*2.2.2 Nutritional starvation responses*

Plants exposed to nutrient limitations exhibit a wide range of responses that include changes to the quantity and composition of the compounds released by roots. In maize, N-deficit causes a reduced exudation of amino acids; P-deficiency

**Figure 2.** *Putative chemical response to abiotic stress.*

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

stimulates the release of gamma-aminobutyric acid (GABA) and carbohydrates; whilst K-deficient plants release less sugars. Moreover, Fe deficiency causes increased release of glutamate, glucose, ribitol, and citrate [26]. Accordingly, plants exposed to different nutrient limitations show differences in the microbial structure composition. For example, P-deficient plants release compounds involved in bacterial chemotaxis and motility, whilst exudates released by Fe and K-deficient plants did not cause dramatic changes in bacterial composition [4, 26]. Interestingly, native maize landraces from *Los Tuxtlas*, Mexico show varying mycorrhizal dependency for P uptake, but there is still no data on bacterial composition [27].

#### *2.2.3 High or low-temperature responses*

Plants growing under high or low-temperature stress exhibit responses such as a decline in photochemical efficiency, stomatal conductance, and net CO2 fixation. High temperatures cause changes in the plasma membrane, water content (transpiration), impaired photosynthesis activity, enzyme functioning, cell division, and plant growth. Some strategies to overcome this stress include the production and accumulation of enzymes and osmolytes. Temperature plays a significant role in the regulation of physiological and metabolic responses. Bacterial endophytes also possess effective mechanisms to protect the structure of proteins, membranes, and nucleic acid molecules, and in this way, they can survive under high temperatures or low temperatures. These phenomena have been studied in genera like *Pseudomonas cedrina*, *Brevundimonas terrae*, and *Arthrobacter nicotianae*, among others [14].

#### *2.2.4 Waterlogging and water deficit responses*

Waterlogging stress adversely impacts the physiology and photosynthetic capacity of the plant, and prolonged exposure generates severe damage to plant growth or productivity. Some strategies that are adopted by plants under water deficit response are reduction in transpiration loss through altering stomatal conductance and distribution, leaf rolling, root-to-shoot ratio dynamics, root length increment, accumulation of compatible solutes, enhancement in transpiration efficiency, osmotic and hormonal regulation, and delayed senescence [28]. In addition, bacterial endophytes can enhance plant tolerance through the maintenance of cell homeostasis and diminishing the adverse effects of oxidative stress [29].

#### *2.2.5 Drought responses*

Drought stress cause as responses a decline in turgor and water potentials, a suppression in photosynthesis, a decrease in the contents of the chlorophyll, and increased accumulation of proline in most plants [30]. Several studies suggest that the use of distinct endophytic bacteria could produce beneficial effects on their host plants if their co-inoculation does not generate antagonistic responses. They show mechanisms that involve the maintenance of the cell water homeostasis under drought conditions, allowing diminished water loss and increasing water inlet, carbon sequestration, nutrient cycling, resulting in health of crops, and rhizosphere ecosystem functioning [31, 32]. Many bacterial groups have been related to these mechanisms, for example, *Acinetobacter*, *Azospirillum*, *Azotobacter*, *Arthrobacter*, *Bacillus*, *Beijerinckia*, *Brevundimonas*, *Burkholderia*, *Clostridium*, *Delftia*, *Duganella*, *Erwinia*, *Enterobacter*, *Flavobacterium*, *Hydrogenophaga*, *Methylobacterium*, *Paenibacillus*,

*Pantoea*, *Proteus*, *Providencia*, *Pseudomonas*, *Psychrobacter*, *Rhizobium*, *Serratia*, *Stenotrophomonas*, *Streptococcus*, and *Streptomyces* [33, 34].

#### *2.2.6 Salinity responses*

Salinity stress induce a condition that prevents water uptake by the plant and relate to a decline in photosynthesis, growth, and uptake of other nutrients. Salinity adversely affects plant growth and development. Halophytic bacteria have several adaptations to mitigate salinity stress that include a reduced stomatal conductance of the host, lower water potential, uptake of inorganic ions, a salt discharge from roots, and accumulation of organic acids, among others [16]. Salinity can disrupt water uptake and ion equilibrium and lead to oxidative damage due to the production of ROS. Halophytic bacteria can keep these ROS at minimal levels due to the presence of an antioxidant system that consists of enzymes like catalase (CAT), peroxidase (POD), and superoxide dismutase (SOD). The salt-tolerant endophytes include genera like *Pseudomonas*, *Kocurias*, *Cronobacter*, *Gracilibacillus*, *Staphylococcus*, *Virgibacillus*, *Salinicoccus*, *Bacillus*, *Zhihengliuella*, *Brevibacterium*, *Oceanobacillus*, *Exiguobacterium*, *Arthrobacter*, and *Halomonas*. These bacterial groups possess an ACC deaminase activity with the potential to ameliorate plant salinity stress [16, 35, 36].

#### *2.2.7 Heavy metal responses*

Heavy metal stress is characterized by the inhibition of processes like photosynthesis, respiration, nitrogen and protein metabolism, and nutrient uptake [37]. There is a wide range of heavy metal-tolerant microorganisms and plantassociated microbes that involve various mechanisms such as efflux, impermeability to metals, volatilization, EPS sequestration, metal complexation, and enzymatic detoxification [14, 38]. The microorganisms with tolerance to heavy metals include genera such as *Rhizobacteria* and the phylum *Firmicutes* that promote plant growth and development during metal stress conditions. They carry out mechanisms to reduce ethylene concentration, production of plant growth regulators such as auxin indole-3-acetic acid (IAA), ACC deaminase, and disease suppression [14].

#### *2.2.8 Pathogen responses*

Biotic stress can be caused by different pathogens or plagues, such as bacteria, viruses, fungi, nematodes, protists, insects, and viroids. These result in a significant reduction in plant growth and development. Endophytic bacteria have been used as antagonists against plant pathogens and species like *Bacillus* spp. and *Pseudomonas* sp., produce a wide variety of compounds such as antibiotics, antifungal compounds, antivirals, and so on [39]. In plants, some defense mechanisms are activated by pathogenic or non-pathogenic invasion, that results in the activation of enzymes, such as chitinase, β-1, 3-glucanase, phenylalanine ammonia-lyase, polyphenol oxidase, peroxidase, lipoxygenase, SOD, CAT, and ascorbate peroxidase (APX). After these encounters, plants remain primed, which means that they are better prepared for future attacks by pathogens. PGPEs promote plant growth by producing metabolites that control phytopathogenic agents. These metabolites include β-1,3 glucanase, ACC-deaminase, and chitinase, which are generally involved in lysing cell

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

walls and neutralizing pathogens [40]. Finally, species from diverse genera, including *Pseudomonas*, *Bacillus*, *Arthrobacter*, *Stenotrophomonas*, and *Serratia* can produce VOCs that impact plant growth and development [13].

### **2.3 Abiotic stress amelioration by plant growth-promoting endophytes**

The use of bacterial strains from rhizosphere, phyllosphere, or endosphere has been suggested to promote an amelioration of abiotic stress. Endophytes promote plant growth through nitrogen fixation, phytohormone production, nutrient acquisition, and by conferring tolerance to abiotic and biotic stresses. These mechanisms have been reported across many genera such as *Bacillus*, *Pantoea*, *Klebsiella*, *Burkholderia*, *Gluconobacter*, and *Pseudomonas*, among others [13, 41, 42]. Specifically, for maize endophytes, these functions have been associated with genera, such as *Massilia*, *Burkholderia*, *Ralstonia*, *Dyella*, *Chitinophaga,* and *Sphingobium*. However, the bacterial community structure significantly changes through different growth or development process. For example, *Massilia*, *Flavobacterium*, *Arenimonas,* and *Ohtaekwangia* were enrichment at early growth stages, whilst genera like *Burkholderia*, *Ralstonia*, *Dyella*, *Chitinophaga*, *Sphingobium*, *Bradyrhizobium,* and *Variovorax* were dominant at later stages [43]. In *milpa*, studies have reported the presence of genera such as *Flavitalea*, *Sphingomonas*, *Blastococcus*, *Luteitalea,* and *Vicinamibacter*, among others groups that are uncommon in hybrid maize [5]. Moreover, endophytes bacteria like *Bacillus*, *Enterobacter*, *Pseudomonas*, *Azotobacter*, *Arthrobacter*, *Streptomyces*, and *Isoptericola* were related to the alleviation of drought, heat, and salt stress in different crop plants, **Figure 3** [11, 44].

#### **Figure 3.** *Potential bacterial endophytes from Arabidopsis, Rice, and maize.*

Several studies have proposed that different rhizosphere bacterial types may serve as initial inoculum populations. It was shown that bacterial communities, such as epiphytic and endophytic, are highly similar in both leaves and roots, respectively, supporting the hypothesis that the communities are recruited from the soil [12]; (**Figure 3**). Firstly, rhizosphere microbial could be defined by exudates released from the host; this is because concentration gradients of carbon sources and phytochemicals function as attractants, while the modulation of oxygen and pH in the soil acts as limiter strategy, and finally, nutrient depletion works as selective mode [9] (**Figure 1**). Secondly, rhizoplane microbes could be recruited by favoring specific functions like attachment or adhesion. Swimming and other types of motility and chemotaxis are the first step to colonization. These depend on cell structures, such as flagella or pili, while colonization requires biofilm formation and adhesins. These are all important features for gaining access to the plant surface and to colonize in susceptible areas caused by wounds or mechanical injuries [42]; (**Figure 1**), Endophytic microbes are found in inter or intracellular spaces in the plant, and it is hypothesized that they require properties such as flagella and twitching motility that contribute to access and colonize at the host. On the other hand, lipopolysaccharide production (LPS), ROS detoxification, plant polymer degradation, quorum sensing, and type VI secretion system are important for the establishment inside the plant host [9, 42]. Finally, it has been reported that some mechanisms are central features in abiotic stress alleviation by plant growthpromoting endophytes. One important example is ACC deaminase activity to keep the stress ethylene concentration below growth inhibitory effects [16, 45–47].
