ChsVb, a class VII chitin synthase involved in septation, is critical for pathogenicity in Fusarium oxysporum

Gene disruption and characterization of a class V chitin synthase in Botrytis cinerea

Cell-wall chitin biosynthesis in the phytopathogenic fungus Botrytis cinerea involves 7 classes of chitin synthases. In this study, we disrupted a gene encoding a chitin synthase with a myosin-like motor domain (BcchsV) through Agrobacterium tumefaciens mediated transformation. The resulting mutant was not significantly affected in either growth characteristics or pathogenicity on tomato leaves. Surprisingly, the BcchsV mutant exhibited a 31% (m/m) increase in its chitin content compared with the wild-type strain. In addition, the BcchsV mutant showed increased sensitivity to Calcofluor White and slightly enhanced tolerance to cell-wall disturbing substances and osmosis regulators, including SDS, sorbitol, and NaCl. These results suggest that Bcchs does not play an essential role in the synthesis of cell-wall chitin in B. cinerea. However, disruption of this gene provoked a compensatory mechanism regulating the cellular response to cell-wall damage.

Transcription levels of CHS5 and CHS4 genes in Paracoccidioides brasiliensis mycelial phase, respond to alterations in external osmolarity, oxidative stress and glucose concentration

The complete sequence of Paracoccidioides brasiliensis CHS5 gene, encoding a putative chitin synthase revealed a 5583nt open reading frame, interrupted by three introns of 82, 87 and 97bp (GenBank Accession No EF654132). The deduced protein contains 1861 amino acids with a predicted molecular weight of 206.9kDa. Both its large size and the presence of a N-terminal region of approx. 800 residues with a characteristic putative myosin motor-like domain, allow us to include PbrChs5 into class V fungal chitin synthases. Sequence analysis of over 4kb from the 5' UTR region in CHS5, revealed the presence of a previously reported CHS4 gene in P. brasiliensis, arranged in a head-to-head configuration with CHS5. A motif search in this shared region showed the presence of stress response elements (STREs), three binding sites for the transcription activators Rlm1p (known to be stimulated by hypo-osmotic stress) and clusters of Adr1 (related to glucose repression). A quantitative RT-PCR analysis pointed to changes in transcription levels for both genes following oxidative stress, alteration of external osmolarity and under glucose-repressible conditions, suggesting a common regulatory mechanism of transcription.

Dynamics of cell wall components of Magnaporthe grisea during infectious structure development

Oligosaccharides derived from cell wall of fungal pathogens induce host primary immune responses. To understand fungal strategies circumventing the host plant immune responses, cell wall polysaccharide localization was investigated using fluorescent labels during infectious structure differentiation in the rice blast fungus Magnaporthe grisea. alpha-1,3-glucan was labelled only on appressoria developing on plastic surfaces, whereas it was detected on both germ tubes and appressoria on plant surfaces. Chitin, chitosan and beta-1,3-glucan were detected on germ tubes and appressoria regardless of the substrate. Major polysaccharides labelled at accessible surface of infectious hyphae were alpha-1,3-glucan and chitosan, but after enzymatic digestion of alpha-1,3-glucan, beta-1,3-glucan and chitin became detectable. Immunoelectron microscopic analysis showed alpha-1,3-glucan and beta-1,3-glucan intermixed in the cell wall of infectious hyphae; however, alpha-1,3-glucan tended to be distributed farther from the fungal cell membrane. The fungal cell wall became more tolerant to chitinase digestion upon accumulation of alpha-1,3-glucan. Accumulation of alpha-1,3-glucan was dependent on the Mps1 MAP kinase pathway, which was activated by a plant wax derivative, 1,16-hexadecanediol. Taken together, alpha-1,3-glucan spatially and functionally masks beta-1,3-glucan and chitin in the cell wall of infectious hyphae. Thus, a dynamic change of composition of cell wall polysaccharides occurs during plant infection in M. grisea.

Pathogen profile update: Fusarium oxysporum

TAXONOMY

Kingdom Fungi; Phylum Ascomycota; Class Sordariomycetes; Order Hypocreales; Family Nectriaceae; genus Fusarium.

HOST RANGE

Very broad at the species level. More than 120 different formae speciales have been identified based on specificity to host species belonging to a wide range of plant families.

DISEASE SYMPTOMS

Initial symptoms of vascular wilt include vein clearing and leaf epinasty, followed by stunting, yellowing of the lower leaves, progressive wilting, defoliation and, finally, death of the plant. On fungal colonization, the vascular tissue turns brown, which is clearly visible in cross-sections of the stem. Some formae speciales are not primarily vascular pathogens, but cause foot and root rot or bulb rot.

ECONOMIC IMPORTANCE

Can cause severe losses in many vegetables and flowers, field crops, such as cotton, and plantation crops, such as banana, date palm and oil palm.

CONTROL

Use of resistant varieties is the only practical measure for controlling the disease in the field. In glasshouses, soil sterilization can be performed.

USEFUL WEBSITES

http://www.broad.mit.edu/annotation/genome/fusarium_group/MultiHome.html; http://www.fgsc.net/Fusarium/fushome.htm; http://www.phi-base.org/query.php

Class III chitin synthase ChsB of Aspergillus nidulans localizes at the sites of polarized cell wall synthesis and is required for conidial development

Class III chitin synthases play important roles in tip growth and conidiation in many filamentous fungi. However, little is known about their functions in those processes. To address these issues, we characterized the deletion mutant of a class III chitin synthase-encoding gene of Aspergillus nidulans, chsB, and investigated ChsB localization in the hyphae and conidiophores. Multilayered cell walls and intrahyphal hyphae were observed in the hyphae of the chsB deletion mutant, and wavy septa were also occasionally observed. ChsB tagged with FLAG or enhanced green fluorescent protein (EGFP) localized mainly at the tips of germ tubes, hyphal tips, and forming septa during hyphal growth. EGFP-ChsB predominantly localized at polarized growth sites and between vesicles and metulae, between metulae and phialides, and between phalides and conidia in asexual development. These results strongly suggest that ChsB functions in the formation of normal cell walls of hyphae, as well as in conidiophore and conidia development in A. nidulans.

Insight into the molecular requirements for pathogenicity of Fusarium oxysporum f. sp. lycopersici through large-scale insertional mutagenesis

BACKGROUND

Fusarium oxysporum f. sp. lycopersici is the causal agent of vascular wilt disease in tomato. In order to gain more insight into the molecular processes in F. oxysporum necessary for pathogenesis and to uncover the genes involved, we used Agrobacterium-mediated insertional mutagenesis to generate 10,290 transformants and screened the transformants for loss or reduction of pathogenicity.

RESULTS

This led to the identification of 106 pathogenicity mutants. Southern analysis revealed that the average T-DNA insertion is 1.4 and that 66% of the mutants carry a single T-DNA. Using TAIL-PCR, chromosomal T-DNA flanking regions were isolated and 111 potential pathogenicity genes were identified.

CONCLUSIONS

Functional categorization of the potential pathogenicity genes indicates that certain cellular processes, such as amino acid and lipid metabolism, cell wall remodeling, protein translocation and protein degradation, seem to be important for full pathogenicity of F. oxysporum. Several known pathogenicity genes were identified, such as those encoding chitin synthase V, developmental regulator FlbA and phosphomannose isomerase. In addition, complementation and gene knock-out experiments confirmed that a glycosylphosphatidylinositol-anchored protein, thought to be involved in cell wall integrity, a transcriptional regulator, a protein with unknown function and peroxisome biogenesis are required for full pathogenicity of F. oxysporum.

The Magnaporthe grisea class VII chitin synthase is required for normal appressorial development and function

The plant pathogenic fungus Magnaporthe grisea is able to enter its host via appressorium-mediated penetration. Earlier investigations have shown that these infection structures are rich in the cell wall polysaccharide chitin. Previously, we have described how the transcription of a class VII chitin synthase-encoding gene CHS7 is completely dependent on the putative transcription factor Con7p during the germination of conidia, and how con7(-) mutants are unable to form appressoria under any conditions tested. Because of the pleiotropic effects of the con7(-) mutation, we examined the consequences of the targeted deletion of CHS7. The chs7(-) mutants generated were unable to form appressoria on artificial surfaces, except following the application of the exogenous inducers 1,16-hexadecanediol and cyclic adenosine monophosphate. The appressoria formed had a reduced chitin content and were often found to be smaller and misshapen compared with the wild-type. chs7(-) mutants were significantly reduced in their ability to enter rice plants, but growth in planta was not affected. Reverse transcriptase-polymerase chain reaction analysis demonstrated that CHS7 transcription was strongly induced on germination of spores, and a green fluorescent protein-tagged Chs7p protein was found to be produced abundantly during infection-related morphogenesis. Together, these data suggest that the class VII chitin synthase Chs7p of M. grisea is required for normal appressorium formation and function.

Identification of virulence genes in Fusarium oxysporum f. sp. lycopersici by large-scale transposon tagging

Forward genetic screens are efficient tools for the dissection of complex biological processes, such as fungal pathogenicity. A transposon tagging system was developed in the vascular wilt fungus Fusarium oxysporum f. sp. lycopersici by inserting the novel modified impala element imp160::gfp upstream of the Aspergillus nidulans niaD gene, followed by transactivation with a constitutively expressed transposase. A collection of 2072 Nia(+) revertants was obtained from reporter strain T12 and screened for alterations in virulence, using a rapid assay for invasive growth on apple slices. Seven strains exhibited reduced virulence on both apple slices and intact tomato plants. Five of these were true revertants showing the re-insertion of imp160::gfp within or upstream of predicted coding regions, whereas the other two showed either excision without re-insertion or no excision. Linkage between imp160::gfp insertion and virulence phenotype was determined in four transposon-tagged loci using targeted deletion in the wild-type strain. Knockout mutants in one of the genes, FOXG_00016, displayed significantly reduced virulence, and complementation of the original revertant with the wild-type FOXG_00016 allele fully restored virulence. FOXG_00016 has homology to the velvet gene family of A. nidulans. The high rate of untagged virulence mutations in the T12 reporter strain appears to be associated with increased genetic instability, possibly as a result of the transactivation of endogenous transposable elements by the constitutively expressed transposase.