**2.2. Phosphate solubilization and mineralization**

Soil stores several structures and forms of phosphate, both organic and inorganic. Phosphorus plays a key role in photosynthesis, respiration, root development, signal transduction, energy transfer, macromolecular biosynthesis and the resistance ability of plants to diseases and adverse conditions. However, majority of soil phosphorus is insoluble that is not available to plants. The secondary significant contributing factor to promoted growth is the availability of phosphorous in the rhizospheric region, as a result of phosphate solubilization by the PGPR [41].

PGPRs serve as phosphate (and zinc) solubilizer (PSB). This is due to the decreased pH of the medium, indicating the possible involvement of organic acids such as gluconic acid. Plant growth promotion can be achieved through solubilization of inorganic phosphates by these organic acids. de Werra et al. showed that this happens with not only gluconate but also malate [42, 43]. These results were consistent with earlier report on the P and Zn solubilizing properties of *Acinetobacter* sp. [44]. Nearly all the *Acinetobacter* species isolated from rhizosphere soil of the three wheat varieties in the present study were efficient phosphate and zinc solubilizers and produced iron chelating siderophores [45]. Phosphate solubilizing bacteria (PSB) belong largely to the genera pseudomonads, bacilli and rhizobia [46].

Phosphorus-solubilizing *Bacillus* strains have been reported to increase the plant biomass and yield of wheat as well as uptake of nutrients [47]. Similar results have been reported by Afzal et al. when a combination of nitrogen-fixing Rhizobium leguminosarum with P-solubilizing *Pseudomonas* sp. strain 54RB have been used [48]. Similarly, several *Pseudomonas* spp. strains have been tested in the field for their efficacy to increase growth and yield of wheat [49]. Four P solubilizer (*Arthrobacter* WP-2, *Bacillus* MP5, *Rhodococcus* M28 and *Serratia* 5D) and one phytohormone producer (*Azospirillum* WS1) strains tested as single-strain inocula resulted in improved growth of wheat plants [50]. Some *Bacillus* species can improve phosphate solubilization of the soil [51, 52]. On the other hand, Baig et al. reported a positive correlation between P-concentration in soil, P-solubilization activity of the Bacillus strains and P uptake by wheat plants [53]. Along the same line, improvement of growth and yield of wheat was observed and reported upon inoculation with P-solubilizing microorganisms. Both PGPR (*Bacillus* and *Pseudomonas* spp.) are similar in effectively solubilizing phosphate. A short list of phosphate-solubilizing bacteria (PSB) includes *P. fluorescens* 153, *P. fluorescens* 169, *P. putida* 4 and *P. putida* 108 together with their capability in natural soil ecosystem to synthesize ACC deaminase and IAA-like products [54].

Combined application of PSB with conventional fertilizer (50% PSB, 25 kg/ha P<sup>2</sup> O5 ) improves plant growth. Similarly, a combination of PGPRs are more effective when compared with isolated applications as reported by Hassan et al. for wheat crops and by Baig et al. for wheat yield and P uptake [53, 55].

#### *2.2.1. Mineralization*

**2. Mechanisms of plant growth promotion**

PGPR improve plant growth by multiple mechanisms. A well-established mechanism is the biological nitrogen fixation (BNF), as described in extensive literature available on diazotrophic association in wheat and subsequent addition of nitrogen to the ecosystem [24], con-

direct plant growth-promoting trait and the nitrogen-fixing rhizobacteria provide an alterna-

*Azospirillum* is a kind of nitrogen-fixing bacterium that lives in close association with plants in the rhizosphere. Its beneficial effects on wheat yields in both greenhouse and field conditions have been reported [28, 29]. Balandreau found that *Azosprillum lipoferum* inoculation increased yield around 1.8 t/ha and wheat grain by up to 30% [30, 31]; Okon and Labandera-Gonzalez by inoculation with *Azospirillum brasilense* [31]. In an earlier study, Boddey et al. were unable to observe fixed N in wheat from similar organisms [32]. Further, Ruppel and Merbach investigated the dinitrogen-fixing ability strain of *Pantoea agglomerans* and *Azospirillum* spp. and in hydroponic experiments with wheat found that bacterial strain inoculation affected plant growth, by nitrogen uptake and the amount of biologically fixed dinitrogen. In this sense, when *Azospirillum* brasilense is inoculated using seed inoculation, it increases the productiv-

Ruppel et al. reported *P. agglomerans* to be superior strain for winter wheat, reporting a grain yield increase for different wheat cultivars ([37], also in Ref. [38]). Moreover, a nitrogen-fixing *P. agglomerans* Lma2 was isolated from wheat rhizosphere, it was found to have the ability to produce IAA, siderophores and solubilize P, and growth performance of plant was signifi-

*Acinetobacter* strains also possessed BNF properties, siderophore and ammonia production as well as mineral solubilization. Rana et al. reported a positive correlation of BNF potential of *Providencia* spp. AW4 and *Brevundimonas diminuta* AW7 strains with panicle weight and plant

Soil stores several structures and forms of phosphate, both organic and inorganic. Phosphorus plays a key role in photosynthesis, respiration, root development, signal transduction, energy transfer, macromolecular biosynthesis and the resistance ability of plants to diseases and adverse conditions. However, majority of soil phosphorus is insoluble that is not available to plants. The secondary significant contributing factor to promoted growth is the availability of phosphorous

PGPRs serve as phosphate (and zinc) solubilizer (PSB). This is due to the decreased pH of the medium, indicating the possible involvement of organic acids such as gluconic acid. Plant growth promotion can be achieved through solubilization of inorganic phosphates by these

in the rhizospheric region, as a result of phosphate solubilization by the PGPR [41].

ity of wheat [33–35]. *P. agglomerans*, as a diazotroph, is able to fix molecular N<sup>2</sup>

height in wheat, indicating the enhancing plant growth role of BNF [40].

requirement of wheat [25–27]. Nitrogen fixation is considered to be a

with wheat [36].

**2.1. Biological nitrogen fixation**

122 Wheat Improvement, Management and Utilization

tive source to inorganic nitrogen fertilizers.

cantly better in the presence of salt [39].

**2.2. Phosphate solubilization and mineralization**

tributing to the total N<sup>2</sup>

Mineralization of most organic phosphorous compounds is carried out by means of phosphatase enzymes. The conversion of insoluble inorganic P to a form accessible by plants is achieved by PSB via organic acids, chelation and exchange reactions [56]. However, organic P forms, particularly phytates, are predominant in most soils (10–50% of total P) and must be mineralized by phytases (myo-inositol hexakisphosphate phosphohydrolases) to be available P for plants [57, 58]. Previous research has shown that *Bacillus* sp., *Providencia* sp., *Brevundimonas* and *Alcaligenes* were recorded positive for P solubilization [40, 59].

Singh et al. reported that phytase-producing bacteria from Himalayan soils showed ability to solubilize inorganic phosphate, producing phytase, siderophores, ammonia and IAA and increased availability of P, IAA and ammonia leading to increased plant growth [57]. The role of PGPR in production of phosphataes, β-gluconase, dehydroginase, antibiotic, solubilization of phosphates and other nutrients, stabilization of soil aggregates, improved soil structure and organic matter contents has been recognized.
