specifically causes release of P from phytic acid. Phytate is a major component of
organic P in soil. Though the ability of plants to obtain P directly from phytate is
very limited, the growth and P nutrition of
Arabidopsis
plants supplied with phytate
was improved significantly when they were genetically transformed with the
phytase gene [phyA] derived from
Aspergillus niger
(Richardson
2001
). This led
to the increase in P nutrition to such an extent that the growth and P content of the
plant was equivalent to control plants supplied with inorganic P. Similar increase in
utilization of inositol P by plants in the presence of microbial communities includ-
ing P-solubilizing fungus (
A. niger
) capable of producing phytase is reported
(Richardson
2001
; Vassilev et al.
2007
). Phosphonatases (Kumar et al.
2013
) and
C-P lyases (Salimpour et al.
2010
) are the other enzymes that cleave the C-P of
organophosphonates. Once the inorganic or organic P compound is changed to
soluble P, it can now easily be used up as P nutrient by plants, algae, cyanobacteria
and autotrophic bacteria and thereafter could be immobilized into organic cellular
macromolecules, for example, DNA, RNA and ATP. Considering the critical
impact of such enzymes in dissolution of complex organic compounds into usable
form of P, it is highly desirable to develop the bacterial/fungal inoculants with high
