509crop specific and can be used in a variety of crop plants across different agro-
climatic conditions. Several reports are available on the use of PGPR in agriculture
as compiled by Desai et al. 2012a. For dryland areas, Bacillus species are appealing
PGPR candidates due to their inherent ability to produce stress-resistant endospores.
Some reports on beneficial effects of PGPR as single inoculants or combination
inoculants in dryland agriculture are included in Table 3.
4.3 Nutrient Solubilizers and Mobilizers
4.3.1 Phosphate, Potassium, Zinc Solubilization
Phosphate-solubilizing microorganisms (PSM) are a group of microbes (including
bacteria, fungi and actinomycetes) capable of solubilizing inorganic phosphorus
from insoluble sources. In alkaline and acidic soils, the availability of phosphorus is
low as it is in dynamic equilibrium. PSMs can reverse this process by solubilizing
the insoluble phosphorous through the production of organic acids and phospha-
tases. Soil bacteria belonging to genera Pseudomonas and Bacillus and fungi
belonging to genera Penicillium and Aspergillus are the most common P-solubilizers.
Other genera exhibiting P-solubilizing ability include Rhizobium, Burkholderia,
Achromobacter, Azospirillum, Azotobacter, Agrobacterium, Micrococcus,
Aerobacter, Flavobacterium, Erwinia, Streptomyces and Micromonospora. Also,
algae such as Cyanobacteria and mycorrhiza have shown P solubilization activity
(Sharma et al. 2013 ). PSMs when used with rock phosphate in corn fields could save
about 50 % of the cropʼs requirement for phosphatic fertilizer, and increase grain
yield (Yazdani et al. 2009 ). In rainfed field experiments (1991 and 1992) with soy-
bean on Vertisol, inoculation with Pseudomonas striata increased seed and total dry
matter yields and phosphorus contents in different plant parts at various stages. Seed
treatment with P. striata further enhanced the yields and phosphorus content (%) of
soybean when applied in conjunction with rock phosphate (Dubey 1996 ). Increased
yields and enhanced avaialbility of P after inoculation with PSM has been reported
in different crops including soybean, maize, pulses, wheat, vegetable and millet
(Rodríguez and Fraga 1999 ; Khan et al. 2009 ). Rhizobacteria belonging to genera
Pseudomonas, Bacillus and Burkholderia reportedly solubilize zinc. The microbes
solubilize the metal forms using protons, chelated ligands and oxidoreductive sys-
tems present on the cell surface and membranes. Bacterial inoculation of maize with
Zn-solubilizing Pseudomonas and Bacillus significantly increased zinc content in
maize leaves (Praveen Kumar et al. 2013 ). Microorganisms like Aspergillus niger,
Bacillus extroquens and Clostridium pasteurianum have reportedly solubilized
potassium. A strain of Azospirillum has not only fixed nitrogen associatively but
solubilized phosphorus and zinc (Desai et al. 2012b). Similarly, two rhizobacterial
strains CRIDA-ZnKPSB1 and CRIDA-ZnKPSB3 able to solubilize multiple nutri-
ents (P, Zn, K) have been identified as Burkholderia cepacia and B. cenocepacia at
the Central Research Institute for Dryland Agriculture (data communicated for
Application of Microbiology in Dryland Agriculture