Agriculture Reference
In-Depth Information
5.6.1 Cold-Tolerant Phosphate-Solubilizing Bacteria
During the past 15 years, the search for cold-tolerant PSB has followed two main
routes. The first one was the isolation of native cold-tolerant phosphate-solubilizing
bacteria (CT-PSB) from natural soils collected in mountainous regions (including
alpine and sub-alpine environments, mainly in the IHR); the second path was the
development of cold-tolerant mutants from well-known PS bacterial strains,
irrespective of their ability of colonizing the roots of crops and/or to survive in
natural rhizospheric environments under the prevailing climatic conditions. Para-
doxically, one of the first attempts in this field followed the second approach.
During the mid- to late 1990s, Goel and her team at the G.B. Pant University of
Agriculture and Technology (India) decided to produce cold-tolerant mutants of
Pseudomonas fluorescens
, a well-known PGPR species able to effectively solubi-
lize P
i
, and then to determine their effect on plant growth promotion at low
temperatures. The mutants were developed by nitrosoguanidine treatment of three
different strains of
P. fluorescens
, namely, GRS1, PRS9, and ATCC13525 (Mishra
and Goel
1999
). In their first report, Das et al. (
2003
) showed that some of these
P. fluorescens
mutants were able to solubilize much more P than their respective
native strains, at 10
C. This primary work was complemented by another study
which revealed that two of these mutants could enhance growth of wheat and mung
bean at 10
C under in vitro (gnotobiotic system) and in situ conditions (Katiyar and
Goel
2003
). In a follow-up experiment, Trivedi and Sa (
2008
) isolated a
psychrotrophic strain of
P. corrugata
from IHR soils and generated mutants with
high P-solubilizing abilities using nitrosoguanidine. Of the total 115 mutants ini-
tially identified, only 2 were chosen to further test their PGPR and P-solubilizing
abilities. These mutants were indeed able to solubilize more P at 4, 9, and 28
C
than their native counterpart; this P-solubilizing activity was concomitant to a
drastic acidification of the culture broth, related to organic acid production (both
gluconic and 2-ketogluconic acids). Subsequently, following bacterization of seeds,
the growth of wheat and mung bean increased significantly under in vitro and
greenhouse conditions at 10 and 15
C and in the presence of rock phosphate
(as the sole source of P). Also, the bacterial inoculation had a positive effect on
soil enzymatic activities, especially acid and alkaline phosphatases.
Even though the mutagenesis strategy has shown some promising results, the
alternative approach isolating soil-borne CT-PSB is by far the most popular prac-
tice among researchers. This easy and inexpensive experimental approach is based
on the premise that for isolating competitive and effective bacterial strains, it is the
pool of indigenous soil bacteria that must be screened in the first place. It is
therefore assumed that such microorganisms would be well adapted to the partic-
ular climatic conditions of the particular site (Paau
1989
). The arguments beneath
such an assumption are that (1) dominant, competitive indigenous strains are
specific to a particular geographical region as a result of natural selection by various
biotic and abiotic pressures (e.g., low temperatures, heavy rainfall and snowfall,
food and non-food crop species commonly used in the area), (2) important
