*2.1.3 Phosphate solubilization*

Solubilization and mineralization of phosphorus by phosphate-solubilizing bacteria is an important trait that can be achieved by PGPR. Phosphate solubilizing PGPR includes genera, such as *Arthrobacter*, *Bacillus*, *Beijerinckia*, *Burkholderia*, *Enterobacter*, *Microbacterium*, *Pseudomonas*, *Erwinia*, *Rhizobium*, *Mesorhizobium*,

*Plant-Growth Promoting Endophytic Bacteria and Their Role for Maize Acclimatation to Abiotic… DOI: http://dx.doi.org/10.5772/intechopen.109798*

*Flavobacterium*, *Rhodococcus*, and *Serratia,* and some of these have been found associated to maize [19].

#### *2.1.4 Potassium solubilization*

Potassium solubilizing PGPR, such as *Acidothiobacillus* sp., *Bacillus edaphicus*, *Ferrooxidans* sp., *Bacillus mucilaginosus*, *Pseudomonas* sp., *Burkholderia* sp., and *Paenibacillus* sp., have been reported to release potassium in accessible form from potassium-bearing minerals in soils; some of them have been reported in interaction with maize [20].

#### *2.1.5 Phytohormone production*

One process employed by PGPRs is phytohormones production. Bacterial species, such as *Pantoea agglomerans*, *Rhodospirillum rubrum*, *Pseudomonas fluorescens*, *Bacillus subtilis*, *Paenibacillus polymyxa*, *Pseudomonas* sp., and *Azotobacter* sp., were found to carry out this mechanism and they have been tested in maize development [13].

#### *2.1.6 Siderophore production*

The production of siderophores by microbes is crucial for plant growth since these compounds chelate iron in the soil. This process is performed by a bacterium, such as *Pseudomonas* sp. and *Streptomyces* sp., and it is useful for generating soluble complexes that can be absorbed by plants, and some of these bacteria have been tested in maize [10].

#### *2.1.7 Exopolysaccharide production (EPS)*

EPS-producing PGPR, such as *Azotobacter vinelandii*, *Bacillus drentensis*, *Enterobacter cloacae*, *Agrobacterium* sp., *Xanthomonas* sp., and *Rhizobium* sp., play a vital role in maintaining water potential, aggregating soil particles, and ensuring an obligate contact between plant roots and rhizobacteria [21].

#### **2.2 Acclimatation to abiotic stresses**

It has been proposed that stress conditions cause the recruitment of particular microbial taxa from the soil. In this sense, environmental factors, such as drought, pH, and temperature have a significant impact on the microbiota associated to roots [15]. Advantages of bacterial endophytes on plant growth include protection from competing bacteria and fungi, a constant and reliable source of nutrition, and protection from exposure to a wide range of potentially deleterious environmental conditions, such as extreme temperature and the presence of inhibitory chemicals in the soil. Some of these responses are described below.

#### *2.2.1 Chemical responses*

Plant exudate compounds through their roots and these are key factors for the assembly of microbial communities in the rhizosphere. Some of these compounds are sugars, amino acids, organic acids, fatty acids, and secondary metabolites such as triterpenes [22]. On one hand, the composition of the root exudate profiles

changes in different plant species, genotypes, and developmental stages. Thus, it is suggested that variations in the composition of root-associated endophytic microbial are caused by changes in root exudation [15, 22]. On the other hand, endophytic bacteria also synthesize a varied array of secondary metabolites with unique chemical structures that have been exploited as biocontrol agents. Additionally, these bioactive compounds can be beneficial as they can stimulate plant growth and development. The composition of secondary metabolites produced by endophytic bacteria depends on the physiological status and species of plants and microorganisms. The bacterial genera include *Azotobacter*, *Serratia*, *Azospirillum*, *Bacillus, Caulobacter*, *Chromobacterium*, *Agrobacterium*, *Erwinia*, *Flavobacterium*, *Arthrobacter*, *Micrococcous*, *Pseudomonas*, and *Burkholderia*. These carry out mechanisms like nutrient fixation, neutralizing biotic and abiotic stress, and producing volatile organic compounds (VOCs) and enzymes to prevent diseases. However, the mode of action is different depending on PGPR-types and varies according to the type of host plant [23]. Our unpublished data report that some native maizes have been shown to induce the synthesis of anthocyanin and phenolic compounds in response to drought or waterlogging (unpublished data), cold, high salinity, or nutrient deficiency stresses. This response is a protective strategy to alleviate these adverse impacts [24]; (**Figure 2**). Studies have shown some specific changes in root exudation of primary and secondary metabolites as follows: (1) high sugar levels exuded in early plant developmental stages may attract a wide range of microbes that can consume sugar substrates, and (2) high levels of phenolics exuded in later plant developmental stages induce specialized pathways, where these compounds are used as specific substrates or signaling molecules in ways that vary across taxa [25].
