Cloning and targeted disruption of two polygalacturonase genes in Penicillium olsonii

Sclerotinia sclerotiorum (Lib.) de Bary: Insights into the Pathogenomic Features of a Global Pathogen

Sclerotinia sclerotiorum (Lib.) de Bary is a broad host-range fungus that infects an inclusive array of plant species and afflicts significant yield losses globally. Despite being a notorious pathogen, it has an uncomplicated life cycle consisting of either basal infection from myceliogenically germinated sclerotia or aerial infection from ascospores of carpogenically germinated sclerotia. This fungus is unique among necrotrophic pathogens in that it inevitably colonizes aging tissues to initiate an infection, where a saprophytic stage follows the pathogenic phase. The release of cell wall-degrading enzymes, oxalic acid, and effector proteins are considered critical virulence factors necessary for the effective pathogenesis of S. sclerotiorum. Nevertheless, the molecular basis of S. sclerotiorum pathogenesis is still imprecise and remains a topic of continuing research. Previous comprehensive sequencing of the S. sclerotiorum genome has revealed new insights into its genome organization and provided a deeper comprehension of the sophisticated processes involved in its growth, development, and virulence. This review focuses on the genetic and genomic aspects of fungal biology and molecular pathogenicity to summarize current knowledge of the processes utilized by S. sclerotiorum to parasitize its hosts. Understanding the molecular mechanisms regulating the infection process of S. sclerotiorum will contribute to devising strategies for preventing infections caused by this destructive pathogen.

Genome shuffling for improving the activity of alkaline pectinase in Bacillus subtilis FS105 and its molecular mechanism

Genome shuffling for improving the activity of alkaline pectinase in Bacillus subtilis FS105 and its molecular mechanism were investigated. The fused strain B. subtilis FS105 with the highest activity of alkaline pectinase was obtained after two rounds of genome shuffling. The activity of alkaline pectinase in B. subtilis FS105 was 499 U/ml, which was improved by 1.6 times compared to that in original strain. To elucidate its molecular mechanism, rpsL gene sequences from original and fused strains were cloned and aligned, and the space structure of their coding proteins were also analyzed and compared. The alignment of the rpsL gene sequences indicated that three bases G, G and C were respectively replaced by A, A and G in the positions 52, 408 and 409 after genome shuffling. This resulted in the substitution of two amino acid residues in ribosomal protein S12: D18N and P137A, and therefore improving the biosynthesis of alkaline pectinase. This study lays a foundation for improving the activity of alkaline pectinase by genome shuffling and understanding its molecular mechanism.

Cell-wall-degrading enzymes produced in vitro and in vivo by Rhizoctonia solani, the causative fungus of peanut sheath blight

Rhizoctonia solani causes the disease peanut sheath blight, involving symptoms of maceration and necrosis of infected tissue, mainly caused by cell-wall-degrading enzymes (CWDEs). This study investigated the production of CWDEs including polygalacturonase (PG), polymethyl-galacturonase (PMG), cellulase (Cx) and β-glucosidase by R. solani in vitro (in liquid culture) and in vivo (in peanut plants). Significant PG, PMG, Cx and β-glucosidase activities were detected in infected tissues including stalk and leaves of Baisha and Silihong peanut cultivars. Extracts of healthy tissue showed little or no such activities. In shaken liquid cultures of R. solani in medium containing pectin or pectin plus carboxymethyl cellulose (CMC) as the carbon source(s), PG and PMG were notably active. Significant Cx activity was detected in cultures with CMC or pectin plus CMC as the carbon source(s). However, only a very low level of β-glucosidase activity was observed in cultures with any of the tested carbon sources. An increase of pH was recorded in decayed peanut tissues and liquid culture filtrates; the filtrate pH and fungal growth positively correlated. The fungal growth and/or pH were important factors for the production of PG, PMG and Cx in culture with pectin plus CMC as the carbon source. A single active PG isozyme with isoelectric point around 9.2 was detected in culture filtrates and in infected peanut tissues by the method of isoelectric focusing electrophoresis. The crude enzymes extracted from liquid culture of R. solani induced decay of healthy peanut leaves.

Disruption of the Gene Encoding Endo-β-1, 4-Xylanase Affects the Growth and Virulence of Sclerotinia sclerotiorum

Sclerotinia sclerotiorum (Lib.) de Bary is a devastating fungal pathogen with worldwide distribution. S. sclerotiorum is a necrotrophic fungus that secretes many cell wall-degrading enzymes (CWDEs) that destroy plant's cell-wall components. Functional analyses of the genes that encode CWDEs will help explain the mechanisms of growth and pathogenicity of S. sclerotiorum. Here, we isolated and characterized a gene SsXyl1 that encoded an endo-β-1, 4-xylanase in S. sclerotiorum. The SsXyl1 expression showed a slight increase during the development and germination stages of sclerotia and a dramatic increase during infection. The expression of SsXyl1 was induced by xylan. The SsXyl1 deletion strains produce aberrant sclerotia that could not germinate to form apothecia. The SsXyl1 deletion strains also lost virulence to the hosts. This study demonstrates the important roles of endo-β-1, 4-xylanase in the growth and virulence of S. sclerotiorum.

An evaluation of the proteolytic and lipolytic potential of Penicillium spp. isolated from traditional Greek sausages in submerged fermentation

A number of novel Penicillium strains belonging to Penicillium nalgiovense, Penicillium solitum, Penicillium commune, Penicillium olsonii, and Penicillium oxalicum species, isolated from the surface of traditional Greek sausages, were evaluated for their proteolytic and lipolytic potential in a solid substrate first and next in submerged fermentations, using complex media. Extracellular proteolytic activity was assessed at acid, neutral, and alkaline pH, while the lipolytic activity was assessed using olive oil, the short-chain triacylglycerol tributyrin, and the long-chain triolein, as substrates. The study revealed that although closely related, the tested strains produce enzymes of distinct specificities. P. nalgiovense PNA9 produced the highest alkaline proteolytic activity (13.2 unit (U)/ml) and the highest lipolytic activity with tributyrin (92 U/ml). Comparisons with known sources show that proteases and/or lipases can be secreted effectively by some Penicillia (P. nalgiovense PNA4, PNA7, and PNA9 and P. solitum PSO1), and further investigations on their properties and characteristics would be promising.

Brassica napus possesses an expanded set of polygalacturonase inhibitor protein genes that are differentially regulated in response to Sclerotinia sclerotiorum infection, wounding and defense hormone treatment

Most plants encode a limited set of polygalacturonase inhibitor (PGIP) genes that may be involved in aspects of plant development, but more importantly in the inactivation of polygalacturonases (PG) secreted by pathogens. Previously, we characterized two Brassica napus PGIP genes, BnPgip1 and BnPgip2, which were differentially expressed in response to pathogen infection and wounding. Here we report that the B. napus genome encodes a set of at least 16 PGIP genes that are similar to BnPgip1 or BnPgip2. This is the largest Pgip gene family reported to date. Comparison of the BnPGIPs revealed several sites within the xxLxLxx region of leucine rich repeats that form beta-sheets along the interacting face of the PGIP that are hypervariable and represent good candidates for generating PGIP diversity. Characterization of the regulatory regions and RT-PCR studies with gene-specific primers revealed that individual genes were differentially responsive to pathogen infection, mechanical wounding and signaling molecules. Many of the BnPgip genes responded to infection by the necrotic pathogen, Sclerotinia sclerotiorum; however, these genes were also induced either by jasmonic acid, wounding and salicylic acid or some combination thereof. The large number of PGIPs and the differential manner in which they are regulated likely ensures that B. napus can respond to attack from a broad spectrum of pathogens and pests.

Molecular characterization and expression profile of pectin-lyase-encoding genes from Penicillium griseoroseum

Penicillium griseoroseum has been studied by our group because of its good pectinase production. Attempts have been done to clone pectinolytic genes, aiming to obtain pectinase-overproducing strains for industrial purposes. Here, two genes coding for pectin lyase were isolated from the P. griseoroseum genome. The plg1 gene has an open reading frame of 1341 bp coding for a putative protein of 374 amino acids with a calculated molecular mass of 40.1 kDa. The plg2 gene is characterized by an open reading frame of 1400 nucleotides and codes for a polypeptide of 383 amino acids. The plg1 gene 5'-flanking region contains putative binding sites for the transcription factors involved in regulation by ambient pH and catabolite repression. The primary structure of Plg1 and Plg2 proteins showed a relatively high homology (varying between 32.4% and 74.8%) to fungal pectin lyases characterized to date. Southern blotting analysis revealed that both genes are present as single copies in the fungus genome. Expression studies revealed a differing pattern of gene expression of plg1 and plg2 when mycelium was cultivated on medium containing different pectic components. Citric pectin followed by apple pectin were the carbon sources that best induced plg1 expression, and transcripts were detected from 24 to 76 h. The expression of the plg2 gene was monitored by reverse transcriptase - polymerase chain reaction, since Northern analysis failed to detect hybridization signals. The differential expression of these genes may provide means for the fungus to adapt to various growth conditions.

Interaction of Sclerotinia sclerotiorum with Brassica napus: cloning and characterization of endo- and exo-polygalacturonases expressed during saprophytic and parasitic modes

Five major and several minor PG isoenzymes were identified in a Sclerotinia sclerotiorum isolate from Brassica napus by isoelectric focusing and pectin gel overlays. Using a combination of degenerate PCR and expressed sequence tags (ESTs) four endo-polygalacturonase (PG) genes, designated as sspg1d, sspg3, sspg5, and sspg6, and two exo-PG genes, ssxpg1 and ssxpg2, were identified. SSPG1d is a member of the PG gene family previously described by Fraissinet-Tachet et al. [Curr. Genet. 29 (1995) 96]. The mature SSPG1d is a neutral PG, whereas fully processed SSPG3, SSPG5, and SSPG6 are acidic enzymes. Under saprophytic growth conditions, sspg1d, sspg3, sspg5, and ssxpg1 expression was induced by pectin and galacturonic acid and subject to catabolite repression by glucose. Conditions could not be identified under which sspg6 or ssxpg2 were expressed well. Transfer of mycelia from liquid media to solid substrates induced expression of sspg1d suggesting that it may also be regulated by thigmotrophic interactions. Under pathogenic conditions, sspg1d was highly expressed during infection. sspg3 was also expressed during infection, albeit at lower levels than sspg1d, whereas sspg5, sspg6, and ssxpg1 were expressed only weakly.