**2.1 Phytohormone production**

Plant hormones or plant growth regulators are a wide range of microorganisms within the rhizosphere involved in secretions of substances known as exudates that regulate growth and development of organic materials [32]. They are also referred to as organic compounds that influence physiological changes in the plant at low

#### *Enzymatic Processing of Pigeon Pea Seed Increased Their Techno-Functional Properties DOI: http://dx.doi.org/10.5772/intechopen.101406*

concentrations [35]. Like Shah et al. [35] and Gouda et al. [32] hold the view that at a minimum concentration (<1 mM), growth hormones/regulators can promote, inhibit, or modify growth and development of plants. Similarly, Kumar et al. [36] maintain that hormones are the basic chemical signals that influence plant's ability to respond to the environment even at a low concentration of these active organic compounds [33]. Surprisingly, these growth regulators (also known as exogenous plant hormones) can be induced or activated by certain microbes, such as PGPR in plants [22, 37].

Organic plants hormones are classified into five (5) groups; auxins, ethylene, cytokinins, gibberellins and abscisic acid [23, 32, 33, 35, 36]. Microbial synthesis of the phytohormone; auxin (indole-3-acetic acid/indole acetic acid (IAA)) has a long history. According to Ahemad and Kibret [23], 80% of microorganisms isolated from various crop rhizospheres have the ability to synthesis and release auxins as secondary metabolites. Auxins are a group of plant hormone relevant in the advancement of root formation which in turns leads to increase in absorption of essential nutrients by the roots of plants [34]. IAA has been implicated in almost all aspects of plant growth and development, likewise in defense responses.

Generally, IAA is a commonly produced phytohormone associated with cell division, cell enlargement, cell extension, cell differentiation; stimulates seeds and tuber germination; increases the rate of xylem and root development and adventitious root formation and initiation; controls vegetative growth process; mediates responses to light, gravity and fluorescence; affect photosynthesis, pigment formation, biosynthesis of various metabolites and resistance to stressful conditions. It was also reported that rhizobacterial IAA could increase root exudation by loosening the plant root cell walls, which in turn helps the rhizobacterial colonization and growth [38]. IAA produced by PGPR is also reported to modifying plant morphological functions to uptake more nutrients from the soil [23, 33–35].

Gibberellins can also alter the plant morphology by elongation of stem tissues [34–36]. Ethylene generally is an essential metabolite for the development of plants [23].

According to Sarkar et al. [34]; cytokinin has similar functions as gibberellins in stimulating lateral shoot development, leading to advance plant development. In addition to that, cytokinin encourages tissue expansion, cell division and cell enlargement, enhanced root hair formation, inhibition of root elongation, shoot initiation, or specific physiological responses in plants [35, 39]. Ethylene is the only gaseous hormone [36]. Kumar et al. [36] also referred to it as "wounding hormone" because its production can be induced by physical or chemical disturbance of plant tissues. Ethylene has many effects on plant growth and development, such as rapid seedling death and deprived growth when secreted in excess [33].

Ethylene production in excess can inhibit root growth. Mode of action of some PGPR involves the production of 1-aminocyclopropane-1-carboxylate (ACC), a deaminase enzyme which significantly improves growth parameters. In the same view, Dutta et al. [38] report that ACC deaminase production of PGPR brings about growth and advancement by decreasing the levels of ethylene, inducing salt tolerance [23] and reducing drought stress in plants. ACC is a predecessor of ethylene in the biosynthesis pathway of ethylene in plants [36].

Therefore, it was reported that although ethylene being an essential metabolite for plant growth and development, under different stress conditions such as drought or salinity stress, the ethylene level drastically increases, this could negatively influence plant growth [23, 35]. In a nutshell, several forms of stress are relieved by decreasing ethylene production [40]. Through ACC deaminase producers such as effects of phytopathogenic microorganisms (viruses, bacteria, fungi, etc.), and

resistance to stress by polyaromatic hydrocarbons, heavy metals, radiation, wounding, insect predation, high salt concentration, drought, extremes of temperature (hot/cold), high light intensity and flooding.

The major noticeable effects of seed/root inoculation with ACC deaminase-producing microbe are root elongation, promotion of shoot growth and enhancement in rhizobial nodulation and N, P and K uptake as well as mycorrhizal colonization in various crops [23, 33, 38].

Kachroo and Kachroo [41] make an interesting contribution with regards to the impact of salicylic acid (SA) and jasmonic acid (JA) in plant defense mechanism. It is a widely held view that many phytohormones are now well known to mediate induced defense signaling, SA and JA are two major players that have been traditionally attributed roles in regulating plant defenses. SA and JA activate specific signaling pathways, which can act individually, synergistically or antagonistically, depending upon the pathogen involved. In addition to plant defense, SA and JA also regulate various developmental processes including flowering, root growth, floral nector secretion, senescence, development, cell growth, trichome development and thermogenesis. JA is an important regulator of plant responses to both biotic and abiotic stresses and is particularly well known for its role in plant defense against insects and herbivores. Febble's [11] work on *C. ensiformis*, complemented by Ruiz-Santiago's [14] study of *P. lunatus* taken together these studies provide important insights into the role of legumes in defense against herbivorous insects due to the activities of the vital phytohormones. **Table 1**


#### **Table 1.**

*Plant beneficial rhizosphere microorganisms (PBRMs) as an efficient phytohormone producer in various plants.*

summarizes the essential phytohormones in certain crop species and the associated plant growth promoting rhizomicrobes.
