*3.1.1 Phytohormone production*

Phytohormones or plant hormones are the organic compounds that cause the stimulation of plant growth and development at lower concentrations. They can be produced either naturally by plants in response to some specific stimuli or can be synthesized artificially and utilized for regulating the growth and development of plants [35]. Apart from regulating growth and development, these phytohormones also play an important role in biocontrol responses as they are involved in several synergetic processes between various plants and organisms. Therefore, these plant hormones not only helps in stimulation of plant growth, development, improvement in nutrient uptake, but also act as a shield against various biotic and abiotic stresses, and as such protection of plants from different phytopathogens [36]. Phytohormones include indole-3-acetic (IAA) acid (auxin), cytokinins, gibberellins and abscisic acid. Each of the plant hormones or plant growth regulators possesses specific functions.

a.**Auxin:** This is the phytohormone which is considered as an important hormone that helps in plant protection mostly in the form of indole acetic acid (IAA). It has been suggested that many rhizobia spp. can secrete plant hormones, such as auxin via indole acetic acid formation [37–41]. Tryptophan has been considered as the major precursor of IAA. However, rhizobium spp. can synthesize IAA even if the tryptophan is not present [42]. Soil-beneficial bacteria have the ability to synthesize IAA and are involved in many phyto-stimulations that could be beneficial in relation to the biocontrol. IAA is also reported to loosen the root walls to increase the secretion of various beneficial substance from roots, which can improve the bacterial growth in root zone [22]. Rhizobia producing IAA are reported to directly affect the growth of phytopathogens (44). Rhizobial IAA is able to affect pathogenesis as being involved in various physiological processes of plant like cell division, extension, rate of xylem development, formation of adventitious root and various pigments, photosynthesis, etc. Therefore, can act as an effector molecule in plant microbial interaction. More than 80% of nitrogenfixing bacteria have reportedly resulted in the production of growth substances

*DOI: http://dx.doi.org/10.5772/intechopen.102657 Potential Applications of Rhizobacteria as Eco-Friendly Biological Control, Plant Growth…*

like indole acetic acid [43]. These substances enhance plant defense mechanism against various pathogens and improves the plant growth by increasing the total phenols, calcium content and polyphenol oxidase activity [44]. Rhizobial IAA was reported to have Phyto stimulation activity which resulted in suppression of more than 84% fungus mycelial growth of *S. rolfsii* because of the synergetic relation between in vitro bacterial IAA production and inhibition of *S. rolfsii* mycelial [45]. Treatment of nodules of vetch roots with *R. leguminosarum* bv*. Viciae* resulted in increase of IAA production by about 60 folds [46]. Application of *Pseudomonas* in combination with *Rhizobium galegae* causes increase in IAA production that results in increasing the number of nodules, nitrogen content, growth of shoot and root. However, biosynthesis of IAA was influenced by both environmental stress factors (acidic pH, osmatic stress, matrix stress and carbon limitation) as well as by genetic factors (auxin biosynthesis genes and the mode of expression. The bacterial strain *Mesorhizobium loti* MP6 produces indole acetic acid (IAA) under normal growth conditions inducing curling of root hair, inhibition of *Sclerotiniasclerotiorum* and improves the growth of Indian mustard (*Brassica campestris*) [29].


(ACC-precursor of ethylene) into α-ketobutyrate and ammonia. It has been reported that when rhizobia producing ACC deaminase are inoculated, the ethylene levels in the plant are reduced, resulting in increased nodulation, longer roots as well as improves rhizobial activity and thereby helps in bringing down various stress levels and also protects the plant from various pathogens (**Table 1**). The strains, which are reported to produce ACC deaminase involve *R. leguminosarum*. *Viciae*, *Rhizobium hedysari, Rhizobium japonicum, Rhizobium gallicum, B. japonicum, Bradyrhizobium elkani, M. loti* and *S. meliloti* [59].
