Molecular genetic evidence for the involvement of a specific polygalacturonase, P2c, in the invasion and spread of Aspergillus flavus in cotton bolls

Wanted: pathogenesis-related marker molecules for Fusarium oxysporum

Although Fusarium oxysporum pathogens cause severe wilts in about 80 botanical species, the mechanisms of pathogenicity and symptom induction are poorly understood. Knowledge about the genetic and biochemical pathways involved in the pathogenesis of F. oxysporum would be invaluable in getting targets for both fungicide development and search for biocontrol agents. In this respect, we described the main approaches that have been developed to identify some mechanisms underlying the pathogenesis of F. oxysporum. During the last decades, the potential functions triggering of F. oysporum pathogenicity have mainly been investigated by comparing soilborne pathogenic strains with nonpathogenic ones with regards to the analysis of the pre- and infection stages and of the resulting plant-fungus interactions. The relatively recent progress in the molecular biology of this fungus has allowed complementary approaches to be developed in order to identify key factors involved in F. oxysporum pathogenicity. Screening mutants of F. oxysporum for loss of virulence led to the successful identification of some pathogenesis-related factors, such as hydrophobicity or attachment of germlings. Taken together, the strategies described above support the idea that changes in fungal metabolism is also of importance in triggering of F. oxysporum pathogenesis.

Two Brassica napus polygalacturonase inhibitory protein genes are expressed at different levels in response to biotic and abiotic stresses

Plants encode a distinct set of polygalacturonase inhibitory proteins (PGIPs) that function to inhibit polygalacturonase enzymes produced by soft-rot fungal pathogens. We characterized two PGIP-encoding genes ( Bnpgip1 and Bnpgip2) from Brassica napus DH12075 (a double-haploid line derived from a cross between 'Crésor' and 'Westar'). The two proteins exhibit 67.4% identity at the amino acid level and contain 10 imperfect leucine-rich repeats. The pgip genes are present as a small multigene family in B. napus with at least four members. Bnpgip1 and Bnpgip2 are constitutively expressed in roots, stems, flower buds and open flowers. In mature leaf tissue, different levels of induction were observed in response to biotic and abiotic stresses. Bnpgip1 expression was highly responsive to flea beetle feeding and mechanical wounding, weakly responsive to Sclerotinia sclerotiorum infection and exposure to cold but not to dehydration. Conversely, Bnpgip2 expression was strongly induced by S. sclerotiorum infection and to a lesser degree by wounding but not by flea beetle feeding. Application of jasmonic acid to leaves induced both Bnpgip1 and Bnpgip2 gene expression; however, salicylic acid did not activate either gene. Taken together, these results suggest that separate pathways regulate Bnpgip1 and Bnpgip2, and that their roles in plant development or resistance to biotic and abiotic stress differ.

Two Brassica napus polygalacturonase inhibitory protein genes are expressed at different levels in response to biotic and abiotic stresses

Plants encode a distinct set of polygalacturonase inhibitory proteins (PGIPs) that function to inhibit polygalacturonase enzymes produced by soft-rot fungal pathogens. We characterized two PGIP-encoding genes ( Bnpgip1 and Bnpgip2) from Brassica napus DH12075 (a double-haploid line derived from a cross between 'Crésor' and 'Westar'). The two proteins exhibit 67.4% identity at the amino acid level and contain 10 imperfect leucine-rich repeats. The pgip genes are present as a small multigene family in B. napus with at least four members. Bnpgip1 and Bnpgip2 are constitutively expressed in roots, stems, flower buds and open flowers. In mature leaf tissue, different levels of induction were observed in response to biotic and abiotic stresses. Bnpgip1 expression was highly responsive to flea beetle feeding and mechanical wounding, weakly responsive to Sclerotinia sclerotiorum infection and exposure to cold but not to dehydration. Conversely, Bnpgip2 expression was strongly induced by S. sclerotiorum infection and to a lesser degree by wounding but not by flea beetle feeding. Application of jasmonic acid to leaves induced both Bnpgip1 and Bnpgip2 gene expression; however, salicylic acid did not activate either gene. Taken together, these results suggest that separate pathways regulate Bnpgip1 and Bnpgip2, and that their roles in plant development or resistance to biotic and abiotic stress differ.

Disruption of Botrytis cinerea pectin methylesterase gene Bcpme1 reduces virulence on several host plants

The pectinolytic enzyme pectin methylesterase (PME) hydrolyses pectin in methanol and polygalacturonic acid. In the expressed sequence tag library of Botrytis cinerea T4, we identified a 1,041 bp Bcpme1 cDNA potentially encoding a 346-amino acid protein of 37 kDa showing 46.8% identity with Aspergillus sp. PMEs. Bcpme1 is a single copy gene and is similarly expressed in glucose and pectin containing media. To evaluate the role of Bcpme1 in Botrytis cinerea virulence, a mutant in Bcpme1 was generated by gene disruption. The Bcpme1 mutant showed similar growth on rich medium but reduced growth on pectin medium. Two isozymes of pI 7.4 and 7.1 were detected in pectin liquid-culture supernatants of wild-type strain Bd90 analyzed by isoelectric focusing-polyacrylamide gel electrophoresis, while those of Bcpme1 mutant possessed only the pI 7.1 isozyme. BCPME1, the pI 7.4 isozyme, is the major PME activity, as PME activity is 75% reduced in Bcpme1 mutant. Moreover, the Bcpme1 mutant was less virulent on apple fruits, grapevine, and Arabidopsis thaliana leaves. Those phenotypes were complemented by reintroducing a Bcpme1 copy in the Bcpme1 mutant. These results showed that B. cinerea possessed more than one PME-encoding gene and that BCPME1 is an important determinant of B. cinerea virulence.

Characterization and evolutionary analysis of a large polygalacturonase gene family in the oomycete plant pathogen Phytophthora cinnamomi

Polygalacturonases (PGs) are secreted by fungal pathogens during saprophytic and parasitic growth, and their degradation of pectin in the plant cell wall is believed to play a major role in tissue invasion and maceration. In this study, PG activity was demonstrated in culture filtrates of the oomycete plant pathogen, Phytophthora cinnamomi. A P. cinnamomi pg gene fragment amplified using degenerate primers based on conserved regions in fungal and plant PGs was used to isolate 17 complete P. cinnamomi pg genes and pseudogenes from a genomic library and partial sequence for another two genes. Gel blotting of genomic DNA indicated that there may be even more pg genes in the P. cinnamomi genome. P. cinnamomi pg gene sequences were expressed in PG-deficient yeast and found to confer PG activity, thereby confirming their functional identity. The predicted mature P. cinnamomi PGs fall into subgroups that exhibit large differences in the extent of N-glycosylation, isoelectric points, and N- and C-terminal structure. Evidence for birth-and-death and reticulate evolution in the P. cinnamomi pg gene family was obtained, and some codons for surface exposed residues in the P. cinnamomi PGs were shown to have been subject to diversifying selection. Contrary to accepted phylogenies for other proteins, phylogenetic analysis of the P. cinnamomi PGs revealed a closer relationship with PGs from true fungi than with those from plants.

A cis-acting sequence homologous to the yeast filamentation and invasion response element regulates expression of a pectinase gene from the bean pathogen Colletotrichum lindemuthianum

Phytopathogenic fungi secrete hydrolytic enzymes that degrade plant cell walls, notably pectinases. The signaling pathway(s) that control pectinase gene expression are currently unknown in filamentous fungi. Recently, the green fluorescent protein coding sequence was used as a reporter gene to study the expression of CLPG2, a gene encoding an endopolygalacturonase of the bean pathogen Colletotrichum lindemuthianum. CLPG2 is transcriptionally induced by pectin in the axenic culture of the fungus and during formation of the appressorium, an infection structure specialized in plant tissue penetration. In the present study, promoter deletion and mutagenesis, as well as gel shift mobility assays, allowed for the first time identification of cis-acting elements that bind protein factors and are essential for the regulation of a pectinase gene. We found that two different adjacent DNA motifs are combined to form an active element that shows a strong sequence homology with the yeast filamentation and invasion response element. The same element is required for the transcriptional activation of CLPG2 by pectin and during appressorium development. This study strongly suggests that the control of virulence genes of fungal plant pathogens, such as pectinases, involves the formation of a complex of transcriptional activators similar to those regulating the invasive growth in yeast.

Polygalacturonase-inhibiting proteins in defense against phytopathogenic fungi

Polygalacturonase-inhibiting proteins (PGIPs) are ubiquitous plant cell wall proteins that are directed against fungal polygalacturonases (PGs), which are important pathogenicity factors. The inhibiting activity of PGIPs directly reduces the aggressive potential of PGs. In addition, it causes PGs to form more long-chain oligogalacturonides that are able to induce defense responses, thereby indirectly contributing to the plant defense. Recent evidence demonstrates that PGIPs are efficient defense proteins and limit fungal invasion. PGIPs and the products of many plant resistance genes share a leucine-rich repeat (LRR) structure, which provides specific recognition of pathogen-derived molecules. The high level of polymorphism of both PGIPs and polygalacturonases is an invaluable tool for deciphering the structure, function and evolution of plant LRR proteins and their ligands. Furthermore, information about PGIP structure and evolution paves the way to the development of efficient strategies for crop protection.

Polygalacturonase is a pathogenicity factor in the Claviceps purpurea/rye interaction

Claviceps purpurea is a biotrophic, organ-specific pathogen of grasses and cereals, attacking exclusively young ovaries. We have previously shown that its mainly intercellular growth is accompanied by degradation of pectin, and that two endopolygalacturonase genes (cppg1/cppg2) are expressed throughout all stages of infection. We report here on a functional analysis of these genes using a gene-replacement approach. Mutants lacking both polygalacturonase genes are not affected in their vegetative properties, but they are nearly nonpathogenic on rye. Complementation of the mutants with wild-type copies of cppg1 and cppg2 fully restored pathogenicity, proving that the endopolygalacturonases encoded by cppg1 and cppg2 represent pathogenicity factors in the interaction system C. purpurea/Secale cereale, the first unequivocally identified so far in this system.

Endopolygalacturonase is encoded by a multigene family in the basidiomycete Chondrostereum purpureum

The basidiomycete Chondrostereum purpureum produces several plant cell wall-degrading enzymes, including endopolygalacturonase (endoPG). Degenerate oligonucleotide primers were designed according to conserved regions of endoPG genes from various fungi, plants, and bacteria and used to amplify members of this gene family from C. purpureum. Four different amplification products showed significant similarity to known endoPGs and were used as hybridization probes to screen a library of genomic DNA sequences and to retrieve five full-length endoPG genes (epgA, epgB1, epgB2, epgC, and epgD). The identities between the deduced polypeptides for epgA, epgB1, epgC, and epgD ranged from 61.8 to 80.0%, while the deduced polypeptides for epgB1 and epgB2 shared 97.1% identity. Phylogenetic analysis suggested that the duplication of existing endoPG genes occurred after the divergence of the ascomycetes and basidiomycetes. C. purpureum is the first basidiomycete fungus for which the endoPG gene family has been described.

Comparison of endo-polygalacturonase activities of citrus and non-citrus races of Geotrichum candidum, and cloning and expression of the corresponding genes

summary Geotrichum candidum citrus race, a fungus that causes a sour rot disease in citrus fruits, secretes an endo-polygalacturonase (PG) that may facilitate the disease. There also exists a non-citrus race that is non-pathogenic to citrus fruits. In this research, we found that the PG activity of the citrus race isolates was much higher than that of the non-citrus race isolates in culture medium and inoculated lemon peel, and that there was a significant correlation between the PG activity and pathogenicity. We isolated the two corresponding PG genes, S31pg1 and S63pg1, from citrus race S31 and non-citrus race S63, respectively. S31PG1 and S63PG1 consisted of 368 and 369 amino acids, respectively. The two PG genes showed 68% identity at the amino acid level. In expression studies, S31pg1 transcript was detected in mycelia grown in liquid cultures of citrus race S31 containing either glucose, pectin or lemon peel broth. The transcript was also detected in lemon peel inoculated with the isolate. On the other hand, no transcript of S63pg1 was detected in mycelia grown on any liquid cultures of non-citrus race S63 and lemon peel inoculated with the isolate. These results indicate that PG may play an important role in the development of the sour rot symptom and be involved in the difference of pathogenicity between the two races.

Local modulation of host pH by Colletotrichum species as a mechanism to increase virulence

The phytopathogenic fungus Colletotrichum gloeosporioides produces one pectate lyase (PL) that is a key virulence factor in disease development. During growth of C. gloeosporioides, Colletotrichum acutatum, and Colletotrichum coccodes in acidified yeast extract medium, the fungus secreted ammonia and increased the medium pH. Ammonia accumulation and the consequent pH change increased as a function of initial pH and buffer capacity of the medium. PL secretion by C. gloeosporioides correspondingly increased as the pH of the medium increased. The C. gloeosporioides pelB gene-disrupted mutant was able to increase ammonia accumulation and pH of the media similarly to the wild-type isolate. C. gloeosporioides in avocado, C. coccodes in tomato, and C. acutatum in apple showed ammonia accumulation in the infected area where pH increased to 7.5 to 8 and PL activity is optima. In nonhost interactions where C. gloeosporioides was inoculated in apples, the addition of ammonia-releasing compounds significantly enhanced pathogenicity to levels similar to those caused by the compatible C. acutatum-apple interaction. The results therefore suggest the importance of ammonia secretion as a virulence factor, enhancing environmental pH and pathogenicity of the Colletotrichum species.

Colletotrichum gloeosporioides pelB is an important virulence factor in avocado fruit-fungus interaction

Colletotrichum gloeosporioides is an important pathogen of tropical and subtropical fruits. The C. gloeosporioides pelB gene was disrupted in the fungus via homologous recombination. Three independent isolates, GD-14, GD-23, and GD-29, did not produce or secrete pectate lyase B (PLB) and exhibited 25% lower pectate lyase (PL) and pectin lyase (PNL) activities and 15% higher polygalacturonase (PG) activity than the wild type. The PLB mutants exhibited no growth reduction on glucose, Na polypectate, or pectin as the sole carbon source at pH 3.8 or 6.0, except for a 15% reduction on pectin at pH 6.0. When pelB mutants were inoculated onto avocado fruits, however, a 36 to 45% reduction in estimated decay diameter was observed compared with the two controls, the wild type and undisrupted transformed isolate. In addition, these pelB mutants induced a significantly higher host phenylalanine ammonia lyase activity as well as the antifungal diene, which is indicative of higher host resistance. These results suggest that PLB is an important factor in the attack of C. gloeosporioides on avocado fruit, probably as a result of its virulence factor and role in the induction of host defense mechanisms.

Botrytis cinerea endopolygalacturonase genes are differentially expressed in various plant tissues

Botrytis cinerea, the causal agent of blight, rot, and gray mold on many plant species, secretes various endopolygalacturonases during all stages of infection. The expression pattern of the encoding genes (Bcpg 1-6) was studied on four hosts: tomato, broad bean, apple, and courgette (also known as zucchini). All gene family members are differentially expressed, depending on the stage of infection and the host. Bcpg1 is expressed in all tissues tested although differences in transcript levels occur. Bcpg2 expression is detected early in the infection of three of four plant tissues tested. Bcpg3 and Bcpg5 are expressed in apple fruit tissue, although probably as a result of different regulatory mechanisms. The expression patterns of Bcpg4 and 6 are in agreement with their inducibility by monogalacturonic acid. The pattern of Bcpg gene expression indicates that B. cinerea is equipped with a flexible enzymatic pectate degradation machinery. The studies pinpoint new targets for gene disruption studies.

Pathogenicity genes of phytopathogenic fungi

Summary Recently many fungal genes have been identified that, when disrupted, result in strains with a reduction or total loss of disease symptoms. Such pathogenicity genes are the subject of this review. The large number of pathogenicity genes identified is due to the application of tagged mutagenesis techniques (random or targeted). Genes have been identified with roles in the formation of infection structures, cell wall degradation, overcoming or avoiding plant defences, responding to the host environment, production of toxins, and in signal cascades. Additionally, genes with no database matches and with 'novel' functions have also been found. Improved technologies for mutation analysis and for sequencing and analysing fungal genomes hold promise for identifying many more pathogenicity genes.

Endopolygalacturonase is essential for citrus black rot caused by Alternaria citri but not brown spot caused by Alternaria alternata

Alternaria citri, the cause of Alternaria black rot, and Alternaria alternata rough lemon pathotype, the cause of Alternaria brown spot, are morphologically indistinguishable pathogens of citrus: one causes rot by macerating tissues and the other causes necrotic spots by producing a host-selective toxin. To evaluate the role of endopolygalacturonase (endoPG) in pathogenicity of these two Alternaria spp. pathogens, their genes for endoPG were mutated by gene targeting. The endoPGs produced by these fungi have similar biochemical properties, and the genes are highly similar (99.6% nucleotide identity). The phenotypes of the mutants, however, are completely different. An endoPG mutant of A. citri was significantly reduced in its ability to cause black rot symptoms on citrus as well as in the maceration of potato tissue and could not colonize citrus peel segments. In contrast, an endoPG mutant of A. alternata was unchanged in pathogenicity. The results indicate that a cell wall-degrading enzyme can play different roles in the pathogenicity of fungal pathogens. The role of a cell wall-degrading enzyme depends upon the type of disease but not the taxonomy of the fungus.

Molecular characterization of an endopolygalacturonase from Fusarium oxysporum expressed during early stages of infection

The tomato vascular wilt pathogen Fusarium oxysporum f. sp. lycopersici produces an array of pectinolytic enzymes that may contribute to penetration and colonization of the host plant. Here we report the isolation of pg5, encoding a novel extracellular endopolygalacturonase (endoPG) that is highly conserved among different formae speciales of F. oxysporum. The putative mature pg5 product has a calculated molecular mass of 35 kDa and a pI of 8.3 and is more closely related to endoPGs from other fungal plant pathogens than to PG1, the major endoPG of F. oxysporum. Overexpression of pg5 in a bacterial heterologous system produced a 35-kDa protein with endoPG activity. Accumulation of pg5 transcript is induced by citrus pectin and D-galacturonic acid and repressed by glucose. As shown by reverse transcription-PCR, pg5 is expressed by F. oxysporum in tomato roots during the initial stages of infection. Targeted inactivation of pg5 has no detectable effect on virulence toward tomato plants.

Variation in in vitro alpha-amylase and protease activity is related to the virulence of Aspergillus flavus isolates

Variation in the ability of Aspergillus flavus isolates to spread between cotton boll locules was previously shown to be at least partially related to variation in the production of a specific polygalacturonase (pectinase P2C). To determine if non-pectolytic hydrolase differences between low- and high-virulence isolates exist and, thus, could also potentially contribute to virulence differences, the present investigation was undertaken. Two A. flavus isolates, AF12 with low virulence and lacking pectinase P2C and AF13 with high virulence and producing pectinase P2C, were compared for production of nonpectolytic hydrolases after growth in 10% potato dextrose broth. Activity of amylases, cellulases, xylanases, and proteases was quantified using the radial diffusion/cup plate technique followed by differential staining. Culture filtrates also were subjected to native polyacrylamide gel electrophoresis. Both isolates produced amylases, proteases, and xylanases, whereas cellulases were not detected for either. AF13 produced more amylase than AF12, and this difference was supported by amylase isoform differences between isolates on native polyacrylamide gel electrophoresis gels. AF13 also produced more protease than AF12; however, isoform differences between isolates were inconclusive. These variations in other hydrolytic activities (besides pectinases) may contribute to virulence differences in cotton bolls between AF12 and AF13.

pH signaling in Sclerotinia sclerotiorum: identification of a pacC/RIM1 homolog

Sclerotinia sclerotiorum acidifies its ambient environment by producing oxalic acid. This production of oxalic acid during plant infection has been implicated as a primary determinant of pathogenicity in this and other phytopathogenic fungi. We found that ambient pH conditions affect multiple processes in S. sclerotiorum. Exposure to increasing alkaline ambient pH increased the oxalic acid accumulation independent of carbon source, sclerotial development was favored by acidic ambient pH conditions but inhibited by neutral ambient pH, and transcripts encoding the endopolygalacturonase gene pg1 accumulated maximally under acidic culture conditions. We cloned a putative transcription factor-encoding gene, pac1, that may participate in a molecular signaling pathway for regulating gene expression in response to ambient pH. The three zinc finger domains of the predicted Pac1 protein are similar in sequence and organization to the zinc finger domains of the A. nidulans pH-responsive transcription factor PacC. The promoter of pac1 contains eight PacC consensus binding sites, suggesting that this gene, like its homologs, is autoregulated. Consistent with this suggestion, the accumulation of pac1 transcripts paralleled increases in ambient pH. Pac1 was determined to be a functional homolog of PacC by complementation of an A. nidulans pacC-null strain with pac1. Our results suggest that ambient pH is a regulatory cue for processes linked to pathogenicity, development, and virulence and that these processes may be under the molecular regulation of a conserved pH-dependent signaling pathway analogous to that in the nonpathogenic fungus A. nidulans.

Requirement for either a host- or pectin-induced pectate lyase for infection of Pisum sativum by Nectria hematococca

Fungal pathogens usually have multiple genes that encode extracellular hydrolytic enzymes that may degrade the physical barriers in their hosts during the invasion process. Nectria hematococca, a plant pathogen, has two inducible pectate lyase (PL) genes (pel) encoding PL that can help degrade the carbohydrate barrier in the host. pelA is induced by pectin, whereas pelD is induced only in planta. We show that the disruption of either the pelA or pelD genes alone causes no detectable decrease in virulence. Disruption of both pelA and pelD drastically reduces virulence. Complementation of the double disruptant with pelD gene, or supplementation of the infection droplets of the double disruptant with either purified enzyme, PLA, or PLD, caused a recovery in virulence. These results show that PL is a virulence factor. Thus, we demonstrate that disruption of all functionally redundant genes is required to demonstrate the role of host barrier-degrading enzymes in pathogenesis and that dismissal of the role of such enzymes based on the effects of single-gene disruption may be premature.

Expression of pectate lyase from Colletotrichum gloesosporioides in C. magna promotes pathogenicity

To test the contribution of pectate lyase (PL) to promoting fungal pathogenicity, a pectate lyase gene (pel) from the avocado pathogen Colletotrichum gloeosporioides, isolate Cg-14, was expressed in C. magna isolate L-2.5, a pathogen of cucurbits that causes minor symptoms in watermelon seedlings and avocado fruits. Isolate L-2.5 was transformed with pPCPH-1 containing hph-B as a selectable marker and the 4.1-kb genomic pel clone. Southern hybridization, with the 4.1-kb genomic pel clone or 2.13-kb hph-B cassette as probes, detected integration of pel in transformed C. magna isolates Cm-PL-3 and Cm-PL-10. Western blot (immunoblot) analysis with antibodies against Cg-14 PL detected a single PL secreted by L-2.5 at a molecular mass of 41.5 kDa, whereas the PL of C. gloeosporioides had a molecular mass of 39 kDa. When PL activity was measured 4 days after inoculation in pectolytic enzyme-inducing media (PEIM), transformed isolates Cm-PL-3 and Cm-PL-10 showed additive PL activity relative to both Cg-14 and L-2.5. Transformed isolates also showed additive maceration capabilities on avocado pericarp relative to the wild-type C. magna alone, but did not reach the maceration ability of C. gloeosporioides. However, more severe maceration and damping off developed in watermelon seedlings inoculated with the transformed isolates compared with the two wild-type isolates, which showed no symptom development on these seedlings during the same period. Results clearly show the contribution of a single pel to the pathogenic abilities of C. magna and suggest that PL is a pathogenicity factor required for the penetration and colonization of Colletotrichum species.

Cloning and disruption of pgx4 encoding an in planta expressed exopolygalacturonase from Fusarium oxysporum

Fusarium oxysporum f. sp. lycopersici, the causal agent of tomato vascular wilt, produces an array of pectinolytic enzymes, including at least two exo-alpha1,4-polygalacturonases (exoPGs). A gene encoding an exoPG, pgx4, was isolated with degenerate polymerase chain reaction primers derived from amino acid sequences conserved in two fungal exoPGs. pgx4 encodes a 454 amino acid polypeptide with nine potential N-glycosylation sites and a putative 21 amino acid N-terminal signal peptide. The deduced mature protein has a calculated molecular mass of 47.9 kDa, a pI of 8.0, and 51 and 49% identity with the exoPGs of Cochliobolus carbonum and Aspergillus tubingensis, respectively. The gene is present in a single copy in different formae speciales of F. oxysporum. Expression of pgx4 was detected during in vitro growth on pectin, polygalacturonic acid, and tomato vascular tissue and in roots and stems of tomato plants infected by F. oxysporum f. sp. lycopersici. Two mutants of F. oxysporum f. sp. lycopersici with a copy of pgx4 inactivated by gene replacement were as virulent on tomato plants as the wild-type strain.

A comparison of the pectate lyase genes, pel-1 and pel-2, of Colletotrichum gloeosporioides f.sp. malvae and the relationship between their expression in culture and during necrotrophic infection

Extracellular pectic lyase and polygalacturonase activities of Colletotrichum gloeosporioides f.sp. malvae were detected in broths containing mallow cell wall extract, pectin or glucose as the carbon source. The initial pH of the broth as well as the carbon source had major influences on pectinase enzyme activities. In the host, only pectic lyase activity was detected, which began at the end of the biotrophic phase and increased in the necrotrophic phase of infection. Two full-length pectate lyase cDNAs, pel-1 and pel-2, were cloned from the fungus. Both genes showed similar patterns of expression when the fungus was grown in mallow cell-wall extract and pectin medium, and the only major difference in expression in culture was that only pel-2 was expressed in glucose broth. Expression of pel-1 and pel-2 was also affected by the initial pH of the medium. Expression of pel-2, but not pel-1, was detected during infection of the host, round-leaved mallow, Malva pusilla. Transcripts of pel-2 were first detectable during the necrotrophic phase of infection approx. 24h after the first detection of pectic lyase enzyme activity. A comparison of expression of pel-1 and pel-2 in culture and in planta with other pectinase genes of C. gloeosporioides f.sp. malvae, as well as with other plant pathogenic fungi, indicates that expression during necrotrophic infection correlates with the ability to be expressed in media containing glucose.

The Cochliobolus carbonum SNF1 gene is required for cell wall-degrading enzyme expression and virulence on maize

The production of cell wall-degrading enzymes (wall depolymerases) by plant pathogenic fungi is under catabolite (glucose) repression. In Saccharomyces cerevisiae, the SNF1 gene is required for expression of catabolite-repressed genes when glucose is limiting. An ortholog of SNF1, ccSNF1, was isolated from the maize pathogen Cochliobolus carbonum, and ccsnf1 mutants of HC toxin-producing (Tox2(+)) and HC toxin-nonproducing (Tox2(-)) strains were created by targeted gene replacement. Growth in vitro of the ccsnf1 mutants was reduced by 50 to 95% on complex carbon sources such as xylan, pectin, or purified maize cell walls. Growth on simple sugars was affected, depending on the sugar. Whereas growth on glucose, fructose, or sucrose was normal, growth on galactose, galacturonic acid, maltose, or xylose was somewhat reduced, and growth on arabinose was strongly reduced. Production of HC toxin was normal in the Tox2(+) ccsnf1 mutant, as were conidiation, conidial morphology, conidial germination, and in vitro appressorium formation. Activities of secreted beta-1,3-glucanase, pectinase, and xylanase in culture filtrates of the Tox2(+) ccsnf1 mutant were reduced by 53, 24, and 65%, respectively. mRNA expression was downregulated under conditions that induced the following genes encoding secreted wall-degrading enzymes: XYL1, XYL2, XYL3, XYL4, XYP1, ARF1, MLG1, EXG1, PGN1, and PGX1. The Tox2(+) ccsnf1 mutant was much less virulent on susceptible maize, forming fewer spreading lesions; however, the morphology of the lesions was unchanged. The Tox2(-) ccsnf1 mutant also formed fewer nonspreading lesions, which also retained their normal morphology. The results indicate that ccSNF1 is required for biochemical processes important in pathogenesis by C. carbonum and suggest that penetration is the single most important step at which ccSNF1 is required. The specific biochemical processes controlled by ccSNF1 probably include, but are not necessarily restricted to, the ability to degrade polymers of the plant cell wall and to take up and metabolize the sugars produced.

Lack of host specialization in Aspergillus flavus

Aspergillus spp. cause disease in a broad range of organisms, but it is unknown if strains are specialized for particular hosts. We evaluated isolates of Aspergillus flavus, Aspergillus fumigatus, and Aspergillus nidulans for their ability to infect bean leaves, corn kernels, and insects (Galleria mellonella). Strains of A. flavus did not affect nonwounded bean leaves, corn kernels, or insects at 22 degrees C, but they killed insects following hemocoelic challenge and caused symptoms ranging from moderate to severe in corn kernels and bean leaves injured during inoculation. The pectinase P2c, implicated in aggressive colonization of cotton balls, is produced by most A. flavus isolates, but its absence did not prevent colonization of bean leaves. Proteases have been implicated in colonization of animal hosts. All A. flavus strains produced very similar patterns of protease isozymes when cultured on horse lung polymers. Quantitative differences in protease levels did not correlate with the ability to colonize insects. In contrast to A. flavus, strains of A. nidulans and A. fumigatus could not invade living insect or plant tissues or resist digestion by insect hemocytes. Our results indicate that A. flavus has parasitic attributes that are lacking in A. fumigatus and A. nidulans but that individual strains of A. flavus are not specialized to particular hosts.

The endopolygalacturonase gene Bcpg1 is required for full virulence of Botrytis cinerea

Botrytis cinerea, a fungus that causes diseases in over 200 plant species, secretes a number of endopolygalacturonases that have been suggested to be involved in pathogenesis. However, so far the corresponding genes have not been isolated from this fungus. We cloned Bcpg1, encoding endopolygalacturonase, with the pgaII gene from Aspergillus niger as a heterologous probe. The Bcpg1 gene is expressed to similar levels in liquid cultures of B. cinerea containing either 1% polygalacturonic acid or 1% sucrose, and is expressed during infection of tomato leaves. The Bcpg1 gene was eliminated by partial gene replacement, and the resulting mutants were tested for virulence on tomato leaves and fruits, as well as on apple fruits. Although the mutants were still pathogenic and displayed similar primary infections when compared with control strains, a significant decrease in secondary infection, i.e., growth of the lesion beyond the inoculation spot, was observed on all three host tissues. These results indicate that the Bcpg1 gene is required for full virulence.