A critical analysis of phosphatidic acid mediated resistance response in Sinapis alba against Alternaria brassicicola

The enhanced expression of PLD genes and the quantitative increase in the level of phosphatidic acid (PA) in resistant Sinapis alba compared to the downregulated or unchanged expression of the PLD genes and decreased PA level in susceptible Brassica juncea when challenged with Alternaria brassicicola, indicated a positive relationship between PA-mediated signalling and resistance against this necrotrophic pathogen. Furthermore, spraying PA onto the susceptible species B. juncea increased resistance and enhanced expression of ABA-responsive genes, which was comparable to the expression of these genes in S. alba when challenged with A. brassicicola.

• •The enhanced expression of several PLD genes in resistant S. alba was observed compared to B. juncea upon challenge with A. brassicicola.
• •An increase in the PA level in S. alba compared to B. juncea following interaction with the pathogen.
• •Leaves of B. juncea incubated on PA-soaked filter paper did not show resistance.
• •Spraying B. juncea leaves with PA led to resistance as well as to the enhanced expression of ABA-responsive genes.
• •A temporal increase in the PA level is associated with increased resistance against A. brassicicola.

     

Jasmonic acid and abscisic acid play important roles in host–pathogen interaction between Fusarium graminearum and wheat during the early stages of fusarium head blight

The effect of phytohormones on the defense response of wheat against Fusarium graminearum infection was investigated. Infection of heads with F. graminearum induced accumulation of salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and indole acetic acid (IAA). Exogenous phytohormone treatments showed crosstalk between them and a complex effect on expression of the genes ATB2, ExpB6, LEA Td16, PR1, Pdf1.2, PR4. JA treatment reduced F. graminearum growth and fusarium head blight (FHB) symptoms while an increase in FHB was observed with ABA. Transient down-regulation of allene oxide synthase (AOS) supports a complex role for JA in wheat head.     

Genome-wide analysis and identification of TIR-NBS-LRR genes in Chinese cabbage (Brassica rapa ssp. pekinensis) reveal expression patterns to TuMV infection

TIR-NBS-LRR (TNL) genes greatly affect plant growth and development. Ninety TNL-type genes were identified and characterized in Chinese cabbage (Brassica rapa ssp. pekinensis). Tissue-expression profiling revealed different expression levels in different tissues. qRT-PCR analysis revealed the expression patterns of 69 genes challenged by Turnip mosaic virus (TuMV): 42 genes were up-regulated, and 11 genes down-regulated; genes were grouped according to their different expression patterns. Sixteen candidate genes were identified as responding to TuMV infection. This study supplies information on resistance genes involved in Chinese cabbage's response against TuMV, and furthers the understanding of resistance mechanisms in B. rapa crops.     

Pepper 9- and 13-lipoxygenase genes are differentially activated by two tobamoviruses and by hormone treatments

Two novel pepper 13-lipoxygenase (LOX) genes were cloned and their expressions were compared with those of three 9-LOX genes in pepper leaves inoculated with two different tobamoviruses. Obuda pepper virus (ObPV) inoculation led to a massive induction of pathogenesis-related genes and to the development of hypersensitive reaction (incompatible interaction), while Pepper mild mottle virus (PMMoV) inoculation resulted in a compatible interaction. Both virus infections markedly activated the expression of the two novel 13-LOXs. The magnitudes of induction of 13-LOXs did not differ substantially between the ObPV- and PMMoV-inoculated leaves. The induction of three 9-LOXs was markedly more robust and rapid in ObPV-inoculated leaves than in PMMoV-inoculated ones. LOXs were very differentially activated in pepper leaves treated with defense hormones. A large number of hormone-related cis-regulatory elements were identified in the promoter regions of LOXs. ObPV inoculation resulted also in the substantial up-regulation of an omega-6-fatty acid desaturase gene. Our results suggest that 9-LOX-dependent pathways are more probably involved in the suppression of virus replication than 13-LOX-dependent plant responses.     

Temporal modulation of oxidant and antioxidative responses in Brassica carinata during β-aminobutyric acid-induced resistance against Alternaria brassicae

A preinoculative foliar application of 5 mM BABA significantly inhibited the colonization of Alternaria brassicae on leaves of Brassica carinata susceptible cultivar car6. BABA treatment led to transient but significant increase in H₂O₂ level during early stages of pathogen colonization. A significant increase in superoxide dismutase (SOD), ascorbate peroxidase (APX), guaiacol dependent peroxidase (GDP) contributed to inhibition of the oxidative stress in BABA treated plants in response to pathogen infection. In conclusion BABA treatment led to proper balance of oxidant & antioxidants suitable for expression of resistance resulting in curtail of pathogen ingress during early stages of colonization.     

Identification of allele specific AFLP markers linked with bacterial wilt [Ralstonia solanacearum (Smith) Yabuuchi et al.] resistance in hot peppers (Capsicum annuum L.)

Even though the bacterial wilt is identified as the most destructive disease in hot peppers world-wide, robust molecular markers that facilitate marker assisted selection are absent till date. Kerala Agricultural University (India) has released two hot peppers named Ujwala and Anugraha which show high level field resistance to this pathogen. The variety Anugraha was developed through backcross breeding between a high yielding but highly susceptible variety Pusa Jwala with the highly resistant Ujwala, using Pusa Jwala as a recurrent parent. Thus, Pusa Jwala and Anugraha are near isogenic lines (NILs) differing for the resistance to bacterial wilt only and the resistance is governed by a homozygous recessive (rr) gene action. The F₁s of Anugraha × Pusa Jwala were selfed to generate the segregating F₂ population. The F₂ population has been field screened, 10 highly susceptible and 10 most resistant plants were identified and DNA from these plants were bulked separately. Bulked segregant analysis with AFLP primer combination EcoACT + MseCAC was done using the DNA from donor parent Ujwala, susceptible parent Pusa Jwala, resistant parent Anugraha, bulked susceptible F₂ and bulked resistant F₂ plants. On resolution using capillary electrophoresis system in genetic analyzer, the AFLP products have yielded three polymorphic bands (103, 117, and 161 bp) which were linked with the resistant recessive allele and three polymorphic bands (183, 296, 319 bp) linked with the dominant susceptible allele of the bacterial wilt resistance gene. The results were confirmed through co-segregation analysis in most resistant and susceptible plants of F₂ segregating population.     

Overproduction of reactive oxygen species involved in the pathogenicity of Fusarium in potato tubers

Differences in virulence between Fusarium sulphureum and Fusarium sambucinum were compared. Changes in reactive oxygen species production and metabolism in inoculated slices of potato tubers were also compared. The result showed that Fusarium infection induced significant production of ROS, lipid peroxidation and loss of cell membrane integrity, but low activity of superoxide dismutase (SOD) and ascorbate peroxidase (APX). Compared to F. sambucinum, F. sulphureum led larger lesion diameters on potato tubers and slices. It resulted in more superoxide anion ($O_2^{-}$) and earlier peak of hydrogen peroxide (H₂O₂), but lower activity of catalase (CAT) and APX, and accompanied with higher malondialdehyde (MDA) content and lower cell membrane integrity. These findings suggested that overproduction of ROS involved in the pathogenicity of Fusarium in potato tubers.     

Heat shock-induced resistance in strawberry against crown rot fungus Colletotrichum gloeosporioides

This present study investigated the effects of heat shock treatments in strawberry seedlings against crown rot caused by Colletotrichum gloeosporioides. Heat shock treatment at 50 °C for 20 s reduced the disease index of strawberry crown rot and increased chitinase 2-1 gene expression as well as free salycilic acid accumulation. Heat shock treatment did not reduce mycelial growth of C. gloeosporioides. BIT(2-benzisothiazol-3(2H)-one,1,1-dioxide) treatment could not protect strawberry seedlings against crown rot disease. This finding suggests that heat shock induces strawberry resistance against Colletotrichum crown rot and assumes that mechanisms other than SAR probably mediated the protective effect of heat shock-induced resistance.     

Comparative proteomic analysis of cucumber roots infected by Fusarium oxysporum f. sp. cucumerium Owen

Cucumber Fusarium wilt (CFW), caused by the soil-borne fungus Fusarium oxysporum f. sp. cucumerium, is a serious disease in cucumber (Cucumis sativus) production worldwide. For the efficient control of the pathogenic fungi, a better understanding of its interaction and associated resistance mechanisms at the molecular level is required. Here, we report a comparative proteomics analysis of total root protein isolated from infected cucumber root of susceptible bulk (SB) and resistant bulk (RB) of cucumber generation F2. Two-dimensional gel electrophoresis (2-DE) coupled with MS/MS approaches identified 15 over-accumulated proteins from the RB plants. Identified proteins are mainly involved in defense and stress responses, oxidation reduction, metabolism and transport and other process. These proteins are likely to be a part of resistance-related protein network, playing different roles in cucumber disease resistance. Three vital clues regarding wilt resistance of C. sativus are gained from this study. First, jasmonic acid and redox signaling components were found in response to F. oxysporum infection in resistant plants. Second, the LRR family protein may play an important role in the defense reaction against CFW. Third, biotic and abiotic stress-related proteins were induced by the CFW fungus F. oxysporum, indicating the activation of common stress pathway.     

TaULP5 contributes to the compatible interaction of adult plant resistance wheat seedlings-stripe rust pathogen

Adult plant resistance indicates that plant is susceptible to pathogen at seedling stage, but resistant at adult stage. Understanding the mechanism of the interactions between APR wheat plants and Puccinia striiformis f. sp. tritici (Pst) is important for the creation of strategies to improve cultivar disease resistance. In this study, a full-length cDNA was isolated from APR wheat cultivar Xingzi 9104 (XZ), and was designated as ubiquitin-like protein 5 (TaULP5). TaULP5 was likely to be located in the cytoplasm, with a percentage of 75.9% Arabidopsis protoplasts number. The expression of TaULP5 was largely induced in the compatible interaction of wheat seedlings to Pst, while no obvious change was found in the incompatible interaction of wheat adult plants to Pst. Moreover, when TaULP5 was knocked down, the wheat resistance at seedling stage to Pst was improved. In addition, knockdown of TaULP5 increased the expression levels of some biotic stress-related genes, such as PR1 and PR2. It is the first time to confirm that ubiquitin-like protein could contribute to the compatible interaction of XZ to Pst, and the results will lay a foundation for understanding the mechanisms of different interactions between APR wheat plants and Pst at post-translational level.     

Nonhost resistance: Self-inflicted DNA damage by fungal DNase accumulation is a major factor in terminating fungal growth in the pea-Fusarium solani f.sp. phaseoli interaction

Pea endocarp tissue generates a total nonhost resistance response against inappropriate pathogens such as the bean pathogen, Fusarium solani f. sp. phaseoli (Fsph) within 6 h. An array of plant components induced include: Pisatin (a phytoalexin), defensins, PR genes and hydrolytic enzymes in the non-host resistance response. This nonhost resistance response is similar but swifter than the responses induced by the compatible true pathogen, F. solani f. sp. pisi (Fspi). It was previously noted that a DNase released by both fungi is involved in induction of these resistance responses within pea endocarp tissue. This report demonstrates the cytological damage that occurs within nuclear DNA of both compatible and incompatible fungi when in contact with pea endocarp tissue and in the presence of DNase activity. The severity of damage to the bean pathogen exceeds that of the pea pathogen and requires only 2 h of contact with the pea tissue to develop. This accumulation of DNA damage is proposed to be the ultimate termination factor in this and other non-host resistance reactions. An updated DNase signaling scheme of the nonhost resistance of pea is presented.     

Ulvan-induced resistance in Arabidopsis thaliana against Alternaria brassicicola requires reactive oxygen species derived from NADPH oxidase

The present work aimed to study the role of reactive oxygen species derived from NADPH oxidase in the ulvan-induced resistance against Alternaria brassicicola in Arabidopsis thaliana. Foliar spraying of ulvan, a water-soluble algal polysaccharide, reduced the colonization of host tissues and, consequently, the severity of A. brassicicola by 90% in both wild type and AtrbohF plants, and it increased NADPH oxidase activity and hydrogen peroxide levels. Ulvan also tended to enhance the activity of enzymes related to the removal of reactive oxygen species (APX, GSR, CAT and SOD) suggesting a tight control of the antioxidant system. Ulvan did not protect the AtrbohD mutant as well as wild type plants previously infiltrated with diphenyleneiodonium, both impaired in NADPH oxidase activity and hydrogen peroxide accumulation. Based on our results and those available in the literature, we propose a general model for ulvan-induced defense responses in plant tissues. Collectively, our results suggest that ulvan-induced resistance in A. thaliana against A. brassicicola requires reactive oxygen species derived from the respiratory burst oxidase homologue D (RBOHD) NADPH oxidase.     

MoTlg2, a t-SNARE component is important for formation of the Spitzenkörper and polar deposition of chitin in Magnaporthe oryzae

T-SNAREs are a family of conserved proteins involved in intracellular transport of membrane-coated cargo among subcellular compartments. In this study, we identified a putative t-SNARE gene, MoTLG2, in Magnaporthe oryzae via insertion mutagenesis. Deletion of MoTLG2 resulted in slower vegetative growth and less conidiation relative to the wild-type strain, but the ΔMotlg2 null mutant was as virulent as the wild-type strain. MoTlg2 has 30% overall amino acid identity with Saccharomyces cerevisiae Tlg2, and rescued the defect of monensin de-sensitivity in the yeast strain where TLG2 had been deleted. More importantly, apical regions of the hyphae of the ΔMotlg2 null mutant were only weakly stained by FM4-64, which was reported as an excellent vesicle tracer, suggesting that the Spitzenkörper was not well formed in the ΔMotlg2 null mutant. In addition, more uneven lateral deposition of chitin was observed in the cell wall of vegetative hyphae of the ΔMotlg2 null mutant. Taken together, this study shows that the t-SNARE Tlg2 is important for both vegetative hyphal growth and conidiation, but dispensable for plant infection in filamentous fungi, and suggests that Tlg2 is important for formation of the Spitzenkörper and polar distribution of chitin.     

The N-acetyltransferase gene-implicated iron acquisition contributes to host specificity of Pseudomonas cichorii strain SPC9018 and its virulence

A pathogenicity island within the genome of a multi-host plant bacterium, Pseudomonas cichorii strain SPC9018, comprises the hrp genes encoding a type III secretion system and the pat gene encoding an N-acetyltransferase proposed to play a role in virulence. However, the function of the N-acetyltransferase remains poorly characterized. Interestingly, limiting the iron condition using a phytosiderophore, mugineic acid, resulted in reduced virulence of strain SPC9018 on respective host plants, including eggplant, similar to the reduced virulence observed with a pat gene-deletion mutant. Spectroscopic analyses showed that the pat deletion reduced the concentration of pyoverdine, which is the main siderophore produced by strain SPC9018, leading to a reduction in pyoverdine-mediated iron acquisition. Furthermore, the pat gene deletion mutant showed enhanced expression of the fecA, pvdL, and pvdR genes, whose expression is induced under deficient siderophore-mediated iron uptake. The pat-deletion mutant showed a hyper-swarming phenotype, and the addition of iron decreased this swarming motility. The pat deletion also reduced the adhesion ability of the bacteria, similar to the effect of iron-limited conditions. Furthermore, deletion of the pat gene enhanced expression of the hrp genes. These findings suggested that the pat gene encoding the N-acetyltransferase may be implicated in iron acquisition, contributing to host specificity of P. cichorii strain SPC9018 and its virulence.     

Activity of polygalacturonases from Moniliophthora perniciosa depends on fungus culture conditions and is enhanced by Theobroma cacao extracts

We report the first analysis of polygalacturonase regulation in the basidiomycete Moniliophthora perniciosa. Non-secreted and secreted polygalacturonase activity was obtained from M. perniciosa cultivated on bran-based solid medium or liquid media containing additional carbon sources or cacao extracts (infected or not by the fungus). Polygalacturonase activity assays were carried out under different temperatures and incubation periods. The best secreted polygalacturonase activity was obtained when the enzymatic assay was made at 50 °C for 10 min. Moreover, the polygalacturonase activity was enhanced when the fungus was cultivated on potato dextrose medium, in the presence of additional fermentable carbon sources, in the presence of cacao pulp or non infected cacao extracts.