Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...

272j 14 Salt-Tolerant Rhizobacteria: Plant Growth Promoting Traits
and nutrients from the subsoil [119,120]. Therefore, if the increase in the amount of
nutrients taken up by the PGPB-inoculated plants was simply owing to an increase
in root surface area, the effect of PGPB would entirely depend on root development
and not on the uptake function [23]. Furthermore, the PGPB inoculants can help
plants acquire nutrients through greater mobilization. A range of diazotrophs (e.g.
Azospirillum, Azotobacter, Azorhizobium, Bacillus, Herbaspirillum and Klebsiella) can
supplement the use of urea in wheat production either by nitrogen fixation or growth
promotion [121]. A number of studies have reported that bacterial inoculation can
lead to an increase in nitrogen content in the biomass of peas and barley
[110,111,121].
Both potassium and phosphorus are important macronutrients for plant growth.
Since the availability of potassium and phosphorus in arid saline soil is limited, the
most appropriate approach to address this situation is to use bioinoculants. These
bioinoculants are able to enhance solubilization of applied phosphorus fertilizers
and insoluble forms of potassium- and phosphorus-bearing minerals by excreting
organic acids (e.g. citric acid) in soil [122–125]. For example, the Bacillus edaphicus
NBT strain was found to increase potassium content of cotton and rape seed plants
by 30% when the soil was treated with insoluble potassium sources [6]. On the
contrary, Bacillus megaterium is considered to be the most effective among the
phosphorus-solubilizing bacteria (PSB) [137]. A number of phosphate-solubilizing
bacteria, such as NBRI0603, NBRI2601, NBRI3246 and NBRI4003 were isolated
from the rhizosphere of alkaline soils used for growing chickpeas. All four strains
have demonstrated diverse levels of phosphate solubilizing activity under in vitro
conditions in the presence of various carbon and nitrogen sources [137]. The
synergistic interactions of plants with coinoculation of PSB were identified to be
Azospirillum species [38].
Application of rock phosphate along with PSB in phosphorus-deficient soil has
reportedly improved phosphorus uptake by plants and enhanced crop yields, thus
suggesting that PSB are effective for solubilizing phosphates and mobilizing available
phosphorus to crops [126]. Phosphorus biofertilizers in the form of microorganisms
can increase the solubility and availability of phosphates for plant growth
[127,128]. It is reported that PSB-plant inoculations resulted in 10–15% increases in
crop yields and phosphorus uptake in 10 out of 37 experiments [129]. Ectorhizospheric
strains from Pseudomonades and Bacilli and endosymbiotic rhizobia have
been described as effective phosphate solubilizers in soils [130,131]; however, their
performance was influenced by environmental factors [132]. Highly saline soils may
cause poor survival and growth of PSB. Weak organic acids, the end-products of
microbial metabolism, often result in decrease of soil pH that probably plays a major
role in solubilization of phosphorus-containing minerals or compounds [122,128].
In our previous work [133], the inoculation of cotton seeds with the salt-tolerant
phosphate-solubilizing bacteria R. meliloti URM1 combined with phosphate had a
significant effect on total dry matter, shoot and root production of plants. The yield of
cotton was increased by 77%. The phosphorus content was significantly increased in
cotton plants with treatment of PSB combined with phosphate (Table 14.10). The
standard treatment with fertilizer did not affect phosphorus uptake by plants.