Non-host Pathogen Research Articles

Molecular Interplay between Non-Host Resistance, Pathogens and Basal Immunity as a Background for Fatal Yellowing in Oil Palm (Elaeis guineensis Jacq.) Plants.

An oil palm (Elaeis guineensis Jacq.) bud rod disorder of unknown etiology, named Fatal Yellowing (FY) disease, is regarded as one of the top constraints with respect to the growth of the palm oil industry in Brazil. FY etiology has been a challenge embraced by several research groups in plant pathology throughout the last 50 years in Brazil, with no success in completing Koch's postulates. Most recently, the hypothesis of having an abiotic stressor as the initial cause of FY has gained ground, and oxygen deficiency (hypoxia) damaging the root system has become a candidate for stress. Here, a comprehensive, large-scale, single- and multi-omics integration analysis of the metabolome and transcriptome profiles on the leaves of oil palm plants contrasting in terms of FY symptomatology-asymptomatic and symptomatic-and collected in two distinct seasons-dry and rainy-is reported. The changes observed in the physicochemical attributes of the soil and the chemical attributes and metabolome profiles of the leaves did not allow the discrimination of plants which were asymptomatic or symptomatic for this disease, not even in the rainy season, when the soil became waterlogged. However, the multi-omics integration analysis of enzymes and metabolites differentially expressed in asymptomatic and/or symptomatic plants in the rainy season compared to the dry season allowed the identification of the metabolic pathways most affected by the changes in the environment, opening an opportunity for additional characterization of the role of hypoxia in FY symptom intensification. Finally, the initial analysis of a set of 56 proteins/genes differentially expressed in symptomatic plants compared to the asymptomatic ones, independent of the season, has presented pieces of evidence suggesting that breaks in the non-host resistance to non-adapted pathogens and the basal immunity to adapted pathogens, caused by the anaerobic conditions experienced by the plants, might be linked to the onset of this disease. This set of genes might offer the opportunity to develop biomarkers for selecting oil palm plants resistant to this disease and to help pave the way to employing strategies to keep the safety barriers raised and strong.

Plastid methylerythritol phosphate pathway participates in the hypersensitive response-related cell death in Nicotiana benthamiana.

Programmed cell death (PCD), a characteristic feature of hypersensitive response (HR) in plants, is an important cellular process often associated with the defense response against pathogens. Here, the involvement of LytB, a gene encoding 4-hydroxy-3-methylbut-2-enyl diphosphate reductase that participates in the final step of the plastid methylerythritol phosphate (MEP) pathway, in plant HR cell death was studied. In Nicotiana benthmiana plants, silencing of the NbLytB gene using virus-induced gene silencing (VIGS) caused plant growth retardation and albino leaves with severely malformed chloroplasts. In NbLytB-silenced plants, HR-related cell death mediated by the expression of either the human proapoptotic protein gene Bax or an R gene with its cognate Avr effector gene was inhibited, whereas that induced by the nonhost pathogen Pseudomonas syringae pv. syringae 61 was enhanced. To dissect the isoprenoid pathway and avoid the pleiotropic effects of VIGS, chemical inhibitors that specifically inhibit isoprenoid biosynthesis in plants were employed. Treatment of N. benthamiana plants with fosmidomycin, a specific inhibitor of the plastid MEP pathway, effectively inhibited HR-related PCD, whereas treatment with mevinolin (a cytoplasmic mevalonate pathway inhibitor) and fluridone (a carotenoid biosynthesis inhibitor) did not. Together, these results suggest that the MEP pathway as well as reactive oxygen species (ROS) generation in the chloroplast play an important role in HR-related PCD, which is not displaced by the cytosolic isoprenoid biosynthesis pathway.

Antimicrobial Terpenes Suppressed the Infection Process of Phytophthora in Fennel-Pepper Intercropping System.

The interactions between non-host roots and pathogens may be key to the inhibition of soilborne pathogens in intercropping systems. Fennel (Foeniculum vulgare) can be intercropped with a wide range of other plants to inhibit soilborne pathogens in biodiversity cultivation. However, the key compounds of fennel root exudates involved in the interactions between fennel roots and pathogens are still unknown. Here, a greenhouse experiment confirmed that intercropping with fennel suppressed pepper (Capsicum annuum) blight disease caused by Phytophthora capsici. Experimentally, the roots and root exudates of fennel can effectively interfere with the infection process of P. capsici at rhizosphere soil concentrations by attracting zoospores and inhibiting the motility of the zoospores and germination of the cystospores. Five terpene compounds (D-limonene, estragole, anethole, gamma-terpenes, and beta-myrcene) that were identified in the fennel rhizosphere soil and root exudates were found to interfere with P. capsica infection. D-limonene was associated with positive chemotaxis with zoospores, and a mixture of the five terpene compounds showed a strong synergistic effect on the infection process of P. capsici, especially for zoospore rupture. Furthermore, the five terpene compounds can induce the accumulation of reactive oxygen species (ROS), especially anethole, in hyphae. ROS accumulation may be one of the antimicrobial mechanisms of terpene compounds. Above all, we proposed that terpene compounds secreted from fennel root play a key role in Phytophthora disease suppression in this intercropping system.

Functional role of formate dehydrogenase 1 (FDH1) for host and nonhost disease resistance against bacterial pathogens.

Nonhost disease resistance is the most common type of plant defense mechanism against potential pathogens. In the present study, the metabolic enzyme formate dehydrogenase 1 (FDH1) was identified to associate with nonhost disease resistance in Nicotiana benthamiana and Arabidopsis thaliana. In Arabidopsis, AtFDH1 was highly upregulated in response to both host and nonhost bacterial pathogens. The Atfdh1 mutants were compromised in nonhost resistance, basal resistance, and gene-for-gene resistance. The expression patterns of salicylic acid (SA) and jasmonic acid (JA) marker genes after pathogen infections in Atfdh1 mutant indicated that both SA and JA are involved in the FDH1-mediated plant defense response to both host and nonhost bacterial pathogens. Previous studies reported that FDH1 localizes to mitochondria, or both mitochondria and chloroplasts. Our results showed that the AtFDH1 mainly localized to mitochondria, and the expression level of FDH1 was drastically increased upon infection with host or nonhost pathogens. Furthermore, we identified the potential co-localization of mitochondria expressing FDH1 with chloroplasts after the infection with nonhost pathogens in Arabidopsis. This finding suggests the possible role of FDH1 in mitochondria and chloroplasts during defense responses against bacterial pathogens in plants.

Activation of the VQ Motif-Containing Protein Gene VQ28 Compromised Nonhost Resistance of Arabidopsis thaliana to Phytophthora Pathogens.

Nonhost resistance refers to resistance of a plant species to all genetic variants of a non-adapted pathogen. Such resistance has the potential to become broad-spectrum and durable crop disease resistance. We previously employed Arabidopsis thaliana and a forward genetics approach to identify plant mutants susceptible to the nonhost pathogen Phytophthora sojae, which resulted in identification of the T-DNA insertion mutant esp1 (enhanced susceptibility to Phytophthora). In this study, we report the identification of VQ motif-containing protein 28 (VQ28), whose expression was highly up-regulated in the mutant esp1. Stable transgenic A. thaliana plants constitutively overexpressing VQ28 compromised nonhost resistance (NHR) against P. sojae and P. infestans, and supported increased infection of P. parasitica. Transcriptomic analysis showed that overexpression of VQ28 resulted in six differentially expressed genes (DEGs) that are involved in the response to abscisic acid (ABA). High performance liquid chromatography-mass spectrometry (HPLC-MS) detection showed that the contents of endogenous ABA, salicylic acid (SA), and jasmonate (JA) were enriched in VQ28 overexpression lines. These findings suggest that overexpression of VQ28 may lead to an imbalance in plant hormone homeostasis. Furthermore, transient overexpression of VQ28 in Nicotiana benthamiana rendered plants more susceptible to Phytophthora pathogens. Deletion mutant analysis showed that the C-terminus and VQ-motif were essential for plant susceptibility. Taken together, our results suggest that VQ28 negatively regulates plant NHR to Phytophthora pathogens.

Comparative analysis of defence responses Triticum aestivum L in response to the endophytic Pseudomonas aeruginosa PM389 and the non-host microbial pathogens Erwinia carotovora and Fusarium monaliforme

In the present study, Pseudomonas aeruginosa PM389 has been studied for comparing the defense responses in wheat plant against non-host pathogens Fusarium monaliforme and Erwinia caratovora. The comparative studies of defense enzymes produced on pretreatment of plants in the presence and absence of PM389 were also studied. Apart from it, disease incidence as well as plant growth promotion under in vitro and pot condition was recorded in the presence or absence of PM389 on challenging wheat plant with the non-host pathogen. Induction in all defense enzymes was observed on pretreatment of the plant with PM389 before pathogenic challenge with non-host. It was suggested that PM389 can generate induced systemic resistance in plants against pathogens. A less pathogenic population was recorded in the presence of PM389 than in its absence. In addition, it also showed plant growth promotion in the wheat under in vitro and pot studies. It was observed that PM389 was more effective against fungal pathogen than bacterial pathogen in both plant growth promotions as well as in decreasing disease incidence. Thus, PM389 can serve as a potential candidate as biofertilizer and biocontrol agent for non-host pathogens as well as having chances of cross-infection among various crops.

Arabidopsis non-host resistance PSS30 gene enhances broad-spectrum disease resistance in the soybean cultivar Williams 82.

Non-host resistance (NHR), which protects all members of a plant species from non-adapted or non-host plant pathogens, is the most common form of plant immunity. NHR provides the most durable and robust form of broad-spectrum immunity against non-adaptive pathogens pathogenic to other crop species. In a mutant screen for loss of Arabidopsis (Arabidopsis thaliana) NHR against the soybean (Glycine max (L.) Merr.) pathogen Phytophthora sojae, the Phytophthora sojae-susceptible 30 (pss30) mutant was identified. The pss30 mutant is also susceptible to the soybean pathogen Fusarium virguliforme. PSS30 encodes a folate transporter, AtFOLT1, which was previously localized to chloroplasts and implicated in the transport of folate from the cytosol to plastids. We show that two Arabidopsis folate biosynthesis mutants with reduced folate levels exhibit a loss of non-host immunity against P. sojae. As compared to the wild-type Col-0 ecotype, the steady-state folate levels are reduced in the pss1, atfolt1 and two folate biosynthesis mutants, suggesting that folate is required for non-host immunity. Overexpression of AtFOLT1 enhances immunity of transgenic soybean lines against two serious soybean pathogens, the fungal pathogen F. virguliforme and the soybean cyst nematode (SCN) Heterodera glycines. Transgenic lines showing enhanced SCN resistance also showed increased levels of folate accumulation. This study thus suggests that folate contributes to non-host plant immunity and that overexpression of a non-host resistance gene could be a suitable strategy for generating broad-spectrum disease resistance in crop plants.

The Arabidopsis Iron-Sulfur (Fe-S) Cluster Gene MFDX1 Plays a Role in Host and Nonhost Disease Resistance by Accumulation of Defense-Related Metabolites.

Until recently, genes from the iron-sulfur (Fe-S) cluster pathway were not known to have a role in plant disease resistance. The Nitrogen Fixation S (NIFS)-like 1 (NFS1) and Mitochondrial Ferredoxin-1 (MFDX1) genes are part of a set of 27 Fe-S cluster genes induced after infection with host and nonhost pathogens in Arabidopsis. A role for AtNFS1 in plant immunity was recently demonstrated. In this work, we showed that MFDX1 is also involved in plant defense. More specifically, Arabidopsis mfdx1 mutants were compromised for nonhost resistance against Pseudomonas syringae pv. tabaci, and showed increased susceptibility to the host pathogen P. syringae pv. tomato DC3000. Arabidopsis AtMFDX1 overexpression lines were less susceptible to P. syringae pv. tomato DC3000. Metabolic profiling revealed a reduction of several defense-related primary and secondary metabolites, such as asparagine and glucosinolates in the Arabidopsis mfdx1-1 mutant when compared to Col-0. A reduction of 5-oxoproline and ornithine metabolites that are involved in proline synthesis in mitochondria and affect abiotic stresses was also observed in the mfdx1-1 mutant. In contrast, an accumulation of defense-related metabolites such as glucosinolates was observed in the Arabidopsis NFS1 overexpressor when compared to wild-type Col-0. Additionally, mfdx1-1 plants displayed shorter primary root length and reduced number of lateral roots compared to the Col-0. Taken together, these results provide additional evidence for a new role of Fe-S cluster pathway in plant defense responses.

A Novel Role of Salt- and Drought-Induced RING 1 Protein in Modulating Plant Defense Against Hemibiotrophic and Necrotrophic Pathogens.

Many plant-encoded E3 ligases are known to be involved in plant defense. Here, we report a novel role of E3 ligase SALT- AND DROUGHT-INDUCED RING FINGER1 (SDIR1) in plant immunity. Even though SDIR1 is reasonably well-characterized, its role in biotic stress response is not known. The silencing of SDIR1 in Nicotiana benthamiana reduced the multiplication of the virulent bacterial pathogen Pseudomonas syringae pv. tabaci. The Arabidopsis sdir1 mutant is resistant to virulent pathogens, whereas SDIR1 overexpression lines are susceptible to both host and nonhost hemibiotrophic bacterial pathogens. However, sdir1 mutant and SDIR1 overexpression lines showed hypersusceptibility and resistance, respectively, against the necrotrophic pathogen Erwinia carotovora. The mutant of SDIR1 target protein, i.e., SDIR-interacting protein 1 (SDIR1P1), also showed resistance to host and nonhost pathogens. In SDIR1 overexpression plants, transcripts of NAC transcription factors were less accumulated and the levels of jasmonic acid (JA) and abscisic acid were increased. In the sdir1 mutant, JA signaling genes JAZ7 and JAZ8 were downregulated. These data suggest that SDIR1 is a susceptibility factor and its activation or overexpression enhances disease caused by P. syringae pv. tomato DC3000 in Arabidopsis. Our results show a novel role of SDIR1 in modulating plant defense gene expression and plant immunity.[Formula: see text] Copyright © 2021 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.

INTERFERENCE BY NON-HOST PLANT ROOTS AND ROOT EXUDATES IN THE INFECTION PROCESSES OF <I>PHYTOPHTHORA NICOTIANAE</I>

<List><ListItem><ItemContent> • The roots of non-host plant interfere infection of <i>Phytophthora nicotianae.</i> </ItemContent></ListItem><ListItem><ItemContent> • Vanillin and other compounds play key roles in antimicrobial activity of fennel roots. </ItemContent></ListItem><ListItem><ItemContent> • ROS accumulation is the potentially mechanism involved in inhibition of antimicrobial compounds on <i>P. nicotianae</i>. </ItemContent></ListItem></List> Crop rotations are widely used because they can significantly reduce the incidence of pests and diseases. The interactions between non-host roots and pathogens may be key in the inhibition of soilborne pathogens in crop rotations. Interactions between fennel (<i>Foeniculum vulgare</i>) roots/root exudates and <i>Phytophthora nicotianae</i> were investigated because of the known allelopathy between fennel and tobacco (<i>Nicotiana tabacum</i>). The effects of the key compounds in the fennel rhizosphere on the mycelial growth and zoospore behavior of <i>P. nicotianae</i> were assessed. The roots of fennel attracted <i>P. nicotianae</i> zoospores and inhibited their motility and the germination of cystospores, with some cystospores rupturing. 4-ethylacetophenone, vanillin and N-formylpiperidine were consistently identified in the fennel rhizosphere and were found to interfere with the infection of <i>P. nicotianae</i>, especially vanillin. Hyphae treated with these compounds produced more abnormal branches and accumulated reactive oxygen species. These interspecific interactions between non-host roots and pathogens were found to be an important factor in the inhibition by fennel of infection by <i>P. nicotianae</i>.

Antioxidant reactions in winter wheat seedlings of different cultivars exposed to the Pseudomonas syringae and its lipopolysaccharides in vitro

Pseudomonas syringae is the most common phytopathogenic bacterium with a wide range of target plants, which include important cereals such as wheat. One of the main pathogens of bacterial diseases of wheat is Pseudomonas syringae pv. atrofaciens. In some countries, wheat yield losses caused by this phytopathogenic bacterium reach 50%. Currently, the taxonomy of P. syringae includes more than 50 pathovars with varying degrees of adaptation to wheat lesions. One of them is Pseudomonas syringae pv. сoronafaciens. P. syringae pv. Coronafaciens is non-host pathogen for wheat. However, the infectionsof a wide range of crops, including wheat, with this pathogen attracts the attention of both researchers and specialiss of the agro-industrial complex. The study of the mechanisms of wheat resistance to host and non-host pathovars of P. syringae is of great interest, both in terms of in-depth study of the pathogen and in the perspective of selection of bacterial disease-resistant varieties of this strategically important grain crop for Ukraine. The aim of the study was to compare the antioxidant reactions of wheat seedlings of different winter wheat varieties under the grain exposition to P. syringae of different pathovars and their lipopolysaccharides (LPS). It was found that reactive oxygen species generation, as a mechanism of plant immune protection against phytopathogenic pseudomonads, is equally activated in the case of exposure to both host and nonhost pathovars and to a lesser extent in the case of the exposure with LPS of both pathovars. In grains of Favoritka variety (most sensitive to phytopathogenic pseudomonads) exposed to host pathovar, significant activation of antioxidant enzymes was observed. Exposure to the non-host pathovar causes sharp proline accumulation. Thus, the sensitivity of wheat seedlings to phytopathogenic host and non-host pathovars of phytopathogenic pseudomonads largely depends on the balanced functioning of the antioxidant defense system. Taken together, these data indicate the wheat cell oxidative metabolism as a target for selection of varieties resistant to phytopathogenic bacteria.

Pattern-triggered immunity restricts host colonization by endophytic fusaria, but does not affect endophyte-mediated resistance.

Fusarium oxysporum (Fo) is best known as a host‐specific vascular pathogen causing major crop losses. Most Fo strains, however, are root endophytes potentially conferring endophyte‐mediated resistance (EMR). EMR is a mechanistically poorly understood root‐specific induced resistance response induced by endophytic or nonhost pathogenic Fo strains. Like other types of induced immunity, such as systemic acquired resistance or induced systemic resistance, EMR has been proposed to rely on the activation of the pattern‐triggered immunity (PTI) system of the plant. PTI is activated upon recognition of conserved microbe‐associated molecular patterns (MAMPs) of invading microbes. Here, we investigated the role of PTI in controlling host colonization by Fo endophytes and their ability to induce EMR to the tomato pathogen Fo f. sp. lycopersici (Fol). Transgenic tomato and Arabidopsis plants expressing the Fo effector gene Avr2 are hypersusceptible to bacterial and fungal infection. Here we show that these plants are PTI‐compromised and are nonresponsive to bacterial‐ (flg22) and fungal‐ (chitosan) MAMPs. We challenged the PTI‐compromised tomato mutants with the EMR‐conferring Fo endophyte Fo47, the nonhost pathogen Fom (a melon pathogen), and with Fol. Compared to wild‐type plants, Avr2‐tomato plants became hypercolonized by Fo47 and Fom. Surprisingly, however, EMR towards Fol, induced by either Fo47 or Fom, was unaffected in these plants. These data show that EMR‐based disease resistance is independent from the conventional defence pathways triggered by PTI, but that PTI is involved in restricting host colonization by nonpathogenic Fo isolates.

Ribosomal protein QM/RPL10 positively regulates defence and protein translation mechanisms during nonhost disease resistance.

Ribosomes play an integral part in plant growth, development, and defence responses. We report here the role of ribosomal protein large (RPL) subunit QM/RPL10 in nonhost disease resistance. The RPL10‐silenced Nicotiana benthamiana plants showed compromised disease resistance against nonhost pathogen Pseudomonas syringae pv. tomato T1. The RNA‐sequencing analysis revealed that many genes involved in defence and protein translation mechanisms were differentially affected due to silencing of NbRPL10. Arabidopsis AtRPL10 RNAi and rpl10 mutant lines showed compromised nonhost disease resistance to P. syringae pv. tomato T1 and P. syringae pv. tabaci. Overexpression of AtRPL10A in Arabidopsis resulted in reduced susceptibility against host pathogen P. syringae pv. tomato DC3000. RPL10 interacts with the RNA recognition motif protein and ribosomal proteins RPL30, RPL23, and RPS30 in the yeast two‐hybrid assay. Silencing or mutants of genes encoding these RPL10‐interacting proteins in N. benthamiana or Arabidopsis, respectively, also showed compromised disease resistance to nonhost pathogens. These results suggest that QM/RPL10 positively regulates the defence and translation‐associated genes during nonhost pathogen infection.

Iron-Sulfur Cluster Protein NITROGEN FIXATION S-LIKE1 and Its Interactor FRATAXIN Function in Plant Immunity.

Iron-sulfur (Fe-S) clusters are inorganic cofactors that are present in all kingdoms of life as part of a large number of proteins involved in several cellular processes, including DNA replication and metabolism. In this work, we demonstrate an additional role for two Fe-S cluster genes in biotic stress responses in plants. Eleven Fe-S cluster genes, including the NITROGEN FIXATION S-LIKE1 (NFS1) and its interactor FRATAXIN (FH), when silenced in Nicotiana benthamiana, compromised nonhost resistance to Pseudomonas syringae pv. tomato T1. NbNFS1 expression was induced by pathogens and salicylic acid. Arabidopsis (Arabidopsis thaliana) atnfs and atfh mutants, with reduced AtNFS1 or AtFH gene expression, respectively, showed increased susceptibility to both host and nonhost pathogen infection. Arabidopsis AtNFS1 and AtFH overexpressor lines displayed decreased susceptibility to infection by host pathogen P syringae pv. tomato DC3000. The AtNFS1 overexpression line exhibited constitutive upregulation of several defense-related genes and enrichment of gene ontology terms related to immunity and salicylic acid responses. Our results demonstrate that NFS1 and its interactor FH are involved not only in nonhost resistance but also in basal resistance, suggesting a new role of the Fe-S cluster pathway in plant immunity.

Organized Disassembly of Photosynthesis During Programmed Cell Death Mediated By Long Chain Bases

In plants, pathogen triggered programmed cell death (PCD) is frequently mediated by polar lipid molecules referred as long chain bases (LCBs) or ceramides. PCD interceded by LCBs is a well-organized process where several cell organelles play important roles. In fact, light-dependent reactions in the chloroplast have been proposed as major players during PCD, however, the functional aspects of the chloroplast during PCD are largely unknown. For this reason, we investigated events that lead to disassembly of the chloroplast during PCD mediated by LCBs. To do so, LCB elevation was induced with Pseudomonas syringae pv. tomato (a non-host pathogen) or Fumonisin B1 in Phaseolus vulgaris. Then, we performed biochemical tests to detect PCD triggering events (phytosphingosine rises, MPK activation and H2O2 generation) followed by chloroplast structural and functional tests. Observations of the chloroplast, via optical phenotyping methods combined with microscopy, indicated that the loss of photosynthetic linear electron transport coincides with the organized ultrastructure disassembly. In addition, structural changes occurred in parallel with accumulation of H2O2 inside the chloroplast. These features revealed the collapse of chloroplast integrity and function as a mechanism leading to the irreversible execution of the PCD promoted by LCBs.

Arabidopsis exhibits differential response in basal immunity and proline metabolism during defense against host and nonhost pathogen infection

A host-pathogen surpasses the plant defense machinery and successfully infects the plant to serve its needs. In contrast, a nonhost pathogen is restricted by plant immune responses. In this study, we deciphered the differential responses of Arabidopsis against the host-pathogen (Pseudomonas syringae pv. maculicola, Psm) and the nonhost pathogen (P. syringae pv. tabaci, Pst) infection. The Pst multiplication was restricted in Arabidopsis plant coinciding with the absence of any disease symptoms which was also associated with the increase in defense associated, pathogenesis related gene1 (PR1) expression and callose deposition. Host-pathogen infection, on the other hand, caused chlorotic symptoms with much less activation of the defense marker genes compared to the nonhost infection. Proline content was decreased in plants infected with Psm but not in case of Pst infection. Proline is a crucial plant metabolite, and osmolyte suggested to be involved in plant defense responses. However, the differential regulation of proline pathway under a susceptible or resistance response has not yet been unravelled. The expression profile of proline metabolism genes in a time course post-infection revealed drastic differences after Psm and Pst pathogen infection. The elevated expression of proline catabolic genes, AtProDH and AtP5CDH were noted under Psm infection. In contrast, plants infected with Pst showed upregulation of AtProDH but a decline in AtP5CDH transcripts along with an upregulation of the biosynthesis genes i.e. AtP5CS and AtP5CR. Our study shows that proline metabolism is tightly regulated under host and nonhost pathogen infection and impacts susceptibility and resistance of the plants, respectively.

Nitrite and nitric oxide are important in the adjustment of primary metabolism during the hypersensitive response in tobacco.

Nitrate and ammonia deferentially modulate primary metabolism during the hypersensitive response in tobacco. In this study, tobacco RNAi lines with low nitrite reductase (NiRr) levels were used to investigate the roles of nitrite and nitric oxide (NO) in this process. The lines accumulate NO2-, with increased NO generation, but allow sufficient reduction to NH4+ to maintain plant viability. For wild-type (WT) and NiRr plants grown with NO3-, inoculation with the non-host biotrophic pathogen Pseudomonas syringae pv. phaseolicola induced an accumulation of nitrite and NO, together with a hypersensitive response (HR) that resulted in decreased bacterial growth, increased electrolyte leakage, and enhanced pathogen resistance gene expression. These responses were greater with increases in NO or NO2- levels in NiRr plants than in the WT under NO3- nutrition. In contrast, WT and NiRr plants grown with NH4+ exhibited compromised resistance. A metabolomic analysis detected 141 metabolites whose abundance was differentially changed as a result of exposure to the pathogen and in response to accumulation of NO or NO2-. Of these, 13 were involved in primary metabolism and most were linked to amino acid and energy metabolism. HR-associated changes in metabolism that are often linked with primary nitrate assimilation may therefore be influenced by nitrite and NO production.

Trichoderma asperellum T42 induces local defense against Xanthomonas oryzae pv. oryzae under nitrate and ammonium nutrients in tobacco.

Trichoderma has been explored and found to play a vital role in the defense mechanism of plants. However, its effects on host disease management in the presence of N nutrients remains elusive. The present study aimed to assess the latent effects of Trichoderma asperellum T42 on oxidative burst-mediated defense mechanisms against Xanthomonas oryzae pv. oryzae (Xoo) in tobacco plants fed 10 mM NO3− and 3 mM NH4+ nutrients. The nitrate-fed tobacco plants displayed an increased HR when Xoo infected, which was enhanced in the Trichoderma-treated plants. This mechanism was enhanced by the involvement of Trichoderma, which elicited NO production and enhanced the expression pattern of NO-modulating genes (NR, NOA and ARC). The real-time NO fluorescence intensity was alleviated in the NH4+-fed tobacco plants compared to that fed NO3− nutrient, suggesting the significant role of Trichoderma-elicited NO. The nitrite content and NR activity demonstration further confirmed that nitrate metabolism led to NO generation. The production of ROS (H2O2) in the plant leaves well-corroborated that the disease resistance was mediated through the oxidative burst mechanism. Nitrate application resulted in greater ROS production compared to NH4+ nutrient after Xoo infection at 12 h post-infection (hpi). Additionally, the mechanism of enhanced plant defense under NO3− and NH4+ nutrients mediated by Trichoderma involved NO, ROS production and induction of PR1a MEK3 and antioxidant enzyme transcription level. Moreover, the use of sodium nitroprusside (100 μM) with Xoo suspension in the leaves matched the disease resistance mediated via NO burst. Altogether, this study provides novel insights into the fundamental mechanism behind the role of Trichoderma in the activation of plant defense against non-host pathogens under N nutrients.

Involvement of lipid transfer proteins in resistance against a non-host powdery mildew in Arabidopsis thaliana.

Non-specific lipid transfer proteins (LTPs) are involved in the transport of lipophilic compounds to the cuticular surface in epidermal cells and in the defence against pathogens. The role of glycophosphatidylinositol (GPI)-anchored LTPs (LTPGs) in resistance against non-host mildews in Arabidopsis thaliana was investigated using reverse genetics. Loss of either LTPG1, LTPG2, LTPG5 or LTPG6 increased the susceptibility to penetration of the epidermal cell wall by Blumeria graminis f. sp. hordei (Bgh). However, no impact on pre-penetration defence against another non-host mildew, Erysiphe pisi (Ep), was observed. LTPG1 was localized to papillae at the sites of Bgh penetration. This study shows that, in addition to the previously known functions, LTPGs contribute to pre-invasive defence against certain non-host powdery mildew pathogens.

Transcript analysis of some defense genes of tomato in response to host and non-host bacterial pathogens.

The transcript levels of six defense genes including pathogenesis-related gene 1 (PR-1), pathogenesis-related gene 2 (PR-2), pathogenesis-related gene 5 (PR-5), lipoxygenase (LOX), phenylalanine ammonia-lyase (PAL) and catalase (CAT) were investigated in tomato plants inoculated with Xanthomonas axonopodis pv. phaseoli as a non-host pathogen and X. euvesicatoria as a host pathogen. Activation of all the genes was confirmed in both host and non-host treatments. Additionally, the results showed stronger expression of majority of the genes (PR-1, PR-2, LOX, PAL and CAT) in non-host treatment compared to host treatment at least at early hours after inoculation. These data suggest that faster and more expression of PR-1, PR-2, LOX, PAL and CAT might have a role in non-host resistance of tomato against X. axonopodis pv. phaseoli.