Enhanced Green Fluorescent Protein Expression in Pleurotus ostreatus for In Vivo Analysis of Fungal Laccase Promoters

Establishment of an Efficient Polyethylene Glycol (PEG)-Mediated Transformation System in Pleurotus eryngii var. ferulae Using Comprehensive Optimization and Multiple Endogenous Promoters

Pleurotus eryngii var. ferulae, a fungus of the genus Pleurotus, efficiently degrades lignin, especially during co-cultivation with other fungi. However, low transformation efficiency and heterologous gene expression restrict systematic studies of the molecular mechanisms and metabolic control of natural products in this mushroom. In this study, the homologous resistance marker carboxin (cbx) was used to establish a polyethylene glycol-mediated transformation (PMT) system in P. eryngii var. ferulae. Optimization of the transformation process greatly improved the number of positive transformants. In particular, we optimized: (i) protoplast preparation and regeneration; (ii) screening methods; and (iii) transformation-promoting factors. The optimized transformation efficiency reached 72.7 CFU/μg, which is higher than the average level of Pleurotus sp. (10–40 CFU/μg). Moreover, three endogenous promoters (Ppfgpd1, Ppfgpd2, and Ppfsar1) were screened and evaluated for different transcription initiation characteristics. A controllable overexpression system was established using these three promoters that satisfied various heterologous gene expression requirements, such as strong or weak, varied, or stable expression levels. This study lays the foundation for recombinant protein expression in P. eryngii var. ferulae and provides a method to investigate the underlying molecular mechanisms and secondary metabolic pathway modifications.

Identification and characterization of a novel light-induced promoter for recombinant protein production in Pleurotus ostreatus

A lectin gene (plectin) with a high level of expression was previously identified by comparative transcriptome analysis of Pleurotus ostreatus. In this study, we cloned a 733-bp DNA fragment from the start codon of the plectin gene. Sequence analysis showed that the plectin promoter (Plp) region contained several eukaryotic transcription factor binding motifs, such as the TATA-box, four possible CAAT-box, light respon-siveness motifs and MeJA-responsiveness motifs. To deter-mine whether the Plp promoter was a light-regulated promoter, we constructed an expression vector with the fused egfp-hph fragment under the control of the Plp promoter and transformed P. ostreatus mycelia via Agrobacterium tunte-faciens. PCR and Southern blot analyses confirmed the Plp-egfp-hph fragment was integrated into the chromosomal DNA of transformants. qRT-PCR, egfp visualization, and intracellular egfp determination experiments showed the Plp promoter could be a light-induced promoter that may be suitable for P. ostreatus genetic engineering. This study lays the foundation for gene homologous expression in P. ostreatus.

Exploring molecular tools for transformation and gene expression in the cultivated edible mushroom Agrocybe aegerita

Agrocybe aegerita is a cultivated edible mushroom in numerous countries, which also serves as a model basidiomycete to study fruiting body formation. Aiming to create an easily expandable customised molecular toolset for transformation and constitutive gene of interest expression, we first created a homologous dominant marker for transformant selection. Progeny monokaryons of the genome-sequenced dikaryon A. aegerita AAE-3 used here were identified as sensitive to the systemic fungicide carboxin. We cloned the wild-type gene encoding the iron-sulphur protein subunit of succinate dehydrogenase AaeSdi1 including its up- and downstream regions, and introduced a single-point mutation (His237 to Leu) to make it confer carboxin resistance. PEG-mediated transformation of protoplasts derived from either oidia or vegetative monokaryotic mycelium with the resulting carboxin resistance marker (Cbx^R) plasmid pSDI1E3 yielded carboxin-resistant transformants in both cases. Plasmid DNA linearised within the selection marker resulted in transformants with ectopic multiple insertions of plasmid DNA in a head-to-tail repeat-like fashion. When circular plasmid was used, ectopic single integration into the fungal genome was favoured, but also gene conversion at the homologous locus was seen in 1 out of 11 analysed transformants. Employing Cbx^R as selection marker, two versions of a reporter gene construct were assembled via Golden Gate cloning which allows easy recombination of its modules. These consisted of an eGFP expression cassette controlled by the native promoter P_AaeGPDII and the heterologous terminator T_nos, once with and once without an intron in front of the eGFP start codon. After protoplast transformation with either construct as circular plasmid DNA, GFP fluorescence was detected with either transformants, indicating that expression of eGFP is intron-independent in A. aegerita. This paves the way for functional genetics approaches to A. aegerita, e.g., via constitutive expression of fruiting-related genes.

A native promoter and inclusion of an intron is necessary for efficient expression of GFP or mRFP in Armillaria mellea

Armillaria mellea is a significant pathogen that causes Armillaria root disease on numerous hosts in forests, gardens and agricultural environments worldwide. Using a yeast-adapted pCAMBIA0380 Agrobacterium vector, we have constructed a series of vectors for transformation of A. mellea, assembled using yeast-based recombination methods. These have been designed to allow easy exchange of promoters and inclusion of introns. The vectors were first tested by transformation into basidiomycete Clitopilus passeckerianus to ascertain vector functionality then used to transform A. mellea. We show that heterologous promoters from the basidiomycetes Agaricus bisporus and Phanerochaete chrysosporium that were used successfully to control the hygromycin resistance cassette were not able to support expression of mRFP or GFP in A. mellea. The endogenous A. mellea gpd promoter delivered efficient expression, and we show that inclusion of an intron was also required for transgene expression. GFP and mRFP expression was stable in mycelia and fluorescence was visible in transgenic fruiting bodies and GFP was detectable in planta. Use of these vectors has been successful in giving expression of the fluorescent proteins GFP and mRFP in A. mellea, providing an additional molecular tool for this pathogen.

The ligninolytic peroxidases in the genus Pleurotus: divergence in activities, expression, and potential applications

Mushrooms of the genus Pleurotus are comprised of cultivated edible ligninolytic fungi with medicinal properties and a wide array of biotechnological and environmental applications. Like other white-rot fungi (WRF), they are able to grow on a variety of lignocellulosic biomass substrates and degrade both natural and anthropogenic aromatic compounds. This is due to the presence of the non-specific oxidative enzymatic systems, which are mainly consisted of lacasses, versatile peroxidases (VPs), and short manganese peroxidases (short-MnPs). Additional, less studied, peroxidase are dye-decolorizing peroxidases (DyPs) and heme-thiolate peroxidases (HTPs). During the past two decades, substantial information has accumulated concerning the biochemistry, structure and function of the Pleurotus ligninolytic peroxidases, which are considered to play a key role in many biodegradation processes. The production of these enzymes is dependent on growth media composition, pH, and temperature as well as the growth phase of the fungus. Mn(2+) concentration differentially affects the expression of the different genes. It also severs as a preferred substrate for these preoxidases. Recently, sequencing of the Pleurotus ostreatus genome was completed, and a comprehensive picture of the ligninolytic peroxidase gene family, consisting of three VPs and six short-MnPs, has been established. Similar enzymes were also discovered and studied in other Pleurotus species. In addition, progress has been made in the development of molecular tools for targeted gene replacement, RNAi-based gene silencing and overexpression of genes of interest. These advances increase the fundamental understanding of the ligninolytic system and provide the opportunity for harnessing the unique attributes of these WRF for applied purposes.

The white-rot fungus pleurotus ostreatus transformant overproduced intracellular cAMP and laccase

Transformation of Pleurotus ostreatus PC9 with the mutated heterotrimeric G protein alpha subunit (Gα) gene resulted in higher laccase (Lac) activity and intracellular cyclic adenosine monophosphate (cAMP) concentrations as compared to those in wild-type PC9. The transformant also exhibited higher Lac activity than the wild type when cultured in a medium containing known Lac inducers CuSO4 and ferulic acid.