An Overview on Identification and Regulatory Mechanisms of Long Non-coding RNAs in Fungi

Discovering the hidden function in fungal genomes

New molecular technologies have helped unveil previously unexplored facets of the genome beyond the canonical proteome, including microproteins and short ORFs, products of alternative splicing, regulatory non-coding RNAs, as well as transposable elements, cis-regulatory DNA, and other highly repetitive regions of DNA. In this Review, we highlight what is known about this 'hidden genome' within the fungal kingdom. Using well-established model systems as a contextual framework, we describe key elements of this hidden genome in diverse fungal species, and explore how these factors perform critical functions in regulating fungal metabolism, stress tolerance, and pathogenesis. Finally, we discuss new technologies that may be adapted to further characterize the hidden genome in fungi.

Non-Coding RNAs: Regulators of Stress, Ageing, and Developmental Decisions in Yeast?

Cells must change their properties in order to adapt to a constantly changing environment. Most of the cellular sensing and regulatory mechanisms described so far are based on proteins that serve as sensors, signal transducers, and effectors of signalling pathways, resulting in altered cell physiology. In recent years, however, remarkable examples of the critical role of non-coding RNAs in some of these regulatory pathways have been described in various organisms. In this review, we focus on all classes of non-coding RNAs that play regulatory roles during stress response, starvation, and ageing in different yeast species as well as in structured yeast populations. Such regulation can occur, for example, by modulating the amount and functional state of tRNAs, rRNAs, or snRNAs that are directly involved in the processes of translation and splicing. In addition, long non-coding RNAs and microRNA-like molecules are bona fide regulators of the expression of their target genes. Non-coding RNAs thus represent an additional level of cellular regulation that is gradually being uncovered.

Identification of lncRNA and weighted gene coexpression network analysis of germinating Rhizopus delemar causing mucormycosis

Rhizopus delemar, an opportunistic fungal pathogen, causes a highly fatal disease, mucormycosis. Spore germination is a crucial mechanism for disease pathogenesis. Thus, exploring the molecular mechanisms of fungal germination would underpin our knowledge of such transformation and, in turn, help control mucormycosis. To gain insight into the developmental process particularly associated with cell wall modification and synthesis, weighted gene co-expression network analysis (WGCNA) was performed including both coding and non-coding transcripts identified in the current study, to find out the module of interest in the germination stages. The module-trait relationship identified a particular module to have a high correlation only at the resting phase and further analysis revealed the module to be enriched for protein phosphorylation, carbohydrate metabolic process, and cellular response to stimulus. Moreover, co-expression network analysis of highly connected nodes revealed cell wall modifying enzymes, especially those involved in mannosylation, chitin-glucan crosslinking, and polygalacturonase activities co-expressing and interacting with the novel lncRNAs among which some of them predicted to be endogenous target mimic (eTM) lncRNAs. Hence, the present study provides an insight into the onset of spore germination and the information on the novel non-coding transcripts with key cell wall-related enzymes as potential targets against mucormycosis.

Long noncoding RNAs emerge from transposon-derived antisense sequences and may contribute to infection stage-specific transposon regulation in a fungal phytopathogen

BACKGROUND

The genome of the obligate biotrophic phytopathogenic barley powdery mildew fungus Blumeria hordei is inflated due to highly abundant and possibly active transposable elements (TEs). In the absence of the otherwise common repeat-induced point mutation transposon defense mechanism, noncoding RNAs could be key for regulating the activity of TEs and coding genes during the pathogenic life cycle.

RESULTS

We performed time-course whole-transcriptome shotgun sequencing (RNA-seq) of total RNA derived from infected barley leaf epidermis at various stages of fungal pathogenesis and observed significant transcript accumulation and time point-dependent regulation of TEs in B. hordei. Using a manually curated consensus database of 344 TEs, we discovered phased small RNAs mapping to 104 consensus transposons, suggesting that RNA interference contributes significantly to their regulation. Further, we identified 5,127 long noncoding RNAs (lncRNAs) genome-wide in B. hordei, of which 823 originated from the antisense strand of a TE. Co-expression network analysis of lncRNAs, TEs, and coding genes throughout the asexual life cycle of B. hordei points at extensive positive and negative co-regulation of lncRNAs, subsets of TEs and coding genes.

CONCLUSIONS

Our work suggests that similar to mammals and plants, fungal lncRNAs support the dynamic modulation of transcript levels, including TEs, during pivotal stages of host infection. The lncRNAs may support transcriptional diversity and plasticity amid loss of coding genes in powdery mildew fungi and may give rise to novel regulatory elements and virulence peptides, thus representing key drivers of rapid evolutionary adaptation to promote pathogenicity and overcome host defense.

Epigenetic mechanisms of lncRNA in response to thermal stress during embryogenesis of allotetraploid Cyprinus carpio

Embryogenesis and epigenetic mechanisms of lncRNA may play an important role in the formation of temperature tolerance in allotetraploid Cyprinus carpio. To investigate the response of lncRNA to thermal stress during embryogenesis of C. carpio, transcriptome sequencing was performed on 81 embryo or larva samples from different early development stages and temperatures. We identified 45,097 lncRNAs and analyzed transcriptome variation during embryogenesis. Stage-specific and temperature-specific DE lncRNAs and DEGs were screened. GO and KEGG analysis identified numerous pathways involved in thermal stress. Temperature-specific regulation of cis-/trans-/antisense lncRNAs was analyzed. Interaction network analysis identified 6 hub lncRNAs and many hub genes, such as cdk1 and hsf1. Decreased expression of many essential genes regulated by lncRNAs may lead to the death of embryos at 33 °C. Our findings provide new insights into the regulation of lncRNA in thermal stress response during embryogenesis and contribute to the understanding of environmental adaptation of allotetraploid species.

Discovery of long non-coding RNAs in Aspergillus flavus response to water activity, CO2 concentration, and temperature changes

Although the role of long non-coding RNAs (lncRNAs) in key biological processes in animals and plants has been confirmed for decades, their identification in fungi remains limited. In this study, we discovered and characterized lncRNAs in Aspergillus flavus in response to changes in water activity, CO₂ concentration, and temperature, and predicted their regulatory roles in cellular functions. A total of 472 lncRNAs were identified in the genome of A. flavus, consisting of 470 novel lncRNAs and 2 putative lncRNAs (EFT00053849670 and EFT00053849665). Our analysis of lncRNA expression revealed significant differential expression under stress conditions in A. flavus. Our findings indicate that lncRNAs in A. flavus, particularly down-regulated lncRNAs, may play pivotal regulatory roles in aflatoxin biosynthesis, respiratory activities, cellular survival, and metabolic maintenance under stress conditions. Additionally, we predicted that sense lncRNAs down-regulated by a temperature of 30 °C, osmotic stress, and CO₂ concentration might indirectly regulate proline metabolism. Furthermore, subcellular localization analysis revealed that up-and down-regulated lncRNAs are frequently localized in the nucleus under stress conditions, particularly at a water activity of 0.91, while most up-regulated lncRNAs may be located in the cytoplasm under high CO₂ concentration.

Identification of Long Intergenic Noncoding RNAs in Rhizoctonia cerealis following Inoculation of Wheat

Wheat sharp eyespot caused by Rhizoctonia cerealis is primarily a severe threat to worldwide wheat production. Currently, there are no resistant wheat cultivars, and the use of fungicides is the primary method for controlling this disease. Elucidating the mechanisms of R. cerealis pathogenicity can accelerate the pace of the control of this disease. Long intergenic noncoding RNAs (lincRNAs) that function in plant-pathogen interactions might provide a new perspective. We systematically analyzed lincRNAs and identified a total of 1,319 lincRNAs in R. cerealis. We found that lincRNAs are involved in various biological processes, as shown by differential expression analysis and weighted correlation network analysis (WGCNA). Next, one of nine hub lincRNAs in the blue module that was related to infection and growth processes, MSTRG.4380.1, was verified to reduce R. cerealis virulence on wheat by a host-induced gene silencing (HIGS) assay. Following that, RNA sequencing (RNA-Seq) analysis revealed that the significantly downregulated genes in the MSTRG.4380.1 knockdown lines were associated mainly with infection-related processes, including hydrolase, transmembrane transporter, and energy metabolism activities. Additionally, 23 novel microRNAs (miRNAs) were discovered during small RNA (sRNA) sequencing (sRNA-Seq) analysis of MSTRG.4380.1 knockdown, and target prediction of miRNAs suggested that MSTRG.4380.1 does not act as a competitive endogenous RNA (ceRNA). This study performed the first genome-wide identification of R. cerealis lincRNAs and miRNAs. It confirmed the involvement of a lincRNA in the infection process, providing new insights into the mechanism of R. cerealis infection and offering a new approach for protecting wheat from R. cerealis. IMPORTANCE Rhizoctonia cerealis, the primary causal agent of wheat sharp eyespot, has caused significant losses in worldwide wheat production. Since no resistant wheat cultivars exist, chemical control is the primary method. However, this approach is environmentally unfriendly and costly. RNA interference (RNAi)-mediated pathogenicity gene silencing has been proven to reduce the growth of Rhizoctonia and provides a new perspective for disease control. Recent studies have shown that lincRNAs are involved in various biological processes across species, such as biotic and abiotic stresses. Therefore, verifying the function of lincRNAs in R. cerealis is beneficial for understanding the infection mechanism. In this study, we reveal that lincRNAs could contribute to the virulence of R. cerealis, which provides new insights into controlling this pathogen.

Genome-wide screening and characterization of long noncoding RNAs involved in flowering/bolting of Lactuca sativa

BACKGROUND

Lettuce (Lactuca sativa L.) is considered the most important vegetable in the leafy vegetable group. However, bolting affects quality, gives it a bitter taste, and as a result makes it inedible. Bolting is an event induced by the coordinated effects of various environmental factors and endogenous genetic components. Although bolting/flowering responsive genes have been identified in most sensitive and non-sensitive species, non-coding RNA molecules like long non-coding RNAs (lncRNAs) have not been investigated in lettuce. Hence, in this study, potential long non-coding RNAs that regulate flowering /bolting were investigated in two lettuce strains S24 (resistant strain) and S39 (susceptible strain) in different flowering times to better understand the regulation of lettuce bolting mechanism. For this purpose, we used two RNA-seq datasets to discover the lncRNA transcriptome profile during the transition from vegetative to reproductive phase.

RESULTS

For identifying unannotated transcripts in these datasets, a 7-step pipeline was employed to filter out these transcripts and terminate with 293 novel lncRNAs predicted by PLncPRO and CREMA. These transcripts were then utilized to predict cis and trans flowering-associated targets and Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analysis. Computational predictions of target gene function showed the involvement of putative flowering-related genes and enrichment of the floral regulators FLC, CO, FT, and SOC1 in both datasets. Finally, 17 and 18 lncRNAs were proposed as competing endogenous target mimics (eTMs) for novel and known lncRNA miRNAs, respectively.

CONCLUSION

Overall, this study provides new insights into lncRNAs that control the flowering time of plants known for bolting, such as lettuce, and opens new windows for further study.

Evidence for further non-coding RNA genes in the fungal rDNA region

Non-coding RNA (ncRNA) genes play important, but incompletely understood, roles in various cellular processes, notably translation and gene regulation. A recent report on the detection of the ncRNA Signal Recognition Particle gene in the nuclear ribosomal internal transcribed spacer region of several species of three genera of ectomycorrhizal basidiomycetes prompted a more thorough bioinformatics search for additional ncRNA genes in the full fungal ribosomal operon. This study reports on the detection of three ncRNA genes hitherto not known from the fungal ribosomal region: nuclear RNase P RNA, RNase MRP RNA, and a possible snoRNA U14 in a total of five species of Auricularia and Inocybe. We verified their presence through resequencing of independent specimens. Two completed Auricularia genomes were found to lack these ncRNAs elsewhere than in the ribosomal operon, suggesting that these are functional genes. It seems clear that ncRNA genes play a larger role in fungal ribosomal genetics than hitherto thought.

Biochemical Evaluation of Agaricus and Pleurotus Strains in Batch Cultures for Production Optimization of Valuable Metabolites

The production of various biochemical compounds such as proteins, glucans and glucanases, from the mycelium of four strains of Basidiomycetes species, Agaricus bisporus, Agaricus subrufescens, Pleurotus eryngii and Pleurotus ostreatus, during batch culture in shaking flasks, was studied. Fungi were cultured for 26 days in defined media with glucose as carbon source and were primarily evaluated for their ability to consume glucose and produce mycelial mass and intracellular polysaccharides (IPS). Results showed that on the 26th day of cultivation, P. ostreatus produced the maximum biomass (16.75 g/L), whereas P. eryngii showed the maximum IPS concentration (3.82 g/L). All strains presented a similar pattern in total protein production, with A. bisporus having the highest percentage of total proteins (36%, w/w). The calculated correlation coefficients among ribonucleic acid (RNA) vs. biomass (0.97) and RNA vs. protein (0.97) indicated a very strong relation between RNA and biomass/protein synthesis. The studied strains exhibited an increase in total glucan and glucanase (β-1,6) production during cultivation, with A. bisporus reaching the highest glucan percentage (8%, w/w) and glucanase activity (12.7 units/g biomass). Subsequently, processed analytical data were used in contour-graph analysis for data extrapolation to optimize future continuous culture.

Long Non-Coding RNAs in Cryptococcus neoformans: Insights Into Fungal Pathogenesis

Long non-coding RNAs (lncRNAs) are highly expressed and can modulate multiple cellular processes including transcription, splicing, translation, and many diverse signaling events. LncRNAs can act as sponges for miRNAs, RNA and DNA binding proteins, functioning as competitive endogenous RNAs. The contribution of lncRNAs to microbial pathogenesis is largely neglected in eukaryotic pathogens despite the abundance of RNA sequencing datasets encompassing conditions of stress, gene deletions and conditions that mimic the host environment. The human fungal pathogen Cryptococcus neoformans encodes 6975 (84%) protein-coding and 1359 (16%) non-protein-coding RNAs, of which 1182 (14.2%) are lncRNAs defined by a threshold of greater than 200 nucleotides in length. Here, we discuss the current state of knowledge in C. neoformans lncRNA biology. Utilizing existing RNA seq datasets, we examine trends in lncRNA expression and discuss potential implications for pathogenesis.

Genome-wide profiling of long non-coding RNA of the rice blast fungus Magnaporthe oryzae during infection

Background

Long non-coding RNAs (lncRNAs) play essential roles in developmental processes and disease development at the transcriptional and post-transcriptional levels across diverse taxa. However, only few studies have profiled fungal lncRNAs in a genome-wide manner during host infection.

Results

Infection-associated lncRNAs were identified using lncRNA profiling over six stages of host infection (e.g., vegetative growth, pre-penetration, biotrophic, and necrotrophic stages) in the model pathogenic fungus, Magnaporthe oryzae. We identified 2,601 novel lncRNAs, including 1,286 antisense lncRNAs and 980 intergenic lncRNAs. Among the identified lncRNAs, 755 were expressed in a stage-specific manner and 560 were infection-specifically expressed lncRNAs (ISELs). To decipher the potential roles of lncRNAs during infection, we identified 365 protein-coding genes that were associated with 214 ISELs. Analysis of the predicted functions of these associated genes suggested that lncRNAs regulate pathogenesis-related genes, including xylanases and effectors.

Conclusions

The ISELs and their associated genes provide a comprehensive view of lncRNAs during fungal pathogen-plant interactions. This study expands new insights into the role of lncRNAs in the rice blast fungus, as well as other plant pathogenic fungi.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08380-4.