*2.2.3 Induced systemic resistance*

There is an important feature of biocontrol that helps plant growth based on two defensive response mechanisms against various external agents known as mechanisms of induced systemic resistance (ISR) and mechanism of acquired systemic resistance (ASR) [56]. ISR is a physiological state of defensive capacity that plants present in response to an environmental stimulus [43] in which nonpathogenic microorganisms, including various PGPB, reduce the negative effects of pathogens of plants by activating a resistance mechanism without the need to target a specific pathogen and can develop this resistance in response to infection by pathogens, to attack by insects, or to a chemical treatment [42].

To stimulate this defensive response mechanism, the ISR uses phytohormones as jasmonic acid (JA) and ethylene (ET) that act as signaling molecules [64], although it has been shown that some organelles (such as flagella) and bacterial molecules (such as lipopolysaccharides of the outer membrane or antibiotics produced by bacteria) can also act as inducing agents activating ISR and generating a rapid accumulation of pathogenesis-related enzymes such as chitinase, β-1,3-glucanase, peroxidase, or liases, among others [85]. It is important to note that the ISR prepares plant to fight the pathogen with an improved defense [56]. A clear example of the ISR elicitor is the effect of *Bradyrhizobium japonicum* in soybean crops, where systemic redox changes are induced in plants [86]. The induction of ISR by *Bacillus* sp. CHEP5 and *Bradyrhizobium* sp. SEMIA 6144 in peanut plants against *S. rolfsii* also has been demonstrated [87]. However, this microbial induction could be limited by abiotic stress like the ISR induction of *B. amyloliquefaciens* S499 in different crops under heat and drought conditions where the response against *Botrytis cinerea* infection was prejudiced [88].
