*4.6. Microbial Inoculants*

The agriculture system is heavily dependent on chemical inputs such as pesticides, herbicides, and fertilizers to enhance yield [72,73]. Due to the thrilling use of these chemicals, the quality of soil and the health of plants are being deteriorated, ultimately affecting human health. Therefore, there is a need to develop a sustainable approach to minimize the harmful effects of these chemicals and promote plant growth, and also improve soil quality without disturbing the natural ecosystem. The use of microbial inoculants is an alternative to these chemical inputs, which can act as biofertilizers, bioherbicides, biopesticides, and biocontrol agents [73]. Microbes are also one of the important categories of biostimulants for plants.

The formulations of beneficial microorganisms, which play an affirmative role in the soil biome in an eco-friendly manner, are called microbial inoculants [74]. Natural soil contains a variety of agriculturally important microorganisms that have a beneficial effect on soil and plants by providing nutrients and also protecting the plant from pests and diseases. Generally, there are two groups of microbial inoculants (i.e., biofertilizers and biopesticides), but those that function as biofertilizers are grouped under biostimulants [8,75]. They are also known as bioinoculants, which contain living organisms and promote plant growth through a variety of mechanisms, such as, increasing root growth and biomass, suppling nutrients, and also enhancing the capacity of nutrient uptake when applied to seeds, plants, or soil [76]. Microorganisms acting as biostimulants mainly belong to beneficial fungi groups including arbuscular mycorrhizal fungi and free-living bacteria [76–78]. Kloepper et al. [79] reported that plant growth-promoting bacteria (PGPB) and plant growth-promoting rhizobacteria (PGPR) are free-living bacteria, isolated from the rhizosphere of plants that can act as biofertilizers and stimulate plant growth. Many factors are responsible for the development of microbial inoculants as biofertilizers, such as the variety of plants [80,81], compatibility with chemical fertilizers, types of soil, and environmental conditions [8]. The activity of microbial inoculants is mostly influenced by root exudates (extracellular secretions by plants) and also serves as a substrate for the formation of biologically active substances [82]. Based on their biostimulatory effect, microbial inoculants (bioinoculants) can be divided into two groups, which are discussed below:

#### 4.6.1. Plant Growth Promoting Bacteria

Plant growth-promoting bacteria (PGPB) are beneficial, free-living, rhizoplanic, rhizospheric, and phylospheric bacteria and play a dynamic role in plant growth [83]. They belong to diverse genera such as *Bacillus* sp., *Pseudomonas* sp., *Azotobacter* sp., *Enterobacter* sp., *Azospirillum* sp., etc. Bashan and de Bashan suggested various positive effects of plant growth that include improved plant nutrition, abiotic and biotic stress [84,85] tolerance, increased growth and yield [85–87], and also control of plant pathogens. They are either applied either directly to the seed and plant or mixed with a carrier material such as compost, peat, sawdust, vermiculite, or compost, which provides a suitable environment for their growth [88,89]. A lot of studies have been conducted by many researchers to demonstrate the role of PGPB on plants. They promote plant growth by various mechanisms and provide nutrients to plants. These mechanisms include: (i) biological nitrogen fixation [90,91]; (ii) solubilization of inorganic P [92]; (iii) production of iron chelating compounds (iv); and phytohormones production, which is discussed in detail below:
