*3.1.2 Antibiotics*

Biologicals are an effective way of combating pathogens in plants [60]. Antibiotics and other antipathogenic compounds may be secreted by beneficial rhizobacteria. Antibiotics are among the most important pathways for biocontrol [61]. Pathogens also acquire antibiotic resistance and other biological control mechanisms to prevent complete long-term control. A systematic strategy of numerous monitoring mechanisms is definitely safer than undue reliance on one solution while confronting pathogens. Pathogen-antagonistic bacteria can therefore adapt their mode of


**Table 1.** *Phytohormone production.*

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

operation in the long-term to combat pathogens. In order to inhibit pathogens, PGPR produces antibiotics, such as lipopeptides, polyketides, and antifungal metabolites [62]. PGPR generates antibiotics that prohibit "saprophytic pathogens" from developing in the root zone; Combining strains that strengthen resistance to other antibiotics and biocontrol strains that modulate one or more antibiotics [61]. Rhizobia produces (TFX) tridolitoxin, an antibiotic narrow-spectrum peptide, and was found responsible for changes in microbial diversity in bean plant rhizosphere. Trifolitoxin (TFX) antibiotic by *R. Leguminosarum*bv. *Trifolii* T24 was documented for disease control. *B. Japonicum* produces rhizobiotoxin which protects Soya from *M. Phaseolina* [63]. *R. Leguminosarum* produces bacteriocins which have different assumed size characteristics (small, medium or large). *Trifolii* and *B. Japonicum* secrete antibiotics that could inhibit several phytopathogens have been documented [3].

### *3.1.3 Phytoalexins*

Plants exist in dynamic ecosystems which are subject to frequent changes. They survive on a host of chemicals called secondary metabolites [64], which are essential for regulating secondary metabolism. Plants have a normal immune system to withstand biotic stress which can be activated by different agents. The plants have a unique potential condition called "priming" which is triggered in the plant before the pathogen challenge. The plants defensive mechanism against biotic stress involves the agglomeration of molecules (phyto-anticipins), which are converted to phytoalexins [65]. Phytoalexins are antimicrobial compounds generated by plants or some organisms as a response of the biotic and abiotic factors. These are "low molecular weight, anti-microbial" compounds synthesized after micro-organism or abiotic exposure in plants. Furthermore, elucidating the biosynthesis of different phytoalexins allowed the use of molecular biology methods to investigate genes encoding enzymes involved in their synthesis. This has led to new technologies to improve plant resistance. Phytoalexins show enormous diversity in various chemical groups, such as terpenoids, phenolics, steroid glycoalkaloids, compounds containing sulfur and indoles [66].

#### *3.1.4 Induced systemic resistance*

In addition to its role in N fixation, rhizobium serves as a tool for biocontrol of plant pathogens by triggering systemic resistance in plants. This is referred to as Induced Systemic Resistance [67]. The latter prepares the plant for defense against various phytopathogens [68]. The mechanism by which a non-exposed part of a plant imparts resistance to pathogenic microbes etc. by earlier exposure with the former is termed as induced resistance, thus it is triggered by an inducer that can be a biological or chemical agent. This induced resistance is not only activated at the site of pathogen attack but also at the parts that are very far from the site of induction so called induced systemic resistance (ISR) (**Figure 2**) and this ISR provides resistance to broad spectrum pathogens. Systemic resistance provided by ISR is regulated by signaling pathways in which different hormones are involved [69].

Rhizobial species inducing systemic resistance are *Pseudomonas*, *Bacillus, Trichoderma* and *Mycorrhiza*. Stringlis et al. [70] observed that these rhizobia are involved in the biosynthesis of antibiotics, flagella, siderophores and other volatile compounds which in turn stimulate microbe associated molecular pattern triggered immunity (MTI). A signaling pathway is generated in response to the perception of any of the above-mentioned substances. This is followed by another signaling

### **Figure 2.**

*Graphical representation of biologically induced disease resistance generated by beneficial microbes (ISR). It involves transport of long-distance signals in form of Jasmonic acid- salicylic acid (J/A & SA) and systemically circulate an improved defensive potential against a broad-spectrum pathogen in other plant parts and helps in plant growth promotion (PGP) as well.*

pathway resulting in a systemic defense response [71]. Pattern-recognition receptors (PRRs) serve as sensors that have been evolved to differentiate and recognize bacterial and fungal products called pathogen associated molecular patterns (PAMPS). Moreover, in case of the damage/invasion caused by the pathogen attack an endogenous signal is produced. The ISR imitation in plants requires microbes that can be beneficial as well as able to effectively colonize the plants root system [72]. Recently microbial aspects around the root micro-sites harboring bacteria and fungi slowly gained interest because of their potential to trigger resistance (induced systemic resistance ISR in case of bacteria/systemic resistance in case of other microbes) in plants as a measure of biocontrol [17]. For instance, 22 kDa xylanase isolate of fungal endophyte *Trichoderma* when introduced into the plant cells evokes the plant's defensive response including potassium, hydrogen ions, calcium ion movements, PR protein synthesis, ethylene formation, glycosylation of phytosterols and fatty acid acylation [17]. Among the prominent changes taking place during ISR are:


Plant responds to a number of biochemical signals induced by soil and plant-associated microbes. The strength and stability of its cross-talk signal play key role in determining the quality of resistance against pathogens. The interactions with these microbes can be in the form of different relationship possibilities

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

(symbiosis, mutualism competition, predation, commensalism, etc. and host. At the initial stage, hypersensitive response gets active, a mechanism used by plants to prevent the spread of local infection by microbial pathogens [73]. While as for a positive mutual association both the host and the microbe must have to respond to the signals equally so that there is mutual benefit for both. In the association between the rhizobium and mycorrhiza, it has been studied that the host secretes strigolactones and flavonoids. Strigolactones are a class of plant hormones which are responsible for stimulation of branching and growth of mycorrhizal fungi. These strigolactones and flavonoids are also responsible for activation and production of symbiosis (sym) and Nodulaton (Nod) factors by microbes. The manipulated entry of rhizobium systematically triggers the whole downstream molecular defense system [67]. Which in turn builds a successful symbiotic relationship by activating common signaling pathways. By modifying the transcriptional programing many free-living plant growths promoting rhizobacteria (PGPR) positively respond to the root exudates that are involved in chemotaxis, energy metabolism etc. [74]. The mode of action of ISR is priming for enhanced defense, it does not cause direct activation of systemic resistance. Elevated transcript levels of various transcription factors were found in *Arabidopsis*eg. AP2/ERF were highly expressed. Among these several members are involved in regulation of jasmonic acid (JA) and ethylene (ET) defensive pathways. ISR by soilborne microbes is mostly regulated by JA/ET pathway. In the rhizosphere ISR is responsible for microbial antagonism, any host pathogen interaction enriches the microbiome and thus provides protection against diseases. The production of elicitors by beneficial microbes is also required in order to result in the onset of systemic immunity [69] so that there is a balance between the costs and benefits of mutualism. Plant-growth-promoting rhizobacteria (PGPR) were successful in managing complex diseases such as anthracnose (*Colletotrichum spp.*), angular leaf spot and bacterial wilt (*Erwinia tracheiphila*). Oxidative changes were observed in soyabean roots after inoculation with *Bradyrhizobium japonicum* [75]. With advancement of next generation sequencing technologies, it has been very easy to study the vast microbial diversity in the rhizosphere. Earlier studies have shown that there are different subsets of diversity in soil bulk, thus type of soil is an important factor for determining rhizosphere microbial community.
