Posts Tagged ‘CAZymes’

Surviving the Ant Gut

March 12th, 2011

leaf-cutting-ant-1403Leaf-cutting ants of the genera Acromyrmex and Atta (Family Formicidae: Subfamily Myrmicinae: Tribe Attini) live in mutualistic symbiosis with the basidiomycete Leucocoprinus gongylophorus (Agaricaceae). The ants cultivate the mycobiont mycelium in ‘fungal gardens’ where they brought freshly cut and chew leaves. They apply fecal droplets to the leaf pulp before depositing this mixed substrate to the top of the garden. The fecal fluid contains a large range of hydrolytic enzymes (proteases, pectinases, carbohydrate degrading enzymes) able to efficiently degrade the plant cell wall and cell material. Released carbohydrates serve as a primary source of nutrient for the fungus which then differenciate clusters of a unique tissular structure so-called the ‘gongylidia‘. This massive hyphal swelling are the main food source of the farming leaf-cutting ants. In ant agriculture,the attine ants actively propagate, nurture and defend the basidiomycete cultivar. This mutualistic symbiosis is thought to have originated in the basin of the Amazon rainforest some 50–65 million years ago. The molecular mechanisms driving this ant-fungus mutualism are poorly know.

In their study published in BMC Biology, Schiøtt et al. showed that the pectinolytic enzymes present in the ant fecal droplets are produced by the fungus. The genes encoding the hydrolytic enzymes are  induced in the gongylidia mycelium, ingested by the feeding ants, transported throughout the ant gut before being released in fecal fluids on the top of the fungal garden. It is suggested by the authors that the fungal enzymes evolved to survive the harsh conditions of the ant gut. The on-going sequencing of the genome of Leucocoprinus gongylophorus will undoubtly provide novel insights on the evolution from saprotrophism to this unique mutualistic symbiosis.

ant

Figure by Schiøtt et al. BMC Biology 2010 8:156   doi:10.1186/1741-7007-8-156

Schiøtt et al. (2010 Leaf-cutting ant fungi produce cell wall degrading pectinase complexes reminiscent of phytopathogenic fungi. BMC Biology 2010, 8:156

Recommended reading: Fungus-Ant mutualism

Photo: © http://www.zsl.org/zsl-london-zoo/animals/inverts/leaf-cutting-ant,59,AN.html

How to Crunch Plant Walls? … by Gene Transfer

October 2nd, 2010

11752Lateral gene transfer (LGT) between bacteria has largely been documented. The transmission of genes between fungi (e.g., Supernumerary chromosomes in a root-rot fungus, In Vino Veritas, Next-Generation Sequencing of Sordaria Genome), and between fungi and insects, such as aphids (see Tap the Vein that Bleeds), have also been reported. In contrast, data on LGT in animals is scarce. In the last issue of PNAS, Pierre Abad’s group from INRA is publishing their study of LGT in plant-parasitic nematodes. Their phylogenetic analysis of  genes coding for degrading enzymes acting on plant cell walls  (e.g., GH28 polygalacturonase, PL3 pectate lyase, GH43 arabinase, …)  from root-knot nematodes (such as Meloidogyne incognita and M. hapla) shows that these nematode enzymes were likely acquired from several independent bacterial sources. The authors hypothesized a series of acquisition through soil bacteria feeding or gene transfers from endosymbiotic bacteria. The  observed abundance of multigenic families (cellulases, pectate lyases, and expansins) in these plant-parasitic nematodes is likely due to a series of duplications that started after acquisition by LGT events. Selective advantage associated with transfer of these CAZyme genes probably has driven their duplications and facilitated fixation in the different populations and species of plant-parasitic nematodes.

In brief, when ‘worms gobble up genes from bugs’ (S. Kamoun) they were able to get access to the largest store of carbon in soil — is this fast-track evolution?

Danchin et al. (2010) Multiple lateral gene transfers and duplications have promoted plant parasitism ability in nematodes. Proc. Ntl. Acad. Sci., published online before print September 27, 2010, doi: 10.1073/pnas.1008486107.

Photo: Root-knot nematode © www.minlnv.nl

Friend or foe? … a Sweet Story

August 16th, 2010

Sans titreThe sky is overcast with clouds and the rain is ceaseless‘ in Gitanjali by Rabindranath Tagore.

Mid-Summer: Across Lorraine, this is generally the hottest month of the year, and when most everyone is headed to a beach or the mountains to look for a little respite. No scorching mid-Summer heat this year. Over the week-end, the sky was pouring with rain and the sun never shone from dawn to dusk. It poured with rain the entire night.

Well, I take this opportunity to get back to outstanding papers. The study of GH32 invertases in fungi from Jerri Parent, together with Tim James and Andy Taylor, was worth re-reading. In this BMC Evolutionary Biology paper, they tested for occurrence of the glycosyl hydrolase family 32 (GH32) genes in all available fungal genomes and an additional 149 species representing a broad phylogenetic and ecological range of biotrophic fungi. This GH32 family, containing mostly invertases, is crucial for plant-interacting fungi. Sucrose is the primary metabolite used by most plants to translocate carbon throughout their tissues, and its abundance within plants makes it a valuable carbon source for the many fungi that are obligate plant associates. To acquire the host sucrose, colonizing fungi must possess the necessary enzymes, such as extracellular invertase(s), to split sucrose into its constituent monosaccharides, glucose and fructose.

Ancestral state reconstruction of GH32 gene abundance showed a strong correlation with nutritional mode (saprobic, endophytic, mutualist, pathogenic). Expansion of gene families was observed in several clades of pathogenic filamentous Ascomycota species. GH32 gene number was negatively correlated with animal pathogenicity and positively correlated with plant biotrophy (e.g. Puccinia graminis), with the notable exception of mycorrhizal taxa (e.g. Laccaria bicolor). Few mycorrhizal species were found to have GH32 genes as compared to other guilds of plant-associated fungi, such as pathogens, endophytes and lichen-forming fungi. GH32 genes were also more prevalent in the Ascomycota than in the Basidiomycota.

We noticed in our Nature paper the lack of invertase in the ectomycorrhizal L. bicolor suggesting that this symbiont depends on its host plant to provide glucose in exchange for nitrogen. I have checked the presence of the extracellular invertase in our draft genome sequences of L. amethystina and Glomus intraradices, and the 454 transcripts of Lactarius quietus and Pisolithus microcarpus. None of these ECM fungi have a gene coding for this enzyme, whereas the poplar rust, Melampsora larici-populina has two sequences similar to the wheat rust Puccinia graminis GH32s. Intriguingly, the genome of the ectomycorrhizal ascomycete, Tuber melanosporum — the Black Truffe of Perigord — contains a gene encoding a GH32 enzyme, suggesting that the truffle may act as a scavenger instead of being a true mutualist. However, the corresponding transcript is lowly expressed in free-living mycelium, fruiting body and ectomycorrhiza according to our NimbleGen oligoarray and RNA-Seq transcript profilings.

As stressed by Parent et al. “Reliance on plant GH32 enzyme activity for C acquisition in these [ECM] symbionts supports earlier predictions of a general absence of invertase in mycorrhizal fungi, and a highly evolved mutualistic relationship between plants and mycorrhizal fungi, a remarkable scenario in light of the high degree of phylogenetic diversity spanned by mycorrhizal fungal taxa. Whether the plant host is able to detect fungal invertase activity and use such a signal to differentiate antagonistic from mutualistic biotrophic symbionts is a completely speculative, though plausible hypothesis.”

Parrent et al. (2009) Friend or foe? Evolutionary history of glycoside hydrolase family 32 genes encoding for sucrolytic activity in fungi and its implications for plant-fungal symbioses. BMC Evolutionary Biology 9:148 doi:10.1186/1471-2148-9-148.

Photo: Lorraine Big Sky © F Martin