Summary
• Hydrogen peroxide (H2O2) is reported to inhibit biotrophic but benefit necrotrophic pathogens. Infection by necrotrophs can result in a massive accumulation of H2O2 in hosts. Little is known of how pathogens with both growth types are affected (hemibiotrophs). The hemibiotroph, Septoria tritici, infecting wheat (Triticum aestivum) is inhibited by H2O2 during the biotrophic phase, but a large H2O2 accumulation occurs in the host during reproduction.
• Here, we infiltrated catalase, H2O2 or water into wheat during the biotrophic or the necrotrophic phase of S. tritici and studied the effect of infection on host physiology to get an understanding of the survival strategy of the pathogen.
• H2O2 removal by catalase at both early and late stages made plants more susceptible, whereas H2O2 made them more resistant. H2O2 is harmful to S. tritici throughout its life cycle, but it can be tolerated.
• The late accumulation of H2O2 is unlikely to result from down-regulation of photosynthesis, but probably originates from damage to the peroxisomes during the general tissue collapse, which is accompanied by release of soluble sugars in a susceptible cultivar.

Role of hydrogen peroxide during the interaction between the hemibiotrophic fungal pathogen Septoria tritici and wheat.

Analysis of the fully sequenced genome of the wheat leaf-specific fungal pathogen Mycosphaerella graminicola identified only a single gene encoding a member of the necrosis- and ethylene-inducing peptide 1 (Nep1)-like protein family (NLP). NLP proteins have frequently been shown to trigger cell death and the activation of defense signaling reactions in dicotyledonous plants. However, complete loss-of-function reverse genetics analyses for their importance in the virulence of eukaryotic plant pathogens are generally lacking. Real-time quantitative polymerase chain reaction on MgNLP demonstrated the gene to be specifically expressed in planta. Peak expression was observed during the immediate presymptomatic phase of colonization of a susceptible host genotype. This was followed by a dramatic decrease during disease lesion formation which, in this system, exhibits characteristics of host programmed cell death (PCD). No comparable peak in transcript levels was seen during an incompatible interaction with a host genotype exhibiting gene-for-gene-based disease resistance. Heterologously expressed MgNLP protein induced necrotic cell death and the activation of defense-related genes when infiltrated into Arabidopsis leaves but not in leaves of a susceptible wheat genotype. MgNLP infiltration also failed to stimulate wheat mitogen-activated protein kinase activities. Finally, targeted deletion of M. graminicola MgNLP caused no detectable reduction in plant pathogenicity or virulence, suggesting that this protein is not a major virulence determinant during fungal infection of its host plant. To our knowledge, this represents the first complete loss-of-function analysis of NLP in a eukaryotic plant pathogen and we discuss our findings in the context of possible functions for NLP in pathogens which only infect monocotyledonous plants.

https://apsjournals.apsnet.org/doi/pdf/10.1094/MPMI-22-7-0790

Molecular characterization and functional analysis of MgNLP, the sole NPP1 domain-containing protein, from the fungal wheat leaf pathogen Mycosphaerella graminicola.

The accumulation of the pathogenesis-related (PR) proteins beta-1,3-glucanase and chitinase and structural defence responses were studied in leaves of wheat either resistant or susceptible to the hemibiotrophic pathogen Septoria tritici. Resistance was associated with an early accumulation of beta-1,3-glucanase and chitinase transcripts followed by a subsequent reduction in level. Resistance was also associated with high activity of beta-1,3-glucanase, especially in the apoplastic fluid, in accordance with the biotrophic/endophytic lifestyle of the pathogen in the apoplastic spaces, thus showing the highly localized accumulation of defence proteins in the vicinity of the pathogen. Isoform analysis of beta-1,3-glucanase from the apoplastic fluid revealed that resistance was associated with the accumulation of an endo-beta-1,3-glucanase, previously implicated in defence against pathogens, and a protein with identity to ADPG pyrophosphatase (92%) and germin-like proteins (93%), which may be involved in cell wall reinforcement. In accordance with this, glycoproteins like extensin were released into the apoplast and callose accumulated to a greater extent in cell walls, whereas lignin and polyphenolics were not found to correlate with defence. Treatment of a susceptible wheat cultivar with purified beta-1,3-glucan fragments from cell walls of S. tritici gave complete protection against disease and this was accompanied by increased gene expression of beta-1,3-glucanase and the deposition of callose. Collectively, these data indicate that resistance is dependent on a fast, initial recognition of the pathogen, probably due to beta-1,3-glucan in the fungal cell walls, and this results in the accumulation of beta-1,3-glucanase and structural defence responses, which may directly inhibit the pathogen and protect the host against fungal enzymes and toxins.

/pdf/effects-of-b-1-3-glucan-from-septoria-tritici-on-structural-2pxqe5x7vp.pdf

Effects of β-1,3-glucan from Septoria tritici on structural defence responses in wheat

Methyl jasmonate induced gene expression in wheat delays symptom development by the crown rot pathogen Fusarium pseudograminearum

The Puccinia graminis f. sp. tritici (Pgt) Ug99 race group is virulent to most stem rust resistance genes currently deployed in wheat and poses a threat to global wheat production. The durum wheat (Triticum turgidum ssp. durum) gene Sr13 confers resistance to Ug99 and other virulent races, and is more effective at high temperatures. Using map-based cloning, we delimited a candidate region including two linked genes encoding coiled-coil nucleotide-binding leucine-rich repeat proteins designated CNL3 and CNL13. Three independent truncation mutations identified in each of these genes demonstrated that only CNL13 was required for Ug99 resistance. Transformation of an 8-kb genomic sequence including CNL13 into the susceptible wheat variety Fielder was sufficient to confer resistance to Ug99, confirming that CNL13 is Sr13CNL13 transcripts were slightly down-regulated 2-6 days after Pgt inoculation and were not affected by temperature. By contrast, six pathogenesis-related (PR) genes were up-regulated at high temperatures only when both Sr13 and Pgt were present, suggesting that they may contribute to the high temperature resistance mechanism. We identified three Sr13-resistant haplotypes, which were present in one-third of cultivated emmer and durum wheats but absent in most tested common wheats (Triticum aestivum). These results suggest that Sr13 can be used to improve Ug99 resistance in a large proportion of modern wheat cultivars. To accelerate its deployment, we developed a diagnostic marker for Sr13 The identification of Sr13 expands the number of Pgt-resistance genes that can be incorporated into multigene transgenic cassettes to control this devastating disease.

/pdf/identification-and-characterization-of-sr13-a-tetraploid-3bspv9p2ay.pdf

Identification and characterization of Sr13, a tetraploid wheat gene that confers resistance to the Ug99 stem rust race group

Mycosphaerella graminicola, incitant of septoria tritici blotch, is a widespread and significant pathogen of wheat that is not closely related to other fungi being developed as genetic models for host-pathogen interactions. Several resistance genes in wheat have been identified, yet the molecular mechanisms of resistance are unknown. To identify host genes involved in the resistance response, expression profiles of the wheat line Tadinia (containing the Stb4 gene for resistance) and the susceptible line Yecora Rojo, non-inoculated or inoculated with M. graminicola, were compared by differential-display polymerase chain reaction (DD-PCR). Among the differentially expressed genes was a protein disulfide isomerase (PDI), which is well known as a molecular chaperone and component of signal-transduction pathways in animal systems but had not been implicated previously in plant defense response. Real-time quantitative reverse-transcription PCR and northern analysis revealed that PDI was induced within 3 h of inoculation with highest induction in the pathogen-treated resistant lines. These responses of PDI were similar to the early and strong resistance-related responses displayed by the pathogenesis-related (PR) proteins, PR-1, PR-2 and PR-5. In contrast, a wheat lipoxygenase was down-regulated in the resistant lines at time points corresponding with peak induction of the PR genes. Thus, part of the resistance mechanism may involve repression of a gene that could otherwise aid fungal growth. Wheat responds much more rapidly than believed previously to signals produced by M.graminicola. These early responses begin prior to penetration of the host and appear to determine the outcome of the host-pathogen interaction.

Rapid induction of a protein disulfide isomerase and defense-related genes in wheat in response to the hemibiotrophic fungal pathogen Mycosphaerella graminicola

Suparna Ray

Papers

Rapid induction of a protein disulfide isomerase and defense-related genes in wheat in response to the hemibiotrophic fungal pathogen Mycosphaerella graminicola