**Abstract**

In order to grow, reproduce, and defend themselves, maize plants use various strategies to obtain adaptive advantages in varying conditions, for example, to tolerate abiotic stress (e.g., drought or heat due to climate change). One of these strategies is the establishment of interactions with plant-growth-promoting bacteria. Bacteria can be associated with plants in the rhizosphere, rhizoplane, or as endophytes. Recent evidence suggest that modern agricultural practices are detrimental to these beneficial plant-microbe interactions, and reservoirs like traditional agroecosystems called *milpas*, emerge as sources of microbiota associated with maize crops, with increased diversity and beneficial functions. Particularly, bacterial endophytes associated with native maize from *milpas* show promising features for their use as plant-growth-promoting inoculates, however, it is necessary to first understand the mechanisms known for beneficial functions of endophytes associated with maize and other plants. Here, we review the mechanisms of beneficial interactions between plants and endophytic bacteria, with emphasis on maize and with mentions of recent findings on maize landraces from *milpa* systems.

**Keywords:** plant associated-microbial communities, *milpa*, stress responses, amelioration, climate change

### **1. Introduction**

Plants are constantly challenged with a plethora of stressful conditions and require several response mechanisms, including the interactions between the roots and soil microbes, which allow for nutrient availability, growth promotion, and disease suppression. Microbiota can be found associated to plants in different degrees and locations, which are divided into (1) endosphere (inter or intracellular tissues), (2) rhizoplane microbial (on the root surface and possibly attached to root hairs), and rhizosphere microbial (soil close to the root surface). The composition of each of these communities is influenced by the host genotype, soil source, cultivation practice, and so on [1–4] (**Figure 1**). Bacteria present in the endosphere have recently

been considered as potential agents for acclimatation to abiotic stress response. One example is endophytic bacteria isolated from native maize from *milpa* traditional systems, which has been proposed as a bacteriome fraction that could be useful for obtaining products toward the bio-fertilization of maize crops [5]. *Milpas* are polyculture systems that include domesticated, semi-domesticated, and tolerated plant species that combine native maize landraces with almost any other crop, tree,

#### **Figure 1.**

*Localization of maize associated-microbial communities. (A) Endosphere; here, communities are affected by plant genotype, its defenses system, and intrinsic responses; (B) in the rhizoplane, extrinsic factors like carbon sources or phytochemicals, [O2], pH, or nutrient depletion, affect microbial community composition; (C) in the rhizosphere, communities are strongly affected by soil source, cultivation practice, and others.*

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

or shrub species. *Milpas* constitute a dynamic system with diverse genetic resources, used as the main crop of native maize (*Zea mays* L.) and as associated crops to beans (*Phaseolus* spp.), squashes (*Cucurbita* sp.), chili peppers (*Capsicum* sp.), tomatoes (*Solanum lycopersicum*), among others. Modern practices of agriculture affect bacterial diversity and functions, therefore, *milpas* have retained unique beneficial microorganisms that interact with native maize, which could have been lost in modern hybrid maize with monocultures, high-tillage, and large agrochemical input [6].

With the current worldwide agricultural practices trend, it is necessary to promote a reduction of agrochemical use and a reintroduction of ancestral agricultural practices and/or the use of microorganism-based bio-formulations that generate a beneficial interaction between plants and microbes and thereby influence plant growth. One option is the use of plant-growth-promoting endophytes, which could be regarded as a new approach to mitigate the detrimental effect of pests and/or diseases, low rainfall, and current climate change [5].

Endophytic bacterial species have been identified in a vast number of plants; notably, they are symbionts that usually do not cause negative effects on plant growth [1, 7, 8]. The endophyte community within the plant is very as dynamic but usually is enriched with specialized types of bacteria with features such as flagella, plant-polymer-degrading enzymes, type V and VI protein secretion systems, iron acquisition and storage, quorum sensing, detoxification of reactive oxygen species (ROS), degradation of aromatic compounds, among others [9]. Colonization of endophytic bacteria can be also classified into three main categories in accordance with lifestyle strategies as (1) obligate endophytes, unable to proliferate outside of plants and are likely transmitted via seed; (2) facultative endophytes, as free living in soil but will colonize plants once conditions are appropriate, and (3) passive endophytes, as do not actively seek to colonize the plant [7, 9, 10].

Recent work suggests that microorganisms from the bulk soil in *milpas* are selected by native maize roots. Native maize selectively recruits, including strains from phyla like Acidobacteria, Actinobacteria, and Bacteroidetes, with a higher relative abundance in comparison to soil adjacent to the roots [6]. Recently, Gastélum *et al*. reported a greater presence of endophytic microbial load in native maize vs. hybrid landraces, which include more bacterial strains with antagonistic activity against soil-borne bacteria, and overall harbor more diverse bacterial communities [5]. There are many factors influencing endophytes to colonize and penetrate the root tissue, as well as the resulting interaction. After colonization, the role of microbes could modulate plant growth and development by diverse mechanisms [11]. In order to grasp maize endophytic microbes from *milpas* for plant growth promotion, we first need to understand the mechanisms for growth promotion. Here, we will discuss how maize-related endophytes can potentially have used in the alleviation of abiotic stress and/or climate change.

### **2. Endophytic bacteria**

Bacterial endophytes inhabit the tissues of plants for at least a part of their life cycle without harming or inhibiting the growth of the plant and establishing symbiotic associations than can result in great benefits for plant health. Endophytes are microorganisms that survive within healthy plant tissues and promote plant growth under stress. A large proportion of endophytic bacteria groups are shared between leaves and roots, suggesting that they are inoculated from the soil. These communities are also dynamic, as they shift when the plants age [12]. Reinhold-Hurek et al. defined a community of least complexity where specific bacterial traits are required for internalization and establishment inside of this compartment, and host genotype likely has the strongest influence on community structure here, in comparison with other compartments such as the roots [9].
