Induced systemic resistance (ISR) and Systemic acquired resistance (SAR) are two different phenomena but both represent active plant defense responses to phytopathogen attack. ISR is similar to hypersensitive response while as SAR is alike inherent immunity of plant system. The terms were first time coined in 1961 by Ross (Virology, 14:329-339 and 340-358) during his research on interactions between tobacco and its mosaic virus (TMV). Induction of ISR is the function of non-pathogenic plant growth promoting rhizobacteria (PGPR). However SAR is triggered by infection of a pathogen.
ISR (Induced Systemic Resistance):
It is the generalized (systemic) resistance which is naturally present in plant but induced or enhanced by plant associated non-pathogenic rhizobacteria or plant growth promoting rhizobacteria. It is independent of salicylic acid and hence no PR-pathogenesis related proteins are synthesized but it is plant specific and is dependent on plant genotype. Here plant specific means host specificity shown by rhizobacteria in activating resistance response. These bacteria, in fact 'recognize' their host plant before setting on ISR. Plant hormones, jasmonic acid and ethylene are required in ISR. Jasmonic acid regulates plants' responses to biotic and abiotic stresses including pathogen attack. Sometimes it is formed as volatile compound which can reach to plant parts and nearby plants to warn off pathogen attack and trigger plant defense responses. Wounding or pathogen attack also stimulates the production of ethylene which then onset defense responses in favor of plant. Typical ethylene induced visual defense responses include rapid senescence, ripening and abscission of infected tissue like sudden fall of leaves or fruits.
SAR (Systemic Acquired Resistance):
It is the resistance which is once acquired remains generalized (systemic) in plant body. It is inherent resistance capacity of a plant which is activated on exposure to pathogen. It is similar to our immune response followed by vaccination. Plants cellular defense system use recognition receptor proteins to identify microbial pathogen which may have been plant's pathogen in the past. Induction of SAR requires pathogen borne tissue necrosis, SAR once induced remains active against broad range of pathogens for prolonged time and it not only resist pathogen attack but also cure disease if occurred. SAR develops primarily from tissue necrosis (infection site) caused by pathogen. Necrosis is followed by release and accumulation of salicylic acid in phloem tissue that triggers first hypersensitive response and induction of SAR. Salicylic acid is plant hormone required for the production of PR proteins, inhibits induction of virulence factors and also known to control another plant hormone, ethylene. These proteins are products of pathogen activated PR genes. They are necessary to induce plant defense, some of them function as antimicrobials degrading cell wall of pathogen; some PRs are specifically antibacterial, antifungal and antiviral. They also possess lytic chitinase, lysozyme and peroxidase enzyme activities which are functional against different pathogens. They also act as messengers to signal pathogen attack. This signaling activates lignin formation and deposition creating efficient barrier to infecting agents. Biochemical genetics of signaling pathway is still not fully known. Onset of SAR determines plant's defending ability, which is found to be very high and elaborated as various plant parts including distal leaves/branches/stems/roots, also acquires immunity to pathogen. Every plant part need not be a site of infection or necrosis or in contact with pathogen.
Non-pathogenic rhizobacteria and plant growth promoting rhizobacteria isolated from soil and rhizosphere are potential natural elicitors. Apart from rhizobacteria, fungi, viruses and nematodes also induce ISR. The residence of PGPR either in symbiosis or free living state in plant rhizosphere is generally enough to induce ISR in host plant naturally; such plants are naturally resistant to their pathogens. This is because antagonistic but plant growth promoting properties such as hormone production, antibiotic and siderophore production of these bacteria are also found to be ISR inducers. Thus PGPR not only promote plant growth but also biocontrol. Three PGPR Bacillus, Pseudomonas and Rhizobium are ideal among natural elicitors. Salicylic acid and pathogenic organisms are the only known natural chemical and biological elicitors of SAR.
Artificial elicitation of ISR and SAR:
Elicitors or activators of ISR and SAR are in use successfully for biocontrol applications. Conventional agriculture uses both biological and chemical form of elicitors. They are injected into stems or applied as leaf sprays on plant body. Biological elicitors are formulations (inoculum) of cell extracts, living bacteria, viruses and fungi, of which bacteria or precisely rhizobacteria are routinely utilized to induce ISR. Chemical elicitors which are commonly used are Beta-aminobutyric acid, 2, 6-dichloroisonicotinic acid, Benzothiadiazole, Silicon dioxide, paraquat, polyacrylate, salicylate etc. which induce SAR. They have been used successfully in integrated pest and disease management either solely (only chemical or biological elicitors) or in combination (both). Bonaldo et al (2005, Piracicaba SP. FEALQ, pp. 11-28) have suggested following advantages of artificial elicitors in controlled field conditions like greenhouse: stability, systemicity, economical, utilization of different mechanisms of resistance and genetic potential of plants.
Comparison of ISR and SAR:
Both ISR and SAR are dissimilar with respect to their induction, signaling pathways and their co-ordination, level of defense elicited in host plant and protein expression. ISR is induced by jasmonic acid and ethylene molecules coordinated signaling pathway; proteins other than pathogenesis related are too expressed. Level of defense generated is lesser and also dependent on host plant genotype and presence of non-pathogenic bacteria. SAR is induced by salicylic acid. SAR and PR proteins are induced together. The defense level produced by SAR is high, long lasting, broad spectrum and independent of plant genotype but requires necrotic pathogens or presence of inducer analogs. It has been indicated that ISR and SAR show phenotypic similarity and together can produce protection of higher level than they produce alone.
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