Identification of genes up-regulated during conidiation of Fusarium oxysporum through expressed sequence tag analysis

Fusarium pathogenomics

Fusarium is a genus of filamentous fungi that contains many agronomically important plant pathogens, mycotoxin producers, and opportunistic human pathogens. Comparative analyses have revealed that the Fusarium genome is compartmentalized into regions responsible for primary metabolism and reproduction (core genome), and pathogen virulence, host specialization, and possibly other functions (adaptive genome). Genes involved in virulence and host specialization are located on pathogenicity chromosomes within strains pathogenic to tomato (Fusarium oxysporum f. sp. lycopersici) and pea (Fusarium 'solani' f. sp. pisi). The experimental transfer of pathogenicity chromosomes from F. oxysporum f. sp. lycopersici into a nonpathogen transformed the latter into a tomato pathogen. Thus, horizontal transfer may explain the polyphyletic origins of host specificity within the genus. Additional genome-scale comparative and functional studies are needed to elucidate the evolution and diversity of pathogenicity mechanisms, which may help inform novel disease management strategies against fusarial pathogens.

Mutation of FVS1, encoding a protein with a sterile alpha motif domain, affects asexual reproduction in the fungal plant pathogen Fusarium oxysporum

Fusarium oxysporum produces three kinds of asexual spores: microconidia, macroconidia and chlamydospores. We previously analysed expressed sequence tags during vegetative growth and conidiation in F. oxysporum and found 42 genes that were markedly upregulated during conidiation compared to vegetative growth. One of the genes, FVS1, encodes a protein with a sterile alpha motif (SAM) domain, which functions in protein-protein interactions that are involved in transcriptional or post-transcriptional regulation and signal transduction. Here, we made FVS1-disrupted mutants from the melon wilt pathogen F. oxysporum f. sp. melonis. Although the mutants produced all three kinds of asexual spores with normal morphology, they formed markedly fewer microconidia and macroconidia than the wild type. The mutants appeared to have a defect in the development of the conidiogenesis cells, conidiophores and phialides, required for the formation of microconidia and macroconidia. In contrast, chlamydospore formation was dramatically promoted in the mutants. The growth rates of the mutants on media were slightly reduced, indicating that FVS1 is also involved in, but not essential for, vegetative growth. We also observed that mutation of FVS1 caused defects in conidial germination and virulence, suggesting that the Fvs1 has pleiotropic functions in F. oxysporum.

Spatial and temporal dynamics of the colonization of Pinus radiata by Fusarium circinatum, of conidiophora development in the pith and of traumatic resin duct formation

· Fusarium circinatum causes pitch canker disease in a wide range of pine trees, including Pinus radiata, with devastating economic consequences. · To assess the spatial and temporal dynamics of growth of this pathogen in radiata pine, we examined the process of infection using both real-time PCR to quantify fungal biomass inside the plant host, and confocal microscopy using a green fluorescent protein (GFP)-tagged strain of F. circinatum. · Pathogen growth exhibited three distinct phases: an initial exponential increase in fungal biomass, concomitant with pathogen colonization of the cortex and phloem; a slowdown in fungal growth coincident with sporulating hyphae deep within the host; and stabilization of the fungal biomass when the first wilting symptoms appeared. The number of resin ducts in the xylem was found to increase in response to infection and the fungus grew inside both constitutive and traumatic resin ducts. · These results indicate that conidiation may contribute to the spatial or temporal dissemination of the pathogen. Moreover, the present findings raise the intriguing possibility that the generation of traumatic resin ducts may be of more benefit to the fungus than to the plant.

OsBADH1 is possibly involved in acetaldehyde oxidation in rice plant peroxisomes

Although rice (Oryza sativa L.) produces little glycine betaine (GB), it has two betaine aldehyde dehydrogenase (BADH; EC 1.2.1.8) gene homologs (OsBADH1 and OsBADH2). We found that OsBADH1 catalyzes the oxidation of acetaldehyde efficiently, while the activity of OsBADH2 is extremely low. The accumulation of OsBADH1 mRNA decreases following submergence treatment, but quickly recovers after re-aeration. We confirmed that OsBADH1 localizes in peroxisomes. In this paper, a possible physiological function of OsBADH1 in the oxidation of acetaldehyde produced by catalase in rice plant peroxisomes is discussed.

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

Nitrite reductase gene upregulated during conidiation is involved in macroconidium formation in Fusarium oxysporum

Fusarium oxysporum produces three kinds of asexual spores, microconidia, macroconidia, and chlamydospores. We previously found that the transcript level of the nitrite reductase gene of F. oxysporum, named FoNIIA, was markedly upregulated during conidiation compared with during vegetative growth. FoNIIA was also found to be positively regulated by Ren1 that is a transcription regulator controlling development of microconidia and macroconidia. In this study, we analyzed the function of FoNIIA in conidiation of F. oxysporum. Conidiation cultures showed markedly higher level of accumulation of FoNiiA protein as well as FoNIIA mRNA than vegetative growth cultures. FoNIIA protein was significantly decreased in cultures of the REN1 disruption mutant compared with that of the wild type. These results confirmed that FoNIIA expression is upregulated during conidiation and is positively regulated by REN1. The FoNIIA disruption mutants produced microconidia, macroconidia, and chlamydospores, which were morphologically indistinguishable from those of the wild type. The mutants, however, produced significantly fewer macroconidia than the wild type, although the wild type and mutant strains produced similar numbers of microconidia and chlamydospores. These results demonstrate that nitrite reductase is involved in quantitative control of macroconidium formation as well as nitrate utilization in F. oxysporum.

Stage-specific gene expression during urediniospore germination in Puccinia striiformis f. sp tritici

Background

Puccinia striiformis f. sp. tritici is an obligate biotrophic pathogen that causes leaf stripe rust on wheat. Although it is critical to understand molecular mechanisms of pathogenesis in the wheat stripe rust fungus for developing novel disease management strategies, little is known about its genome and gene functions due to difficulties in molecular studies with this important pathogen. To identify genes expressed during early infection stages, in this study we constructed a cDNA library with RNA isolated from urediniospores of P. striiformis f. sp. tritici germinated for 10 h.

Results

A total of 4798 ESTs were sequenced from the germinated urediniospore library and assembled into 315 contigs and 803 singletons. About 23.9% and 13.3% of the resulting 1118 unisequences were homologous to functionally characterized proteins and hypothetical proteins, respectively. The rest 62.8% unisequences had no significant homologs in GenBank. Several of these ESTs shared significant homology with known fungal pathogenicity or virulence factors, such as HESP767 of the flax rust and PMK1, GAS1, and GAS2 of the rice blast fungus. We selected six ESTs (Ps28, Ps85, Ps87, Ps259, Ps261, and Ps159) for assaying their expression patterns during urediniospore germination and wheat infection by quantitative real-time PCR. All of them had the highest transcript level in germinated urediniospores and a much less transcript level in un-germinated urediniospores and infected wheat tissues (1–7 dpi). The transcript level of Ps159 increased at later infection stages (6–7 dpi). Our data indicated that these genes were highly expressed in germinated urediniospores and may play important roles in fungal-plant interactions during early infection stages in the wheat stripe rust fungus.

Conclusion

Genes expressed in germinated urediniospores of P. striiformis f. sp. tritici were identified by EST analysis. Six of them were confirmed by quantitative real-time PCR assays to be highly expressed in germinated urediniospores.

Gene discovery and transcript analyses in the corn smut pathogen Ustilago maydis: expressed sequence tag and genome sequence comparison

Background

Ustilago maydis is the basidiomycete fungus responsible for common smut of corn and is a model organism for the study of fungal phytopathogenesis. To aid in the annotation of the genome sequence of this organism, several expressed sequence tag (EST) libraries were generated from a variety of U. maydis cell types. In addition to utility in the context of gene identification and structure annotation, the ESTs were analyzed to identify differentially abundant transcripts and to detect evidence of alternative splicing and anti-sense transcription.

Results

Four cDNA libraries were constructed using RNA isolated from U. maydis diploid teliospores (U. maydis strains 518 × 521) and haploid cells of strain 521 grown under nutrient rich, carbon starved, and nitrogen starved conditions. Using the genome sequence as a scaffold, the 15,901 ESTs were assembled into 6,101 contiguous expressed sequences (contigs); among these, 5,482 corresponded to predicted genes in the MUMDB (MIPS Ustilago maydis database), while 619 aligned to regions of the genome not yet designated as genes in MUMDB. A comparison of EST abundance identified numerous genes that may be regulated in a cell type or starvation-specific manner. The transcriptional response to nitrogen starvation was assessed using RT-qPCR. The results of this suggest that there may be cross-talk between the nitrogen and carbon signalling pathways in U. maydis. Bioinformatic analysis identified numerous examples of alternative splicing and anti-sense transcription. While intron retention was the predominant form of alternative splicing in U. maydis, other varieties were also evident (e.g. exon skipping). Selected instances of both alternative splicing and anti-sense transcription were independently confirmed using RT-PCR.

Conclusion

Through this work: 1) substantial sequence information has been provided for U. maydis genome annotation; 2) new genes were identified through the discovery of 619 contigs that had previously escaped annotation; 3) evidence is provided that suggests the regulation of nitrogen metabolism in U. maydis differs from that of other model fungi, and 4) Alternative splicing and anti-sense transcription were identified in U. maydis and, amid similar observations in other basidiomycetes, this suggests these phenomena may be widespread in this group of fungi. These advances emphasize the importance of EST analysis in genome annotation.

Fow2, a Zn(II)2Cys6-type transcription regulator, controls plant infection of the vascular wilt fungus Fusarium oxysporum

The filamentous fungus Fusarium oxysporum is a soil-borne parasite that causes vascular wilts in a wide variety of crops by directly penetrating roots and colonizing the vascular tissue. In previous work, we generated the non-pathogenic mutant B137 of the melon wilt pathogen F. oxysporum f. sp. melonis by using restriction enzyme-mediated integration (REMI) mutagenesis. Molecular characterization of B137 revealed that this mutant has a single-copy plasmid insertion in a gene, designated FOW2, which encodes a putative transcription regulator belonging to the Zn(II)2Cys6 family. The REMI mutant B137 and other FOW2-targeted mutants completely lost pathogenicity, but were not impaired in vegetative growth and conidiation in cultures. Microscopic observation of infection behaviours of green fluorescent protein (GFP)-marked wild-type and mutant strains revealed that the mutants were defective in their abilities to invade roots and colonize plant tissues. FOW2 is conserved in F. oxysporum pathogens that infect different plants. The FOW2-targeted mutants of the tomato wilt pathogen F. oxysporum f. sp. lycopersici also lost pathogenicity. Nuclear localization of Fow2 was verified using strains expressing Fow2-GFP and GFP-Fow2 fusion proteins. These data strongly suggest that FOW2 encodes a transcription regulator controlling the plant infection capability of F. oxysporum pathogens.