Landscape, complexity and regulation of a filamentous fungal transcriptome

Alternative splicing (AS) and alternative polyadenylation (APA) of pre-mRNAs contribute greatly to transcriptome complexity and gene expression regulation in higher eukaryotes. Their biological impact in filamentous fungi, however, has been poorly studied. Here we combine PacBio Isoform Sequencing and strand-specific RNA-Seq of multiple tissues together with mutant characterization to reveal the landscape, complexity and regulation of AS and APA in the filamentous plant pathogenic fungus Fusarium graminearum. We updated the reference genome and generated a comprehensive annotation comprising 51,617 transcript isoforms from 17,189 genes. Majority of the transcripts represent novel isoforms, including 2,998 undiscovered protein-coding genes. In total, 42.7% of multi-exonic genes and 64.8% of genes have AS and APA isoforms, respectively, suggesting AS and APA increase previously unrecognized transcriptome complexity in fungi. Nonsense-mediated mRNA decay factor FgUPF1 may not degrade AS transcripts with premature-stop codons but regulate ribosome biogenesis. Distal polyadenylation sites have a strong signal but proximal polyadenylation isoforms are high expressed. The core 3-end processing factors FgRNA15, FgHRP1, and FgFIP1 play important roles in promoting proximal polyadenylation site usage and also intron splicing. Genome-wide increase in the abundance of transcripts with retained introns and long 3-UTRs and downregulation of the spliceosomal and 3-end processing factors are found in older tissues and quiescent conidia, indicating that intron retention and 3-UTR lengthening may be a transcriptional signature of aging and dormancy in fungi. Overall, our study generates a comprehensive full-length transcript annotation for F. graminearum and provides new insights into the complexity and regulation of transcriptome in filamentous fungi.