Melampsora larici-populina is an obligate biotrophic plant pathogen responsible for the poplar rust disease. This fungus belongs to the taxonomical order Pucciniales and exhibits a complex heteroecious and macrocyclic life cycle, i.e. it has the capacity to infect two unrelated host plants, larch and poplar, and to form five distinct spore types through the year. The M. larici-populina genome has been sequenced and annotated in 2011 and since, different transcriptomic analyses were conducted at different stages with oligoarrays and later on with RNA-Seq covering most of its life cycle. Here, we collected published transcriptome data for the poplar rust fungus, transposed them for the version 2 of the genome now available at the Joint Genome Institute Mycocosm website and performed normalization of different oligoarray datasets on one hand, and of RNA-Seq on the other hand. We report a comprehensive normalized dataset for this fungus, made available for the community, which allows life cycle transcriptomics analysis.

Nutrients acquisition by rust fungi during their biotrophic growth has been assigned to a few transporters expressed in haustorial infection structures. We performed a comparative genomic analysis of all transporter genes (hereafter termed transportome) classified according to the Transporter Classification Database (TCDB) focusing specifically on rust fungi (order Pucciniales) versus other species in the Dikarya. We also surveyed expression of transporter genes in the poplar rust fungus for which transcriptomics data are available across the whole life cycle. Despite a significant increase in gene number, rust fungi presented a reduced transportome compared to a vast majority of fungi in the Dikarya. However, a few transporter families in the subclass Porters showed significant expansions. Noteworthy, three metal transport-related families involved in the import, export and sequestration of metals were expanded in Pucciniales and expressed at various stages of the rust life cycle suggesting a tight regulation of metal homeostasis. The most remarkable gene expansion in the Pucciniales was observed for the oligopeptide transporter (OPT) family with 25 genes in average compared to seven to 14 genes in the other surveyed taxonomical ranks. A phylogenetic analysis showed several specific expansion events at the root of the order Pucciniales with subsequent expansions in rust taxonomical families. The OPT genes showed dynamic expression patterns along the rust life cycle and more particularly during infection of the poplar host tree, suggesting a specialization for the acquisition of nitrogen and sulfur through the transport of oligopeptides from the host during biotrophic growth.

Nutrients acquisition by rust fungi during their biotrophic growth has been assigned to a few transporters expressed in haustorial infection structures. We performed a comparative genomic analysis of all transporter genes (hereafter termed transportome) classified according to the Transporter Classification Database (TCDB) focusing specifically on rust fungi (order Pucciniales) versus other species in the Dikarya. We also surveyed expression of transporter genes in the poplar rust fungus for which transcriptomics data are available across the whole life cycle. Despite a significant increase in gene number, rust fungi presented a reduced transportome compared to a vast majority of fungi in the Dikarya. However, a few transporter families in the subclass Porters showed significant expansions. Noteworthy, three metal transport-related families involved in the import, export and sequestration of metals were expanded in Pucciniales and expressed at various stages of the rust life cycle suggesting a tight regulation of metal homeostasis. The most remarkable gene expansion in the Pucciniales was observed for the oligopeptide transporter (OPT) family with 25 genes in average compared to seven to 14 genes in the other surveyed taxonomical ranks. A phylogenetic analysis showed several specific expansion events at the root of the order Pucciniales with subsequent expansions in rust taxonomical families. The OPT genes showed dynamic expression patterns along the rust life cycle and more particularly during infection of the poplar host tree, suggesting a possible specialization for the acquisition of nitrogen and sulfur through the transport of oligopeptides from the host during biotrophic growth.

Fungi of the order Pucciniales are obligate plant biotrophs causing rust diseases. They exhibit a complex life cycle with the production of up to five spore types, infection of two unrelated hosts and an overwintering stage. Transcription factors (TFs) are key regulators of gene expression in eukaryote cells. In order to better understand genetic programs expressed during major transitions of the rust life cycle, we surveyed the complement of TFs in fungal genomes with an emphasis on Pucciniales. We found that despite their large gene numbers, rust genomes have a reduced repertoire of TFs compared to other fungi. The proportions of C2H2 and Zinc cluster - two of the most represented TF families in fungi-indicate differences in their evolutionary relationships in Pucciniales and other fungal taxa. The cold shock protein (CSP) family showed a striking expansion in Pucciniomycotina with specific duplications in the order Pucciniales. The survey of expression profiles collected by transcriptomics along the life cycle of the poplar rust fungus revealed TF genes related to major biological transitions, e.g. response to environmental cues and host infection. Particularly, poplar rust CSPs were strongly expressed in basidia produced after the overwintering stage suggesting a possible role in dormancy exit. Expression during transition from dormant telia to basidia confirmed the specific expression of the three poplar rust CSP genes. Their heterologous expression in yeast improved cell growth after cold stress exposure, strengthening their implication in dormancy exit. This study addresses for the first time TF involved in developmental transition in the rust life cycle opening perspectives to further explore molecular regulation in the biology of the Pucciniales.

No abstract