Bacteria that inhabit the rhizosphere and roots of the plant, and improve the growth of the plant through any mechanism
are referred to as plant growth-promoting rhizobacteria (PGPR). These bacteria aggressively colonize the rhizospere
(Gholami et al. 2009). PGPR are known to improve plant health by increasing the availability of micronutrients and by
secretion of growth- promoting substances. At the beginning, studies of PGPR were only performed with crop roots, for
example, on the roots of potato (Solanum tuberosum L.), radish (Raphanus sativus L.), and sugar beet (Beta vulgaris L.)
(Kloepper et al. 1999). After- ward, studies of PGPR were carried out on a wide range of hosts, together with cereals,
legumes and trees. PGPR directly enhance the growth of plants by producing plant hormones and by rising nutrient uptake
or by inducing resistance of plants towards disease-causing microorganisms (Gupta et al. 2015). PGPR can also motivate
plant growth, ultimately by the eradication of deleterious microorganisms or pathogens present in the rhizosphere
(Grobelak et al. 2015).
The extensive use of chemical fertilizers and pesticides in the farming is at present under discussion due to ecological
concern and consumers’ health fear. Many fluorescent pseudomonads produce a number of antibiotics, which also have a
role in plant growth promoting activity (Thomashow and Macrodi, 1997). The PGPR influence the enlargement and improvement
of various crops because they release growth regulators and improve the morphological traits of roots inoculated
with PGPR, which also helped them in nutrient uptake. The ability of rhizobacteria to promote growth and development
of crops may be due to the production of phytohormones, N fixation and more efficient use of nutrients (Ramamoorthy
et al. 2001).
PGPR can improve the growth and nutrition of the plant, the pattern of root growth, plant competitiveness and
responses to external pressure. PGPR have also been revealed to encourage systematic resistance (ISR) to various microorganisms
in various crops (Zhang et al. 2002). Van Peer et al. (1991) found that Pseudomonas fluorescens causes induced
confrontation and phytoalexin accretion in carnation against fungal strain Fusarium sp. Besides the improvement
of plant growth, they are solidly concerned with synthesis of plant hormones, increased uptake of nitrogen, solubilization
of mineral deposits, such as phosphorus, and creation of siderophores that chelate iron to make it accessible to the root of
the plant. PGPR have also the ability to solubilize inorganic and organic phosphates present in soil (Gupta et al. 2015).
Besides, PGPRs, endophytic fungi is a dominant group of microorganisms that colonizing internal tissues of the plant
with agricultural and industrial application. All plants appear to have mutualistic association with endophytes in natural
ecosystems. Fungal endophytes can have profound impact on plant communities, through increasing biomass, reduce water
consumption and enhance fitness of the host plant by conferring stress tolerance in extreme conditions (Waqas et al.
2011). Approximately, all vascular plant species established association with endophytic bacteria and fungi. Moreover,
endophytes colonization has already been recognized in marine algae, mosses and ferns (Verma et al. 2009).
Soil fertility is also influenced by microbial community through soil process viz. mineralizing, decomposition, and
storage or liberation of nutrients. Microorganisms also mineralizeorganic phosphorous in soil and increase the P accessibility
to plants (Chen et al. 2006). These bacteria serve as a sink for phosphorous in the presence of labile carbon by
quickly immobilizing it compared to soils having low phosphorous contents. The bacteria with a potential to solubilize
phosphate increase the accessibility of soluble phosphates and can also enhance the growth of plants by increasing the
effectiveness of biological nitrogen fixation and enhancing the availability of trace elements (Reddy, 2014).
Many rhizobacteria are solubilized sparingly by soluble phosphates by releasing chelating organic aicds. The solubilization
effect of these microorganisms is generally due to the production of organic acids. Rhizospheres are concentrated
due to high proportion of PSM, and they are metabolically more active than PSM from other sources (Vazquez et al. 2000).
Generally, one gram of productive soil contains 101-1010 bacteria, and their live mass may exceed 2,000 kg ha 49-1. Soil bacteria
are in bacilli (rod, 0.5-0.3 μm), cocci (sphere, 0.5 μm), or spiral (1-100 μm) shapes. The PSB is ever-present with differences
in the form and population in diverse soils. The population of PSB varies in different soils depending on the physical and
chemical properties, phosphorus content and organic matter of soil (Kim et al. 1997). Pseudomonas inoculation result had a
favourable effect on salt tolerance of Zea mays L., under NaCl stress (Bano and Fatima, 2009). Due to the increase of international
concern for foodstuff and environmental excellence, the utilization of PGPR for dipping chemical inputs in farming is a
potentially vital issue. Some PGPR have been commercialized and some are applied to a range of crops to improve growth,
seed appearance and crop yield (Minorsky, 2008).
In summary, this special issue addresses current advances in the understanding of the effects of various biofertilizers
on plant growth, water conservation and availability of micro- and macronutrients. Besides this, it also emphasizes the
changes in plant functional traits under various stresses and the role of PGPR/fungi in drought tolerance of plants.
