Ultrastructural studies of Phellinus sulphurascens infection of Douglas-fir roots and immunolocalization of host pathogenesis-related proteins

The Apoplast: A Key Player in Plant Survival

The apoplast comprises the intercellular space, the cell walls, and the xylem. Important functions for the plant, such as nutrient and water transport, cellulose synthesis, and the synthesis of molecules involved in plant defense against both biotic and abiotic stresses, take place in it. The most important molecules are ROS, antioxidants, proteins, and hormones. Even though only a small quantity of ROS is localized within the apoplast, apoplastic ROS have an important role in plant development and plant responses to various stress conditions. In the apoplast, like in the intracellular cell compartments, a specific set of antioxidants can be found that can detoxify the different types of ROS produced in it. These scavenging ROS components confer stress tolerance and avoid cellular damage. Moreover, the production and accumulation of proteins and peptides in the apoplast take place in response to various stresses. Hormones are also present in the apoplast where they perform important functions. In addition, the apoplast is also the space where microbe-associated molecular Patterns (MAMPs) are secreted by pathogens. In summary, the diversity of molecules found in the apoplast highlights its importance in the survival of plant cells.

Functional analysis of a pathogenesis-related thaumatin-like protein gene TaLr35PR5 from wheat induced by leaf rust fungus

BACKGROUND

Plants have evolved multifaceted defence mechanisms to resist pathogen infection. Production of the pathogenesis-related (PR) proteins in response to pathogen attack has been implicated in plant disease resistance specialized in systemic-acquired resistance (SAR). Our earlier studies have reported that a full length TaLr35PR5 gene, encoding a protein exhibiting amino acid and structural similarity to a sweet protein thaumatin, was isolated from wheat near-isogenic line TcLr35. The present study aims to understand the function of TaLr35PR5 gene in Lr35-mediated adult resistance to Puccinia triticina.

RESULTS

We determined that the TaLr35PR5 protein contained a functional secretion peptide by utilizing the yeast signal sequence trap system. Using a heterologous expression assay on onion epidermal cells we found that TaLr35PR5 protein was secreted into the apoplast of onion cell. Expression of TaLr35PR5 was significantly reduced in BSMV-induced gene silenced wheat plants, and pathology test on these silenced plants revealed that Lr35-mediated resistance phenotype was obviously altered, indicating that Lr35-mediated resistance was compromised.

CONCLUSIONS

All these findings strongly suggest that TaLr35PR5 is involved in Lr35-mediated adult wheat defense in response to leaf rust attack.

Gene expression profiling of candidate virulence factors in the laminated root rot pathogen Phellinus sulphurascens

BACKGROUND

Phellinus sulphurascens is a fungal pathogen that causes laminar root rot in conifers, one of the most damaging root diseases in western North America. Despite its importance as a forest pathogen, this fungus is still poorly studied at the genomic level. An understanding of the molecular events involved in establishment of the disease should help to develop new methods for control of this disease.

RESULTS

We generated over 4600 expressed sequence tags from two cDNA libraries constructed using either mycelia grown on cellophane sheets and exposed to Douglas-fir roots or tissues from P. sulphurascens-infected Douglas-fir roots. A total of 890 unique genes were identified from the two libraries, and functional classification of 636 of these genes was possible using the Functional Catalogue (FunCat) annotation scheme. cDNAs were identified that encoded 79 potential virulence factors, including numerous genes implicated in virulence in a variety of phytopathogenic fungi. Many of these putative virulence factors were also among 82 genes identified as encoding putatively secreted proteins. The expression patterns of 86 selected fungal genes over 7 days of infection of Douglas-fir were examined using real-time PCR, and those significantly up-regulated included rhamnogalacturonan acetylesterase, 1,4-benzoquinone reductase, a cyclophilin, a glucoamylase, 3 hydrophobins, a lipase, a serine carboxypeptidase, a putative Ran-binding protein, and two unknown putatively secreted proteins called 1 J04 and 2 J12. Significantly down-regulated genes included a manganese-superoxide dismutase, two metalloproteases, and an unknown putatively secreted protein called Ps0058.

CONCLUSIONS

This first collection of Phellinus sulphurascens EST sequences and its annotation provide an important resource for future research aimed at understanding key virulence factors of this forest pathogen. We examined the expression patterns of numerous fungal genes with potential roles in virulence, and found a collection of functionally diverse genes that are significantly up- or down-regulated during infection of Douglas-fir seedling roots by P. sulphurascens.

Pathogenesis related-10 proteins are small, structurally similar but with diverse role in stress signaling

Pathogenesis related-10 proteins are small proteins with cytosolic localization, conserved three dimensional structures and single intron at 185 bp position. These proteins have a broad spectrum of roles significantly in biotic and abiotic stresses. The RNase activity, ligand binding activity, posttranslational modification (phosphorylation) and phytohormone signaling provide some information into the mechanism of the regulation of PR-10 proteins, however the presence of isoforms makes it difficult to decipher its exact mode of function. The involvement of phosphorylation/dephosphorylation events in its activation is interesting and provides unique and unbiased insights into the complexity of its regulation. Studies on upstream region of different PR-10 genes indicate the presence of cis-acting elements for WRKY, RAVI, bZ1P, ERF, SEBF and Pti4 transcription factors indicating their role in regulating PR-10 promoter. In this review, we discuss in detail the structure and mechanism of regulation of PR-10 proteins.

Gene expression profiling of a compatible interaction between Douglas-Fir and the root rot fungal pathogen Phellinus sulphurascens

Douglas-fir (DF) (Pseudotsuga menziesii) is one of the largest and most economically important coniferous species in western North America. Its productivity is greatly affected by the root rot fungus Phellinus sulphurascens Pilát. Evidence of resistance by DF to fungal root pathogens such as P. sulphurascens has been reported but mechanisms of resistance in this compatible pathosystem are not yet known. To better understand the DF-P. sulphurascens interaction, especially at the molecular level, we selected 12 diverse plant genes already identified as defense-related from a cDNA library constructed using root tissues from P. sulphurascens-infected DF seedlings. Using quantitative reverse-transcriptase polymerase chain reaction on infected DF root samples collected at five different time points after inoculation, we found that P. sulphurascens infection significantly elevated expression of the 12 selected genes. In most cases the highest expression level was recorded within 2 to 3 days after inoculation. The constructed cDNA library, which is enriched with defense-related host genes and a number of fungal genes, will continue to serve as a useful resource for future larger-scale gene discovery and functional research on the P. sulphurascens and DF pathosystem.

Genome-wide analysis of eukaryote thaumatin-like proteins (TLPs) with an emphasis on poplar

BACKGROUND

Plant inducible immunity includes the accumulation of a set of defense proteins during infection called pathogenesis-related (PR) proteins, which are grouped into families termed PR-1 to PR-17. The PR-5 family is composed of thaumatin-like proteins (TLPs), which are responsive to biotic and abiotic stress and are widely studied in plants. TLPs were also recently discovered in fungi and animals. In the poplar genome, TLPs are over-represented compared with annual species and their transcripts strongly accumulate during stress conditions.

RESULTS

Our analysis of the poplar TLP family suggests that the expansion of this gene family was followed by diversification, as differences in expression patterns and predicted properties correlate with phylogeny. In particular, we identified a clade of poplar TLPs that cluster to a single 350 kb locus of chromosome I and that are up-regulated by poplar leaf rust infection. A wider phylogenetic analysis of eukaryote TLPs - including plant, animal and fungi sequences - shows that TLP gene content and diversity increased markedly during land plant evolution. Mapping the reported functions of characterized TLPs to the eukaryote phylogenetic tree showed that antifungal or glycan-lytic properties are widespread across eukaryote phylogeny, suggesting that these properties are shared by most TLPs and are likely associated with the presence of a conserved acidic cleft in their 3D structure. Also, we established an exhaustive catalog of TLPs with atypical architectures such as small-TLPs, TLP-kinases and small-TLP-kinases, which have potentially developed alternative functions (such as putative receptor kinases for pathogen sensing and signaling).

CONCLUSION

Our study, based on the most recent plant genome sequences, provides evidence for TLP gene family diversification during land plant evolution. We have shown that the diverse functions described for TLPs are not restricted to specific clades but seem to be universal among eukaryotes, with some exceptions likely attributable to atypical protein structures. In the perennial plant model Populus, we unravelled the TLPs likely involved in leaf rust resistance, which will provide the foundation for further functional investigations.

Identification, characterization, and expression analyses of class II and IV chitinase genes from Douglas-fir seedlings infected by Phellinus sulphurascens

Laminated root rot (LRR) disease, caused by the fungus Phellinus sulphurascens, is a major threat to coastal Douglas-fir (DF) (Pseudotsuga menziesii) forests in western North America. Understanding host-pathogen interactions of this pathosystem is essential to manage this important conifer root disease. Our research objectives were to identify DF pathogenesis-related (PR) genes and analyze their expression patterns over the course of infection. We constructed a cDNA library of Phellinus sulphurascens-infected DF seedling roots and sequenced a total of 3,600 random cDNA clones from this library. One of the largest groups of identified genes (203 cDNA clones) matched with chitinase genes reported in other plant species. We identified at least three class II and six class IV chitinase genes from DF seedlings. Quantitative reverse-transcriptase polymerase chain reaction analyses showed significant differential expression patterns locally in root tissues and systemically in needle tissues after fungal invasion. Nonetheless, there was a common trend in gene expression patterns for most of the chitinase genes: an upregulation within 12 h of pathogen inoculation followed by down-regulation within 2 to 3 days postinoculation (dpi), and then further upregulation within 5 to 7 dpi. Western immunoblot data showed differential accumulation of class IV chitinases in Phellinus sulphurascens-infected DF seedlings. Further detailed functional analyses will help us to understand the specific role of DF chitinases in defense against Phellinus sulphurascens infection.