Agriculture Reference
In-Depth Information
extremely important to find microorganisms with varying plant growth-promoting
(PGP) abilities and potential to adapt well to low temperatures [
cold-tolerant
(CT) or cold-loving], in order to develop proficient CT-biofertilizers for use in
mountain agriculture.
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5.5 Plant Growth-Promoting Rhizobacteria,
Plant Growth-Promoting Bacteria, and
Phosphate-Solubilizing Bacteria
Soil microorganisms do play a significant role in the biogeochemical cycling of
elements, regulating the dynamics of organic and inorganic matter and increasing
the availability of plant nutrients in the rhizosphere. The beneficial effects of some
primitive “environmentally friendly” agricultural practices on plant growth and
development were indeed observed and recorded centuries ago by the Ancient
Greeks and Romans. At the time, it was proposed that mixing different soil samples
or adding organic manures to the farmland might improve soil fertility and,
consequently, crop yields (Tisdale and Nelson
1975
). Subsequently, Hellriegel
and Wilfarth (
1888
) and Beijerinck (
1888
) reported that some soil bacteria may
convert atmospheric nitrogen (N) into plant usable forms of N. Thereafter,
Kloepper and Schroth (
1978
) introduced the term “rhizobacteria” referring to the
soil bacterial community competent in colonizing plant roots and able to stimulate
plant growth. Later on, Kloepper and Schroth (
1981
) termed such beneficial
rhizobacteria as plant growth-promoting rhizobacteria (PGPR). Considering the
fact that not all plant-beneficial bacteria are inhabitants of the rhizosphere, some-
times the term PGPB (for plant growth-promoting bacteria) is used instead of PGPR
(Andrews and Harris
2003
).
Mechanistically, some PGPB enhance crop growth and development indirectly:
for example, many of them inhibit plant pathogens (these are termed
“bioprotectants,” “biocontrollers,” or “biopesticides”); others degrade toxic xeno-
biotics (“bioremediators”) or trigger the induced systemic resistance (ISR) in
plants. Additionally, some PGPB act directly and promote plant growth by releas-
ing phytostimulators (“biostimulants”) or by providing essential nutrients, such as
N and P (“biofertilizers”) (Glick
2012
). Phosphate-solubilizing bacteria (or PSB)
belong to the latter group. This heterogeneous group of PGPB is characterized by
their ability to readily and efficiently solubilize mineral forms of inorganic P (P
i
).
The mechanisms employed by PSB to perform this Pi solubilization are as diverse
as their phylogeny (see below). Another group of PGPB includes organic-P (P
o
)
mineralizing bacteria, which hydrolyze organic forms of P (phosphate esters,
phosphonates, and anhydrides) through the action of specific enzymes (mainly
phosphatases) (Turner et al.
2006
; Richardson and Simpson
2011
). This process,
usually called “substrate mineralization,” is of fundamental importance because it
releases plant-available orthophosphate (PO
4
2
). Many PSB have been tested both
