Induced systemic resistance (ISR) in plants: mechanism of action

Plants possess a range of active defense apparatuses that can be actively expressed in response to biotic stresses (pathogens and parasites) of various scales (ranging from microscopic viruses to phytophagous insect). The timing of this defense response is critical and reflects on the difference between coping and succumbing to such biotic challenge of necrotizing pathogens/parasites. If defense mechanisms are triggered by a stimulus prior to infection by a plant pathogen, disease can be reduced. Induced resistance is a state of enhanced defensive capacity developed by a plant when appropriately stimulated. Systemic acquired resistance (SAR) and induced systemic resistance (ISR) are two forms of induced resistance wherein plant defenses are preconditioned by prior infection or treatment that results in resistance against subsequent challenge by a pathogen or parasite. Selected strains of plant growth-promoting rhizobacteria (PGPR) suppress diseases by antagonism between the bacteria and soil-borne pathogens as well as by inducing a systemic resistance in plant against both root and foliar pathogens. Rhizobacteria mediated ISR resembles that of pathogen induced SAR in that both types of induced resistance render uninfected plant parts more resistant towards a broad spectrum of plant pathogens. Several rhizobacteria trigger the salicylic acid (SA)-dependent SAR pathway by producing SA at the root surface whereas other rhizobacteria trigger different signaling pathway independent of SA. The existence of SA-independent ISR pathway has been studied in Arabidopsis thaliana, which is dependent on jasmonic acid (JA) and ethylene signaling. Specific Pseudomonas strains induce systemic resistance in viz., carnation, cucumber, radish, tobacco, and Arabidopsis, as evidenced by an enhanced defensive capacity upon challenge inoculation. Combination of ISR and SAR can increase protection against pathogens that are resisted through both pathways besides extended protection to a broader spectrum of pathogens than ISR/SAR alone. Beside Pseudomonas strains, ISR is conducted by Bacillus spp. wherein published results show that several specific strains of species B. amyloliquifaciens, B. subtilis, B. pasteurii, B. cereus, B. pumilus, B. mycoides, and B.sphaericus elicit significant reduction in the incidence or severity of various diseases on a diversity of hosts.     

Endophytes from Argemone mexicana and Datura metel activate induced-systemic resistance in multiple hosts and show host- and pathogen-specific protection

Endophytes are known to activate induced systemic resistance (ISR) in plants. However, detailed studies regarding the location, host, and pathogen specificity are limited. To investigate the influence of the source of endophytes on ISR, we collected Argemone mexicana and Datura metel plants from two different locations in northeast India. We isolated endophytes from these plants and tested their ability to confer ISR-mediated protection. We found that endophytes from D. metel from both locations activated ISR in D. metel plants. However, endophytes from A. mexicana from only one place triggered ISR in A. mexicana. We also observed cross-protection by the endophytes from these plants in Arabidopsis thaliana in a host- and pathogen-specific manner. The results altogether demonstrated that endophytes found in plants under the natural environment play a significant role in generating broad-spectrum resistance against soil microbes. These endophytes might be selectively used to induce ISR in plants.

     

Colonization of the <Emphasis Type="Italic">Arabidopsis</Emphasis> rhizosphere by fluorescent <Emphasis Type="Italic">Pseudomonas</Emphasis> spp. activates a root-specific,ethylene-responsive <Emphasis Type="Italic">PR-5</Emphasis> gene in the vascular bundle

Plants of which the roots are colonized by selected strains of non-pathogenic, fluorescent Pseudomonas spp. develop an enhanced defensive capacity against a broad spectrum of foliar pathogens. In Arabidopsis thaliana, this rhizobacteria-induced systemic resistance (ISR) functions independently of salicylic acid but requires responsiveness to jasmonic acid and ethylene. In contrast to pathogen-induced systemic acquired resistance (SAR), ISR is not associated with systemic changes in the expression of genes encoding pathogenesis-related (PR) proteins. To identify genes that are specifically expressed in response to colonization of the roots by ISR-inducing Pseudomonas fluorescens WCS417r bacteria, we screened a collection of Arabidopsis enhancer trap and gene trap lines containing a transposable element of the Ac/Ds system and the GUS reporter gene. We identified an enhancer trap line (WET121) that specifically showed GUS activity in the root vascular bundle upon colonization of the roots by WCS417r. Fluorescent Pseudomonas spp. strains P. fluorescens WCS374r and P. putida WCS358r triggered a similar expression pattern, whereas ISR-non-inducing Escherichia coli bacteria did not. Exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylate (ACC) mimicked the rhizobacteria-induced GUS expression pattern in the root vascular bundle, whereas methyl jasmonic acid and salicylic acid did not, indicating that the Ds element in WET121 is inserted in the vicinity of an ethylene-responsive gene. Analysis of the expression of the genes in the close vicinity of the Ds element revealed AtTLP1 as the gene responsible for the in cis activation of the GUS reporter gene in the root vascular bundle. AtTLP1 encodes a thaumatin-like protein that belongs to the PR-5 family of PR proteins, some of which possess antimicrobial properties. AtTLP1 knockout mutant plants showed normal levels of WCS417r-mediated ISR against the bacterial leaf pathogen Pseudomonas syringae pv. tomato DC3000, suggesting that expression of AtTLP1 in the roots is not required for systemic expression of ISR in the leaves. Together, these results indicate that induction of AtTLP1 is a local response of Arabidopsis roots to colonization by non-pathogenic fluorescent Pseudomonas spp. and is unlikely to play a role in systemic resistance.     

Fluorescent pseudomonad mixtures mediate disease resistance in rice plants against sheath rot (<Emphasis Type="Italic">Sarocladium oryzae</Emphasis>) disease

Plant growth-promoting rhizobacterial (PGPR) strains were isolated from different agro-ecosystems of Tamil Nadu, India, and were tested for their efficacy against the sheath rot pathogen Sarocladium oryzae under in vitro, glasshouse and field conditions. Vigour and a relative performance index (RPI) were used to assay the growth promotion and antagonistic activity of Pseudomonas strains against S. oryzae under in vitro conditions. The results revealed the significant performance by strains Pf1, TDK1 and PY15 compared to other strains. Further, the combination of Pseudomonas strains Pf1, TDK1 and PY15 was more effective in reducing sheath rot disease in rice plants compared to individual strains under glasshouse and field conditions. Quantitative and native polyacrylamide gel electrophoresis (PAGE) analysis of peroxidase (PO), polyphenol oxidase (PPO) and chitinase activity in rice plants showed an increased accumulation of defence enzymes in the treatment with a combination of Pf1, TDK1 and PY15 compared to the treatment with individual strains and untreated controls. The present study revealed the probable influence of antagonism, plant growth promotion and induced systemic resistance (ISR) by the mixture of Pseudomonas bioformulations in enhancing the disease resistance in rice plants against sheath rot disease.

Evaluation of plant growth-promoting Rhizobacteria for their efficiency to promote growth and induce systemic resistance in pearl millet against downy mildew disease

Six strains of Plant growth promoting Rhizobacteria (PGPR) were tested for their ability to promote growth and induce resistance in pearl millet against downy mildew disease. All the PGRP strains showed a significant (P < 0.01) increase in growth promotion in laboratory as well as greenhouse conditions. Only two strains of Pseudomonas spp., UOM ISR 17 and UOM ISR 23, were capable of protecting pearl millet against downy mildew significantly. Pseudomonas UOM ISR 17 and UOM ISR 23 were able to offer 56.3 and 47.5%, respectively against downy mildew disease. When tested for the time gap needed to offer maximum protection, it was found that both the strains needed four days to offer maximum protection of 73.3% and 59.7%, respectively. While both the Acetobacter strains UOM Ab9 and Ab11 and Azospirillum strain UOM Az3 were able to promote growth and offered disease protection of 39.2, 22.3 and 17.40% respectively, they were not as efficient as the two Pseudomonas strains in protecting pearl millet against downy mildew. Maximum growth promotion was recorded by Pseudomonas spp. UOM ISR 17 with 33.9 cm height which was 44, 45, 42 and 46.8% more in height, fresh weight, dry weight and leaf area over the control which recorded 27 cm height, 8.1 g fresh weight, 2.1 g dry weight and 29 cm² leaf area, respectively.     

Systemic resistance and lipoxygenase-related defence response induced in tomato by <Emphasis Type="Italic">Pseudomonas putida</Emphasis>strain BTP1

Background  

Previous studies showed the ability of Pseudomonas putida strain BTP1 to promote induced systemic resistance (ISR) in different host plants. Since ISR is long-lasting and not conducive for development of resistance of the targeted pathogen, this phenomenon can take part of disease control strategies. However, in spite of the numerous examples of ISR induced by PGPR in plants, only a few biochemical studies have associated the protective effect with specific host metabolic changes.     

Inoculation of tomato plants (Solanum lycopersicum) with growth-promoting Bacillus subtilis retards whitefly Bemisia tabaci development

Root inoculation of tomato (Solanum lycopersicum) plants with a Bacillus subtilis strain BEB-DN (BsDN) isolated from the rhizosphere of cultivated potato plants was able to promote growth and to generate an induced systemic resistance (ISR) response against virus-free Bemisia tabaci. Growth promotion was evident 3 weeks after inoculation. No changes in oviposition density, preference and nymphal number in the early stages of B. tabaci development were observed between BsDN-treated plants and control plants inoculated with a non-growth promoting Bs strain (PY-79), growth medium or water. However, a long-term ISR response was manifested by a significantly reduced number of B. tabaci  pupae developing into adults in BsDN-treated plants. The observed resistance response appeared to be a combination of jasmonic acid (JA) dependent and JA-independent responses, since the BsDN-related retardation effect on B. tabaci development was still effective in the highly susceptible spr2 tomato mutants with an impaired capacity for JA biosynthesis. A screening of 244 genes, 169 of which were previously obtained from subtractive-suppressive-hybridization libraries generated from B. tabaci-infested plants suggested that the BsDN JA-dependent ISR depended on an anti-nutritive effect produced by the simultaneous expression of genes coding principally for proteases and proteinase inhibitors, whereas the JA-independent ISR observed in the spr2 background curiously involved the up-regulation of several photosynthetic genes, key components of the phenyl-propanoid and terpenoid biosynthetic pathways and of the Hsp90 chaperonin, which probably mediated pest resistance response(s), in addition to the down-regulation of pathogenesis and hypersensitive response genes.     

Nitric oxide functions as a signal in induced systemic resistance

The study was aimed to search out the probable molecule behind the activation of a broad spectrum resistance during Pseudomonas aeruginosa WS-1 mediated induced systemic resistance (ISR) in Capsicum annuum where plants were challenged inoculated with its pathogen Colletotrichum capsici 24 h after induction of ISR. On the fourth day after pathogen inoculation a significant increase of pathogenesis-related (PR) proteins, other defence enzymes and phenolics as well as a two-fold increase of nitric oxide (NO) a potent defence signalling molecule were observed. Treatment of the host with NO donor also induced the same defence molecule in a similar manner. Results suggest the possible signalling role of NO in ISR during crosstalk between ISR inducing agent and pathogen within the host system.     

Plant growth-promoting archaea trigger induced systemic resistance in Arabidopsis thaliana against Pectobacterium carotovorum and Pseudomonas syringae

Archaea have inhabited the earth for a long period of time and are ubiquitously distributed in diverse environments. However, few studies have focused on the interactions of archaea with other organisms, including eukaryotes such as plants, since it is difficult to cultivate sufficient numbers of archaeal cells for analysis. In this study, we investigated the interaction between soil archaea and Arabidopsis thaliana. We demonstrate for the first time that soil archaea promote plant growth and trigger induced systemic resistance (ISR) against the necrotrophic bacterium Pectobacterium carotovorum subsp. carotovorum SCC1 and biotrophic bacterium Pseudomonas syringae pv. tomato DC3000. Ammonia-oxidizing archaeon Nitrosocosmicus oleophilus MY3 cells clearly colonized the root surface of Arabidopsis plants, and increased resistance against both pathogenic species via the salicylic acid-independent signalling pathway. This mechanism of bacterial resistance resembles that underlying soil bacteria- and fungi-mediated ISR signalling. Additionally, volatile emissions from N. oleophilus MY3 were identified as major archaeal determinants that elicit ISR. Our results lay a foundation for archaea–plant interactions as a new field of research.     

Amino Acids, Iron, and Growth Rate as Key Factors Influencing Production of the Pseudomonas Putida BTP1 Benzylamine Derivative Involved in Systemic Resistance Induction in Different Plants

The biological control bacterium Pseudomonas putida BTP1 exerts its protective effect mostly by inducing an enhanced state of resistance in the host plant against pathogen attack [induced systemic resistance (ISR)]. We previously reported that a specific compound derived from benzylamine may be involved in the elicitation of the ISR phenomenon by this Pseudomonas strain. In this article, we provide further information about the N,N-dimethyl-N-tetradecyl-N-benzylammonium structure of this determinant for ISR and show that the benzylamine moiety may be important for perception of the molecule by root cells of different plant species. We also investigated some regulatory aspects of elicitor production with the global aim to better understand how in situ expression of these ISR elicitors can be modulated by physiological and environmental factors. The biosynthesis is clearly related to secondary metabolism, and chemostat experiments showed that the molecule is more efficiently produced at low cell growth rate. Interestingly, the presence of free amino acids in the environment is necessary for optimal production, and a specific positive effect of phenylalanine was evidenced in pulsed continuous cultures. The influence of other abiotic factors, such as mineral content, oxygen concentration, or pH, on elicitor production is also reported and discussed with respect to the specific conditions that the producing strain undergoes in the rhizosphere environment.     

Role of ethylene signalling in growth and systemic resistance induction by the plant growth‐promoting fungus Penicillium viridicatum in Arabidopsis

The plant growth‐promoting fungi (PGPF) have long been known to improve plant growth and suppress plant diseases. The PGPF Penicillium viridicatum GP15‐1 elicited plant growth and induced systemic resistance (ISR) in Arabidopsis thaliana against Pseudomonas syringae pv. tomato DC3000 (Pst), leading to a restriction of pathogen growth and disease development. Examination of local and systemic genes indicated that GP15‐1 did not modulate the expression of any of the tested defence‐related marker genes involved in salicylic acid (SA), jasmonic acid (JA) and ethylene signalling pathways. Subsequent challenge of GP15‐1‐colonized plants with Pst bacterium primed Arabidopsis plants for enhanced activation of the JA‐inducible Atvsp (vegetative storage protein) gene at a later stage of infection. To assess the contribution of different signalling pathways in GP15‐1‐elicited plant growth and ISR, Arabidopsis genotypes implicated in SA signalling expressing the nahG transgene (NahG) or carrying disruption in NPR1 (npr1), JA signalling (jar1) and ethylene signalling (ein2) were tested. The GP15‐1‐induced plant growth and ISR were fully compromised in an ein2 mutation. Root colonization assay revealed that the inability of the ein2 mutant to express GP15‐1‐induced plant growth and ISR was not associated with reduced root colonization by GP15‐1. In conclusion, our results demonstrate the ethylene signalling pathway is involved in plant growth promotion and ISR elicitation by the PGPF P. viridicatum GP15‐1 in Arabidopsis. These results provide evidence that ethylene signalling has a substantial role in plant growth and disease resistance.     

<Emphasis Type="SmallCaps">l</Emphasis>-Alanine augments rhizobacteria-induced systemic resistance in cucumber

Bacillus vallismortis strain EXTN-1 is a proven biotic elicitor of systemic resistance in many crops against various pathogens. l-Alanine (Ala) was tested in cucumber as a chemical elicitor of induced systemic resistance (ISR) against Colletotrichum orbiculare. In the greenhouse, both Ala and EXTN-1 induced significant levels of disease suppression in cucumber against anthracnose. When cucumber plants were treated with EXTN-1 and Ala together, augmentative disease suppression was observed. Experiments with transgenic tobacco plants carrying pathogenesis-related genes fused with the β-glucuronidase (GUS) reported gene (PR-1a::GUS & PDF 1.2::GUS) showed an enhanced activation of both PR-1a and PDF 1.2 genes upon combined treatment with Ala and EXTN-1. RT-PCR analysis with transgenic (PR-1a or PDF 1.2 over expressing) Arabidopsis plant showed more enhanced expression of resistance genes PR-1a and PDF 1.2 upon combined treatment with Ala and EXTN-1 than either alone. An augmentative ISR effect, when the bacterial elicitor and chemical elicitor were combined together, was confirmed.     

The role of phytohormones in basal resistance and Trichoderma-induced systemic resistance to Botrytis cinerea in Arabidopsis thaliana

Thirty-six phytohormone-affected mutants of Arabidopsis thaliana (L.) Heynh. and their parental ecotypes were tested for resistance/susceptibility to Botrytis cinerea Pers.; Fr. and ability to develop Trichoderma-mediated induced systemic resistance (ISR). Ecotype Colombia-0 (Col-0) was relatively resistant to B. cinerea, and Trichoderma harzianum Rifai T39 application at sites spatially separated (roots) from the B. cinerea inoculation (leaves) resulted in reduction of grey mold symptoms. Ecotypes Wassilewskija-4, Nossen-0 and Landsberg-0 had low levels of basal resistance to B. cinerea and were unable to express ISR. Mutants derived from ISR-non-inducible ecotypes displayed ISR-non-inducible phenotypes, whereas the ISR inducibility of mutants derived from the ISR-inducible genotype Col-0 varied according to the type of mutant. Thus, salicylic acid (SA)-impaired mutants derived from Col-0 were ISR-inducible, while ethylene/jasmonic acid (ethylene/JA)-impaired mutants of the same origin were ISR-non-inducible. SA-impaired mutants retained basal level of resistance to B. cinerea, while most ethylene/JA-impaired mutants were highly susceptible. Abscisic acid- and gibberellin-impaired mutants were highly susceptible to B. cinerea and showed ISR-non-inducible phenotypes irrespective of their lines of origin. Auxin-resistant mutants derived from Col-0 were ISR-inducible; mutant originating from Landsberg-0 and mutants which were resistant to both auxin and ethylene were ISR-non-inducible. Most of the arabidopsis genotypes which were unable to express Trichoderma-mediated ISR against B. cinerea exhibited enhanced susceptibility to this pathogen. T. harzianum treatments enhanced the growth of arabidopsis plants regardless of genotype or ISR inducibility.     

Members only: induced systemic resistance to herbivory in a clonal plant network

The stoloniferous herb Trifolium repens was used to study the expression of induced systemic resistance (ISR) to the generalist caterpillar Spodoptera exigua in interconnected ramets of clonal fragments. The ISR was assessed as caterpillar preference in dual choice tests between control and systemically induced plants. The ISR was detected in young ramets, after inducing older sibling ramets on the same stolon by a controlled herbivore attack. However, older ramets did not receive a defense induction signal from younger ramets unless the predominant phloem flow was reversed by means of basal shading. This provides evidence for the notion that in T. repens the clone-internal expression of ISR is coupled to phloem transport and follows source–sink gradients. The inducibility of the genotypes was not linked to their constitutive ability to produce cyanide, implying the absence of a trade-off between these two defense traits. To our knowledge, this is the first study that explores ISR to herbivory in the context of physiological integration in potentially extensive clonal plant networks.     

Effects of Plant Growth Promoting Rhizobacteria (PGPRs) on the Biological Nitrogen Fixation,Nodulation, and Growth of Lupinus albus l. cv. Multolupa

The effects of five plant growth promoting rhizobacteria on the biological nitrogen fixation (BNF), nodulation, and growth promotion of plants of Lupinus albus cv. Multolupa were investigated. The plants were selected for their capacity to use 1‐aminocyclopropane‐1‐carboxylic acid (ACC) as the sole source of nitrogen. Four strains belonged to the genus Pseudomonas (Luc 1, Luc 2, Luc 3, and Luc 4) and one (Luc 5) belonged to the genus Bacillus. Three patterns of inoculation were examined. In the first pattern, PGPRs were inoculated seven days before being inoculated with B. japonicum. In the second pattern, PGPRs and B. japonicum were co‐inoculated, and in the last pattern, PGPRs were inoculated seven days after being inoculated with B. japonicum. The plants were sampled 30 and 45 days after being inoculated with B. japonicum (T1 and T2). In the first pattern of inoculation, Luc 5 significantly increased the biological nitrogen fixation compared to the control at the first sampling time, as did Luc 1 and Luc 4 at the second sampling time. In the second pattern of inoculation, Luc 5 negatively effected the biological nitrogen fixation at both sampling times. In the third pattern of inoculation, all PGPRs caused a decrease in the nitrogen content of the plants compared to the control. The results obtained according to the patterns of inoculation showed that the mechanisms of action of the effects and routes used by Gram‐negative and Gram‐positive strains were clearly different. Competition between PGPRs and B. japonicum, competition for the niches in the rhizoplane, production of auxins, and induction of systemic resistance (ISR) by the production of siderophores or by lipopolysaccharides present in the outer membrane (LPS) are discussed as probable reasons for the effects observed.     

Differential effectiveness of Serratia plymuthica IC1270-induced systemic resistance against hemibiotrophic and necrotrophic leaf pathogens in rice

Background  

Induced resistance is a state of enhanced defensive capacity developed by a plant reacting to specific biotic or chemical stimuli. Over the years, several forms of induced resistance have been characterized, including systemic acquired resistance, which is induced upon localized infection by an avirulent necrotizing pathogen, and induced systemic resistance (ISR), which is elicited by selected strains of nonpathogenic rhizobacteria. However, contrary to the relative wealth of information on inducible defense responses in dicotyledoneous plants, our understanding of the molecular mechanisms underlying induced resistance phenomena in cereal crops is still in its infancy. Using a combined cytomolecular and pharmacological approach, we analyzed the host defense mechanisms associated with the establishment of ISR in rice by the rhizobacterium Serratia plymuthica IC1270.     

Signal transduction downstream of salicylic and jasmonic acid in herbivory-induced parasitoid attraction by Arabidopsis is independent of JAR1 and NPR1

Plants can defend themselves indirectly against herbivores by emitting a volatile blend upon herbivory that attracts the natural enemies of these herbivores, either predators or parasitoids. Although signal transduction in plants from herbivory to induced volatile production depends on jasmonic acid (JA) and salicylic acid (SA), the pathways downstream of JA and SA are unknown. Use of Arabidopsis provides a unique possibility to study signal transduction by use of signalling mutants, which so far has not been exploited in studies on indirect plant defence. In the present study it was demonstrated that jar1‐1 and npr1‐1 mutants are not affected in caterpillar (Pieris rapae)‐induced attraction of the parasitoid Cotesia rubecula. Both JAR1 and NPR1 (also known as NIM1) are involved in signalling downstream of JA in induced defence against pathogens such as induced systemic resistance (ISR). NPR1 is also involved in signalling downstream of SA in defence against pathogens such as systemic acquired resistance (SAR). These results demonstrate that signalling downstream of JA and SA differs between induced indirect defence against herbivores and defence against pathogens such as SAR and ISR. Furthermore, it was demonstrated that herbivore‐derived elicitors are involved in induced attraction of the parasitoid Cotesia rubecula