Agriculture Reference
In-Depth Information
6.3.2
In Vitro Screening of Phosphate-Solubilizing
Actinomycetes
The phosphate-solubilizing activity of the actinomycetal strains is assessed gener-
ally on the Pikovskaya (PVK) medium (Pikovskaya
1948
) which contains (g/l):
glucose 10; Ca
3
(PO
4
)
2
, 5; (NH
4
)
2
SO
4
, 0.5; NaCl, 0.2; MgSO
4
·7H
2
O, 0.1; KCl, 0.2;
yeast extract, 0.5; MnSO
4
·H
2
O, 0.002; and FeSO
4
·7H
2
O, 0.002, agar 15. The
streaked/spot inoculated plates are incubated at 22
C for 14 days and observed
for halo formation. The development of a clear zone around the colony on the
culture plates is taken as an index of P solubilization. Halo size is calculated by
subtracting colony diameter from the total diameter (Valverde et al.
2006
). There-
after, the halo-forming actinobacteria is grown on SRSM-1 medium which has the
same composition as that of the PVK medium but is supplemented with
bromocresol purple as a pH indicator for determining the involvement of organic
acid production in PS activity.
6.3.3 Role of Actinobacteria in Rock Phosphate
Solubilization
Nearly 20 % of the actinomycetes including
Streptomyces
and
Micromonospora
possess profound phosphate-solubilizing potential (Barreto et al.
2008
; El-Tarabily
et al.
2008
; Hamdali et al.
2008b
). Actinomycetes as P solubilizer have received
greater attention for two reasons: (1) ability to produce agro-active metabolites, for
example, phytohormones, siderophores, and antibiotics, etc. (Hoster et al.
2005
;
Errakhi et al.
2007
; Errakhi et al.
2009
), and (2) ability to tolerate different stressor
molecules (Fabre et al.
1988
; Keiser et al.
2000
; Hamdali et al.
2008d
). Also, the
inoculation with actinomycetal strains has been found to increase the plant growth
(Hamdali et al.
2008c
). Plants take up the orthophosphorus form of P as nutrient, the
availability of which depends on soil characteristics (Nath and Borah
1983
).
However, Fe and Al at high pH and Ca at low pH fix the soluble form into insoluble
form rendering it unavailable to the plants (Rengel and Marschner
2005
; Johnson
and Loepper
2006
). The PS actinomycetes, however, through the release of organic
acid (acidification) (Hoberg et al.
2005
; Abdulla
2009
) and by other mechanisms
such as chelation, exchange reactions, and polymeric substances formation
(Delvasto et al.
2006
) convert the insoluble forms of P into soluble forms. The
organic anions assist P solubilization mainly in two ways: (a) lowering pH and
(b) ligand exchange reaction (Beunemann et al.
2011
; Balemi and Negisho
2012
).
However, the organic anions in some cases have been found not to acidify the
medium, probably because once they are released in soil, they already exist in the
dissociated form due to their low acid dissociation constants (pKa). In fact, it is
assisted by the proton extrusion accompanying respiration and NH
4+
assimilation
(Illmer and Schinner
1992
), which compensates for the losses of negative charge
