Archive for March, 2013

Fungal Carbon Sequestration

March 29th, 2013

 

By  K. E. Clemmensen, A. Bahr, O. Ovaskainen, A. Dahlberg, A. Ekblad, H. Wallander, J. Stenlid, R. D. Finlay, D. A. Wardle, B. D. Lindahl

 

Abstract. Boreal forest soils function as a terrestrial net sink in the global carbon cycle. The prevailing dogma has focused on aboveground plant litter as a principal source of soil organic matter. Using 14 C bomb-carbon modeling, we show that 50 to 70% of stored carbon in a chronosequence of boreal forested islands derives from roots and root-associated microorganisms. Fungal biomarkers indicate impaired degradation and preservation of fungal residues in late successional forests. Furthermore, 454 pyrosequencing of molecular barcodes, in conjunction with stable isotope analyses, highlights root-associated fungi as important regulators of ecosystem carbon dynamics. Our results suggest an alternative mechanism for the accumulation of organic matter in boreal forests during succession in the long-term absence of disturbance]

Read also the linked Commentary by Kathleen K. Treseder and Sandra R. HoldenFungal Carbon Sequestration.

Photo: One of the investigated island situated in the two adjacent lakes Uddjaure and Hornavan in the Northern boreal zone of Sweden (from Björn Lindahl’s home page).

 

March 13th, 2013

Genomics of Fungal Drug Producers

March 2nd, 2013

In a breakthrough paper, Schardl’s group and collaborators have published 15 genomes of diverse species of Clavicipitaceae plant endophytes and parasites in the last issue of PloS Genetics. The Clavicipitaceae (PezizomycotinaSordariomycetes, Hypocreales) includes “ergot” fungi that parasitize ears of cereals and produce  the toxic ergoline derivatives; ergot fungi have historically caused epidemics of gangrenous poisonings, the ergotism, also known as the Saint Anthony’s Fire. The ascomycetous family also includes plant endophytic symbionts that produce several psychoactive and bioprotective alkaloids. The family includes grass symbionts in the epichloae clade (Epichloë and Neotyphodium species), which are extraordinarily diverse both in their host interactions and in their alkaloid profiles. They synthesize alkaloids with chemical similarities to biogenic amines that deter insects, livestock, and wildlife from feeding on the fungus or plant. Thanks to this chemical warfare, Epichloae protect their hosts from cattle grazing. The lysergic acid diethylamide (LSD), a semisynthetic ergot alkaloid originally developed as an antidepressant, is the most potent known hallucinogen.

In this study, they sequenced the genomes of 10 epichloae, three ergot fungi (Claviceps species), a morning-glory symbiont (Periglandula ipomoeae), and the bamboo witch’s broom pathogen (Aciculosporium take), profiled the alkaloids in these species and compared the gene clusters for four classes of alkaloids. The genomes were primarily sequenced by shotgun 454 pyrosequencing, but paired-end and mate-pair reads were used to scaffold several assemblies. Size of the assembled genome among the sequenced strains varied 2-fold from 29.2 to 58.7 Mb, with wide ranges even within the genera Claviceps (31–52.3 Mb) and Epichloë (29.2–49.3 Mb). This genome size variation is mainly resulting from the abundance of repeated elements, which ranged from 4.7 to 56.9%. Annotated genome sequences have been posted at www.endophyte.uky.edu.

In the epichloae, the clusters of genes coding for enzymes of alkaloid biosynthesis contain very large blocks of repetitive elements which promote gene losses, mutations, and even the evolution of new genes. Two striking features emerged from the detailed analysis of alkaloid biosynthesis gene clusters. Firstly, in most alkaloid loci in most species, the periphery of each cluster was enriched in genes that by virtue of their presence, absence, or sequence variations determined the diversity of alkaloids within the respective chemical class. Second, alkaloid gene loci of the epichloae had extraordinarily large and pervasive blocks of AT- rich repeats derived from retroelements, DNA transposons, and MITEs. This finding suggests that these plant-interacting fungi are under selection for alkaloid diversification.

In their conclusions, the authors suggest that this selection of chemotypes is related to the variable life histories of the epichloae, their protective roles as symbionts, and their associations with the ecologically diverse cool-season grasses.

Schardl CL, Young CA, Hesse U, Amyotte SG, Andreeva K, et al. (2013) Plant-Symbiotic Fungi as Chemical Engineers: Multi-Genome Analysis of the Clavicipitaceae Reveals Dynamics of Alkaloid Loci. PLoS Genet 9(2): e1003323. doi:10.1371/journal.pgen.1003323

Image: Claviceps purpurea -Franz Eugen Köhler, Köhler’s Medizinal-Pflanzen (Wikimedia Commons).