**2. Role of plant growth promoting microbes in sustainable crop production**

PGPR play a pivotal role in promoting plant growth either directly (by enabling resource acquisition of essential nutrients and elements like nitrogen, phosphorus), moderating plant hormone levels, or indirectly serving as biocontrol agents by decreasing the inhibitory effects of various pathogens on plant growth and development [19, 20].

According to Finkel et al. [21], growing crops continuously in agricultural soils can result in pathogen build up as well as bring about the emergence of disease suppressive soils. These soils in the long run convey resistance to plant pathogens and contain biocontrol agents within their resident bacterial community. Hassan et al. [22] points out that plant growth promoting rhizobacteria may affect plant growth, development and disease suppression by one or more direct or indirect mechanisms.

Ahemad and Kibret [23], in their study on the current prospect of mechanisms and application of plant growth promoting rhizobacteria point out that 80% of

microorganisms isolated from the rhizosphere of various crops possesses the ability to synthesis and release of auxins as secondary metabolites. Phytohormones like auxins, gibberellins, cytokinin, ethylene and abscisic acid facilitate plant cell enlargement and extension in symbiotic and non-symbiotic roots [24]. It is now well established from a variety of studies that indole acetic acid (IAA) is involved in multiple processes in plant growth and development such as aiding cell division, differentiation and vascular formation which are the three most essential processes involved in nodule formation. Therefore, it was suggested that auxins levels in host legume plants are necessary for nodule formation [23–30]. Supporting this view is the study by Camerini et al. [31] that introducing IAA biosynthesis pathway in the inoculation with *Rhizobium leguminosarum* bv Viciae produced potential nitrogen fixing root nodules containing up to 60-fold more IAA than nodules formed by wild-type counterpart in *Vicia hirsute*. Therefore, drawing from the extensive range of sources, *Rhizobium* sp. have shown to produce IAA.

Gouda et al. [32] claims that plants have always been in symbiotic relationship with soil microbes (bacteria and fungi). The leguminous crops and beneficial microbes create a mutual relationship in the rhizosphere to fix the atmospheric nitrogen into plant available form. Mehmood's et al. [33] work on plant growth promoting rhizobacteria is complemented by Hassan's [22] study of the interactions of rhizo deposits with plant growth-promoting rhizobacteria in the rhizosphere, holds the view that plant release certain exudates such as sugars, sterols, growth factors, etc. which stimulate the movement of beneficial microbes towards plant roots. On reaching the root, the beneficial microbes are known to form nodules.

It is documented that 80% of plant available nitrogen comes from biological nitrogen fixation (BNF), while remaining 20% is contributed by other nonsymbiotic organisms [34]. Nitrogen fixing organisms are generally categorized as symbiotic N2 fixing bacteria including members of the family rhizobiaceae which forms symbiosis with leguminous plants (e.g. *rhizobia*) [28]. Non-leguminous trees (e.g. Frankia) and non-symbiotic (free living, associative and endophytes) nitrogen fixing forms such as cyanobacteria (*Anabaena*, *Nostoc*), *Azospirillum*, *Azotobacter*, and symbiotic nitrogen fixing bacteria provide only a small amount of the fixed nitrogen that the bacterially-associated host plant requires [29]. On a global scale, BNF provides the largest input of nitrogen to agricultural soils. Inoculation of these efficient plant beneficial rhizo-microbe species usually increases plant's productivity. If *Rhizobium* is inoculated as biofertilizer in the crops such as groundnut, pigeon pea, soybean, etc., it can supply ~19–22 kg ha−1 which can raise the production by ~17–33% [34]. Gouda et al. [32] in a similar study on 'revitalization of plant growth promoting rhizobacteria for sustainable development in agriculture' reported that plant growth promoting rhizobacteria fix atmospheric nitrogen in the soil including the strains of *Azoarus* sp., *Beijinrinka* sp., *Pantoea agglomerans* and *Klebsiella pneumoniae*. Symbiotic nitrogen fixing rhizobia within the rhizobiaceae family (a-proteobacteria) infect and establish symbiotic relationship with the roots of leguminous plants. On the other hand, arbuscular mycorrhizal fungi (AMF) were similarly reported to improve growth, nodulation and nitrogen fixation in legume-Rhizobium symbiosis [26].
