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Researchers eavesdrop on the internal communication system plants use to warn they are under attack

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26.09.2002

An international team of scientists have today reported the discovery of a protein, called DIR1, that is a key step in the pathways that enable plants to protect themselves against disease. DIR1 is involved in the transmission of a warning signal from plant cells infected by disease. The signal alerts cells, in areas remote from the infected site, that the plant is under attack and switches on defence mechanisms that prevent the disease establishing further infection sites. The report, from scientists at the John Innes Centre (JIC) Norwich, University of Edinburgh University of Toronto and The Noble Foundation, appears in the international journal Nature.

“As part of their defence against disease attack plants use internal communication systems to transmit warning signals around the plant”, said Professor Chris Lamb (JIC Director and leader of the research team at the JIC). “The protein we have discovered is essential to this warning system. It enables cells attacked by disease to transmit a signal that alerts other parts of the plant to the presence of infection. The signal stimulates cells, remote from the attack site, to activate defences that render them resistant to disease infection and this prevents the disease spreading through the plant”.

The induction of disease resistance, in areas of a plant remote from the site of initial infection, is called systemic acquired resistance (SAR) and is an important element in plants’ strategies to protect themselves from disease attack. DIR1 is essential to the SAR response.

The research team searched among 11,000 mutant lines of thale cress (Arabidopsis thaliana) and found one line that had lost the ability to develop SAR when inoculated with the bacterial disease Pseudomonas syringae. (This failure was due to a mutation in a single gene – DIR1[1]). Inoculation of A. thaliana with P. syringae typically results in the appearance of new proteins in the cells at the inoculation site, so-called PR (pathogenesis-related) proteins. The inoculated plants also develop SAR, and PR proteins are produced in cells distant from the inoculation site. When the mutant line was inoculated with P. syringae the cells at the inoculation points behaved normally, producing PR proteins, but the plants failed to develop SAR and no PR proteins were produced in cells remote from the inoculation site. When other parts of these plants were inoculated with P. syringae the bacteria were able to establish infections, unlike the situation in the normal plants, where SAR prevents further infections occurring. This shows that the mutation specifically affects the transmission of the signal responsible for SAR.

Further experiments demonstrated that plants of the mutant line were unable to produce the signal that is responsible for switching on SAR. This may be because the DIR1 gene is involved in production of the signal itself or affects the export of the signal molecule from infected cells and its transport around the plant.

“This newly discovered gene is central to the plants ability to develop long lasting immunity that is effective against a wide range of different diseases, two highly desirable characteristics for agricultural crops,” noted Professor Lamb. “Insights from this work could lead to powerful new genetic strategies for disease protection in crops that will reduce the need for pesticides but maintain high yields with lowered inputs of agrochemicals” he continued.

[1] DIR1 is the abbreviated form of ‘defective in induced resistance’ the name given to the single gene that has been identified as the basis of the loss of systemic acquired resistance response in the A. thaliana mutant line studied here. DIR1 is a 306 nucleotide open reading frame that encodes a 102 amino acid peptide of 10.6 kDa. Dir1 shares a region of 54 nucleotides with a Phaesolus vulgaris non-specific lipid transfer-like protein and has 53% sequence similarity throughout the protein. The DIR1 protein contains a hydrophobic N-terminal signal sequence and the eight cysteine residues conserved in all lipid transfer proteins – this suggest that DIR1 encodes an apoplastic lipid transfer protein that is involved in export and/or transport of an apoplastic signal molecule.

Ray Mathias | Source: alphagalileo
Further information: www.nature.com/nature

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