**1. Introduction**

Wheat (*Triticum aestivum* L.) is one of the three major cereals (together with maize and rice), major source of energy, renewable resource for food, feed and industrial raw material, protein and fibre source in human diet, staple food crop for more than one-third of the world population [1], grown both as a spring and winter crop.

Plant growth-promoting bacteria (PGPR), typically colonizing at the rhizosphere, is known to increase the yield and help alleviating the effects of biotic or abiotic stresses [2]. The practice of PGPRs is promising in reducing the use of chemical fertilisers, at the same time maintaining yields at commercially viable levels and/or maintaining grain protein content [3]. As such, PGPR contributes to the improvement of both local and global environments, reducing dependence on non-renewable resources while still being economically competitive (both price and quality aspect) [4–6].

Several beneficial free-living rhizobacteria have been termed as PGPR, including, but not limited to, *Acinetobacter, Acetobacter, Alcaligenes, Arthrobacter, Azotobacter, Azospirillum, Bacillus, Burkholderia, Beijerinckia, Enterobacter, Flavobacterium, Methylobacterium, Pseudomonas, Rhizobium, Paenibacillus* and *Pantoea* [7–10]. These bacteria enhance growth through numerous mechanisms [2, 11–15]. A short list of mechanisms cover:


A non-exhaustive list of Plant Growth Promoting Rhizobacteria (PGPR)s used to alleviate various stresses is given in **Table 1**, and the various other uses of these bacteria are listed in **Table 2**. Two important mechanisms employed by PGPR are the production of different phytohormones,


**1. Introduction**

116 Wheat Improvement, Management and Utilization

tion [1], grown both as a spring and winter crop.

mechanisms [2, 11–15]. A short list of mechanisms cover:






price and quality aspect) [4–6].

lins and ethylene




and soil

Wheat (*Triticum aestivum* L.) is one of the three major cereals (together with maize and rice), major source of energy, renewable resource for food, feed and industrial raw material, protein and fibre source in human diet, staple food crop for more than one-third of the world popula-

Plant growth-promoting bacteria (PGPR), typically colonizing at the rhizosphere, is known to increase the yield and help alleviating the effects of biotic or abiotic stresses [2]. The practice of PGPRs is promising in reducing the use of chemical fertilisers, at the same time maintaining yields at commercially viable levels and/or maintaining grain protein content [3]. As such, PGPR contributes to the improvement of both local and global environments, reducing dependence on non-renewable resources while still being economically competitive (both

Several beneficial free-living rhizobacteria have been termed as PGPR, including, but not limited to, *Acinetobacter, Acetobacter, Alcaligenes, Arthrobacter, Azotobacter, Azospirillum, Bacillus, Burkholderia, Beijerinckia, Enterobacter, Flavobacterium, Methylobacterium, Pseudomonas, Rhizobium, Paenibacillus* and *Pantoea* [7–10]. These bacteria enhance growth through numerous



 - Exhibiting antifungal activity, exhibition of antagonistic activity against phytopathogenic microorganisms by producing siderophores, b-1,3-glucanase, chitinases and antibiotics

A non-exhaustive list of Plant Growth Promoting Rhizobacteria (PGPR)s used to alleviate various stresses is given in **Table 1**, and the various other uses of these bacteria are listed in **Table 2**. Two important mechanisms employed by PGPR are the production of different phytohormones,


#### **Table 1.** PGPB-mediated IST against abiotic stress.



KFP9-F, *Paenibacillus alvei* NAS6G-6

**Stress type Bacterial inoculate Properties of the crop Reference**

Salinity *B. subtilis, Arthrobacter* sp. Wheat (*T. aestivum*) [97]

**PGPR Source Plant growth regulation Results of addition of bacteria** 

*Azospirillum* sp. Wheat rhizospheric N2 fixation Grain yield, dry matter, N

(IAA)

*Cyanobacteria* Rhizospheric N2 fixation Root dry weight, N content

*Azorhizobium caulinodans* Wheat N2 fixation Dry weight, nitrogen content [192]

fixation, IAA

*Paenibacillus polymyxa* Wheat Cytokinin, N2 fixation Plant growth [208]

Wheat Rhizospheric N2 fixation Growth [73]

N2 fixation Root-hair deformation

Wheat [204]

Wheat (*T. aestivum*) [46]

[140]

[205]

[1]

[138]

**References**

[32]

[206]

[207]

[114]

[209]

[99]

P solubilization, indole acetic acid (IAA), siderophore, ammonia,proline accumulation, salt tolerance, choline oxidase

activity

status

weight

Wheat (*T. aestivum*)

growth and yield

**to plants**

content

hairs.

root and hoot

plumule length

 fixation Activities of a-glucosidase, b-glucosidase and b-galactosidase in wheat-seedling

colonization

Germination rate percentage and index and improved nutrient

Root length, root elongation, dry

Number of tillers, grain weight,

Number and length of lateral roots, distribution of root

Seed emergence radicle and

*Streptomyces* sp Wheat (*T. aestivum*) [85]

*Azospirillum* sp. Wheat (*T. aestivum*) [95]

Salinity *Pseudomonas putida, Enterobacter cloacae,* 

118 Wheat Improvement, Management and Utilization

Salinity *Bacillus pumilus, Pseudomonas mendocina,* 

Salinity *Pseudomonas putida, Enterobacter cloacae,* 

*halodenitrificans* PU62

*Azospirillum brasilense* Mutant Indole-3-acetic acid

*Azotobacter chroococcum* Wheat Rhizospheric P solubilization, N2

Non-sterilised and surface-sterilised wheat roots

Digitaria decumbens Lectins, N<sup>2</sup>

Salinity *Hallobacillus* sp. SL3 and *Bacillus* 

Salinity *Enterobacter asburiae, Moraxella* 

**Table 1.** PGPB-mediated IST against abiotic stress.

*Azotobacter* sp. *Azotobacter chroococcum*

Sp7

*Azospirillum brasilense*

*Azospirillum brasilense* 75, 80 and Sp245

*Serratia ficaria* and *P. fluorescens*

*pluranimalium, Pseudomonas stutzeri*

*Nitrinicola lacisaponensis*

*Serratia ficaria*, and *Pseudomonas fluorescens*

*Bacillus, Burkholderia, Enterobacter, Microbacterium, Paenibacillus*

*Arthrobacter* sp., *Halomonas* sp., and



**PGPR Source Plant growth regulation Results of addition of bacteria** 

Wheat roots P solubilization,

sorghum Phytohormones(IAA,

N2

ACC deaminase, siderophores, IAA

IAA, siderophores, ACC deaminase, diacetyl-phloroglucinol

GA), HCN, ammonia, Siderophore, P-solubilization

P-solubilization,

ACC deaminase, IAA-like products, P solubilization

HCN, siderophores

deaminase. Protease

Rubus and wheat P solubilization Shoot length, root and shoot

ACC deaminase, IAA, HCN, siderophores, P solubilization,

 fixation, P solubilization

HCN, IAA, P solubilization Zn solubilization

phytate

N2 fixation P solubilization,

Barley Siderophore ACC

 fixation, ACC deaminase siderophore, ammonia, HCN

*Pseudomonas* sp. Wheat P solubilization,

120 Wheat Improvement, Management and Utilization

Rhizosphere of wheat

*Pseudomonas lurida* Radish P solubilization IAA,

*Providencia sp*. PW5 Wheat rhizosphere Ammonia siderophore,

Wheat N<sup>2</sup>

Naturally saline habitats

*Azospirillum brasilense* Wheat N<sup>2</sup>

*Pseudomonas jessenii* R62; *Pseudomonas synxantha* R81 and arbuscular *mycorrhizal* fungi (AMF)

*Pseudomonas putida*

*Bacillus* sp. (AW1), *Providencia* sp. (AW5), *Brevundimonas diminuta*

*Pseudomonas fluorescens* 153 and 169, *P. putida* 4

*Pseudomonas fluorescens*

*Azospirillum sp., Azotobacter* sp. *Bacillus megaterium*

*P. fluorescens* and *Serratia* sp.

*Hallobacillus* sp. SL3, *Bacillus halodenitrificans*

*A. chroococcum* (W5), *Mesorhizobium ciceri* (F 75), *P.striata* (P27), *S.marcescens* (L11) *A.torulosa*

PU62

AKMP7

(AW7)

and 108

MKB37

**to plants**

assimilation

Grain yield

content

root weight

parameters

yield

 fixation Agronomic performance and yield of wheat

Increased soil enzyme activities, total productivity, and nutrient uptake, nutrient

Protein and mineral nutrient concentration (P, K, Cu, Fe, Zn, Mn) alkaline and acid phosphatase, urease, dehydrogenase.

Increased root, shoot length, dry biomass, chlorophyll

Seedling length, germination, plant height, panicle weight,

Height, tillers, number of grains/spike, garain and straw yield, N, P and K uptake

Growth and nutrient uptake

N uptake in wheat grain. protein content grain Fe, Zn, Mn, and Cu content

Grain number, weight and

Plant height, number spikes, grain yield, protein content

dry weight, P uptake

root biomass

P uptake

Seed germination, root length, root elongation, dry weight

Nutrient status of soil and plants, plant biomass, N and **References**

[102]

[163]

[98]

[40]

[54]

[64]

[161]

[23]

[213]

[35]

[214]

[1]

[177]

**Table 2.** Examples of plant growth-promoting substances released by some commonly employed PGPR.

including auxins, cytokinins and gibberellins, and the synthesis of several enzymes, such as phosphatase and catalase, modulating plant growth and development as well as strengthening their immune system [16, 17]. In a review, Palacios et al. compiled many molecules facilitating interactions of PGPB with plants [18]. The list includes plant hormones, hydrolytic enzymes, antibiotics, flavonoids, other signal molecules, toxic molecules, siderophores, exopolysaccharide, volatiles, polyamines, lectins and vitamins. The PGPR efficiency, in turn, depends upon a number of factors like soil mineral content, type of crop and its genotype, specific PGPR strain and its combination with the plant, competition with indigenous strains, environmental conditions and the growth parameters evaluated, as illustrated in greenhouse and field trials [3] and other studies [19–22].

Despite the promising features from agronomic efficiency and crop yield perspective, the key bottleneck for the commercial use of PGPRs is their varying performance under field conditions: the results obtained in a field are not always similar to those of laboratory [23], which calls for immediate further research on the agricultural use of these PGPRs.
