Effects of jasmonic acid, ethylene, and salicylic acid signaling on the rhizosphere bacterial community of Arabidopsis thaliana

Systemically induced resistance is a promising strategy to control plant diseases, as it affects numerous pathogens. However, since induced resistance reduces one or both growth and activity of plant pathogens, the indigenous microflora may also be affected by an enhanced defensive state of the plant. The aim of this study was to elucidate how much the bacterial rhizosphere microflora of Arabidopsis is affected by induced systemic resistance (ISR) or systemic acquired resistance (SAR). Therefore, the bacterial microflora of wild-type plants and plants affected in their defense signaling was compared. Additionally, ISR was induced by application of methyl jasmonate and SAR by treatment with salicylic acid or benzothiadiazole. As a comparative model, we also used wild type and ethylene-insensitive tobacco. Some of the Arabidopsis genotypes affected in defense signaling showed altered numbers of culturable bacteria in their rhizospheres; however, effects were dependent on soil type. Effects of plant genotype on rhizosphere bacterial community structure could not be related to plant defense because chemical activation of ISR or SAR had no significant effects on density and structure of the rhizosphere bacterial community. These findings support the notion that control of plant diseases by elicitation of systemic resistance will not significantly affect the resident soil bacterial microflora.

Differential effectiveness of microbially induced resistance against herbivorous insects in Arabidopsis

Rhizobacteria-induced systemic resistance (ISR) and pathogen-induced systemic acquired resistance (SAR) have a broad, yet partly distinct, range of effectiveness against pathogenic microorganisms. Here, we investigated the effectiveness of ISR and SAR in Arabidopsis against the tissue-chewing insects Pieris rapae and Spodoptera exigua. Resistance against insects consists of direct defense, such as the production of toxins and feeding deterrents and indirect defense such as the production of plant volatiles that attract carnivorous enemies of the herbivores. Wind-tunnel experiments revealed that ISR and SAR did not affect herbivore-induced attraction of the parasitic wasp Cotesia rubecula (indirect defense). By contrast, ISR and SAR significantly reduced growth and development of the generalist herbivore S. exigua, although not that of the specialist P. rapae. This enhanced direct defense against S. exigua was associated with potentiated expression of the defense-related genes PDF1.2 and HEL. Expression profiling using a dedicated cDNA microarray revealed four additional, differentially primed genes in microbially induced S. exigua-challenged plants, three of which encode a lipid-transfer protein. Together, these results indicate that microbially induced plants are differentially primed for enhanced insect-responsive gene expression that is associated with increased direct defense against the generalist S. exigua but not against the specialist P. rapae.

Kinetics of salicylate-mediated suppression of jasmonate signaling reveal a role for redox modulation

Cross talk between salicylic acid (SA) and jasmonic acid (JA) signaling pathways plays an important role in the regulation and fine tuning of induced defenses that are activated upon pathogen or insect attack. Pharmacological experiments revealed that transcription of JA-responsive marker genes, such as PDF1.2 and VSP2, is highly sensitive to suppression by SA. This antagonistic effect of SA on JA signaling was also observed when the JA pathway was biologically activated by necrotrophic pathogens or insect herbivores, and when the SA pathway was triggered by a biotrophic pathogen. Furthermore, all 18 Arabidopsis (Arabidopsis thaliana) accessions tested displayed SA-mediated suppression of JA-responsive gene expression, highlighting the potential significance of this phenomenon in induced plant defenses in nature. During plant-attacker interactions, the kinetics of SA and JA signaling are highly dynamic. Mimicking this dynamic response by applying SA and methyl jasmonate (MeJA) at different concentrations and time intervals revealed that PDF1.2 transcription is readily suppressed when the SA response was activated at or after the onset of the JA response, and that this SA-JA antagonism is long lasting. However, when SA was applied more than 30 h prior to the onset of the JA response, the suppressive effect of SA was completely absent. The window of opportunity of SA to suppress MeJA-induced PDF1.2 transcription coincided with a transient increase in glutathione levels. The glutathione biosynthesis inhibitor l-buthionine-sulfoximine strongly reduced PDF1.2 suppression by SA, suggesting that SA-mediated redox modulation plays an important role in the SA-mediated attenuation of the JA signaling pathway.

Four weeks' corticosteroid inhalation does not augment maximal power output in endurance athletes

OBJECTIVE

To assess possible ergogenic properties of corticosteroid administration.

DESIGN

A balanced, double-blind, placebo-controlled design was used.

PARTICIPANTS

28 well-trained cyclists and rowers.

INTERVENTION

4 weeks' daily inhalation of 800 microg budesonide or placebo.

MAIN OUTCOME MEASUREMENTS

The subjects performed three incremental cycle ergometer tests until exhaustion, before and after 2 and 4 weeks of placebo or budesonide administration, to measure maximal power output (W(max)). Once a week they filled in a profile of mood state (POMS) questionnaire.

RESULTS

There was no significant difference in W(max) between the placebo (376 (SD 25) W) and the corticosteroid group (375 (36) W) during the preintervention test, and there were no significant changes in either group after 2 and 4 weeks of intervention. No effect of the intervention on mood state was found.

CONCLUSION

4 weeks of corticosteroid or placebo inhalation in healthy, well-trained athletes did not affect maximal power output or mood state. Hence no ergogenic properties of 4 weeks' corticosteroid administration could be demonstrated, which corroborates previous studies of short-term corticosteroid administration.

MYB72 is required in early signaling steps of rhizobacteria-induced systemic resistance in Arabidopsis

Colonization of Arabidopsis thaliana roots by nonpathogenic Pseudomonas fluorescens WCS417r bacteria triggers a jasmonate/ethylene-dependent induced systemic resistance (ISR) that is effective against a broad range of pathogens. Microarray analysis revealed that the R2R3-MYB-like transcription factor gene MYB72 is specifically activated in the roots upon colonization by WCS417r. Here, we show that T-DNA knockout mutants myb72-1 and myb72-2 are incapable of mounting ISR against the pathogens Pseudomonas syringae pv tomato, Hyaloperonospora parasitica, Alternaria brassicicola, and Botrytis cinerea, indicating that MYB72 is essential to establish broad-spectrum ISR. Overexpression of MYB72 did not result in enhanced resistance against any of the pathogens tested, demonstrating that MYB72 is not sufficient for the expression of ISR. Yeast two-hybrid analysis revealed that MYB72 physically interacts in vitro with the ETHYLENE INSENSITIVE3 (EIN3)-LIKE3 transcription factor EIL3, linking MYB72 function to the ethylene response pathway. However, WCS417r activated MYB72 in ISR-deficient, ethylene-insensitive ein2-1 plants. Moreover, exogenous application of the ethylene precursor 1-aminocyclopropane-1-carboxylate induced wild-type levels of resistance in myb72-1, suggesting that MYB72 acts upstream of ethylene in the ISR pathway. Collectively, this study identified the transcriptional regulator MYB72 as a novel ISR signaling component that is required in the roots during early signaling steps of rhizobacteria-mediated ISR.

Transcription factor MYC2 is involved in priming for enhanced defense during rhizobacteria-induced systemic resistance in Arabidopsis thaliana

Upon appropriate stimulation, plants can develop an enhanced capacity to express infection-induced cellular defense responses, a phenomenon known as the primed state. Colonization of the roots of Arabidopsis thaliana by the beneficial rhizobacterial strain Pseudomonas fluorescens WCS417r primes the leaf tissue for enhanced pathogen- and insect-induced expression of jasmonate (JA)-responsive genes, resulting in an induced systemic resistance (ISR) that is effective against different types of pathogens and insect herbivores. Here the molecular mechanism of this rhizobacteria-induced priming response was investigated using a whole-genome transcript profiling approach. Out of the 1879 putative methyl jasmonate (MeJA)-responsive genes, 442 genes displayed a primed expression pattern in ISR-expressing plants. Promoter analysis of ISR-primed, MeJA-responsive genes and ISR-primed, Pseudomonas syringae pv. tomato DC3000 (Pst DC3000)-responsive genes revealed over-representation of the G-box-like motif 5'-CACATG-3'. This motif is a binding site for the transcription factor MYC2, which plays a central role in JA- and abscisic acid-regulated signaling. MYC2 expression was consistently up-regulated in ISR-expressing plants. Moreover, mutants impaired in the JASMONATE-INSENSITIVE1/MYC2 gene (jin1-1 and jin1-2) were unable to mount WCS417r-ISR against Pst DC3000 and the downy mildew pathogen Hyaloperonospora parasitica. Together, these results pinpoint MYC2 as a potential regulator in priming for enhanced JA-responsive gene expression during rhizobacteria-mediated ISR.

Herbivore-induced resistance against microbial pathogens in Arabidopsis

Caterpillars of the herbivore Pieris rapae stimulate the production of jasmonic acid (JA) and ethylene (ET) in Arabidopsis (Arabidopsis thaliana) and trigger a defense response that affects insect performance on systemic tissues. To investigate the spectrum of effectiveness of P. rapae-induced resistance, we examined the level of resistance against different pathogens. Although the necrotrophic fungus Alternaria brassicicola is sensitive to JA-dependent defenses, herbivore-induced resistance was not effective against this pathogen. By contrast, caterpillar feeding significantly reduced disease caused by the bacterial pathogens Pseudomonas syringae pv tomato and Xanthomonas campestris pv armoraciae. However, this effect was apparent only locally in caterpillar-damaged tissue. Arabidopsis mutants jar1, coi1, ein2, sid2, eds5, and npr1 showed wild-type levels of P. rapae-induced protection against P. syringae pv tomato, suggesting that this local, herbivore-induced defense response does not depend exclusively on either JA, ET, or salicylic acid (SA). Resistance against the biotroph Turnip crinkle virus (TCV) requires SA, but not JA and ET. Nevertheless, herbivore feeding strongly affected TCV multiplication and TCV lesion formation, also in systemic tissues. Wounding alone was not effective, but application of P. rapae regurgitate onto the wounds induced a similar level of protection. Analysis of SA-induced PATHOGENESIS RELATED-1 (PR-1) expression revealed that P. rapae grazing primed Arabidopsis leaves for augmented expression of SA-dependent defenses. Pharmacological experiments showed that ET acts synergistically on SA-induced PR-1, suggesting that the increased production of ET upon herbivore feeding sensitizes the tissue to respond faster to SA, thereby contributing to an enhanced defensive capacity toward pathogens, such as TCV, that trigger SA-dependent defenses upon infection.

Signal signature and transcriptome changes of Arabidopsis during pathogen and insect attack

Plant defenses against pathogens and insects are regulated differentially by cross-communicating signaling pathways in which salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) play key roles. To understand how plants integrate pathogen- and insect-induced signals into specific defense responses, we monitored the dynamics of SA, JA, and ET signaling in Arabidopsis after attack by a set of microbial pathogens and herbivorous insects with different modes of attack. Arabidopsis plants were exposed to a pathogenic leaf bacterium (Pseudomonas syringae pv. tomato), a pathogenic leaf fungus (Alternaria brassicicola), tissue-chewing caterpillars (Pieris rapae), cell-content-feeding thrips (Frankliniella occidentalis), or phloem-feeding aphids (Myzus persicae). Monitoring the signal signature in each plant-attacker combination showed that the kinetics of SA, JA, and ET production varies greatly in both quantity and timing. Analysis of global gene expression profiles demonstrated that the signal signature characteristic of each Arabidopsis-attacker combination is orchestrated into a surprisingly complex set of transcriptional alterations in which, in all cases, stress-related genes are overrepresented. Comparison of the transcript profiles revealed that consistent changes induced by pathogens and insects with very different modes of attack can show considerable overlap. Of all consistent changes induced by A. brassicicola, Pieris rapae, and E occidentalis, more than 50% also were induced consistently by P. syringae. Notably, although these four attackers all stimulated JA biosynthesis, the majority of the changes in JA-responsive gene expression were attacker specific. All together, our study shows that SA, JA, and ET play a primary role in the orchestration of the plant's defense response, but other regulatory mechanisms, such as pathway cross-talk or additional attacker-induced signals, eventually shape the highly complex attacker-specific defense response.

NPR1: the spider in the web of induced resistance signaling pathways

The plant hormones salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are major players in the regulation of signaling networks that are involved in induced defense responses against pathogens and insects. During the past two years, significant progress has been made in understanding the function of NON-EXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1), a key regulator of systemic acquired resistance (SAR), that is essential for transducing the SA signal to activate PATHOGENESIS-RELATED (PR) gene expression. SA-mediated redox changes in Arabidopsis cells regulate both the functioning of NPR1 and its binding to TGA1, a member of the TGA family of transcription factors that activate SA-responsive elements in the promoters of PR genes upon binding with NPR1. Apart from its role in regulating SAR in the nucleus, a novel cytosolic function of NPR1 in cross-communication between SA- and JA-dependent defense signaling pathways has been identified. Other advances in induced resistance signaling, such as the implication that ET is involved in the generation of systemic signal molecules, the suggestion of the involvement of lipid-derived molecules in long-distance signaling, and the identification of new components of various systemic defense signaling pathways, shed new light on how plants actively defend themselves against harmful organisms.

Leaf position prevails over plant age and leaf age in reflecting resistance to late blight in potato

ABSTRACT The effects of plant age, leaf age, and leaf position on race-nonspecific resistance against Phytophthora infestans were investigated in a series of field and controlled environment experiments with five different potato (Solanum tuberosum) cultivars. Leaf position proved to be the most significant factor; apical leaves were far more resistant to late blight than basal leaves. Plant age and leaf age had only minor effects; therefore, the resistance of a specific leaf remained about the same during its entire lifetime. The gradual increase in late blight resistance from basal leaves to apical leaves appeared to be a general effect, irrespective of cultivar, growing conditions, or resistance test. Therefore, it is important to consider leaf position in tests for late blight resistance, because contrasts in resistance may be ascribed erroneously to differences between genotypes or treatments, whereas they are actually caused by differences in leaf position.

NPR1 modulates cross-talk between salicylate- and jasmonate-dependent defense pathways through a novel function in the cytosol

Plant defenses against pathogens and insects are regulated differentially by cross-communicating signal transduction pathways in which salicylic acid (SA) and jasmonic acid (JA) play key roles. In this study, we investigated the molecular mechanism of the antagonistic effect of SA on JA signaling. Arabidopsis plants unable to accumulate SA produced 25-fold higher levels of JA and showed enhanced expression of the JA-responsive genes LOX2, PDF1.2, and VSP in response to infection by Pseudomonas syringae pv tomato DC3000, indicating that in wild-type plants, pathogen-induced SA accumulation is associated with the suppression of JA signaling. Analysis of the Arabidopsis mutant npr1, which is impaired in SA signal transduction, revealed that the antagonistic effect of SA on JA signaling requires the regulatory protein NPR1. Nuclear localization of NPR1, which is essential for SA-mediated defense gene expression, is not required for the suppression of JA signaling, indicating that cross-talk between SA and JA is modulated through a novel function of NPR1 in the cytosol.

Characterization of Arabidopsis enhanced disease susceptibility mutants that are affected in systemically induced resistance

In Arabidopsis, the rhizobacterial strain Pseudomonas fluorescens WCS417r triggers jasmonate (JA)- and ethylene (ET)-dependent induced systemic resistance (ISR) that is effective against different pathogens. Arabidopsis genotypes unable to express rhizobacteria-mediated ISR against the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) exhibit enhanced disease susceptibility towards this pathogen. To identify novel components controlling induced resistance, we tested 11 Arabidopsis mutants with enhanced disease susceptibility (eds) to pathogenic P. syringae bacteria for WCS417r-mediated ISR and pathogen-induced systemic acquired resistance (SAR). Mutants eds4-1, eds8-1 and eds10-1 failed to develop WCS417r-mediated ISR, while mutants eds5-1 and eds12-1 failed to express pathogen-induced SAR. Whereas eds5-1 is known to be blocked in salicylic acid (SA) biosynthesis, analysis of eds12-1 revealed that its impaired SAR response is caused by reduced sensitivity to this molecule. Analysis of the ISR-impaired eds mutants revealed that they are non-responsive to induction of resistance by methyl jasmonate (MeJA) (eds4-1, eds8-1 and eds10-1), or the ET precursor 1-aminocyclopropane-1-carboxylate (ACC) (eds4-1 and eds10-1). Moreover, eds4-1 and eds8-1 showed reduced expression of the plant defensin gene PDF1.2 after MeJA and ACC treatment, which was associated with reduced sensitivity to either ET (eds4-1) or MeJA (eds8-1). Although blocked in WCS417r-, MeJA- and ACC-induced ISR, eds10-1 behaved normally for several other responses to MeJA or ACC. The results indicate that EDS12 is required for SAR and acts downstream of SA, whereas EDS4, EDS8 and EDS10 are required for ISR acting either in JA signalling (EDS8), ET signalling (EDS4), or downstream JA and ET signalling (EDS10) in the ISR pathway.

Differential effectiveness of salicylate-dependent and jasmonate/ethylene-dependent induced resistance in Arabidopsis

Salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) are each involved in the regulation of basal resistance against different pathogens. These three signals play important roles in induced resistance as well. SA is a key regulator of pathogen-induced systemic acquired resistance (SAR), whereas JA and ET are required for rhizobacteria-mediated induced systemic resistance (ISR). Both types of induced resistance are effective against a broad spectrum of pathogens. In this study, we compared the spectrum of effectiveness of SAR and ISR using an oomycete, a fungal, a bacterial, and a viral pathogen. In noninduced Arabidopsis plants, these pathogens are primarily resisted through either SA-dependent basal resistance (Peronospora parasitica and Turnip crinkle virus [TCV]), JA/ET-dependent basal resistance responses (Alternaria brassicicola), or a combination of SA-, JA-, and ET-dependent defenses (Xanthomonas campestris pv. armoraciae). Activation of ISR resulted in a significant level of protection against A. brassicicola, whereas SAR was ineffective against this pathogen. Conversely, activation of SAR resulted in a high level of protection against P. parasitica and TCV, whereas ISR conferred only weak and no protection against P. parasitica and TCV, respectively. Induction of SAR and ISR was equally effective against X. campestris pv. armoraciae. These results indicate that SAR is effective against pathogens that in noninduced plants are resisted through SA-dependent defenses, whereas ISR is effective against pathogens that in noninduced plants are resisted through JA/ET-dependent defenses. This suggests that SAR and ISR constitute a reinforcement of extant SA- or JA/ET-dependent basal defense responses, respectively.

Identification of a locus in arabidopsis controlling both the expression of rhizobacteria-mediated induced systemic resistance (ISR) and basal resistance against Pseudomonas syringae pv. tomato

Selected nonpathogenic rhizobacteria with biological disease control activity are able to elicit an induced systemic resistance (ISR) response that is phenotypically similar to pathogen-induced systemic acquired resistance (SAR). Ten ecotypes of Arabidopsis thaliana were screened for their potential to express rhizobacteria-mediated ISR and pathogen-induced SAR against the leaf pathogen Pseudomonas syringae pv. tomato DC3000 (Pst). All ecotypes expressed SAR. However, of the 10 ecotypes tested, ecotypes RLD and Wassilewskija (Ws) did not develop ISR after treatment of the roots with nonpathogenic Pseudomonas fluorescens WCS417r bacteria. This nonresponsive phenotype was associated with relatively high susceptibility to Pst infection. The F1 progeny of crosses between the non-responsive ecotypes RLD and Ws on the one hand, and the responsive ecotypes Columbia (Col) and Landsberg erecta (Ler) on the other hand, were fully capable of expressing ISR and exhibited a relatively high level of basal resistance, similar to that of their WCS417r-responsive parent. This indicates that the potential to express ISR and the relatively high level of basal resistance against Pst are both inherited as dominant traits. Analysis of the F2 and F3 progeny of a Col x RLD cross revealed that inducibility of ISR and relatively high basal resistance against Pst cosegregate in a 3:1 fashion, suggesting that both resistance mechanisms are monogenically determined and genetically linked. Neither the responsiveness to WCS417r nor the relatively high level of basal resistance against Pst were complemented in the F1 progeny of crosses between RLD and Ws, indicating that RLD and Ws are both affected in the same locus, necessary for the expression of ISR and basal resistance against Pst. The corresponding locus, designated ISR1, was mapped between markers B4 and GL1 on chromosome 3. The observed association between ISR and basal resistance against Pst suggests that rhizobacteria-mediated ISR against Pst in Arabidopsis requires the presence of a single dominant gene that functions in the basal resistance response against Pst infection.

Systemic resistance in Arabidopsis induced by rhizobacteria requires ethylene-dependent signaling at the site of application

Root colonization of Arabidopsis thaliana by the nonpathogenic, rhizosphere-colonizing, biocontrol bacterium Pseudomonas fluorescens WCS417r has been shown to elicit induced systemic resistance (ISR) against Pseudomonas syringae pv. tomato (Pst). The ISR response differs from the pathogen-inducible systemic acquired resistance (SAR) response in that ISR is independent of salicylic acid and not associated with pathogenesis-related proteins. Several ethylene-response mutants were tested and showed essentially normal symptoms of Pst infection. ISR was abolished in the ethylene-insensitive mutant etr1-1, whereas SAR was unaffected. Similar results were obtained with the ethylene-insensitive mutants ein2 through ein7, indicating that the expression of ISR requires the complete signal-transduction pathway of ethylene known so far. The induction of ISR by WCS417r was not accompanied by increased ethylene production in roots or leaves, nor by increases in the expression of the genes encoding the ethylene biosynthetic enzymes 1-aminocyclopropane-1-carboxylic (ACC) synthase and ACC oxidase. The eir1 mutant, displaying ethylene insensitivity in the roots only, did not express ISR upon application of WCS417r to the roots, but did exhibit ISR when the inducing bacteria were infiltrated into the leaves. These results demonstrate that, for the induction of ISR, ethylene responsiveness is required at the site of application of inducing rhizobacteria.

Enhanced expression and activation of the alternative oxidase during infection of Arabidopsis with Pseudomonas syringae pv tomato

Cyanide-resistant ("alternative") respiration was studied in Arabidopsis during incompatible and compatible infection with Pseudomonas syringae pv tomato DC3000. Total leaf respiration increased as the leaves became necrotic, as did the cyanide-resistant component that was sensitive to salicylhydroxamic acid. Infiltration of leaves with an avirulent strain rapidly induced alternative oxidase (AOX) mRNA, whereas the increase was delayed in the compatible combination. The increase in mRNA correlated with the increase in AOX protein. Increased expression was confined to the infected leaves, in contrast to the pathogenesis-related protein-1, which was induced systemically. Virtually all of the AOX protein was in the reduced (high-activity) form. Using transgenic NahG and mutant npr1-1 and etr1-1 plants, we established that the rapid induction of the AOX was associated with necrosis and that ethylene, but not salicylic acid, was required for its induction. Increased pyruvate levels in the infected leaves suggested that increased substrate levels were respired through the alternative pathway; however, in the control leaves and the infected leaves, respiration was not inhibited by salicylhydroxamic acid alone. Increased respiration appeared to be associated primarily with symptom expression rather than resistance reactions.

Differential induction of systemic resistance in Arabidopsis by biocontrol bacteria

Selected nonpathogenic, root-colonizing bacteria are able to elicit induced systemic resistance (ISR) in plants. To elucidate the molecular mechanisms underlying this type of systemic resistance, an Arabidopsis-based model system was developed in which Pseudomonas syringae pv. tomato and Fusarium oxysporum f. sp. raphani were used as challenging pathogens. In Arabidopsis thaliana ecotypes Columbia and Landsberg erecta, colonization of the rhizosphere by P. fluorescens strain WCS417r induced systemic resistance against both pathogens. In contrast, ecotype RLD did not respond to WCS417r treatment, whereas all three ecotypes expressed systemic acquired resistance upon treatment with salicylic acid (SA). P. fluorescens strain WCS374r, previously shown to induce ISR in radish, did not elicit ISR in Arabidopsis. The opposite was found for P. putida strain WCS358r, which induced ISR in Arabidopsis but not in radish. These results demonstrate that rhizosphere pseudomonads are differentially active in eliciting ISR in related plant species. The outer membrane lipopolysaccharide (LPS) of WCS417r is the main ISR-inducing determinant in radish and carnation, and LPS-containing cell walls also elicit ISR in Arabidopsis. However, mutant WCS417rOA-, lacking the O-antigenic side chain of the LPS, induced levels of protection similar to those induced by wild-type WCS417r. This indicates that ISR-inducing bacteria produce more than a single factor that trigger ISR in Arabidopsis. Furthermore, WCS417r and WCS358r induced protection in both wild-type Arabidopsis and SA-nonaccumulating NahG plants without activating pathogenesis-related gene expression. This suggests that elicitation of an SA-independent signaling pathway is a characteristic feature of ISR-inducing biocontrol bacteria.

Degradation of tobacco pathogenesis-related proteins : evidence for conserved mechanisms of degradation of pathogenesis-related proteins in plants

Tobacco (Nicotiana tabacum L.) leaves were found to contain an extracellular proteinase that endoproteolytically cleaves tobacco pathogenesis-related (PR) proteins. This proteinase was partially purified from tobacco leaves and characterized as an aspartyl proteinase with a pH optimum around pH 3 and a molecular mass of 36,000 to 40,000 daltons. In vitro, the enzyme cleaved purified tobacco and tomato PR proteins into discrete fragments. The characteristics of this proteinase were similar to pepsin and identical to those displayed by a previously described tomato 37-kilodalton aspartyl proteinase active against tomato PR proteins (I Rodrigo, P Vera, V Conejero [1989] Eur J Biochem 184: 663-669), suggesting that these extracellular proteases could play a role in a conserved mechanism for PR protein turnover in plants.

Molecular characterization of messenger RNAs for 'pathogenesis related' proteins la, lb and lc, induced by TMV infection of tobacco

A cDNA library was made to poly(A)-containing RNA from tobacco mosaic virus (TMV)-infected Samsun NN tobacco plants and clones corresponding to mRNAs for the 'pathogenesis-related' (PR) proteins la, lb and lc were identified.One clone was found to contain a complete copy of PR-lb mRNA. The structural organization of this RNA is: a leader sequence of 29 nucleotides, an open reading frame of 504 nucleotides encoding a 30 amino acid long signal peptide and a 138 amino acid long mature protein, and a 3'-non-coding region of 235 nucleotides. Two other clones were found to contain partial copies of PR-la and PR-lc mRNAs. The data indicate an -90% homology between the amino acid sequences of PR-la, -lb and -lc. Using one of the clones as probe it was shown that in the TMV-inoculated lower leaves and the non-inoculated upper leaves of a tobacco plant, the PR-1 mRNAs become detectable from 2 and 8 days after inoculation,respectively.