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Monday, August 13, 2012

Different Genomes Of Pathogens Determine Lifestyles Transition

(a) Schematic representation of the four C. higginsianum developmental stages selected for RNA sequencing. Gray indicates polystyrene, green indicates living plant cell, and brown indicates dead plant cell. Hpi, hours post-inoculation. (b) Heatmaps of gene expression showing the 100 most highly expressed and significantly regulated genes (log2 fold change >2, P < 0.05) in five functional categories. Overrepresented (pale red to dark red) and underrepresented transcripts (pale blue to dark blue) are shown as log2 fold changes relative to the mean expression measured across all four stages. The arrow indicates the CSEP-encoding gene ChEC6 (CH063_01084). (c) The statistical significance of gene induction (y axis) in five functional categories during fungal developmental transitions (x axis). The P values were calculated using a one-sided Fisher's exact test and represent the probability of observing the number of significantly induced genes for a specific category during a transition given the total number of significantly induced genes during that transition (log2 fold change >2, P < 0.05) and the total number of genes in the category. (d) Transcriptional regulation of the effector gene ChEC6 by plant-derived signals. Confocal micrographs showing C. higginsianum expressing the mCherry reporter gene under the native ChEC6 promoter (overlays of bright-field and fluorescence channels). Appressoria (A) formed on polystyrene are unlabeled (top left), whereas those on the leaf surface (top right) have fluorescent cytoplasm. After host penetration, labeling is visible in young biotrophic hyphae (YH) but not older biotrophic hyphae (OH) (bottom). Scale bars, 10 μm. C, conidium.
Colletotrichum fungi have different lifestyles. In just a few hours, the fungus is able to adapt from benign to destructive plant cells. This pathogen also has other different lifestyles, some species attack different plants, while other species attack only on certain plants. One species preferentially attacks crucifers, including thale cress (Arabidopsis thaliana). The unique thing is, the pathogen is able to promote themselves to the corn to be able to coexist without symptoms. While in other places, produces proteins to destroy the plant cells. Which means, these fungi has a different lifestyle spatially. Colletotrichum fungus spread by the wind and rain, their presence very detrimental to agriculture and biodiesel.

O'Connell and colleagues have studied a different life style in these fungus. They found that the transcriptome determine when and which genes are active.  Several other fungal genomes have already been decoded, but never with such detailed information about if and when each gene is used during plant infection", says O'Connell. For example, both genomes have similar numbers of genes for hemicellulase enzymes, with which the plant cell wall is decomposed. However, the maize fungus switches on many more of these genes because the cell walls of maize contain more hemicellulose than do plants attacked by the Arabidopsis fungus. "This difference could not have been identified simply from cataloguing the numbers of such genes in the genome: transcriptome data are essential to obtain this information", explains O'Connell.

The genomes of the two pathogens are similar in size, but the Arabidopsis fungus accommodates more genes in its genome, probably as a result of its broader host range. A pathogen that attacks a single plant requires fewer genes than one which colonizes many different plants. This is especially true for "effector" genes, which are required by the fungus to protect itself from the plant's defense responses. Both fungi have remarkably large numbers of genes for producing secondary metabolites, which are small molecules with potential roles during infection. "We are not aware of any other phytopathogenic fungi that produce so many secondary metabolites", says Jochen Kleemann who, together with other colleagues from the Max Planck Institute for Plant Breeding Research in Cologne, was also involved the study. "The genes for these products are switched on very early on during infection and are therefore potential targets for plant protection. But first we need to understand more about the functions of these molecules", continues Kleemann.

The scientists also discovered previously unknown functions of the fungal adhesion organ, the appressorium. The appressorium is formed after a fungal spore lands on the leaf surface and builds up a high pressure, with which the fungus pushes itself into the interior of the plant cell, like a finger into an inflated balloon. "On a leaf, the adhesion organ switches on completely different genes than when it is located on a plastic surface. It must in some way recognize where it is", says O'Connell. The adhesion organ would thus appear not only to open the door into the plant cell, but also to sense the presence of the plant. "Appressoria were discovered almost 130 years ago, but it is only from our research that it has become clear that they also have a sensing function", says Kleemann.

This article had edited by authors of threelas
Source: Max Planck Gesellschaft
Publication: DOI: 10.1038/ng.2372

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