Induced Systemic Resistance
Authors: PRIYANKA CHANDRA & POONAM JASROTIA


Several researches have shown that soil microorganisms can ameliorate abiotic stresses very effectively in various crops. Along with, if these soil microorganisms have the capability to promote growth, they become more beneficial. Plant-growth-promoting rhizobacteria (PGPR) colonize the rhizosphere of many plant species and confer beneficial effects and also elicit physical or chemical changes related to plant defense, this mechanism is known as “induced systemic resistance” (ISR).


According to the studies, ISR elicited by PGPR has suppressed several plant diseases caused by a range of pathogens in both the greenhouse as well as on field trails. Yang et al. (2009) proposed the term ‘induced systemic tolerance’ (IST) for PGPR which induces physical and chemical changes in plants that result into enhanced tolerance and resistant to abiotic stress. These include tolerance to drought, salt, water stress and nutrient deficiency. Azospirillum sp., Pseudomanas syringae, P. fluorescens, Bacillus sp, has been reported to ameliorate salt and water stress while B. Polymyxa and Pseudomonas alacaligenes has been reported to reduce stress during nutrient deficiency.

ISR has been reported as one of the mechanisms by which PGPR reduces plant disease modulating the physical and biochemical properties of host plants. The first studies on ISR were carried out by van Peer et al. (1991). In this study, non-pathogenic Pseudomonas spp. was inoculated in the roots of plant and observation was carried to see the trigger of a plant-mediated resistance response in plant parts. Since then, the ISR elicitation by PGPR as a bio-control method has been studied in many crops such as bean, tomato, tobacco, radish, cucumber and carnation. ISR is characterized by a specificity relationship between plant and PGPR species i.e. PGPR that produces ISR in one plant species may not do it in another. Several strains from Pseudomonas, Bacillus and Azospirillum genera are the major group of PGPR that have been described eliciting ISR response. Induced systemic resistance (ISR) are activated by certain microorganism molecules referred to as elicitors. This defence response is dependent on ethylene and jasmonic acid signalling in the plant.
Elicitors are the molecules which includes, Cell wall polysaccharides, flagella, salicylic acid, cyclic lipopeptides, siderophores, antibiotics, the signal molecule AHL or volatile compounds.

PGPR mitigate the effect of drought stress on plants by following mechanisms:
(a) The production of cytokinins causes the accumulation of abscisic acid (ABA) in leaves, which in its turn results in the closing of stomata.
(b) The production of antioxidants (e.g., the enzyme catalase) causes the degradation of reactive forms of oxygen, which neutralize the toxic effects of ROS in plant cells, reducing damage to cells and biomolecules to a minimum.
(c) The bacterial-produced ACC deaminase degrades the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC), by reducing the level of ethylene, the plant becomes more resistant to stress conditions.

References:

1. van Peer, R., Niemann, G.J., Schippers, B., 1991. Induced resistance and phytoalexin accumulation in biological control of Fusarium wilt of carnation by Pseudomonas sp. strain WCS417r. Phytopathology 81, 728-734.
2. Yang Jungwook, Kloepper Joseph W. and Ryu Choong-Min., 2009. Rhizosphere bacteria help plants tolerate abiotic stress. Trends in Plant Science Vol.14 No.1


About Author / Additional Info:
I am working as Scientist in ICAR-Indian Institute of Wheat and Barley Research . My specialization is in Agricultural Microbiology. The Co-author of the article is Poonam Jasrotia is agricultural entomologist in the same institute.