Induced Systemic Resistance by Microbes

Induced Systemic Resistance (ISR) is a defense priming mechanism in plants activated by non-pathogenic microbes, resulting in a heightened ability to respond to biotic and abiotic stress. Unlike Systemic Acquired Resistance (SAR), which is triggered by pathogenic attack and mediated primarily by salicylic acid, ISR is commonly initiated by plant growth-promoting rhizobacteria (PGPR) and beneficial fungi through signaling pathways involving jasmonic acid (JA) and ethylene (ET). Microbes such as Pseudomonas fluorescens, Bacillus subtilis, and Trichoderma harzianum colonize the rhizosphere or root surfaces without causing harm, and in doing so, stimulate host immune pathways that protect against a broad spectrum of pathogens and pests.

The mechanism of ISR involves complex cross-talk between microbial elicitors such as flagellin, lipopolysaccharides, siderophores, and lipopeptides and plant receptors. These microbial signals do not trigger direct resistance responses but rather prime the plant’s immune system, leading to faster and stronger activation of defense-related genes upon subsequent pathogen challenge. This “primed” state results in enhanced production of pathogenesis-related (PR) proteins, reactive oxygen species (ROS), phytoalexins, and cell wall fortifications only when needed, minimizing the metabolic cost to the plant. Additionally, ISR has been associated with improved tolerance to abiotic stresses, including drought, salinity, and nutrient deficiency, through modulation of stress-responsive pathways and hormone balance.

Harnessing ISR through microbial inoculants offers a promising and sustainable strategy for plant protection in integrated crop management systems. By incorporating ISR-inducing microbes into seed treatments, soil amendments, or foliar sprays, crops gain systemic, long-lasting defense without the need for continuous chemical inputs. Moreover, ISR can complement other biological control strategies and contribute to disease-suppressive soil development, reducing pathogen load over time. The effectiveness of ISR varies depending on plant genotype, microbial strain, and environmental context, underscoring the need for tailored microbiome management approaches. As research into ISR advances, it holds great potential for enhancing crop resilience, reducing agrochemical dependency, and promoting environmentally sound agricultural practices.