Establishment of Agrobacterium tumefaciens-Mediated Transformation of Cladonia macilenta, a Model Lichen-Forming Fungus

Identifying pathogenicity-related genes in the pathogen Colletotrichum magnum causing watermelon anthracnose disease via T-DNA insertion mutagenesis

Fruit rot caused by Colletotrichum magnum is a crucial watermelon disease threatening the production and quality. To understand the pathogenic mechanism of C. magnum, we optimized the Agrobacterium tumefaciens-mediated transformation system (ATMT) for genetic transformation of C. magnum. The transformation efficiency of ATMT was an average of around 245 transformants per 100 million conidia. Southern blot analysis indicated that approximately 75% of the mutants contained a single copy of T-DNA. Pathogenicity test revealed that three mutants completely lost pathogenicity. The T-DNA integration sites (TISs) of three mutants were Identified. In mutant Cm699, the TISs were found in the intron region of the gene, which encoded a protein containing AP-2 complex subunit σ, and simultaneous gene deletions were observed. Two deleted genes encoded the transcription initiation protein SPT3 and a hypothetical protein, respectively. In mutant Cm854, the TISs were found in the 5'-flanking regions of a gene that was similar to the MYO5 encoding Myosin I of Pyricularia oryzae (78%). In mutant Cm1078, the T-DNA was integrated into the exon regions of two adjacent genes. One was 5'-3' exoribonuclease 1 encoding gene while the other encoded a WD-repeat protein retinoblastoma binding protein 4, the homolog of the MSl1 of Saccharomyces cerevisiae.

Development of a Transformation System for the Medicinal Fungus Sanghuangporus baumii and Acquisition of High-Value Strain

To further explore the molecular mechanism of triterpenoid biosynthesis and acquire high-value strain of Sanghuangporus baumii, the Agrobacterium tumefaciens-mediated transformation (ATMT) system was studied. The key triterpenoid biosynthesis-associated gene isopentenyl diphosphate isomerase (IDI) was transformed into S. baumii by ATMT system. Then, the qRT-PCR technique was used to analyze gene transcript level, and the widely targeted metabolomics was used to investigate individual triterpenoid content. Total triterpenoid content and anti-oxidant activity were determined by spectrophotometer. In this study, we for the first time established an efficient ATMT system and transferred the IDI gene into S. baumii. Relative to the wild-type (WT) strain, the IDI-transformant (IT) strain showed significantly higher transcript levels of IDI and total triterpenoid content. We then investigated individual triterpenoids in S. baumii, which led to the identification of 10 distinct triterpenoids. The contents of individual triterpenoids produced by the IT2 strain were 1.76-10.03 times higher than those produced by the WT strain. The triterpenoid production showed a significant positive correlation with the IDI gene expression. Besides, IT2 strain showed better anti-oxidant activity. The findings provide valuable information about the biosynthetic pathway of triterpenoids and provide a strategy for cultivating high-value S. baumii strains.

The coming golden age for lichen biology

Lichens are a diverse group of organisms. They are both commonly observed but also mysterious. It has long been known that lichens are composite symbiotic associations of at least one fungus and an algal or cyanobacterial partner, but recent evidence suggests that they may be much more complex. We now know that there can be many constituent microorganisms in a lichen, organized into reproducible patterns that suggest a sophisticated communication and interplay between symbionts. We feel the time is right for a more concerted effort to understand lichen biology. Rapid advances in comparative genomics and metatranscriptomic approaches, coupled with recent breakthroughs in gene functional studies, suggest that lichens may now be more tractable to detailed analysis. Here we set out some of the big questions in lichen biology, and we speculate about the types of gene functions that may be critical to their development, as well as the molecular events that may lead to initial lichen formation. We define both the challenges and opportunities in lichen biology and offer a call to arms to study this remarkable group of organisms.

A comparative genomic analysis of lichen-forming fungi reveals new insights into fungal lifestyles

Lichen-forming fungi are mutualistic symbionts of green algae or cyanobacteria. We report the comparative analysis of six genomes of lichen-forming fungi in classes Eurotiomycetes and Lecanoromycetes to identify genomic information related to their symbiotic lifestyle. The lichen-forming fungi exhibited genome reduction via the loss of dispensable genes encoding plant-cell-wall-degrading enzymes, sugar transporters, and transcription factors. The loss of these genes reflects the symbiotic biology of lichens, such as the absence of pectin in the algal cell wall and obtaining specific sugars from photosynthetic partners. The lichens also gained many lineage- and species-specific genes, including those encoding small secreted proteins. These genes are primarily induced during the early stage of lichen symbiosis, indicating their significant roles in the establishment of lichen symbiosis.Our findings provide comprehensive genomic information for six lichen-forming fungi and novel insights into lichen biology and the evolution of symbiosis.

Modeling in yeast how rDNA introns slow growth and increase desiccation tolerance in lichens

We connect ribosome biogenesis to desiccation tolerance in lichens, widespread symbioses between specialized fungi (mycobionts) and unicellular phototrophs. We test whether the introns present in the nuclear ribosomal DNA of lichen mycobionts contribute to their anhydrobiosis. Self-splicing introns are found in the rDNA of several eukaryotic microorganisms, but most introns populating lichen rDNA are unable to self-splice, being either catalytically impaired group I introns, or spliceosomal introns ectopically present in rDNA. Although the mycobiont’s splicing machinery removes all introns from rRNA, Northern analysis indicates delayed post-transcriptional removal during rRNA processing, suggesting interference with ribosome assembly. To study the effects of lichen introns in a model system, we used CRISPR to introduce a spliceosomal rDNA intron from the lichen fungus Cladonia grayi into all nuclear rDNA copies of Saccharomyces cerevisiae, which lacks rDNA introns. Three intron-bearing yeast mutants were constructed with the intron inserted either in the 18S rRNA genes, the 25S rRNA genes, or in both. The mutants removed the introns correctly but had half the rDNA genes of the wildtype, grew 4.4–6 times slower, and were 40–1700 times more desiccation tolerant depending on intron position and number. Intracellular trehalose, a disaccharide implicated in desiccation tolerance, was detected at low concentration. Our data suggest that the interference of the splicing machinery with ribosome assembly leads to fewer ribosomes and proteins and to slow growth and increased desiccation tolerance in the yeast mutants. The relevance of these findings for slow growth and desiccation tolerance in lichens is discussed.