The Gα1-cAMP signaling pathway controls conidiation, development and secondary metabolism in the taxol-producing fungus Pestalotiopsis microspora

Recent advances on Pestalotiopsis genus: chemistry, biological activities, structure-activity relationship, and biosynthesis

Strains of the fungal genus Pestalotiopsis are reported as large promising sources of structurally varied biologically active metabolites. Many bioactive secondary metabolites with diverse structural features have been derived from Pestalotiopsis. Moreover, some of these compounds can potentially be developed into lead compounds. Herein, we have systematically reviewed the chemical constituents and bioactivities of the fungal genus Pestalotiopsis, covering a period ranging from January 2016 to December 2022. As many as 307 compounds, including terpenoids, coumarins, lactones, polyketides, and alkaloids, were isolated during this period. Furthermore, for the benefit of readers, the biosynthesis and potential medicinal value of these new compounds are also discussed in this review. Finally, the perspectives and directions for future research and the potential applications of the new compounds are summarized in various tables.

Pestalotiopsis Diversity: Species, Dispositions, Secondary Metabolites, and Bioactivities

Pestalotiopsis species have gained attention thanks to their structurally complex and biologically active secondary metabolites. In past decades, several new secondary metabolites were isolated and identified. Their bioactivities were tested, including anticancer, antifungal, antibacterial, and nematicidal activity. Since the previous review published in 2014, new secondary metabolites were isolated and identified from Pestalotiopsis species and unidentified strains. This review gathered published articles from 2014 to 2021 and focused on 239 new secondary metabolites and their bioactivities. To date, 384 Pestalotiopsis species have been discovered in diverse ecological habitats, with the majority of them unstudied. Some may contain secondary metabolites with unique bioactivities that might benefit pharmacology.

The Arf-GAP Proteins AoGcs1 and AoGts1 Regulate Mycelial Development, Endocytosis, and Pathogenicity in Arthrobotrys oligospora

Small GTPases from the ADP-ribosylation factor (Arf) family and their activating proteins (Arf-GAPs) regulate mycelial development, endocytosis, and virulence in fungi. Here, we identified two orthologous Arf-GAP proteins, AoGcs1 and AoGts1, in a typical nematode-trapping fungus Arthrobotrys oligospora. The transcription of Aogcs1 and Aogts1 was highly expressed in the sporulation stage. The deletion of Aogcs1 and Aogts1 caused defects in DNA damage, endocytosis, scavenging of reactive oxygen species, lipid droplet storage, mitochondrial activity, autophagy, serine protease activity, and the response to endoplasmic reticulum stress. The combined effects resulted in slow growth, decreased sporulation capacity, increased susceptibility to chemical stressors and heat shock, and decreased pathogenicity of the mutants compared with the wild-type (WT) strain. Although deletion of Aogcs1 and Aogts1 produced similar phenotfypic traits, their roles varied in conidiation and proteolytic activity. The ΔAogts1 mutant showed a remarkable reduction in conidial yield compared with the WT strain but not in proteolytic activity; in contrast, the ΔAogcs1 mutant showed an increase in proteolytic activity but not in sporulation. In addition, the growth of ΔAogcs1 and ΔAogts1 mutants was promoted by rapamycin, and the ΔAogts1 mutant was sensitive to H-89. Collectively, the ΔAogts1 mutant showed a more remarkable difference compared with the WT strain than the ΔAogcs1 mutant. Our study further illustrates the importance of Arf-GAPs in the growth, development, and pathogenicity of nematode-trapping fungi.

Deciphering Trichoderma-Plant-Pathogen Interactions for Better Development of Biocontrol Applications

Members of the fungal genus Trichoderma (Ascomycota, Hypocreales, Hypocreaceae) are ubiquitous and commonly encountered as soil inhabitants, plant symbionts, saprotrophs, and mycoparasites. Certain species have been used to control diverse plant diseases and mitigate negative growth conditions. The versatility of Trichoderma’s interactions mainly relies on their ability to engage in inter- and cross-kingdom interactions. Although Trichoderma is by far the most extensively studied fungal biocontrol agent (BCA), with a few species already having been commercialized as bio-pesticides or bio-fertilizers, their wide application has been hampered by an unpredictable efficacy under field conditions. Deciphering the dialogues within and across Trichoderma ecological interactions by identification of involved effectors and their underlying effect is of great value in order to be able to eventually harness Trichoderma’s full potential for plant growth promotion and protection. In this review, we focus on the nature of Trichoderma interactions with plants and pathogens. Better understanding how Trichoderma interacts with plants, other microorganisms, and the environment is essential for developing and deploying Trichoderma-based strategies that increase crop production and protection.

The role of PKAc1 in gene regulation and trichodimerol production in Trichoderma reesei

BACKGROUND

Trichoderma reesei represents a model system for investigation of plant cell wall degradation and its connection to light response. The cyclic adenosine monophosphate pathway (cAMP pathway) plays an important role in both physiological outputs, being crucial for regulation of photoreceptor function as well as for cellulase regulation on different carbon sources. Phosphorylation of photoreceptors and of the carbon catabolite repressor CRE1 was shown in ascomycetes, indicating a relevance of protein kinase A in regulation of the target genes of these transcription factors as well as an impact on regulation of induction specific genes. Moreover, the cAMP pathway impacts growth and development.

RESULTS

Here, we investigated gene regulation by the catalytic subunit of protein kinase A (PKAc1) upon growth on cellulose. We found distinct gene sets for regulation upon growth in light and darkness with an overlap of only 13 genes. PKAc1 regulates metabolic genes as well as transport and defense functions. The overlap of gene regulation by PKAc1 with the genes representing the cAMP dependent regulatory output of the photoreceptor ENV1 indicates an involvement of PKA in this pathway, which counteracts its effects by contrasting regulation. Moreover, we found considerable overlap with the gene sets regulated under cellulase inducing conditions and by the carbon catabolite repressor CRE1. Our analysis also showed that PKAc1 regulates the genes of the SOR cluster associated with the biosynthesis of sorbicillinoids. The homologue of gin4, encoding a CAMK type kinase, which is regulated by PKAc1, CRE1 and YPR2 showed a moderate impact on trichodimerol production. We isolated trichodimerol as representative sorbicillin compound and established a method for its quantification in large sample sets using high performance thin layer chromatography (HPTLC), which can be broadly applied for secondary metabolite screening of mutants or different growth conditions. Due to the high expression levels of the SOR cluster under conditions of sexual development we crosschecked the relevance of PKAc1 under these conditions. We could show that PKAc1 impacts biosynthesis of trichodimerol in axenic growth and upon mating.

CONCLUSIONS

We conclude that PKAc1 is involved in light dependent regulation of plant cell wall degradation, including carbon catabolite repression as well as secondary metabolism and development in T. reesei.

Regulation of the Gα-cAMP/PKA signaling pathway in cellulose utilization of Chaetomium globosum

BACKGROUND

The canonical heterotrimeric G protein-cAMP/PKA pathway regulates numerous cellular processes in filamentous fungi. Chaetomium globosum, a saprophytic fungus, is known for producing many secondary metabolites, including cytotoxic chaetoglobosin A (ChA), as well as abundant cellulase and xylanase.

RESULTS

Here we report on the functional characterization of this signaling pathway in C. globosum. We blocked the pathway by knocking down the putative Gα-encoding gene gna1 (in the pG14 mutant). This led to impaired cellulase production and significantly decreased transcription of the major cellulase and xylanase genes. Almost all the glycohydrolase family genes involved in cellulose degradation were downregulated, including the major cellulase genes, cel7a, cel6a, egl1, and egl2. Importantly, the expression of transcription factors was also found to be regulated by gna1, especially Ace1, Clr1/2 and Hap2/3/5 complex. Additionally, carbon metabolic processes including the starch and sucrose metabolism pathway were substantially diminished, as evidenced by RNA-Seq profiling and quantitative reverse transcription (qRT)-PCR. Interestingly, these defects could be restored by simultaneous knockdown of the pkaR gene encoding the regulatory subunit of cAMP-dependent PKA (in the pGP6 mutant) or supplement of the cAMP analog, 8-Br-cAMP. Moreover, the Gα-cAMP/PKA pathway regulating cellulase production is modulated by environmental signals including carbon sources and light, in which VelB/VeA/LaeA complex and ENVOY probably work as downstream effectors.

CONCLUSION

These results revealed, for the first time, the positive role of the heterotrimeric Gα-cAMP/PKA pathway in the regulation of cellulase and xylanase utilization in C. globosum.

A MYST Histone Acetyltransferase Modulates Conidia Development and Secondary Metabolism in Pestalotiopsis microspora, a Taxol Producer

Reverse genetics is a promising strategy for elucidating the regulatory mechanisms involved in secondary metabolism and development in fungi. Previous studies have demonstrated the key role of histone acetyltransferases in transcriptional regulation. Here, we identified a MYST family histone acetyltransferase encoding gene, mst2, in the filamentous fungus Pestalotiopsis microspora NK17 and revealed its role in development and secondary metabolism. The gene mst2 showed temporal expression that corresponded to the conidiation process in the wild-type strain. Deletion of mst2 resulted in serious growth retardation and impaired conidial development, e.g., a delay and reduced capacity of conidiation and aberrant conidia. Overexpression of mst2 triggered earlier conidiation and higher conidial production. Additionally, deletion of mst2 led to abnormal germination of the conidia and caused cell wall defects. Most significantly, by HPLC profiling, we found that loss of mst2 diminished the production of secondary metabolites in the fungus. Our data suggest that mst2 may function as a general mediator in growth, secondary metabolism and morphological development.

Gα-cAMP/PKA pathway positively regulates pigmentation, chaetoglobosin A biosynthesis and sexual development in Chaetomium globosum

Sensing the environmental signals, the canonical Gα-cAMP/PKA pathway modulates mycelial growth and development, and negatively regulates some secondary metabolism in filamentous fungi, e.g. aflatoxin in Aspergillus nidulans. Here we report the characterization of this signaling pathway in Chaetomium globosum, a widely spread fungus known for synthesizing abundant secondary metabolites, e.g. chaetoglobosin A (ChA). RNAi-mediated knockdown of a putative Gα-encoding gene gna-1, led to plural changes in phenotype, e.g. albino mycelium, significant restriction on perithecium development and decreased production of ChA. RNA-seq profiling and qRT-PCR verified significantly fall in expression of corresponding genes, e.g. pks-1 and CgcheA. These defects could be restored by simultaneous knock-down of the pkaR gene encoding a regulatory subunit of cAMP-dependent protein kinase A (PKA), suggesting that pkaR had a negative effect on the above mentioned traits. Confirmatively, the intracellular level of cAMP in wild-type strain was about 3.4-fold to that in gna-1 silenced mutant pG14, and addition of a cAMP analog, 8-Br-cAMP, restored the same defects, e.g., the expression of CgcheA. Furthermore, the intracellular cAMP in gna-1 and pkaR double silenced mutant was approaching the normal level. The following activity inhibition experiment proved that the expression of CgcheA was indeed regulated by PKA. Down-regulation of LaeA/VeA/SptJ expression in gna-1 mutant was also observed, implying that Gα signaling may crosstalk to other regulatory pathways. Taken together, this study proposes that the heterotrimeric Gα protein-cAMP/PKA signaling pathway positively mediates the sexual development, melanin biosynthesis, and secondary metabolism in C. globosum.

Distinct Roles of Velvet Complex in the Development, Stress Tolerance, and Secondary Metabolism in Pestalotiopsis microspora, a Taxol Producer

The velvet family proteins have been shown to play critical roles in fungal secondary metabolism and development. However, variations of the roles have been observed in different fungi. We report here the observation on the role of three velvet complex components VeA, VelB, and LaeA in Pestalotiopsis microspora, a formerly reported taxol-producing fungus. Deletion of individual members led to the retardation of vegetative growth and sporulation and pigmentation, suggesting critical roles in these processes. The mutant strain △velB appeared hypersensitive to osmotic stress and the dye Congo red, whereas △veA and △laeA were little affected by the pressures, suggesting only velB was required for the integrity of the cell wall. Importantly, we found that the genes played distinct roles in the biosynthesis of secondary metabolites in P. microspora. For instance, the production of pestalotiollide B, a previously characterized polyketide, required velB and laeA. In contrast, the veA gene appeared to inhibit the pestalotiollide B (PB) role in its biosynthesis. This study suggests that the three components of the velvet complex are important global regulators, but with distinct roles in hyphal growth, asexual production, and secondary metabolism in P. microspora. This work provides information for further understanding the biosynthesis of secondary metabolism in the fungus.