Defence Genetics
The Defence Genetics research group is interested in the mechanisms that control and regulate expression of defence against pathogens. Upon pathogen attack, the plant tissue stimulates a complex signalling network. This activates defence mechanisms that collectively have the potential to result in efficient resistance.
The aim of the research is to develop strategies for generating disease resistant crop plants.
Description of research area
Ongoing projects
Recent publications
We focus our research on the economically important powdery mildew fungi, and by using barley and Arabidopsis as models, we seek to understand how powdery mildew fungi attack plants and how plants defend themselves against the attack.
Powdery mildew fungi are obligate biotrophs that proliferate on the leaf surface and take up nutrients from the plant by penetrating epidermal cells in which they place haustoria. These fungal structures become surrounded by a plant-derived membrane, allowing the fungus to acquire nutrients while the plant cell continues living.
Defence is manifested at two stages:
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During penetration, when the host cell forms a papilla, a localised cell wall apposition that potentially stops the fungus.
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At the stage of the haustorium, when the host cell can undergo a programme cell death response, called the hypersensitive reaction (HR). The death of the plant cell also causes the fungus to die.
In the meantime, the powdery mildew fungi secrete effectors to the plant cell where these proteins target and suppress defence components.
The research is organised in the following projects:
Membrane trafficking pathway to penetration resistance against powdery mildew
In barley and Arabidopsis, the HvROR2/AtPEN1 plasma membrane syntaxin, SYP121, plays a major role in penetration resistance. Syntaxins are SNARE proteins involved in membrane fusion events. We recently discovered barley ARFA1b/1c to be another membrane trafficking component on this pathway to penetration resistance. This AFR-GTPase, required for a vesicle budding event, localises to multivesicular bodies that assemble near the site of penetration prior to the formation the papilla. Ongoing research aims at identifying more components in this pathway.
Negative regulators of penetration resistance
Based on a cDNA library from epidermal tissue of powdery mildew-diseased leaves, we have previously identified barley EST sequences not hitherto available in public sequence databases. In order to study how these genes may be involved in the barley-powdery mildew interaction, they have been screened by transient-induced gene silencing. These RNAi experiments indicated that a number of the genes are negative regulators of penetration resistance. The molecular functions of these genes are now being studied in detail.
Signalling network controlling pathogen defence
The Arabidopsis double mutant, syp121 syp122, suffers from severe necroses and dwarfism due to strong defence expression.
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This phenotype, which is not present in the two single mutants, syp121 and syp122, is very advantageous for studies of the defence signalling. syp121 syp122 can be rescued to various degrees by mutations in genes required for defence, such as SID2 and NPR1.
We have therefore made a mutant screen of re-mutagenised syp121 syp122 in order identify novel genes involved in defence signalling. In this screen, we found approximately 240 triple mutant syp121 syp122 ssd-lines, in which mutations in SUPPRESSOR OF SYNTAXIN-RELATED DEFENCE genes partially rescue the syntaxin double mutant. |
Based on these mutants, we have identified several genes encoding defence signalling components using map-based cloning procedures, and assembled a defence signalling network.
Powdery mildew effectors
Powdery mildew fungi have very intimate interactions with plants in that they place haustoria in the leaf epidermal cells. The haustoria secrete effector proteins to the plant cell, where they functions as inhibitors of defence.
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We have recently identified more than 200 barley powdery mildew fungal genes encoding secreted proteins, which share a “YxC” motif in the N-terminal of the mature protein. “YxC” describes the alternative amino acid motifs, YxC, FxC and WxC, initiated by the three existing aromatic amino acid.
We predict that the “YxC” motif is responsible for protein transfer across the extrahaustorial membrane (EHM), and that the more than 200 YxC-proteins are effectors that manipulate the host cell, e.g. by suppressing defence. We are currently searching for plant protein targets of these effector candidates. |
Barley cyanogenic glucosides
Barley leaf epidermal tissue has a high concentration of the cyanogenic glucoside, epiheterodendrin, and we speculate that this and related compounds influences the interaction with the powdery mildew fungus. Therefore, we have initiated a genetic study of these molecules. A barley mutant population has been screened for low cyanogenesis, and mutant candidates are currently being analysed in detail.
Sophie Jagd Rosendal, - last update:16 September 2011