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The largest of them all

Genomes of rust fungi are on average larger than those of other fungi

Oeiras, 11.09.2014

Many plants are affected by rust, which unlike the phenomenon of iron oxidation, is caused by a specific group of fungi belonging to the Order Pucciniales. Despite the agronomic impact of these fungi, little genomic evidence has been gathered. Now, a team of researchers involving the Plant Cell Biology Lab has demonstrated that the genomes of rust fungi are seven times larger than the average fungal genome, making them as a group the largest genomes within fungi. The study, coordinated by Centro de Investigação das Ferrugens do Cafeeiro/Instituto de Investigação Científica Tropical, was published in August in the open access journal Frontiers in Plant Science and featured on the homepage of the Frontiers website.

As the largest group of fungal plant pathogens, rust fungi collectively affect a very wide range of plant hosts. The disease is characterized by dark orange, brown, yellow or red spots or pustules on the undersides of the leaves, which impact plant development and compromise agricultural and forest crops worldwide. In this genome era, sequencing is the best option to gain insight into rust fungi biology but so far only a few species have been sequenced. Most sequencing efforts are hampered by uncertainty about the genome sizes of rust fungi. The present work provides the genome size of 30 unsequenced rust fungi and corroborates the genome size determined by previous sequencing of another two rust species.

Rust fungi can only complete their life cycle in the plant. This means that growing the fungi in the laboratory is not possible and every study must include a step of fungal cultivation in the proper host, which in most cases is very specific. In their work, researchers developed an innovative and simple approach to simultaneously isolate nuclei from the rust and the host plant in order to estimate the genome size of rust species by flow cytometry. Researchers found that the rust fungi genome sizes varied over 10-fold (from 70 to 893 Mbp) with an average genome size value of 380.2 Mbp. One particular species, Gymnosporangium confusum, possesses the largest fungal genome ever reported (893.2 Mbp) and even the smallest rust genome determined in this study is larger than the vast majority of fungal genomes.

Researchers believe that although this study comprises only a very small fraction of the more than 7000 rust species described, with many genera and some families not represented, Pucciniales represent a group where genome size expansion could be a common characteristic, making this group of organisms a subject of interest for genomic research. One possibility raised in the paper is that the fungal genome size is a host driven adaptive trait for overcoming host defenses; interestingly “although the rust genome sizes determined in this study surpass most other fungi and are within the range of genome sizes of many plants, (…) all rust genome sizes in this study are smaller than those of the hosts from where they were obtained.”

This study involves reseachers from ITQB, Instituto de Investigação Científica Tropical (IICT), Instituto Superior de Agronomia, Universidade de Coimbra, and Universität Hohenheim (Germany).


Original article
Front. Plant Sci. 5:422. Doi: 10.3389/fpls.2014.00422

Genome size analyses of Pucciniales reveal the largest fungal genomes

Sílvia Tavares, Ana Paula Ramos, Ana Sofia Pires, Helena G. Azinheira, Patrícia Caldeirinha, Tobias Link, Rita Abranches, Maria do Céu Silva, Ralf T. Voegele, João Loureiro and Pedro Talhinhas

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