**2.3. Production of plant hormones and other beneficial plant metabolite**

There are five groups of plant hormones of well-known PGRs, namely auxins, gibberellins, cytokinins, ethylene and abscisic acid [60]. Direct plant growth promotion includes symbiotic and non-symbiotic PGPR, which functions through production of these plant hormones [11, 61–63]. Much attention has been given on the role of phytohormone auxin. Production of indole-3-ethanol or indole-3-acetic acid (IAA), the compounds belonging to auxins, which is known to stimulate in cell elongation, division and differentiation responses in plants, has been reported for several bacterial genera [12, 17, 64]. PGPR promote root growth by increasing root surface area, which, in turn, promotes nutrient uptake, thereby indirectly stimulating plant growth positively [52, 65]. Khalid et al. reported a correlation between *in vitro* auxin production and increase in early growth parameters of inoculated wheat seeds [66].

Inoculation with *A. brasilense* Cd and the application of pure IAA to the roots both increased root length, number of lateral roots and number of root hairs in wheat as observed by earlier workers [67, 68]. IAA-producing *Azospirillum* sp. also promoted alterations in the growth and development of wheat (*Triticum aestivum* L.) plants [69–72]. Bacteria of the *Azotobacter* genus synthesize auxins, cytokinins and GA-like substances, and these growth materials are the primary substances controlling the enhanced growth [73]. These hormonal substances, which originate from the rhizosphere or root surface, affect the growth of the closely associated higher plants. The highest concentration of IAA is produced by bacterial strain *P. fluorescens* and *Kocuria* varians [74]. Specifically for wheat, the positive effect of PGPR via IAA has been reported [75–78].

When applied in optimum concentrations, bacterial indole-3-acetic acid (IAA), synthesized by gram-positive and -negative, photosynthetic, methylotrophic and cyanobacteria, is reported to stimulate root hair formation, at the same time increasing the length and the number of primary and lateral roots [66, 72, 79]. IAA synthesis by these bacteria is reported to be affected by tryptophan, vitamins, salt and oxygen levels, as well as pH, temperature, carbon and nitrogen source. For example, IAA from *Azospirillum brasilense* Sp245 stimulates early plant development and increases significantly the plants and roots yield (in dry weight) and the N-uptake efficiency of wheat [71, 80]. The ability to synthesize ABA, particularly under stressful conditions, for example, salinity, and to affect the ABA level in plants was detected in PGPB from the genera *Azospirillum, Bacillus, Pseudomonas, Brevibacterium* and *Lysinibacillus* [15, 81, 82]. Both plants and bacteria can be synthesized via several pathways, including the indole-3-pyruvic acid (IPA), indole-3-acetamide (IAM) and indole-3-acetonitrile (IAN) pathways, which are often regulated by tryptophan, carbon and nitrogen availability, a reduction in growth rate and abiotic factors such as temperature, pH and oxygen [79].

As a PGPR application to wheat seedlings, Sachdev et al. reported that IAA producing *Klebsiella* strains significantly increased the root length and shoot height, when compared with the control, in pot experiments [83, 84]. Similarly, Khalid et al. reported up to 28% higher grain yields in wheat grown in field as a result of seed inoculation in peats with high auxinproducing rhizobacteria [66]. The capability of auxin synthesis detected in many bacterial strains from the genera *Azospirillium, Pseudomonas, Bacillus*, etc., is thought to underlie the activation of plant root growth by these microorganisms [81]. Sadeghi et al. demonstrated that a *Streptomyces* isolate increased plant growth in wheat and produced indole acetic acid and auxin in presence of salt [85]. Phytohormone-producing *Bacillus* sp. and *B. subtilis* have potential at field level to improve wheat productivity and may be helpful in formulation of an effective biofertilizer for wheat [52, 79, 86–89]. A complete understanding of the IAA system can further mediate the efficient use of these PGPRs for biofertilizer.

Cytokinins can be produced by representative strains of *Bacillus, Rhizobium, Arthrobacter, Azotobacter, Azospirillium* and *Pseudomonas*. The plants inoculated with cytokinin-producing bacteria *B. subtilis* showed the increased chlorophyll content and cytokinin accumulation, which led to the increase in weight of shoots and roots [90, 91]. On the other hand, treatment of plant with a substance obtained from cytokinin-producing microorganisms, typically colonizing in wheat roots [92, 93], increased chlorophyll content in leaf; in this case, the level of chlorophyll was comparable to that observed in the plants treated with a synthetic cytokinin benzyladenine. Cytokinins can promote stomatal opening, stimulate shoot growth and decrease root growth.
