Agriculture Reference
In-Depth Information
higher, reaching three to four orders of magnitude. Plant-based bioassays-
conducted at 22
C and using wheat as test crop under greenhouse conditions-
indicated that the bacterial isolates did not lose their plant growth promotion
abilities. Furthermore, the bacterial isolates retained the root colonization, antifun-
gal, and enzyme activities in the alginate-based formulation during storage. The
authors noticed, however, some liquefaction at the bottom of the flasks containing
P. corrugata
after 3 years of storage, possibly due to organic acid production. No
such loss of integrity was observed in the case of beads entrapping
B. subtilis
. From
all these studies, it became apparent that sodium alginate might be the best choice
for the large scale production of high-quality CT bacterial inoculants, for commer-
cialization and field application of this microbe-based technology.
5.8 What Lies Ahead?
Even though much work has been done, many aspects concerning the development
of CT-PS bioinoculants require further scientific attention. For instance, almost
nothing is known about the specificities of bacterial-mediated phosphate dissolution
at low temperatures (e.g., enzymes involved, enzyme kinetics, genetics, and regu-
lation). It is widely accepted that the mechanisms adopted by CT bacteria for
mobilizing P from sparingly insoluble P-containing minerals are similar to those
adopted by mesophilic PS organisms. This assumption, which might prove incor-
rect, has been biased in many cases in the search for efficient CT P solubilizers
toward microorganisms only capable of solubilizing tri-calcium phosphate in
agarized media (e.g., Pikovskaya's Medium). Alas, as recently shown by Bashan
et al. (
2013
), this form of reasoning can be misleading and certainly yields many
false PS-microorganisms. Therefore, a different experimental approach-including
the use of a combination of two or three metal-P compounds together or in tandem
according to the end use of these bacteria-has been invoked. In the case of acidic
soils, Fe-P and Al-P compounds should be included in the preliminary tests; for
alkaline soils, Ca-P compounds (including rock phosphates) would be the right
choice. Once a potential candidate is identified following these guidelines, appro-
priate tests must be performed to confirm direct contribution to P plant nutrition and
not to general growth promotion, as commonly done. Indeed, as we have previously
seen, in many cases growth promotion of plants by PS microorganisms can be the
consequence of other-direct or indirect-mechanisms (e.g., phytohormone produc-
tion). According to Bashan and his coauthors, isolates that do not comply with this
general sequence of testing should not be declared as PSB. Also, it is important to
mention here that there are still numerous unexplored natural environments which
can be targeted for isolating potential CT P solubilizers. Perhaps one of the most
evident choices in this regard is glacier ice. Indeed, as shown by a great number of
scientific reports, glaciers are repositories of an almost unknown diversity of
microorganisms (Miteva
2008
). Many of these are true psychrophiles and possess
unexpected metabolic repertoires which, in some cases, have been exploited to
