Expression of exo-polygalacturonases in Botrytis cinerea

Discovery of plant chemical defence mediated by a two-component system involving β-glucosidase in Panax species

Plants usually produce defence metabolites in non-active forms to minimize the risk of harm to themselves and spatiotemporally activate these defence metabolites upon pathogen attack. This so-called two-component system plays a decisive role in the chemical defence of various plants. Here, we discovered that Panax notoginseng, a valuable medicinal plant, has evolved a two-component chemical defence system composed of a chloroplast-localized β-glucosidase, denominated PnGH1, and its substrates 20(S)-protopanaxadiol ginsenosides. The β-glucosidase and its substrates are spatially separated in cells under physiological conditions, and ginsenoside hydrolysis is therefore activated only upon chloroplast disruption, which is caused by the induced exoenzymes of pathogenic fungi upon exposure to plant leaves. This activation of PnGH1-mediated hydrolysis results in the production of a series of less-polar ginsenosides by selective hydrolysis of an outer glucose at the C-3 site, with a broader spectrum and more potent antifungal activity in vitro and in vivo than the precursor molecules. Furthermore, such β-glucosidase-mediated hydrolysis upon fungal infection was also found in the congeneric species P. quinquefolium and P. ginseng. Our findings reveal a two-component chemical defence system in Panax species and offer insights for developing botanical pesticides for disease management in Panax species.

Bioprospecting of microbial strains for biofuel production: metabolic engineering, applications, and challenges

The issues of global warming, coupled with fossil fuel depletion, have undoubtedly led to renewed interest in other sources of commercial fuels. The search for renewable fuels has motivated research into the biological degradation of lignocellulosic biomass feedstock to produce biofuels such as bioethanol, biodiesel, and biohydrogen. The model strain for biofuel production needs the capability to utilize a high amount of substrate, transportation of sugar through fast and deregulated pathways, ability to tolerate inhibitory compounds and end products, and increased metabolic fluxes to produce an improved fermentation product. Engineering microbes might be a great approach to produce biofuel from lignocellulosic biomass by exploiting metabolic pathways economically. Metabolic engineering is an advanced technology for the construction of highly effective microbial cell factories and a key component for the next-generation bioeconomy. It has been extensively used to redirect the biosynthetic pathway to produce desired products in several native or engineered hosts. A wide range of novel compounds has been manufactured through engineering metabolic pathways or endogenous metabolism optimizations by metabolic engineers. This review is focused on the potential utilization of engineered strains to produce biofuel and gives prospects for improvement in metabolic engineering for new strain development using advanced technologies.

Copper induces transcription of BcLCC2 laccase gene in phytopathogenic fungus, Botrytis cinerea

Laccases are one of many groups of inducible enzymes produced by the filamentous fungus, Botrytis cinerea during colonisation of host plant tissues. While the processes involved in laccase induction are not fully understood, Cupric ions (e.g. CuSO₄) and gallic acid (GA) have been reported as laccase inducers. This study investigates laccases activities and the expression of three laccase genes (BcLCC1, BcLCC2, BcLCC3) in three B. cinerea isolates grown in laccase-inducing medium (LIM) supplemented with CuSO₄ and GA. Laccase activity in culture filtrates with CuSO₄ increased after 48 h of growth in LIM at 24°C. The induction of BcLCC2 transcription was greatest at a concentration of 0.6 mM CuSO₄, concentrations greater than 0.6 mM inhibited fungal growth. In contrast, no laccase induction was observed in the presence of GA. Liquid chromatography-mass spectroscopy (NanoLC ESI MS/MS) analysis confirmed the presence of a 63.4 kDa protein, the BcLCC2 isoform in the culture filtrate with 0.6 mM CuSO₄. Analysis of mRNA transcripts further showed BcLCC3 was also inducible and the expression of BcLCC2 and BcLCC3 was isolate-dependent. In conclusion, CuSO₄ induces a 63.4 kDa laccase in B. cinerea by induced transcription of the BcLCC2 gene.

Green Production and Biotechnological Applications of Cell Wall Lytic Enzymes

: Energy demand is constantly growing, and, nowadays, fossil fuels still play a dominant role in global energy production, despite their negative effects on air pollution and the emission of greenhouse gases, which are the main contributors to global warming. An alternative clean source of energy is represented by the lignocellulose fraction of plant cell walls, the most abundant carbon source on Earth. To obtain biofuels, lignocellulose must be efficiently converted into fermentable sugars. In this regard, the exploitation of cell wall lytic enzymes (CWLEs) produced by lignocellulolytic fungi and bacteria may be considered as an eco-friendly alternative. These organisms evolved to produce a variety of highly specific CWLEs, even if in low amounts. For an industrial use, both the identification of novel CWLEs and the optimization of sustainable CWLE-expressing biofactories are crucial. In this review, we focus on recently reported advances in the heterologous expression of CWLEs from microbial and plant expression systems as well as some of their industrial applications, including the production of biofuels from agricultural feedstock and of value-added compounds from waste materials. Moreover, since heterologous expression of CWLEs may be toxic to plant hosts, genetic strategies aimed in converting such a deleterious effect into a beneficial trait are discussed.

Four Arabidopsis berberine bridge enzyme-like proteins are specific oxidases that inactivate the elicitor-active oligogalacturonides

Recognition of endogenous molecules acting as 'damage-associated molecular patterns' (DAMPs) is a key feature of immunity in both animals and plants. Oligogalacturonides (OGs), i.e. fragments derived from the hydrolysis of homogalacturonan, a major component of pectin are a well known class of DAMPs that activate immunity and protect plants against several microbes. However, hyper-accumulation of OGs severely affects growth, eventually leading to cell death and clearly pointing to OGs as players in the growth-defence trade-off. Here we report a mechanism that may control the homeostasis of OGs avoiding their deleterious hyper-accumulation. By combining affinity chromatography on acrylamide-trapped OGs and other procedures, an Arabidopsis thaliana enzyme that specifically oxidizes OGs was purified and identified. The enzyme was named OG OXIDASE 1 (OGOX1) and shown to be encoded by the gene At4g20830. As a typical flavo-protein, OGOX1 is a sulphite-sensitive H₂ O₂ -producing enzyme that displays maximal activity on OGs with a degree of polymerization >4. OGOX1 belongs to a large gene family of mainly apoplastic putative FAD-binding proteins [Berberine Bridge Enzyme-like (BBE-like); 27 members], whose biochemical and biological function is largely unexplored. We have found that at least four BBE-like enzymes in Arabidopsis are OG oxidases (OGOX1-4). Oxidized OGs display a reduced capability of activating the immune responses and are less hydrolysable by fungal polygalacturonases. Plants overexpressing OGOX1 are more resistant to Botrytis cinerea, pointing to a crucial role of OGOX enzymes in plant immunity.

Genome-wide analysis of pectate-induced gene expression in Botrytis cinerea: identification and functional analysis of putative d-galacturonate transporters

The fungal plant pathogen Botrytis cinerea produces a spectrum of cell wall degrading enzymes for the decomposition of host cell wall polysaccharides and the consumption of the monosaccharides that are released. Especially pectin is an abundant cell wall component, and the decomposition of pectin by B. cinerea has been extensively studied. An effective concerted action of the appropriate pectin depolymerising enzymes, monosaccharide transporters and catabolic enzymes is important for complete d-galacturonic acid utilization by B. cinerea. In this study, we performed RNA sequencing to compare genome-wide transcriptional profiles between B. cinerea cultures grown in media containing pectate or glucose as sole carbon source. Transcript levels of 32 genes that are induced by pectate were further examined in cultures grown on six different monosaccharides, by means of quantitative RT-PCR, leading to the identification of 8 genes that are exclusively induced by d-galacturonic acid. Among these, the hexose transporter encoding genes Bchxt15 and Bchxt19 were functionally characterised. The subcellular location was studied of BcHXT15-GFP and BcHXT19-GFP fusion proteins expressed under control of their native promoter, in a B. cinerea wild-type strain. Both genes are expressed during growth on d-galacturonic acid and the fusion proteins are localized in plasma membranes and intracellular vesicles. Target gene knockout analysis revealed that BcHXT15 contributes to d-galacturonic acid uptake at pH 5∼5.6. The virulence of all B. cinerea hexose transporter mutants tested was unaltered on tomato and Nicotiana benthamiana leaves.

Proteomic analysis of ripening tomato fruit infected by Botrytis cinerea

Botrytis cinerea, a model necrotrophic fungal pathogen that causes gray mold as it infects different organs on more than 200 plant species, is a significant contributor to postharvest rot in fresh fruit and vegetables, including tomatoes. By describing host and pathogen proteomes simultaneously in infected tissues, the plant proteins that provide resistance and allow susceptibility and the pathogen proteins that promote colonization and facilitate quiescence can be identified. This study characterizes fruit and fungal proteins solubilized in the B. cinerea-tomato interaction using shotgun proteomics. Mature green, red ripe wild type and ripening inhibited (rin) mutant tomato fruit were infected with B. cinerea B05.10, and the fruit and fungal proteomes were identified concurrently 3 days postinfection. One hundred eighty-six tomato proteins were identified in common among red ripe and red ripe-equivalent ripening inhibited (rin) mutant tomato fruit infected by B. cinerea. However, the limited infections by B. cinerea of mature green wild type fruit resulted in 25 and 33% fewer defense-related tomato proteins than in red and rin fruit, respectively. In contrast, the ripening stage of genotype of the fruit infected did not affect the secreted proteomes of B. cinerea. The composition of the collected proteins populations and the putative functions of the identified proteins argue for their role in plant-pathogen interactions.

The D-galacturonic acid catabolic pathway in Botrytis cinerea

D-galacturonic acid is the most abundant component of pectin, one of the major polysaccharide constituents of plant cell walls. Galacturonic acid potentially is an important carbon source for microorganisms living on (decaying) plant material. A catabolic pathway was proposed in filamentous fungi, comprising three enzymatic steps, involving D-galacturonate reductase, L-galactonate dehydratase, and 2-keto-3-deoxy-L-galactonate aldolase. We describe the functional, biochemical and genetic characterization of the entire D-galacturonate-specific catabolic pathway in the plant pathogenic fungus Botrytis cinerea. The B. cinerea genome contains two non-homologous galacturonate reductase genes (Bcgar1 and Bcgar2), a galactonate dehydratase gene (Bclgd1), and a 2-keto-3-deoxy-L-galactonate aldolase gene (Bclga1). Their expression levels were highly induced in cultures containing GalA, pectate, or pectin as the sole carbon source. The four proteins were expressed in Escherichia coli and their enzymatic activity was characterized. Targeted gene replacement of all four genes in B. cinerea, either separately or in combinations, yielded mutants that were affected in growth on D-galacturonic acid, pectate, or pectin as the sole carbon source. In Aspergillus nidulans and A. niger, the first catabolic conversion only involves the Bcgar2 ortholog, while in Hypocrea jecorina, it only involves the Bcgar1 ortholog. In B. cinerea, however, BcGAR1 and BcGAR2 jointly contribute to the first step of the catabolic pathway, albeit to different extent. The virulence of all B. cinerea mutants in the D-galacturonic acid catabolic pathway on tomato leaves, apple fruit and bell peppers was unaltered.

The nature of tobacco resistance against Botrytis cinerea depends on the infection structures of the pathogen

To protect themselves, plants have evolved an armoury of defences in response to pathogens and other stress situations. These include the production of pathogenesis-related (PR) proteins and the accumulation of antimicrobial molecules such as phytoalexins. Here we report that resistance of tobacco to Botrytis cinerea is cultivar specific. Nicotiana tabacum cv. Petit Havana but not N. tabacum cv. Xanthi or cv. samsun is resistant to B. cinerea. This resistance is correlated with the accumulation of the phytoalexin scopoletin and PR proteins. We also show that this resistance depends on the type of B. cinerea stage. Nicotiana tabacum cv. Petit Havana is more resistant to spores than to mycelium of B. cinerea. This reduced resistance of N. tabacum cv. Petit Havana to the mycelium compared with spores is correlated with the suppression of PR proteins accumulation and the capacity of the mycelium, not the spores, to metabolize scopoletin. These data present an important advance in understanding the strategies used by B. cinerea to establish its disease on tobacco plants.

Plant signalling components EDS1 and SGT1 enhance disease caused by the necrotrophic pathogen Botrytis cinerea

* Botrytis cinerea is a necrotrophic fungus that causes grey mould on a wide range of food plants, especially grapevine, tomato, soft fruits and vegetables. This disease brings about important economic losses in both pre- and postharvest crops. Successful protection of host plants against this pathogen is severely hampered by a lack of resistance genes in the hosts and the considerable phenotypic diversity of the fungus. * The aim of this study was to test whether B. cinerea manipulates the immunity-signalling pathways in plants to restore its disease. * We showed that B. cinerea caused disease in Nicotiana benthamiana through the activation of two plant signalling genes, EDS1 and SGT1, which have been shown to be essential for resistance against biotrophic pathogens; and more interestingly, virus-induced gene silencing of these two plant signalling components enhanced N. benthamiana resistance to B. cinerea. Finally, plants expressing the baculovirus antiapoptotic protein p35 were more resistant to this necrotrophic pathogen than wild-type plants. * This work highlights a new strategy used by B. cinerea to establish disease. This information is important for the design of strategies to improve plant pathogen resistance.

The tetraspanin BcPls1 is required for appressorium-mediated penetration of Botrytis cinerea into host plant leaves

Animal tetraspanins are membrane proteins controlling cell adhesion, morphology and motility. In fungi, the tetraspanin MgPls1 controls an appressorial function required for the penetration of Magnaporthe grisea into host plants. An orthologue of MgPLS1, BcPLS1, was identified in the necrotrophic fungal plant pathogen Botrytis cinerea. We constructed a Bcpls1::bar null mutant by targeted gene replacement. Bcpls1::bar is not pathogenic on intact plant tissues of bean, tomato or rose, but it infects wounded plant tissues. Both wild type and Bcpls1::bar differentiate appressoria on plant and artificial surfaces, a process involving an arrest of polarized growth, apex swelling and its cell wall reinforcement. Although wild-type appressoria allowed the penetration of the fungus into the host plant within 6-12 h, no successful penetration events were observed with Bcpls1::bar, suggesting that its appressoria are not functional. An eGFP transcriptional fusion showed that BcPLS1 was specifically expressed in conidia, germ tubes and appressoria during host penetration. Our results indicate that BcPLS1 is required for the penetration of B. cinerea into intact host plants. The defect in pathogenicity of Bcpls1::bar also demonstrates that functional B. cinerea appressoria are required for a successful penetration process. As Bcpls1::bar and Mgpls1 Delta::hph penetration defects are similar, fungal tetraspanins are likely to be required for an essential appressorial function widespread among fungi.

Agrobacterium-mediated transformation of Botrytis cinerea, simple purification of monokaryotic transformants and rapid conidia-based identification of the transfer-DNA host genomic DNA flanking sequences

The Agrobacterium tumefaciens-mediated transfer of foreign DNA to the phytopathogenic fungus Botrytis cinerea was investigated. Fifteen stable transformants per 10(6) conidia were consistently produced. Monokaryons were purified in a single step and their molecular analysis demonstrated the random integration of predominantly single or tandem copies of the foreign DNA into their genome. Thermal asymmetric interlaced PCR performed directly on conidia led to the rapid identification of the genomic DNA sequences that flanked the integration sites of the transfer-DNA. Transcriptional fusions of green fluorescent protein and beta-glucuronidase-encoding genes to the promoter of the secreted proteolytic enzyme ACP1 were realised to validate the system. We provide herein observations of B. cinerea hyphae producing green fluorescent protein or beta-glucuronidase under growth conditions similar to those known to induce transcription of the acp1 gene.