**2. Microbial interactions**

An existence of unseen host-microbial interaction has predominance from prehistoric times. While microbes are of minute size, they are available in nature in an astonishing majority, interacting directly or indirectly at different hierarchical levels of life. Almost all of these microorganisms are incredibly small, widely recognized by Archaea and Bacteria, although some microscopic forms include handful of fungi and even most protists. From an ecological standpoint, microorganisms are very often found in the soils as complex microbial population groups and have been investigated for several ties of microbiota-host interactions such as mutualists, endosymbionts, antagonists, parasites, and pathogens (**Figure 1**) [7].

Microbial community dynamic trends in the food chains look likely to be beneficial (positive), harmful (negative) or even sometimes neutral, with very little or no effect

*DOI: http://dx.doi.org/10.5772/intechopen.102657 Potential Applications of Rhizobacteria as Eco-Friendly Biological Control, Plant Growth…*

**Figure 1.**

*Types of microbial interactions found in nature.*

on their symbiotic associates [8]. Via physiochemical shifts, signaling mechanismquorum sensing system (chemotaxis), cell transduction signaling through secondary metabolites, siderophores (used for iron acquisition) and gene expression microbial processes always have shown substantial impact on ecological parameters, resulting in established suitable alleles in diverse habitats [9]. Rapid and altered microorganism genetic variation corresponds both to biotic and abiotic sources of stress. Furthermore, atmospheric Nitrogen fixing microbial interaction and AMF symbiotic relationship activates a unique signaling process-CSSP (**C**ommon **S**ymbiosis **S**ignaling **P**athway) with calcium fluctuations in nucleus [10]. Many such strategies lead to an expansive population of microorganisms constantly getting established, culminating in pathogenic or beneficial effects on host plant species.

While many others have shown plants are able to select microbiota from all of diverse plant exudates including certain amino acids, carbohydrates and other biomolecules [11] which could also vary depending upon the plant itself, its stage of development and on biotic or abiotic conditions. Flavonoids, for example, are needed for talks between Legume-Rhizobia while AMF (mycorrhizal arbuscular fungi) rely solely on Strigolactone signaling [12]. In addition, the position of bacterial iron acquisition chelators that enforce a restricted supply of iron in the rhizospheric plane for pathogenic fungi constrains pathogen proliferation and occurrence. Consequently, synergetic microbial populations in the root micro-sites have a critical role to play in cloaking plants from disease deterioration, environmental factors and also ramping up nutrient uptake [13]. It has been well established that plant-associated microorganisms, particularly endophytic and rhizospheric microorganisms, can stimulate plant growth. A typical specified example is that of biotrophic symbioses between rhizobium and legume, such bacteria boost the growth of plant species by fastening atmospheric N2, supplying of essential nutrients, enhance sequestration of minerals, produce phyto-hormones and also act as potential biocontrol against pathogens. Preliminary experiments on some endophytic and pathogen microbe genomes revealed pathogen degrade and displacement of host (host invasion), whereas the endophytic-mutualists express genes that aid in stress amelioration encoding proteins for nitrogen fixation and RubisCO [14]. During genetic interchange in a rhizobial symbiotic relationship, the root cortical cells are populated, making a distinction into nitrogen fixing bacteroids. Studies also show rhizobacter colonization into the root systems of non-leguminous plant species as such can be used as biocontrol in plant species other than legumes. Other popular, well-known, bacterial-based biocontrol method is *Agrobacterium* to prevent infection with *Agrobacterium tumefaciens.*In fact,

myriad microorganisms (in particular belonging to genera *Bacillus, Pseudomonas* and *Trichoderma*) generate few chemicals against plant pathogenic fungi [8]. Bacterial isolates broadly find their application against plant pathogenic bacteria and fungi, whereas fungi are taken as biocontrols for pathogenic protozoans, pathogenic bacteria as well as pathogenic fungi. Juxtaposition between plants and several types of microbes has also been known to help mitigate many toxic metal build-up in plants [15]. While a general mechanism affecting mostly saprotrophs involves enhancement of microbial activity, selective different categories of symbionts can be stimulated in root microsites of plants. On the other hand, disease development by saproptrophs or biotrophs present in root micro-sites takes place only by developing antagonistic symbioses between pathogens and susceptible host plant roots. Importantly, the elimination of disease can sometimes be addressed through manipulating microbiological or physio-chemical surroundings mostly by classical practices- like use of soil refinements, agronomic rotational practices, fumigant use or even soil solarisation. A voluminous literature shows that interactive bacteria both symbiotic and pathogenic develop common signaling molecules to promote their host cell invasion through predominant substances such as conserved PAMP/MAMPs (Microbe-Associated Molecular Patterns) and protein effectors [16]. Organisms have developed recognizing mechanisms which differentiate between pathogens and symbionts and react in different ways to them, but this distinction often is not efficient; as a consequence, recognizing sensitivity also appears to occur both on pathogenic and symbiotic interaction [17, 18] at earlier stages. Thus, evidently microbial associations drive a complex sequence of interdependent metabolisms. In this paradigm of unexpected symbiotic partnership only host species utilize chemical synthesis capacities of symbiotic organisms to inhibit the development of certain environmental major competitors in order to sustain themselves [19]. In modern days, the philosophy of regulation of soilborne diseases through the use of agro-chemicals such as pesticides and fungicides is now being modified through biological management [20]. Currently with the aid of molecular know-how, molecular pathways and processes involved in the interaction of microbes have been immensely explored.
