Acquired Resistance in Barley (The Resistance Mechanism Induced by 2,6-Dichloroisonicotinic Acid Is a Phenocopy of a Genetically Based Mechanism Governing Race-Specific Powdery Mildew Resistance)

Sustainable and efficient control of sunflower downy mildew by means of genetic resistance: a review

The breeding of sunflower (Helianthus annuus L.) for resistance to downy mildew (caused by the oomycete Plasmopara halstedii Farl. Berl. & de Toni) is reviewed in this work under the scope of its sustainability and efficiency. When sunflower turned into an oilseed crop, resistance to the disease was included in its initial breeding strategies. Subsequent development of genomic tools allowed a significant expansion of the knowledge on the diversity of its genetic resistance and its application to the genetic control of the disease. Simultaneously to genetic improvements, and as a consequence of the close interaction between the pathogen and its host plant, an enormous variety of pathotypes has been described in all the sunflower-growing areas worldwide. Thus, the genetic control of sunflower downy mildew is an active research field subjected to continuous evolution and challenge. In practice, genetic resistance constitutes the base tier of Integrated Pest Management against sunflower downy mildew. The second tier is composed of elements related to crop management: rotation, removal of volunteer plants, sowing date, tillage. Biological control alternatives and resistance inducers could also provide additional restraint. Finally, the top tier includes chemical treatments that should only be used when necessary and if the more basal Integrated Pest Management elements fail to keep pathogen populations under the economic threshold.

The rise, fall and resurrection of chemical-induced resistance agents

Since the discovery that the plant immune system could be augmented for improved deployment against biotic stressors through the exogenous application of chemicals that lead to induced resistance (IR), many such IR-eliciting agents have been identified. Initially it was hoped that these chemical IR agents would be a benign alternative to traditional chemical biocides. However, owing to low efficacy and/or a realization that their benefits sometimes come at the cost of growth and yield penalties, chemical IR agents fell out of favour and were seldom used as crop protection products. Despite the lack of interest in agricultural use, researchers have continued to explore the efficacy and mechanisms of chemical IR. Moreover, as we move away from the approach of 'zero tolerance' toward plant pests and pathogens toward integrated pest management, chemical IR agents could have a place in the plant protection product list. In this review, we chart the rise and fall of chemical IR agents, and then explore a variety of strategies used to improve their efficacy and remediate their negative adverse effects. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Bioprospecting of Rhizosphere-Resident Fungi: Their Role and Importance in Sustainable Agriculture

Rhizosphere-resident fungi that are helpful to plants are generally termed as 'plant growth promoting fungi' (PGPF). These fungi are one of the chief sources of the biotic inducers known to give their host plants numerous advantages, and they play a vital role in sustainable agriculture. Today's biggest challenge is to satisfy the rising demand for crop protection and crop yield without harming the natural ecosystem. Nowadays, PGPF has become an eco-friendly way to improve crop yield by enhancing seed germination, shoot and root growth, chlorophyll production, and fruit yield, etc., either directly or indirectly. The mode of action of these PGPF includes the solubilization and mineralization of the essential micro- and macronutrients needed by plants to regulate the balance for various plant processes. PGPF produce defense-related enzymes, defensive/volatile compounds, and phytohormones that control pathogenic microbes' growth, thereby assisting the plants in facing various biotic and abiotic stresses. Therefore, this review presents a holistic view of PGPF as efficient natural biofertilizers to improve crop plants' growth and resistance.

Application of a robust microplate assay to determine induced β-1,3-glucanase and chitinase activity in the cotton plant

Resistance is induced in cotton plants as the result of either viral infection or exogenous application of elicitors. Induced resistance can be evaluated by determining the production of β-1,3-glucanase and chitinase in plants as a biochemical parameter. The assays being used for the determination of chitinase and β-1,3-glucanase activity are laborious and not cost-effective, as the reducing sugars produced by the substrates colloidal chitin and laminarin are very expensive. The concentration of both substrates was standardized and reduced to 0.25% from 4% in a modified microplate assay, which appeared to be more effective. The amount of β-1,3-glucanase and chitinase produced was significant and determined by the new modified assay. The sensitivity of the microplate assay was significantly raised approximately one- to twofold.

Inducible biosynthesis and immune function of the systemic acquired resistance inducer N-hydroxypipecolic acid in monocotyledonous and dicotyledonous plants

Recent work has provided evidence for the occurrence of N-hydroxypipecolic acid (NHP) in Arabidopsis thaliana, characterized its pathogen-inducible biosynthesis by a three-step metabolic sequence from l-lysine, and established a central role for NHP in the regulation of systemic acquired resistance. Here, we show that NHP is biosynthesized in several other plant species in response to microbial attack, generally together with its direct metabolic precursor pipecolic acid and the phenolic immune signal salicylic acid. For example, NHP accumulates locally in inoculated leaves and systemically in distant leaves of cucumber in response to Pseudomonas syringae attack, in Pseudomonas-challenged tobacco and soybean leaves, in tomato inoculated with the oomycete Phytophthora infestans, in leaves of the monocot Brachypodium distachyon infected with bacterial (Xanthomonas translucens) and fungal (Magnaporthe oryzae) pathogens, and in M. oryzae-inoculated barley. Notably, resistance assays indicate that NHP acts as a potent inducer of acquired resistance to bacterial and fungal infection in distinct monocotyledonous and dicotyledonous species. Pronounced systemic accumulation of NHP in leaf phloem sap of locally inoculated cucumber supports a function for NHP as a phloem-mobile immune signal. Our study thus generalizes the existence and function of an NHP resistance pathway in plant systemic acquired resistance.

Pipecolic Acid Is Induced in Barley upon Infection and Triggers Immune Responses Associated with Elevated Nitric Oxide Accumulation

Pipecolic acid (Pip) is an essential component of systemic acquired resistance, priming resistance in Arabidopsis thaliana against (hemi)biotrophic pathogens. Here, we studied the potential role of Pip in bacteria-induced systemic immunity in barley. Exudates of barley leaves infected with the systemic immunity-inducing pathogen Pseudomonas syringae pv. japonica induced immune responses in A. thaliana. The same leaf exudates contained elevated Pip levels compared with those of mock-treated barley leaves. Exogenous application of Pip induced resistance in barley against the hemibiotrophic bacterial pathogen Xanthomonas translucens pv. cerealis. Furthermore, both a systemic immunity-inducing infection and exogenous application of Pip enhanced the resistance of barley against the biotrophic powdery mildew pathogen Blumeria graminis f. sp. hordei. In contrast to a systemic immunity-inducing infection, Pip application did not influence lesion formation by a systemically applied inoculum of the necrotrophic fungus Pyrenophora teres. Nitric oxide (NO) levels in barley leaves increased after Pip application. Furthermore, X. translucens pv. cerealis induced the accumulation of superoxide anion radicals and this response was stronger in Pip-pretreated compared with mock-pretreated plants. Thus, the data suggest that Pip induces barley innate immune responses by triggering NO and priming reactive oxygen species accumulation.

Silencing of RBOHF2 Causes Leaf Age-Dependent Accelerated Senescence, Salicylic Acid Accumulation, and Powdery Mildew Resistance in Barley

Plant RBOH (RESPIRATORY BURST OXIDASE HOMOLOGS)-type NADPH oxidases produce superoxide radical anions and have a function in developmental processes and in response to environmental challenges. Barley RBOHF2 has diverse reported functions in interaction with the biotrophic powdery mildew fungus Blumeria graminis f. sp. hordei. Here, we analyzed, in detail, plant leaf level- and age-specific susceptibility of stably RBOHF2-silenced barley plants. This revealed enhanced susceptibility to fungal penetration of young RBOHF2-silenced leaf tissue but strongly reduced susceptibility of older leaves when compared with controls. Loss of susceptibility in old RBOHF2-silenced leaves was associated with spontaneous leaf-tip necrosis and constitutively elevated levels of free and conjugated salicylic acid. Additionally, these leaves more strongly expressed pathogenesis-related genes, both constitutively and during interaction with B. graminis f. sp. hordei. Together, this supports the idea that barley RBOHF2 contributes to basal resistance to powdery mildew infection in young leaf tissue but is required to control leaf cell death, salicylic acid accumulation, and defense gene expression in older leaves, explaining leaf age-specific resistance of RBOHF2-silenced barley plants.

A Conserved Puccinia striiformis Protein Interacts with Wheat NPR1 and Reduces Induction of Pathogenesis-Related Genes in Response to Pathogens

In Arabidopsis, NPR1 is a key transcriptional coregulator of systemic acquired resistance. Upon pathogen challenge, NPR1 translocates from the cytoplasm to the nucleus, in which it interacts with TGA-bZIP transcription factors to activate the expression of several pathogenesis-related (PR) genes. In a screen of a yeast two-hybrid library from wheat leaves infected with Puccinia striiformis f. sp. tritici, we identified a conserved rust protein that interacts with wheat NPR1 and named it PNPi (for Puccinia NPR1 interactor). PNPi interacts with the NPR1/NIM1-like domain of NPR1 via its C-terminal DPBB_1 domain. Using bimolecular fluorescence complementation assays, we detected the interaction between PNPi and wheat NPR1 in the nucleus of Nicotiana benthamiana protoplasts. A yeast three-hybrid assay showed that PNPi interaction with NPR1 competes with the interaction between wheat NPR1 and TGA2.2. In barley transgenic lines overexpressing PNPi, we observed reduced induction of multiple PR genes in the region adjacent to Pseudomonas syringae pv. tomato DC3000 infection. Based on these results, we hypothesize that PNPi has a role in manipulating wheat defense response via its interactions with NPR1.

Antimalarial chemotherapy: orally curative artemisinin-derived trioxane dimer esters

Eight new artemisinin-derived trioxane dimer esters 5 have been prepared and tested for antimalarial efficacy in malaria-infected mice. At a single oral dose of only 6mg/kg combined with 18mg/kg of mefloquine, each of the dimer esters 5 outperformed the antimalarial drug artemether (2). The most efficacious dimer, dichlorobenzoate ester 5h, prolonged mouse survival past day 30 of infection with three of the four mice in this group having no detectable parasitemia and appearing and acting healthy on day 30.

Linking pattern recognition and salicylic acid responses in Arabidopsis through ACCELERATED CELL DEATH6 and receptors

The Arabidopsis membrane protein ACCELERATED CELL DEATH 6 (ACD6) and the defense signal salicylic acid (SA) are part of a positive feedback loop that regulates the levels of at least 2 pathogen-associated molecular patterns (PAMP) receptors, including FLAGELLIN SENSING 2 (FLS2) and CHITIN ELICITOR RECEPTOR (LYSM domain receptor-like kinase 1, CERK1). ACD6- and SA-mediated regulation of these receptors results in potentiation of responses to FLS2 and CERK1 ligands (e.g. flg22 and chitin, respectively). ACD6, FLS2 and CERK1 are also important for callose induction in response to an SA agonist even in the absence of PAMPs. Here, we report that another receptor, EF-Tu RECEPTOR (EFR) is also part of the ACD6/SA signaling network, similar to FLS2 and CERK1.

Bacteria-triggered systemic immunity in barley is associated with WRKY and ETHYLENE RESPONSIVE FACTORs but not with salicylic acid

Leaf-to-leaf systemic immune signaling known as systemic acquired resistance is poorly understood in monocotyledonous plants. Here, we characterize systemic immunity in barley (Hordeum vulgare) triggered after primary leaf infection with either Pseudomonas syringae pathovar japonica (Psj) or Xanthomonas translucens pathovar cerealis (Xtc). Both pathogens induced resistance in systemic, uninfected leaves against a subsequent challenge infection with Xtc. In contrast to systemic acquired resistance in Arabidopsis (Arabidopsis thaliana), systemic immunity in barley was not associated with NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 or the local or systemic accumulation of salicylic acid. Instead, we documented a moderate local but not systemic induction of abscisic acid after infection of leaves with Psj. In contrast to salicylic acid or its functional analog benzothiadiazole, local applications of the jasmonic acid methyl ester or abscisic acid triggered systemic immunity to Xtc. RNA sequencing analysis of local and systemic transcript accumulation revealed unique gene expression changes in response to both Psj and Xtc and a clear separation of local from systemic responses. The systemic response appeared relatively modest, and quantitative reverse transcription-polymerase chain reaction associated systemic immunity with the local and systemic induction of two WRKY and two ETHYLENE RESPONSIVE FACTOR (ERF)-like transcription factors. Systemic immunity against Xtc was further associated with transcriptional changes after a secondary/systemic Xtc challenge infection; these changes were dependent on the primary treatment. Taken together, bacteria-induced systemic immunity in barley may be mediated in part by WRKY and ERF-like transcription factors, possibly facilitating transcriptional reprogramming to potentiate immunity.

Characterization of Withania somnifera leaf transcriptome and expression analysis of pathogenesis-related genes during salicylic acid signaling

Withania somnifera (L.) Dunal is a valued medicinal plant with pharmaceutical applications. The present study was undertaken to analyze the salicylic acid induced leaf transcriptome of W. somnifera. A total of 45.6 million reads were generated and the de novo assembly yielded 73,523 transcript contig with average transcript contig length of 1620 bp. A total of 71,062 transcripts were annotated and 53,424 of them were assigned GO terms. Mapping of transcript contigs to biological pathways revealed presence of 182 pathways. Seventeen genes representing 12 pathogenesis-related (PR) families were mined from the transcriptome data and their pattern of expression post 17 and 36 hours of salicylic acid treatment was documented. The analysis revealed significant up-regulation of all families of PR genes by 36 hours post treatment except WsPR10. The relative fold expression of transcripts ranged from 1 fold to 6,532 fold. The two families of peroxidases including the lignin-forming anionic peroxidase (WsL-PRX) and suberization-associated anionic peroxidase (WsS-PRX) recorded maximum expression of 377 fold and 6532 fold respectively, while the expression of WsPR10 was down-regulated by 14 fold. Additionally, the most stable reference gene for normalization of qRT-PCR data was also identified. The effect of SA on the accumulation of major secondary metabolites of W. somnifera including withanoside V, withaferin A and withanolide A was also analyzed and an increase in content of all the three metabolites were detected. This is the first report on expression patterns of PR genes during salicylic acid signaling in W. somnifera.

Avocado roots treated with salicylic acid produce phenol-2,4-bis (1,1-dimethylethyl), a compound with antifungal activity

We demonstrated the ability of salicylic acid (SA) to induce a compound in avocado roots that strengthens their defense against Phytophthora cinnamomi. The SA content of avocado roots, before and after the application of exogenous SA, was determined by High-Performance Liquid Chromatography (HPLC). After 4h of SA feeding, the endogenous level in the roots increased to 223 μg g(-1) FW, which was 15 times the amount found in control roots. The methanolic extract obtained from SA-treated avocado roots inhibited the radial growth of P. cinnamomi. A thin layer chromatographic bioassay with the methanolic extract and spores of Aspergillus showed a distinct inhibition zone. The compound responsible for the inhibition was identified as phenol-2,4-bis (1,1-dimethylethyl) by gas chromatography and mass spectrometry. At a concentration of 100 μg/mL, the substance reduced germinative tube length in Aspergillus and radial growth of P. cinnamomi. A commercial preparation of phenol-2,4-bis (1,1-dimethylethyl) caused the same effects on mycelium morphology and radial growth as our isolate, confirming the presence of this compound in the root extracts. This is the first report of the induction of this compound in plants by SA, and the results suggest that it plays an important role in the defense response of avocado.

Lipid metabolism is differentially modulated by salicylic acid and heptanoyl salicylic acid during the induction of resistance in wheat against powdery mildew

Heptanoyl salicylic acid (HSA) is a salicylic acid (SA) derivative obtained by esterification of 2-OH benzoic acid with heptanoic acid. In wheat, the protection levels obtained against Blumeria graminis f. sp. tritici (Bgt) increased from 50% with SA to 95% with HSA. Using molecular, biochemical and cytological approaches, we investigated here how wheat lipid metabolism is differentially activated by SA and HSA in both infectious and non-infectious conditions, and how Bgt infectious process is altered by both inducers. First, in the absence of Bgt, continuous lipoxygenase (LOX)-encoding gene expression and corresponding activity were specifically induced by HSA. Moreover, compared to SA, HSA treatment resulted in earlier up-regulations of the phospholipase C2-encoding gene expression and it specifically affected the expression of a lipid transfer protein-encoding gene. In infectious context, both HSA and SA sprayings impaired penetration events and therefore haustorium formation, leading to less frequent fungal colonies. While this alteration only slowed down the evolution of Bgt infectious process in SA-sprayed leaves, it completely impaired the establishment of successful infectious events in HSA-sprayed leaves. In addition, HSA induced continuous increases of a LOX-encoding gene expression and of the corresponding LOX activity when compared to SA-sprayed leaves. Lipid metabolism is therefore overall highly responsive to HSA spraying and could represent effective defence mechanism triggered during the induction of resistance in wheat toward Bgt. The concepts of priming and energy costs of the defences induced by SA and HSA are also discussed.

Effect of salicylic acid and structurally related compounds in the accumulation of phytoalexins in cotyledons of common bean (Phaseolus vulgaris L.) cultivars

In the present work, isoflavonoid phytoalexin production in response to the application of salicylic acid in cotyledons of four common bean (Phaseolus vulgaris) cultivars (SA) was evaluated. The time-course and dose-response profiles of the induction process were established by quantifying the isoflavonoids by HPLC. Cotyledons of anthracnose-resistant cultivars induced by SA produced substantially higher phytoalexin contents as compared to the susceptible ones. In addition, maximum levels of phytoalexins (50-100 fold increases) were reached between 96 and 144 h, and when a concentration of SA from 3.62 to 14.50 mM was used. The observations also indicate that there was a relatively good correlation between the phytoalexin contents and the inhibitory effect against C. lindemuthianum; the higher antifungal activity was observed during the first 48 hours for extracts from cotyledons treated with SA at 1.45 and 3.62 mM, and between 96 and 144 h after induction. Finally, compounds structurally related to SA (dihydro-quinazolinones and some imines) showed a strong elicitor effect. Moreover, induced extracts from cotyledons treated with these potential elicitors, besides the properly elicitors, displayed a weak to moderated antifungal activity. These compounds may be considered good candidates for developing of new phytoprotectants. Furthermore, phytoalexin-eliciting substances may contribute for selecting disease resistant cultivars.

On the move: induced resistance in monocots

Although plants possess an arsenal of constitutive defences such as structural barriers and preformed antimicrobial defences, many attackers are able to overcome the pre-existing defence layers. In response, a range of inducible plant defences is set up to battle these pathogens. These mechanisms, commonly integrated as induced resistance (IR), control pathogens and pests by the activation of specific defence pathways. IR mechanisms have been extensively studied in the Dicotyledoneae, whereas knowledge of IR in monocotyledonous plants, including the globally important graminaceous crop plants, is elusive. Considering the potential of IR for sustainable agriculture and the recent advances in monocot genomics and biotechnology, IR in monocots is an emerging research field. In the following, current facts and trends concerning basal immunity, and systemic acquired/induced systemic resistance in the defence of monocots against pathogens and herbivores will be summarized.

Temporal accumulation of salicylic acid activates the defense response against Colletotrichum in strawberry

Many authors have reported interactions between strawberry cultivars and pathogenic microorganisms, yet little is known about the mechanisms triggered in the plant. In this paper we examine the participation of the salicylic acid (SA) signaling pathway involved in the response of Fragaria x ananassa cv. Pájaro plants to pathogens. Strawberry plants were challenged with the virulent strain M11 of Colletotrichum acutatum, or with the avirulent strain M23 of Colletotrichum fragariae which confers resistance to the former. Our study showed that the isolate M23 induced a temporal SA accumulation that was accompanied with the induction of PR-1 gene expression in strawberry plants. Such events occured after the oxidative burst, evaluated as the accumulation of hydrogen peroxide and superoxide anion, and many hours before the protection could be detected. Similar results were obtained with exogenously applied SA. Results obtained supports the hypothesis that strawberry plants activate a SA mediated defense mechanisms that is effective against a causal agent of anthracnose. In contrast, plants inoculated with M11 did not show oxidative burst, SA accumulation or PR1 gene induction. This is the first report about a defense response signaling pathway studied in strawberry plants.

Salicylic acid derivatives: synthesis, features and usage as therapeutic tools

INTRODUCTION

In the field of medicinal chemistry, there is a growing interest in the use of small molecules. Although acetyl salicylic acid is well known for medical applications, little is known about other salicylic acid derivatives, and there is serious lack of data and information on the effects and biological evaluation that connect them.

AREAS COVERED

This review covers the synthesis and drug potencies of salicylic acid derivatives. After a brief overview of the information on salicylic acid and its features, a detailed review of salicylic acids as drugs and prodrugs, usage as cyclooxygenase inhibitors, properties in plants, synthesis and recent patents, is developed.

EXPERT OPINION

Salicylic acid research is still an important area and innovations continue to arise, which offer hope for new therapeutics in related fields. It is anticipated that this review will guide the direction of long-term drug/nutraceutical safety trials and stimulate ideas for future research.

Salicylic acid confers resistance to a biotrophic rust pathogen, Puccinia substriata, in pearl millet (Pennisetum glaucum)

Studies were undertaken to assess the induction of defence response pathways in pearl millet (Pennisetum glaucum) in response to infection with the leaf rust fungus Puccinia substriata. Pretreatment of pearl millet with salicylic acid (SA) conferred resistance to a virulent isolate of the rust fungus, whereas methyl jasmonate (MeJA) did not significantly reduce infection levels. These results suggest that the SA defence pathway is involved in rust resistance. In order to identify pearl millet genes that are specifically regulated in response to SA and not MeJA, and thus could play a role in resistance to P. substriata, gene expression profiling was performed. Substantial overlap in gene expression responses between the treatments was observed, with MeJA and SA treatments exhibiting 17% co-regulated transcripts. However, 34% of transcripts were differentially expressed in response to SA treatment, but not in response to MeJA treatment. SA-responsive transcripts represented genes involved in SA metabolism, defence response, signal transduction, protection from oxidative stress and photosynthesis. The expression profiles of pearl millet plants after treatment with SA or MeJA were more similar to one another than to the response during a compatible infection with P. substriata. However, some SA-responsive genes were repressed during P. substriata infection, indicating possible manipulation of host responses by the pathogen.

Salicylic Acid, a multifaceted hormone to combat disease

For more than 200 years, the plant hormone salicylic acid (SA) has been studied for its medicinal use in humans. However, its extensive signaling role in plants, particularly in defense against pathogens, has only become evident during the past 20 years. This review surveys how SA in plants regulates both local disease resistance mechanisms, including host cell death and defense gene expression, and systemic acquired resistance (SAR). Genetic studies reveal an increasingly complex network of proteins required for SA-mediated defense signaling, and this process is amplified by several regulatory feedback loops. The interaction between the SA signaling pathway and those regulated by other plant hormones and/or defense signals is also discussed.

Systemic and local modulation of plant responses by Piriformospora indica and related Sebacinales species

Piriformospora indica is a fungus of the order Sebacinales (Basidiomycota) infesting roots of mono- and dicotyledonous plants. Endophytic fungal colonization leads to enhanced plant growth while host cell death is required for proliferation in differentiated root tissue to form a mutualistic interaction. Colonization of barley roots by P. indica and related Sebacina vermifera strains also leads to systemic resistance against the leaf pathogenic fungus Blumeria graminis f.sp. hordei due to a yet unknown mechanism of induced resistance. In order to elucidate plant response pathways governed by these root endophytes, we analyzed gene expression in barley plants exhibiting an established symbiosis with P. indica 3 weeks after inoculation. P. indica-colonized roots showed no induction of defence-related genes, while other genes showed a differential regulation pattern indicating a faster P. indica-dependent root development. Gene expression analysis of leaves detected only few systemically induced mRNAs. Among differentially regulated transcripts, we characterized the pathogenesis-related gene HvPr17b and the molecular chaperone HvHsp70 in more detail. HvPr17b shows similarity with TaWCI5, a wheat gene inducible by chemical resistance inducers and salicylate, and was previously proven to exhibit antifungal activity against B. graminis. HvHsp70 is the first gene found to systemically indicate root colonization with endophytic fungi of the order Sebacinales. Both genes are discussed as markers for endophytic colonization and resulting systemic responses.

A proteinaceous elicitor Sm1 from the beneficial fungus Trichoderma virens is required for induced systemic resistance in maize

We have previously shown that the beneficial filamentous fungus Trichoderma virens secretes the highly effective hydrophobin-like elicitor Sm1 that induces systemic disease resistance in the dicot cotton (Gossypium hirsutum). In this study we tested whether colonization of roots by T. virens can induce systemic protection against a foliar pathogen in the monocot maize (Zea mays), and we further demonstrated the importance of Sm1 during maize-fungal interactions using a functional genomics approach. Maize seedlings were inoculated with T. virens Gv29-8 wild type and transformants in which SM1 was disrupted or constitutively overexpressed in a hydroponic system or in soil-grown maize seedlings challenged with the pathogen Colletotrichum graminicola. We show that similar to dicot plants, colonization of maize roots by T. virens induces systemic protection of the leaves inoculated with C. graminicola. This protection was associated with notable induction of jasmonic acid- and green leaf volatile-biosynthetic genes. Neither deletion nor overexpression of SM1 affected normal growth or development of T. virens, conidial germination, production of gliotoxin, hyphal coiling, hydrophobicity, or the ability to colonize maize roots. Plant bioassays showed that maize grown with SM1-deletion strains exhibited the same levels of systemic protection as non-Trichoderma-treated plants. Moreover, deletion and overexpression of SM1 resulted in significantly reduced and enhanced levels of disease protection, respectively, compared to the wild type. These data together indicate that T. virens is able to effectively activate systemic disease protection in maize and that the functional Sm1 elicitor is required for this activity.

Competence of roots for race-specific resistance and the induction of acquired resistance against Magnaporthe oryzae

SUMMARY Generally, Magnaporthe oryzae, the causal agent of rice blast disease, is considered to be a typical leaf-infecting plant pathogenic fungus. However, it was recently reported that M. oryzae shares many characteristics in common with root-infecting pathogens and indeed was able to infect roots. Here, we report on studies testing for the capacity of roots of rice and barley to resist infections with M. oryzae. We established that roots of rice plants were colonized by M. oryzae in a manner which is different from the gene-for-gene specificity seen in leaves for the same genotypes. Furthermore, treatment of rice seedlings with benzo-(1,2,3)-thiadiazole-7-carbothioic acid S-methyl ester (BTH), a chemical that protects leaves effectively against blast by conditioning acquired resistance, was not able to prevent colonization of roots by M. oryzae although a reduction in disease levels was observed. Moreover, BTH was not able to protect barley roots against infection with M. oryzae. Taken together, our results suggest that although roots show intrinsic variation in their ability to resist colonization by M. oryzae, neither gene-for-gene incompatibility nor aquired resistance are as effective at blocking the pathogen as they are in leaves.

Evidence that DL-3-aminobutyric acid and acibenzolar-S-methyl induce resistance against bacterial head rot disease of broccoli

Head rot of broccoli caused by Pseudomonas marginalis (Brown) Stevens and P. fluorescens Migula is a major disease in Brittany (France). To date, no accession with a satisfactory field resistance has been identified, and available pesticides are not effective in controlling the disease. The aim of this study was to test whether acibenzolar-S-methyl (ASM), DL-3-aminobutyric acid (BABA) and potassium phosphonate (K(2)HPO(3)), known to induce resistance against various diseases, can help protect broccoli against head rot. The susceptible broccoli F1 hybrids Marathon and Shogun were grown in a greenhouse until head formation. They were then sprayed with ASM (0.23 mM AI), BABA (20 mM AI) or potassium phosphonate (37.41 mM AI) until runoff. In one experiment, heads from treated plants were excised, inoculated (10(4) cfu ml(-1)) and incubated in Magenta GA7 vessels. In another experiment, heads were inoculated on treated living plants. Disease ratings were made 5 days after inoculation. Antibiotic- and water-treated plants served as controls. Results obtained showed that, on excised treated heads, potassium phosphonate was not protective and disease scores were comparable with those of the water control. BABA- and ASM-treated excised heads were poorly, but significantly, protected. On whole plants with heads attached, the latter two compounds were much more effective. ASM-induced resistance increased in effectiveness over 8 days after inoculation, whilst that induced by BABA decreased. This result suggests that testing disease resistance inducers on excised broccoli heads is not accurate. ASM and BABA may offer alternative methods for controlling head rot of broccoli.

Growth parameters and resistance against Drechslera teres of spring barley (Hordeum vulgare L. cv. Scarlett) grown at elevated ozone and carbon dioxide concentrations

Spring barley ( Hordeum vulgare L. cv. Scarlett) was grown at two CO2 levels (400 vs. 700 ppm) combined with two ozone regimes (ambient vs. double ambient) in climate chambers for four weeks, beginning at seedling emergence. Elevated CO2 concentration significantly increased aboveground biomass, root biomass, and tiller number, whereas double ambient ozone significantly decreased these parameters. These ozone-induced reductions in growth parameters were strongly overridden by 700 ppm CO2. The elevated CO2 level increased C : N ratio of the leaf tissue and leaf starch content but decreased leaf protein levels. Exposure to double ambient ozone did not affect protein content and C : N ratio but dramatically increased leaf starch levels at 700 ppm CO2. Resistance against Drechslera teres (Sacc.) Shoemaker was increased in leaves grown at double ambient ozone but was less obvious at 700 ppm than at 400 ppm CO2. Constitutive activities of beta-1,3-glucanase and chitinase were significantly higher in leaves grown at double ambient ozone compared to ambient ozone levels. The sum of methanol-soluble and alkali-released cell wall-bound aromatic metabolites (i.e., C-glycosylflavones and several structurally unidentified metabolites) and lignin contents did not show any treatment-dependent differences.

A pattern recognition tool for quantitative analysis of in planta hyphal growth of powdery mildew fungi

The development of fungal pathogens can be quantified easily at the level of spore germination or penetration. However, the exact quantification of hyphal growth rates after initial, successful host invasion is much more difficult. Here, we report on the development of a new pattern recognition software (HyphArea) for automated quantitative analysis of hyphal growth rates of powdery mildew fungi on plant surfaces that usually represent highly irregular and noisy image backgrounds. By using HyphArea, we measured growth rates of colonies of the barley powdery mildew, Blumeria graminis f. sp. hordei, on susceptible and induced-resistant host plants. Hyphal growth was not influenced by the resistance state of the plants up to 48 h postinoculation. At later time points, growth rate increased on susceptible plants, whereas it remained restricted on induced-resistant plants. This difference in hyphal growth rate was accompanied by lack of secondary haustoria formation on induced-resistant plants, suggesting that induced resistance in barley against Blumeria graminis is caused mainly by reduced penetration rates of primary as well as secondary appressoria leading, finally, to fewer and less-developed fungal colonies. No evidence was found for reduced nutrient-uptake efficiency of the primary haustoria in induced-resistant leaves, which would be expected to have resulted in reduced hyphal growth rates during the first 48 h of the interaction.

Induced disease resistance and gene expression in cereals

Disease resistance strategies reduce chemical input into the environment and are therefore powerful approaches to sustainable agriculture. Induced resistance (IR) has emerged as a potential alternative, or a complementary strategy, for crop protection. IR signifies the control of pathogens and pests by prior activation of plant defence pathways. A molecular understanding of IR in cereals, including the most important global crops wheat and rice, has been largely missing. Evidence indicating that central elements of IR pathways are conserved among Di- and Monocotyledoneae has only recently been presented, although their regulation and interaction with other plant pathways may be quite divergent. We present here a synopsis of current molecular knowledge of cereal IR mechanisms.

Powdery mildew pathogens can suppress the chitinase gene expression induced in detached inner epidermis of barley coleoptile

Two-step PCR (RT-PCR and nested PCR) was used to detect gene expression in powdery mildew pathogen-infected cells of detached inner epidermis of barley coleoptiles. Cellular contents of infected cells were microscopically suctioned with a micropipette and subjected to PCR. Triosephosphate isomerase and glyceraldehyde-3-phosphate dehydrogenase genes involved in the glycolytic pathway and a stimulus-induced endochitinase gene were targeted, and their expression was determined by detecting cDNAs derived from spliced transcripts. The two gycolysis-related genes were constantly expressed in the tissue irrespective of pathogen inoculation. In contrast, chitinase gene expression was induced in non-infected inner epidermis after detachment. After inoculation, this expression was selectively suppressed in pathogen-invaded cells, in spite of continuous expression in non-invaded cells of the same epidermis. Thus, the present method enabled us to directly analyze transcripts in individual cells at the infection site and assess the capability of the pathogen to regulate host gene expression.

Systemic acquired resistance delays race shifts to major resistance genes in bell pepper

ABSTRACT The lack of durability of host plant disease resistance is a major problem in disease control. Genotype-specific resistance that involves major resistance (R) genes is especially prone to failure. The compatible (i.e., disease) host-pathogen interaction with systemic acquired resistance (SAR) has been studied extensively, but the incompatible (i.e., resistant) interaction less so. Using the pepper-bacterial spot (causal agent, Xanthomonas axonopodis pv. vesicatoria) pathosystem, we examined the effect of SAR in reducing the occurrence of race-change mutants that defeat R genes in laboratory, greenhouse, and field experiments. Pepper plants carrying one or more R genes were sprayed with the plant defense activator acibenzolar-S-methyl (ASM) and challenged with incompatible strains of the pathogen. In the greenhouse, disease lesions first were observed 3 weeks after inoculation. ASM-treated plants carrying a major R gene had significantly fewer lesions caused by both the incompatible (i.e., hypersensitive) and compatible (i.e., disease) responses than occurred on nonsprayed plants. Bacteria isolated from the disease lesions were confirmed to be race-change mutants. In field experiments, there was a delay in the detection of race-change mutants and a reduction in disease severity. Decreased disease severity was associated with a reduction in the number of race-change mutants and the suppression of disease caused by the race-change mutants. This suggests a possible mechanism related to a decrease in the pathogen population size, which subsequently reduces the number of race-change mutants for the selection pressure of R genes. Thus, inducers of SAR are potentially useful for increasing the durability of genotype-specific resistance conferred by major R genes.

Abscisic acid and jasmonic acid affect proteinase inhibitor activities in barley leaves

Proteinase inhibitor (PI) accumulation has been described as a plant defense response against insects and pathogens. The induction of PIs is known to be regulated by endogenous chemical factors including phytohormones. We studied the induction of barley chymotrypsin and trypsin inhibitory activities by aphid infestation, mechanical wounding, abscisic acid (ABA) and jasmonic acid (JA). Wounding experiments led to a minimal accumulation of PI activity (16% over controls) compared to that found in barley seedlings infested by aphids, where chymotrypsin inhibitor activity showed a two-fold increment. No systemic induction could be detected in healthy leaves of an infested or mechanically injured plant. Exogenous ABA applied on barley leaves increased the chymotrypsin inhibitory activity, while JA only increased trypsin inhibitory activity locally and systemically when applied exogenously. Our data suggest that two different mechanisms may be regulating the induction of these two types of inhibitors.

Systemic gene expression in Arabidopsis during an incompatible interaction with Alternaria brassicicola

Pathogen challenge can trigger an integrated set of signal transduction pathways, which ultimately leads to a state of "high alert," otherwise known as systemic or induced resistance in tissue remote to the initial infection. Although large-scale gene expression during systemic acquired resistance, which is induced by salicylic acid or necrotizing pathogens has been previously reported using a bacterial pathogen, the nature of systemic defense responses triggered by an incompatible necrotrophic fungal pathogen is not known. We examined transcriptional changes that occur during systemic defense responses in Arabidopsis plants inoculated with the incompatible fungal pathogen Alternaria brassicicola. Substantial changes (2.00-fold and statistically significant) were demonstrated in distal tissue of inoculated plants for 35 genes (25 up-regulated and 10 down-regulated), and expression of a selected subset of systemically expressed genes was confirmed using real-time quantitative polymerase chain reaction. Genes with altered expression in distal tissue included those with putative functions in cellular housekeeping, indicating that plants modify these vital processes to facilitate a coordinated response to pathogen attack. Transcriptional up-regulation of genes encoding enzymes functioning in the beta-oxidation pathway of fatty acids was particularly interesting. Transcriptional up-regulation was also observed for genes involved in cell wall synthesis and modification and genes putatively involved in signal transduction. The results of this study, therefore, confirm the notion that distal tissue of a pathogen-challenged plant has a heightened preparedness for subsequent pathogen attacks.

Overexpression of barley BAX inhibitor 1 induces breakdown of mlo-mediated penetration resistance to Blumeria graminis

Cell death regulation is linked to pathogen defense in plants and animals. Execution of apoptosis as one type of programmed cell death in animals is irreversibly triggered by cytochrome c release from mitochondria via pores formed by BAX proteins. This type of programmed cell death can be prevented by expression of BAX inhibitor 1 (BI-1), a membrane protein that protects cells from the effects of BAX by an unknown mechanism. In barley, a homologue of the mammalian BI-1 is expressed in response to inoculation with the barley powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh). We found differential expression of BI-1 in response to Bgh in susceptible and resistant plants. Chemical induction of resistance to Bgh by soil drench treatment with 2,6-dichloroisonicotinic acid led to down-regulation of the expression level of BI-1. Importantly, single-cell transient overexpression of BI-1 in epidermal leaf tissue of susceptible barley cultivar Ingrid led to enhanced accessibility, resulting in a higher penetration efficiency of Bgh on BI-1-transformed cells. In Bgh-resistant mlo5 genotypes, which do not express the negative regulator of defense and cell death MLO, overexpression of BI-1 almost completely reconstituted susceptibility to fungal penetration. We suggest that BI-1 is a regulator of cellular defense in barley sufficient to substitute for MLO function in accessibility to fungal parasites.

Overexpression of barley BAX inhibitor 1 induces breakdown of mlo-mediated penetration resistance to Blumeria graminis

Cell death regulation is linked to pathogen defense in plants and animals. Execution of apoptosis as one type of programmed cell death in animals is irreversibly triggered by cytochrome c release from mitochondria via pores formed by BAX proteins. This type of programmed cell death can be prevented by expression of BAX inhibitor 1 (BI-1), a membrane protein that protects cells from the effects of BAX by an unknown mechanism. In barley, a homologue of the mammalian BI-1 is expressed in response to inoculation with the barley powdery mildew fungus Blumeria graminis f.sp. hordei (Bgh). We found differential expression of BI-1 in response to Bgh in susceptible and resistant plants. Chemical induction of resistance to Bgh by soil drench treatment with 2,6-dichloroisonicotinic acid led to down-regulation of the expression level of BI-1. Importantly, single-cell transient overexpression of BI-1 in epidermal leaf tissue of susceptible barley cultivar Ingrid led to enhanced accessibility, resulting in a higher penetration efficiency of Bgh on BI-1-transformed cells. In Bgh-resistant mlo5 genotypes, which do not express the negative regulator of defense and cell death MLO, overexpression of BI-1 almost completely reconstituted susceptibility to fungal penetration. We suggest that BI-1 is a regulator of cellular defense in barley sufficient to substitute for MLO function in accessibility to fungal parasites.

Reactive oxygen intermediates in plant-microbe interactions: who is who in powdery mildew resistance?

Reactive oxygen intermediates (ROIs) such as hydrogen peroxide (H(2)O(2)) and the superoxide anion radical (O*(2)(-)) accumulate in many plants during attack by microbial pathogens. Despite a huge number of studies, the complete picture of the role of ROIs in the host-pathogen interaction is not yet fully understood. This situation is reflected by the controversially discussed question as to whether ROIs are key factors in the establishment and maintenance of either host cell inaccessibility or accessibility for fungal pathogens. On the one hand, ROIs have been implicated in signal transduction as well as in the execution of defence reactions such as cell wall strengthening and a rapid host cell death (hypersensitive reaction). On the other hand, ROIs accumulate in compatible interactions, and there are reports suggesting a function of ROIs in restricting the spread of leaf lesions and thus in suppressing cell death. Moreover, in situ analyses have demonstrated that different ROIs may trigger opposite effects in plants depending on their spatiotemporal distribution and subcellular concentrations. This demonstrates the need to determine the particular role of individual ROIs in distinct stages of pathogen development. The well-studied interaction of cereals with fungi from the genus Blumeria is an excellent model system in which signal transduction and defence reactions can be further elucidated in planta. This review article gives a synopsis of the role of ROI accumulation, with particular emphasis on the pathosystem Hordeum vulgare L.- Blumeria graminis.

Acquired resistance functions in mlo barley, which is hypersusceptible to Magnaporthe grisea

Barley plants carrying a mutation in the Mlo (barley [Hordeum vulgare L.] cultivar Ingrid) locus conferring a durable resistance against powdery mildew are hypersusceptible to the rice blast fungus Magnaporthe grisea. It has been speculated that a functional Mlo gene is required for the expression of basic pathogen resistance and that the loss of Mlo function mediating powdery mildew resistance is an exception for this particular disease. Here, we report that the onset of acquired resistance (AR) after chemical as well as biological treatments is sufficient to overcome the hypersusceptible phenotype of backcross line BCIngridmlo5 (mlo) barley plants against M. grisea. Moreover, even barley plants bearing a functional Mlo gene and thus showing a moderate infection phenotype against rice blast exhibit a further enhanced resistance after induction of AR. Cytological investigations reveal that acquired resistance in mlo genotypes is manifested by the restoration of the ability to form an effective papilla at sites of attempted penetration, similarly to wild-type Mlo plants. In addition, the rate of effective papillae formation in Mlo plants was further enhanced after the onset of AR. These results demonstrate that treatments leading to the AR state in barley function independently of the Mlo/mlo phenotype and suggest that the Mlo protein is not a component of the AR signaling network. Moreover, it seems that only concomitant action of Mlo together with AR permits high level resistance in barley against blast. Higher steady state levels of PR1 and barley chemically induced mRNA correlate with higher disease severity rather than with the degree of resistance observed in this particular interaction.

Benzothiadiazole Activates Resistance in Sunflower (Helianthus annuus) to the Root-Parasitic Weed Orobanche cuman

ABSTRACT The study was conducted to evaluate the potential of induced resistance to infestation of sunflower (Helianthus annuus L.) by the parasitic weed Orobanche cumana Wallr. Treatment of sunflower seeds with 40 ppm of benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH) for 36 h completely prevented infection in root chambers. In pot studies using 2.86 x 10(-4) g of Orobanche seeds per gram of soil as inoculum, the total number of O. cumana shoots was reduced by 84 and 95% in the 60-ppm BTH treatment in the first and second trial, respectively. Evaluation of the disease incidences revealed that attachment of O. cumana at the sunflower root and the stage of early penetration was reduced in the BTH-treated plants. Chemical analysis of root extracts revealed synthesis of the phytoalexin scopoletin and of hydrogen peroxide in the BTH-treated sunflower roots, but no increase in lignification. Western blot analysis demonstrated accumulation of the pathogenesis-related protein chitinase in roots and stems of induced resistant plants. These results show that the phenomenon of induced resistance is not restricted to viral, bacterial, and fungal disease and demonstrate the great potential of this protection strategy as an effective component of future plant production systems.

Expression analysis of genes induced in barley after chemical activation reveals distinct disease resistance pathways

Abstract Salicylic acid (SA) and its synthetic mimics 2,6-dichloroisonicotinic acid (DCINA) and benzo(1,2,3)thiadiazole-7-carbothioic acid S-methyl ester (BTH), protect barley systemically against powdery mildew (Blumeria graminis f.sp. hordei, Bgh) infection by strengthening plant defence mechanisms that result in effective papillae and host cell death. Here, we describe the differential expression of a number of newly identified barley chemically induced (BCI) genes encoding a lipoxygenase (BCI-1), a thionin (BCI-2), an acid phosphatase (BCI-3), a Ca(2+)-binding EF-hand protein (BCI-4), a serine proteinase inhibitor (BCI-7), a fatty acid desaturase (BCI-8) and several further proteins with as yet unknown function. Compared with SA, the chemicals DCINA and BTH were more potent inducers of both gene expression and resistance. Homologues of four BCI genes were detected in wheat and were also differentially regulated upon chemical activation of disease resistance. Except for BCI-4 and BCI-5 (unknown function), the genes were also induced by exogenous application of jasmonates, whereas treatments that raise endogenous jasmonates as well as wounding were less effective. The fact that BCI genes were not expressed during incompatible barley-Bgh interactions governed by gene-for-gene relationships suggests the presence of separate pathways leading to powdery mildew resistance.

Barley coleoptile peroxidases. Purification, molecular cloning, and induction by pathogens

A cDNA clone encoding the Prx7 peroxidase from barley (Hordeum vulgare L.) predicted a 341-amino acid protein with a molecular weight of 36,515. N- and C-terminal putative signal peptides were present, suggesting a vacuolar location of the peroxidase. Immunoblotting and reverse-transcriptase polymerase chain reaction showed that the Prx7 protein and mRNA accumulated abundantly in barley coleoptiles and in leaf epidermis inoculated with powdery mildew fungus (Blumeria graminis). Two isoperoxidases with isoelectric points of 9.3 and 7.3 (P9.3 and P7.3, respectively) were purified to homogeneity from barley coleoptiles. P9.3 and P7.3 had Reinheitszahl values of 3.31 and 2.85 and specific activities (with 2,2'-azino-di-[3-ethyl-benzothiazoline-6-sulfonic acid], pH 5.5, as the substrate) of 11 and 79 units/mg, respectively. N-terminal amino acid sequencing and matrix-assisted laser desorption/ionization time-of-flight mass-spectrometry peptide analysis identified the P9. 3 peroxidase activity as due to Prx7. Tissue and subcellular accumulation of Prx7 was studied using activity-stained isoelectric focusing gels and immunoblotting. The peroxidase activity due to Prx7 accumulated in barley leaves 24 h after inoculation with powdery mildew spores or by wounding of epidermal cells. Prx7 accumulated predominantly in the epidermis, apparently in the vacuole, and appeared to be the only pathogen-induced vacuolar peroxidase expressed in barley tissues. The data presented here suggest that Prx7 is responsible for the biosynthesis of antifungal compounds known as hordatines, which accumulate abundantly in barley coleoptiles.

Hypersensitive cell death and papilla formation in barley attacked by the powdery mildew fungus are associated with hydrogen peroxide but not with salicylic acid accumulation

We analyzed the pathogenesis-related generation of H2O2 using the microscopic detection of 3,3-diaminobenzidine polymerization in near-isogenic barley (Hordeum vulgare L.) lines carrying different powdery mildew (Blumeria graminis f.sp. hordei) resistance genes, and in a line expressing chemically activated resistance after treatment with 2,6-dichloroisonicotinic acid (DCINA). Hypersensitive cell death in Mla12 and Mlg genotypes or after chemical activation by DCINA was associated with H2O2 accumulation throughout attacked cells. Formation of cell wall appositions (papillae) mediated in Mlg and mlo5 genotypes and in DCINA-activated plants was paralleled by H2O2 accumulation in effective papillae and in cytosolic vesicles of up to 2 μm in diameter near the papillae. H2O2 was not detected in ineffective papillae of cells that had been successfully penetrated by the fungus. These findings support the hypothesis that H2O2 may play a substantial role in plant defense against the powdery mildew fungus. We did not detect any accumulation of salicylic acid in primary leaves after inoculation of the different barley genotypes, indicating that these defense responses neither relied on nor provoked salicylic acid accumulation in barley.

Hypersensitive cell death and papilla formation in barley attacked by the powdery mildew fungus are associated with hydrogen peroxide but not with salicylic acid accumulation

We analyzed the pathogenesis-related generation of H2O2 using the microscopic detection of 3,3-diaminobenzidine polymerization in near-isogenic barley (Hordeum vulgare L.) lines carrying different powdery mildew (Blumeria graminis f.sp. hordei) resistance genes, and in a line expressing chemically activated resistance after treatment with 2,6-dichloroisonicotinic acid (DCINA). Hypersensitive cell death in Mla12 and Mlg genotypes or after chemical activation by DCINA was associated with H2O2 accumulation throughout attacked cells. Formation of cell wall appositions (papillae) mediated in Mlg and mlo5 genotypes and in DCINA-activated plants was paralleled by H2O2 accumulation in effective papillae and in cytosolic vesicles of up to 2 μm in diameter near the papillae. H2O2 was not detected in ineffective papillae of cells that had been successfully penetrated by the fungus. These findings support the hypothesis that H2O2 may play a substantial role in plant defense against the powdery mildew fungus. We did not detect any accumulation of salicylic acid in primary leaves after inoculation of the different barley genotypes, indicating that these defense responses neither relied on nor provoked salicylic acid accumulation in barley.

Wheat genes encoding two types of PR-1 proteins are pathogen inducible, but do not respond to activators of systemic acquired resistance

Wheat cDNAs that encode proteins PR-1.1 and PR-1.2 were cloned. Deduced amino acid sequences were homologous to those of pathogen-induced, basic PR-1 proteins from plants. Although expression of PR1.1 and PR1.2 genes was induced upon infection with either compatible or incompatible isolates of the fungal pathogen Erysiphe graminis, these genes did not respond to activators of systemic acquired resistance (SAR), such as salicylic acid (SA), benzothiadiazole (BTH), or isonicotinic acid (INA).

Separate jasmonate-dependent and salicylate-dependent defense-response pathways in Arabidopsis are essential for resistance to distinct microbial pathogens

The endogenous plant hormones salicylic acid (SA) and jasmonic acid (JA), whose levels increase on pathogen infection, activate separate sets of genes encoding antimicrobial proteins in Arabidopsis thaliana. The pathogen-inducible genes PR-1, PR-2, and PR-5 require SA signaling for activation, whereas the plant defensin gene PDF1.2, along with a PR-3 and PR-4 gene, are induced by pathogens via an SA-independent and JA-dependent pathway. An Arabidopsis mutant, coi1, that is affected in the JA-response pathway shows enhanced susceptibility to infection by the fungal pathogens Alternaria brassicicola and Botrytis cinerea but not to Peronospora parasitica, and vice versa for two Arabidopsis genotypes (npr1 and NahG) with a defect in their SA response. Resistance to P. parasitica was boosted by external application of the SA-mimicking compound 2, 6-dichloroisonicotinic acid [Delaney, T., et al. (1994) Science 266, 1247-1250] but not by methyl jasmonate (MeJA), whereas treatment with MeJA but not 2,6-dichloroisonicotinic acid elevated resistance to Alternaria brassicicola. The protective effect of MeJA against A. brassicicola was the result of an endogenous defense response activated in planta and not a direct effect of MeJA on the pathogen, as no protection to A. brassicicola was observed in the coi1 mutant treated with MeJA. These data point to the existence of at least two separate hormone-dependent defense pathways in Arabidopsis that contribute to resistance against distinct microbial pathogens.

Impaired fungicide activity in plants blocked in disease resistance signal transduction

Fungicide action is generally assumed to be dependent on an antibiotic effect on a target pathogen, although a role for plant defense mechanisms as mediators of fungicide action has not been excluded. Here, we demonstrate that in Arabidopsis, the innate plant defense mechanism contributes to the effectiveness of fungicides. In NahG and nim1 (for noninducible immunity) Arabidopsis plants, which normally exhibit increased susceptibility to pathogens, the fungicides metalaxyl, fosetyl, and Cu(OH)2 are much less active and fail to control Peronospora parasitica. In contrast, the effectiveness of these fungicides is not altered in Arabidopsis mutants defective in the ethylene or jasmonic acid signal transduction pathways. Application of the systemic acquired resistance activator benzothiadiazole (BTH) in combination with these fungicides results in a synergistic effect on pathogen resistance in wild-type plants and an additive effect in NahG and BTH-unresponsive nim1 plants. Interestingly, BTH treatment normally induces long-lasting pathogen protection; however, in NahG plants, the protection is transient. These observations suggest that BTH treatment can compensate only partially for an impaired signal transduction pathway and support the idea that pathogen defense mechanisms are under positive feedback control. These observations are strikingly reminiscent of the reduced efficacy of antifungal agents in immunocompromised animals.

Induced resistance responses in maize

Systemic acquired resistance (SAR) is a widely distributed plant defense system that confers broad-spectrum disease resistance and is accompanied by coordinate expression of the so-called SAR genes. This type of resistance and SAR gene expression can be mimicked with chemical inducers of resistance. Here, we report that chemical inducers of resistance are active in maize. Chemical induction increases resistance to downy mildew and activates expression of the maize PR-1 and PR-5 genes. These genes are also coordinately activated by pathogen infection and function as indicators of the defense reaction. Specifically, after pathogen infection, the PR-1 and PR-5 genes are induced more rapidly and more strongly in an incompatible than in a compatible interaction. In addition, we show that monocot lesion mimic plants also express these defense-related genes and that they have increased levels of salicylic acid after lesions develop, similar to pathogeninfected maize plants. The existence of chemically inducible disease resistance and PR-1 and PR-5 gene expression in maize indicates that maize is similar to dicots in many aspects of induced resistance. This reinforces the notion of an ancient plant-inducible defense pathway against pathogen attack that is shared between monocots and dicots.

Gene-Expression Patterns and Levels of Jasmonic Acid in Rice Treated with the Resistance Inducer 2,6-Dichloroisonicotinic Acid

Acquired disease resistance can be induced in rice (Oryza sativa) by a number of synthetic or natural compounds, but the molecular mechanisms behind the phenomenon are poorly understood. One of the synthetic inducers of resistance, 2,6-dichloroisonicotinic acid (INA), efficiently protected rice leaves from infection by the rice blast fungus Magnaporthe grisea (Hebert) Barr. A comparison of gene-expression patterns in plants treated with INA versus plants inoculated with the compatible pathogen M. grisea or the incompatible pathogen Pseudomonas syringae pv syringae revealed only a marginal overlap: 6 gene products, including pathogenesis-related proteins (PR1-PR9), accumulated in both INA-treated and pathogen-attacked leaves, whereas 26 other gene products accumulated only in INA-treated or only in pathogen-attacked leaves. Lipoxygenase enzyme activity and levels of nonconjugated jasmonic acid (JA) were enhanced in leaves of plants treated with a high dose of INA (100 ppm). Exogenously applied JA enhanced the gene induction and plant protection caused by lower doses of INA (0.1 to 10 ppm) that by themselves did not give rise to enhanced levels of endogenous (-)-JA. These data suggest that INA, aside from activating a pathogen-induced signaling pathway, also induces events that are not related to pathogenesis. JA acts as an enhancer of both types of INA-induced reactions in rice.

PROGRAMMED CELL DEATH IN PLANT-PATHOGEN INTERACTIONS

Plants cope with pathogen attacks by using mechanisms of resistance that rely both on preformed protective defenses and on inducible defenses. The latter are the most well studied, and progress is being made in determining which induced responses are responsible for limiting pathogen growth. Many plant-pathogen interactions are accompanied by plant cell death. Recent evidence suggests that this cell death is often programmed and results from an active process on the part of the host. The review considers the roles and possible mechanisms of plant cell death in response to pathogens.