Elicit Defense Responses Research Articles

Minor effects of two elicitors of insect and pathogen resistance on volatile emissions and parasitism of Spodoptera frugiperda in Mexican maize fields

Synthetic elicitors can be used to induce resistance in plants against pathogens and arthropod herbivores. Such compounds may also change the emission of herbivore-induced plant volatiles, which serve as important cues for parasitic wasps to locate their hosts. Therefore, the use of elicitors in the field may affect biological control of insect pests. To test this, we treated maize seedlings growing in a subtropical field in Mexico with methyl jasmonate (MeJA), an elicitor of defense responses against many insects, and benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), an elicitor of resistance against certain pathogens. Volatile emission, herbivore infestation, pathogen infection, and plant performance (growth and grain yield) of treated and untreated maize plants were measured. Application of BTH slightly reduced volatile emission in maize, while MeJA increased the emission compared to control treatments. Despite the apparent changes in volatile emissions, the elicitor application did not consistently affect infestation by Spodoptera frugiperda larvae, the main insect pest found on the maize seedlings, and had only marginal effects on parasitism rates. Similarly, there were no treatment effects on infestation by other herbivores and pathogens. Results for the six replications that stretched over one summer and one winter season were highly variable, with parasitism rates and the species composition of the parasitoids differing significantly between seasons. This variability, as well as the severe biotic and abiotic stresses on young seedlings might explain why we measured only slight effects of elicitor application on pest incidence and biological control in this specific field study. Indeed, an additional field experiment under milder and more standardized conditions revealed that BTH induced significant resistance against Bipolaris maydis, a major pathogen in the experimental maize fields. Similar affects can be expected for herbivory and parasitism rates.

Lipo-chitooligosaccharide Signaling in Endosymbiotic Plant-Microbe Interactions

The arbuscular mycorrhizal (AM) and the rhizobia-legume (RL) root endosymbioses are established as a result of signal exchange in which there is mutual recognition of diffusible signals produced by plant and microbial partners. It was discovered 20 years ago that the key symbiotic signals produced by rhizobial bacteria are lipo-chitooligosaccharides (LCO), called Nod factors. These LCO are perceived via lysin-motif (LysM) receptors and activate a signaling pathway called the common symbiotic pathway (CSP), which controls both the RL and the AM symbioses. Recent work has established that an AM fungus, Glomus intraradices, also produces LCO that activate the CSP, leading to induction of gene expression and root branching in Medicago truncatula. These Myc-LCO also stimulate mycorrhization in diverse plants. In addition, work on the nonlegume Parasponia andersonii has shown that a LysM receptor is required for both successful mycorrhization and nodulation. Together these studies show that structurally related signals and the LysM receptor family are key components of both nodulation and mycorrhization. LysM receptors are also involved in the perception of chitooligosaccharides (CO), which are derived from fungal cell walls and elicit defense responses and resistance to pathogens in diverse plants. The discovery of Myc-LCO and a LysM receptor required for the AM symbiosis, therefore, not only raises questions of how legume plants discriminate fungal and bacterial endosymbionts but also, more generally, of how plants discriminate endosymbionts from pathogenic microorganisms using structurally related LCO and CO signals and of how these perception mechanisms have evolved.

Defense/stress responses activated by chitosan in sycamore cultured cells

Chitosan (CHT) is a natural, non-toxic, and inexpensive compound obtained by partial alkaline deacetylation of chitin, the main component of the exoskeleton of crustaceans and other arthropods. The unique physiological and biological properties of CHT make this polymer useful for a wide range of industries. In agriculture, CHT is used to control numerous pre- and postharvest diseases on various horticultural commodities. In recent years, much attention has been devoted to CHT as an elicitor of defense responses in plants, which include raising of cytosolic Ca(2+), activation of MAP kinases, callose apposition, oxidative burst, hypersensitive response, synthesis of abscisic acid, jasmonate, phytoalexins, and pathogenesis-related proteins. In this work, we investigated the effects of different CHT concentrations on some defense/stress responses of sycamore (Acer pseudoplatanus L.) cultured cells. CHT induced accumulation of dead cells, and of cells with fragmented DNA, production of H(2)O(2) and nitric oxide, release of cytochrome c from the mitochondrion, accumulation of regulative 14-3-3 proteins in the cytosol and of HSP70 molecular chaperone binding protein in the endoplasmic reticulum, accompanied by marked modifications in the architecture of this cell organelle.

Of PAMPs and Effectors: The Blurred PTI-ETI Dichotomy

Typically, pathogen-associated molecular patterns (PAMPs) are considered to be conserved throughout classes of microbes and to contribute to general microbial fitness, whereas effectors are species, race, or strain specific and contribute to pathogen virulence. Both types of molecule can trigger plant immunity, designated PAMP-triggered and effector-triggered immunity (PTI and ETI, respectively). However, not all microbial defense activators conform to the common distinction between PAMPs and effectors. For example, some effectors display wide distribution, while some PAMPs are rather narrowly conserved or contribute to pathogen virulence. As effectors may elicit defense responses and PAMPs may be required for virulence, single components cannot exclusively be referred to by one of the two terms. Therefore, we put forward that the distinction between PAMPs and effectors, between PAMP receptors and resistance proteins, and, therefore, also between PTI and ETI, cannot strictly be maintained. Rather, as illustrated by examples provided here, there is a continuum between PTI and ETI. We argue that plant resistance is determined by immune receptors that recognize appropriate ligands to activate defense, the amplitude of which is likely determined by the level required for effective immunity.

Whitefly infestation of pepper plants elicits defence responses against bacterial pathogens in leaves and roots and changes the below‐ground microflora

Summary1. Upon facing biotic stresses, plants orchestrate defence mechanisms via internal and external mechanisms that are mediated by signalling molecules such as salicylic acid, jasmonic acid, ethylene and various other volatile compounds. Although pathogen‐ and chemical‐induced plant resistance has been studied extensively within the same plant compartment, the effects of above‐ground (AG) insect‐elicited plant defence on the resistance expression in roots and the below‐ground (BG) microbial community are not well understood.2. We assessed the effect of AG whitefly (Bemisia tabaci) attack on the elicitation of induced resistance against a leaf pathogen,Xanthomonas axonopodispv.vesicatoria, a soil‐borne pathogen,Ralstonia solanacearum, and on BG modifications of the rhizosphere microflora in peppers (Capsicum annuum).3. Symptom development caused by the two bacterial pathogens on leaves and roots was significantly reduced in whitefly‐exposed plants as compared to controls. A combined treatment with benzothiadiazole (BTH) and whitefly caused an additive effect on induced resistance, indicating that whitefly‐induced plant defence can utilize salicylic acid (SA)‐dependent signalling. To obtain further genetic evidence of this phenomenon, we evaluated the gene expression ofCapsicum annuum pathogenesis‐related protein(CaPR)1,CaPR4,CaPR10andCa protease inhibitor II, and observed increased expression after BTH and/or whitefly treatment indicating that AG whitefly infestation elicited SA and jasmonic acid signalling in AG and BG. Since the expression pattern of PR genes in the roots differed, we assessed microbial diversity in plants treated with BTH and/or whitefly.4. In addition to eliciting BG defence responses, a whitefly infestation of the leaves augmented the population of root‐associated Gram‐positive bacteria and fungi, which may have positively affected plant growth and induced systemic resistance. Whitefly feeding reduced plant size, which usually occurs as a consequence of the high costs of direct resistance induction.5. Synthesis.Our results demonstrate that whitefly‐induced resistance against bacterial pathogens can cross the AG–BG border and may cause further indirect benefits on future plant development, because it can positively affect the association or plant roots with putatively beneficial microorganisms.

Trichoderma viride cellulase induces resistance to the antibiotic pore-forming peptide alamethicin associated with changes in the plasma membrane lipid composition of tobacco BY-2 cells

BackgroundAlamethicin is a membrane-active peptide isolated from the beneficial root-colonising fungus Trichoderma viride. This peptide can insert into membranes to form voltage-dependent pores. We have previously shown that alamethicin efficiently permeabilises the plasma membrane, mitochondria and plastids of cultured plant cells. In the present investigation, tobacco cells (Nicotiana tabacum L. cv Bright Yellow-2) were pre-treated with elicitors of defence responses to study whether this would affect permeabilisation.ResultsOxygen consumption experiments showed that added cellulase, already upon a limited cell wall digestion, induced a cellular resistance to alamethicin permeabilisation. This effect could not be elicited by xylanase or bacterial elicitors such as flg22 or elf18. The induction of alamethicin resistance was independent of novel protein synthesis. Also, the permeabilisation was unaffected by the membrane-depolarising agent FCCP. As judged by lipid analyses, isolated plasma membranes from cellulase-pretreated tobacco cells contained less negatively charged phospholipids (PS and PI), yet higher ratios of membrane lipid fatty acid to sterol and to protein, as compared to control membranes.ConclusionWe suggest that altered membrane lipid composition as induced by cellulase activity may render the cells resistant to alamethicin. This induced resistance could reflect a natural process where the plant cells alter their sensitivity to membrane pore-forming agents secreted by Trichoderma spp. to attack other microorganisms, and thus adding to the beneficial effect that Trichoderma has for plant root growth. Furthermore, our data extends previous reports on artificial membranes on the importance of lipid packing and charge for alamethicin permeabilisation to in vivo conditions.

Deciphering the structural and biological properties of the lipid A moiety of lipopolysaccharides from Burkholderia cepacia strain ASP B 2D, in Arabidopsis thaliana

Lipopolysaccharides (LPSs) are major, indispensable cell surface components of Gram-negative bacteria that have diverse roles in bacterial pathogenesis of plants. Environmental strains of Burkholderia cepacia have been described as phytopathogens, growth promotors, biocontrol agents and bioremediation agents. We have previously shown that LPSs from B. cepacia can be recognized as microbe-associated molecular pattern molecules, to elicit defense responses in plants. Recent findings suggest that the lipid A moiety might be partially responsible for LPSs perception. These studies were extended by analysis of the structure and biological activity of the lipid A moiety of LPSs of B. cepacia(.) The full structure was determined by a combination of negative/positive-ion matrix-assisted laser desorption ionization (MALDI) mass spectrometry (MS) on intact and partially degraded substrates. B. cepacia lipid A was found to contain a tetra- or penta-acylated, 1,4'-diphosphorylated, β-(1-6)-linked D-GlcN disaccharide and further substituted by L-Ara4N in position 4'. As primary fatty acids, R-configurated 16:0(3-OH) (amide-linked in 2 and 2') and 14:0(3-OH) (ester-linked in 3 and 3', nonstoichiometric) were identified. A secondary 14:0 was located at position 2'. Its biological activity to elicit defense-related responses was subsequently investigated by monitoring the changes in the transcriptome of Arabidopsis thaliana seedlings. Genes found to be upregulated code for proteins involved in signal perception and transduction, transcriptional regulation, defense and stress responses. Furthermore, genes encoding proteins involved in chaperoning, protein interactions and protein degradation were differentially expressed as part of the metabolic reprogramming of cellular activities in support of immunity and defense.

Induction of sesquiterpenes, phytoesterols and extracellular pathogenesis-related proteins in elicited cell cultures of Capsicum annuum

Capsicum annuum suspension cell cultures were used to evaluate the effect of cyclodextrins and methyl jasmonate as elicitors of defence responses. The induced defence responses included the accumulation of sesquiterpenes and phytosterols and the activation of pathogenesis-related proteins, leading to reinforcement and modification of the cell wall architecture during elicitation and protection cells against biotic stress. The results showed that the addition of both cyclodextrins and methyl jasmonate induced the biosynthesis of two sesquiterpenes, aromadendrene and solavetivone. This response was clearly synergistic since the increase in the levels of these compounds was much greater in the presence of both elicitors than when they were used separately. The biosynthesis of phytosterols was also induced in the combined treatment, as the result of an additive effect. Likewise, the exogenous application of methyl jasmonate induced the accumulation of pathogenesis-related proteins. The analysis of the extracellular proteome showed the presence of amino acid sequences homologous to PR1 and 4, NtPRp27-like proteins and class I chitinases, peroxidases and the hydrolytic enzymes LEXYL1 and 2, arabinosidases, pectinases, nectarin IV and leucin-rich repeat protein, which suggests that methyl jasmonate plays a role in mediating defence-related gene product expression in C. annuum. Apart from these methyl jamonate-induced proteins, other PR proteins were found in both the control and elicited cell cultures of C. annuum. These included class IV chitinases, beta-1,3-glucanases, thaumatin-like proteins and peroxidases, suggesting that their expression is mainly constitutive since they are involved in growth, development and defence processes.

Cerato-platanin elicits transcription of defence-related genes earlier than Ceratocystis platani on Platanus acerifolia

The ascomycete Ceratocystis platani (Walter) Engelbrecht & Harrington is the causal agent of canker stain, the most dangerous disease affecting the plane tree. Cerato-platanin (CP) is a protein elicitor from C. platani that acts as a pathogen-associated molecular pattern (PAMP), eliciting defence responses in both host and non-host plants. Using plane leaves as an experimental model, we compared the capacity of CP and C. platani to cause alterations in the transcription level of five Platanus acerifolia (Ait.) Willd. genes, three strictly related to a defence response, and two non-defence related. The results suggest that CP elicits the transcription of defence-related genes earlier than does C. platani. The expression of the cp gene in C. platani inoculated on plane leaves and grown in an axenic culture was also studied. For the first time CP has been shown to be released by C. platani when inoculated on P. acerifolia. The cp gene was expressed sooner when C. platani was inoculated on the leaves, than when it was grown in axenic culture, suggesting that the host-tree has a positive effect on the expression of this elicitor protein.

Inhibition effect on tobacco mosaic virus and regulation effect on calreticulin of oligochitosan in tobacco by induced Ca 2+ influx

Oligochitosan induce plant resistance against plant virus via different signal transduction pathways. Here, the Ca 2+ signaling pathway was investigated by which oligochitosan elicited defense responses to tobacco mosaic virus (TMV) in tobacco ( Nicotiana tabacum cv. Huangmiaoyu nn) plants. The results showed that the multiplication and movement of TMV in tobacco plants were inhibited by oligochitosan. Oligochitosan also increased cytosolic free calcium ions and regulated the expression of calreticulin. Furthermore, the inhibitory effect on TMV and the regulation effect on calreticulin were depressed when blocking the Ca 2+ signaling pathway by EGTA. These results indicated that oligochitosan induced tobacco resistance to TMV through Ca 2+ signaling pathway.

Hydroxyproline-rich glycoproteins accumulate in pearl millet after seed treatment with elicitors of defense responses against Sclerospora graminicola

The accumulation of hydroxyproline-rich glycoproteins (HRGPs) was investigated after induction of resistance in pearl millet against downy mildew caused by Sclerospora graminicola. Treatment of susceptible pearl millet seeds with various biotic and abiotic elicitors resulted in increased HRGP content in the cell walls of coleoptiles at 9 h after inoculation. Similar results with increased accumulation at 4–6 h after inoculation were obtained in suspension cells of pearl millet. Maximum HRGP accumulation was observed in seedlings raised from susceptible seeds treated with chitosan and Pseudomonas fluorescens. Western blot analysis with MAC 265 (a rat monoclonal antibody raised against pea HRGP) identified three proteins of 27, 17 and 14 kDa in resistant cultivars. The absence of the 14 kDa HRGP was observed in susceptible cultivars as reported earlier. The induced accumulation of the 14 kDa HRGP upon elicitor treatments was observed in the present study. Peroxidase and hydrogen peroxide, essential components for HRGP cross-linking, were also increased in samples treated with these elicitors. A tissue specific increase in HRGP at the regions around vascular bundles was observed upon chitosan treatment. The results presented will have a presumed importance in identifying the susceptible pearl millet varieties and improving those using elicitors of defense for field applications.

Data‐independent liquid chromatography/mass spectrometry (LC/MSE) detection and quantification of the secreted Apium graveolens pathogen defense protein mannitol dehydrogenase

Plant cells secrete a wide variety of defense-related proteins into the extracellular space or apoplast in response to pathogen attack. One of these, mannitol dehydrogenase (MTD), is normally a cytoplasmic enzyme whose primary role is the regulation of intracellular levels of the sugar alcohol mannitol in plants. Recent immunological and biochemical evidence, however, suggests that MTD is also secreted into the apoplast in response to pathogen attack, despite lacking a known peptide signal sequence for Golgi-mediated secretion. Because many plant pathogenic fungi secrete mannitol to overcome pathogen-induced generation of reactive oxygen species (ROS) by the plant, extracellular localization of MTD is hypothesized to have a defensive role of catabolizing pathogen-secreted mannitol. In the current study, LC/MS(E) was used to analyze proteins in the secretome of Apium graveolens (celery) following treatment with salicylic acid (SA), an endogenous elicitor of defense responses in plants. Levels of MTD in the secretome of SA-treated celery cell cultures were found to be induced at least 18-fold over secretome samples from cell cultures not exposed to SA. This value is in close agreement with published immunological and biochemical observations. Overall, this report provides the first mass spectrometry identification and quantification measurements supporting the hypothesis that MTD is secreted in response to simulated pathogen attack via a non-classical secretion mechanism. As demonstrated with MTD secretion, LC/MS(E) can be implemented as a discovery-driven MRM-based quantitative approach which can be used to reveal potential post-translational modifications, thus providing a new method in the area of gel-free and label-free proteomic analysis.

The rhizobacterial elicitor acetoin induces systemic resistance in Arabidopsis thaliana

Majority of plant growth promoting rhizobacteria (PGPR) confer plant immunity against a wide range of foliar diseases by activating plant defences that reduce a plant’s susceptibility to pathogen attack. Here we show that Arabidopsis thaliana (Col-0) plants exposed to Bacillus subtilis strain FB17 (hereafter FB17), results in reduced disease severity against Pseudomonas syringae pv. tomato DC3000 (hereafter DC3000) compared to plants without FB17 treatment. Exogenous application of the B. subtilis derived elicitor, acetoin (3-hydroxy-2-butanone), was found to trigger induced systemic resistance (ISR) and protect plants against DC3000 pathogenesis. Moreover, B. subtilis acetoin biosynthetic mutants that emitted reduced levels of acetoin conferred reduced protection to A. thaliana against pathogen infection. Further analysis using FB17 and defense-compromised mutants of A. thaliana indicated that resistance to DC3000 occurs via NPR1 and requires salicylic acid (SA)/ethylene (ET) whereas jasmonic acid (JA) is not essential. This study provides new insight into the role of rhizo-bacterial volatile components as elicitors of defense responses in plants.

Induction of plant defense responses by Ocimum gratissimum L. (Lamiaceae) leaf extracts

Aqueous extracts of the leaves of Ocimum gratissimum at 10, 25, 40 and 50% (w/v) concentrations induced the production of phytoalexins in soybean cotyledons and sorghum mesocotyls. The aqueous extracts also induced systemic resistance in cucumber to Colletotrichum lagenarium, reflected by reduction in disease incidence and an increase in chitinase production. Modes of action and the existence of possible elicitors of defense response in O. gratissimum leaf extracts are discussed.

The Cyst Nematode SPRYSEC Protein RBP-1 Elicits Gpa2- and RanGAP2-Dependent Plant Cell Death

Plant NB-LRR proteins confer robust protection against microbes and metazoan parasites by recognizing pathogen-derived avirulence (Avr) proteins that are delivered to the host cytoplasm. Microbial Avr proteins usually function as virulence factors in compatible interactions; however, little is known about the types of metazoan proteins recognized by NB-LRR proteins and their relationship with virulence. In this report, we demonstrate that the secreted protein RBP-1 from the potato cyst nematode Globodera pallida elicits defense responses, including cell death typical of a hypersensitive response (HR), through the NB-LRR protein Gpa2. Gp-Rbp-1 variants from G. pallida populations both virulent and avirulent to Gpa2 demonstrated a high degree of polymorphism, with positive selection detected at numerous sites. All Gp-RBP-1 protein variants from an avirulent population were recognized by Gpa2, whereas virulent populations possessed Gp-RBP-1 protein variants both recognized and non-recognized by Gpa2. Recognition of Gp-RBP-1 by Gpa2 correlated to a single amino acid polymorphism at position 187 in the Gp-RBP-1 SPRY domain. Gp-RBP-1 expressed from Potato virus X elicited Gpa2-mediated defenses that required Ran GTPase-activating protein 2 (RanGAP2), a protein known to interact with the Gpa2 N terminus. Tethering RanGAP2 and Gp-RBP-1 variants via fusion proteins resulted in an enhancement of Gpa2-mediated responses. However, activation of Gpa2 was still dependent on the recognition specificity conferred by amino acid 187 and the Gpa2 LRR domain. These results suggest a two-tiered process wherein RanGAP2 mediates an initial interaction with pathogen-delivered Gp-RBP-1 proteins but where the Gpa2 LRR determines which of these interactions will be productive.

Characterization of ordered aggregates of cerato-platanin and their involvement in fungus–host interactions

Background The “cerato-platanin family” consists of fungal-secreted proteins that are involved in various stages of the host–fungus interaction and act as phytotoxins, elicitors of defense responses and allergens. Cerato-platanin (CP) is a moderately hydrophobic protein secreted and localized in the cell wall of Ceratocystis platani, the causal agent of a severe disease of Platanus. These properties make CP like the hydrophobins: these are self-assembling proteins that form a surface coating which is involved in the formation of aerial hyphae and in adherence to surfaces. Methods CP aggregation was monitored by ThT, circular dichroism, and AFM. The eliciting activity of CP aggregates was assayed on leaves and cells. Results The CP self-assembles forming amyloid-like aggregates via a nucleated growth mechanism which is joined up with a cleavage of the N-terminus. The ovoidal shape and the lack of a clear transition toward an all-β structure distinguish these aggregates from typical amyloid fibrils. Moreover, CP aggregates interact with hydrophobic surfaces and enhance the hypersensitive response of Platanus. Conclusion and general significance CP forms “ordered aggregates” for which the soluble prefibrillar structures are the end point of the aggregation process, and do not evolve to insoluble fibrils. An involvement in host–microbe interaction is also suggested.

The zig‐zag‐zig in oomycete–plant interactions

In addition to a range of preformed barriers, plants defend themselves against microbial invasion by detecting conserved, secreted molecules, called pathogen-associated molecular patterns (PAMPs). PAMP-triggered immunity (PTI) is the first inducible layer of plant defence that microbial pathogens must navigate by the delivery of effector proteins that act to suppress or otherwise manipulate key components of resistance. Effectors may themselves be targeted by a further layer of defence, effector-triggered immunity (ETI), as their presence inside or outside host cells may be detected by resistance proteins. This 'zig-zag-zig' of tightly co-evolving molecular interactions determines the outcome of attempted infection. In this article, we consider the complex molecular interplay between plants and plant pathogenic oomycetes, drawing on recent literature to illustrate what is known about oomycete PAMPs and elicitors of defence responses, the effectors they utilize to suppress PTI, and the phenomenal molecular 'battle' between effector and resistance (R) genes that dictates the establishment or evasion of ETI.

The effect of cholic acid treatment on the oxidative status of soybean plants

The objective of this work was to study the effect of treatment of young soybean plants with cholic acid of different concentrations on their oxidative status. Young soybean plants, grown hydroponically for two weeks, were treated by adding cholic acid to the nutrient solution at the concentrations 20, 40, 60 and 80 mg/L, the control being without cholic acid. After one week, several parameters of the oxidative status were determined in the leaves and roots of the plants: contents of superoxide (-O?2 ), hydroxyl radicals (?OH) and glutathione (GSH), lipid peroxidation (LP), the superoxide dismutase (SOD) activity and the soluble protein accumulation, as well as the contents of chlorophylls and carotenoids. Treatments with cholic acid increased - O?2, LP, ?OH and GSH in the leaves of the treated plants, while only the OH content increased in the roots at higher cholic acid concentrations. The obtained results support the idea that cholic acid, as an elicitor of defense responses in plants, might act through the generation of an oxidative burst.

Host-derived signals activate plant innate immunity

Oligogalacturonides (OGs) are endogenous elicitors of defense responses released after partial degradation of pectin in the plant cell wall. Despite OGs cannot be considered true pathogen-associated molecular patterns, such as Flg22, they can be considered host-associated molecular patterns that are generated by the host cell during the infection process, and that stimulate the plant innate immune system. We have previously shown that, in Arabidopsis, OGs increase resistance to Botrytis cinerea independently of jasmonate, salicylic acid and ethylene. Recently, we demonstrated that, in Arabidopsis, OGs elicit a robust extracellular oxidative burst that is generated through the NADPH-oxidase AtrbohD. Moreover, we showed that this burst is dispensable either for early expression of OG-induced marker genes or for OG-induced resistance to B. cinerea. Similarly to Flg22, stimulation with OGs leads to the phosphorylation of mitogen activated protein kinase 3 and 6, suggesting that, even though different elicitors are perceived by distinct receptors, the signalling pathways mediated by these molecules converge very early and lead to the stimulation of the innate immune system.Addendum to: Galletti R, Denoux C, Gambetta S, Dewdney J, Ausubel FM, Lorenzo GD, Ferrari S. The AtrbohD-mediated oxidative burst elicited by oligogalacturonides in Arabidopsis thaliana is dispensable for the activation of defense responses effective against Botrytis cinerea. Plant Physiol 2008; In press; PMID: 18790995; DOI: 10.1104/pp.108.127845.

The AtrbohD-mediated oxidative burst elicited by oligogalacturonides in Arabidopsis is dispensable for the activation of defense responses effective against Botrytis cinerea.

Oligogalacturonides (OGs) are endogenous elicitors of defense responses released after partial degradation of pectin in the plant cell wall. We have previously shown that, in Arabidopsis (Arabidopsis thaliana), OGs induce the expression of PHYTOALEXIN DEFICIENT3 (PAD3) and increase resistance to the necrotrophic fungal pathogen Botrytis cinerea independently of signaling pathways mediated by jasmonate, salicylic acid, and ethylene. Here, we illustrate that the rapid induction of the expression of a variety of genes by OGs is also independent of salicylic acid, ethylene, and jasmonate. OGs elicit a robust extracellular oxidative burst that is generated by the NADPH oxidase AtrbohD. This burst is not required for the expression of OG-responsive genes or for OG-induced resistance to B. cinerea, whereas callose accumulation requires a functional AtrbohD. OG-induced resistance to B. cinerea is also unaffected in powdery mildew resistant4, despite the fact that callose accumulation was almost abolished in this mutant. These results indicate that the OG-induced oxidative burst is not required for the activation of defense responses effective against B. cinerea, leaving open the question of the role of reactive oxygen species in elicitor-mediated defense.