Investigating the role of ATR1 in pathogenicity and RPP1 mediated resistance in Arabidopsis
Lead Research Organisation:
University of Warwick
Department Name: Warwick HRI
Abstract
Disease causing organisms, such as bacteria, viruses and fungi, infect their hosts by producing attack proteins that are targeted at suppressing host defence responses. Animals have developed an immune system that recognises the invader and produces a new protein (antibody) that attacks the disease-causing organism. Plants lack this sort of somatically adaptive immune system but face the same invaders. They have produced a different set of highly variable genes called resistance genes. These are thought to recognise the presence of the pathogen attack proteins and respond by causing a plant defence reaction. We have isolated one of these pathogen attack proteins that triggers a particular resistance gene. We now want to find out which plant protein is its target. This will show us which plant protein the pathogen thinks is important and, therefore, it is likely to play an important role in plant defence against disease. We will also learn how this attack protein triggers the resistance response via the plant resistance gene. By assembling the components of this resistance complex we will learn how plants defend themselves against disease. In the future we would hope to use this information to enhance plants' natural defence capability or to develop new products that prevent pathogen growth.
Technical Summary
The oomycete downy mildew pathogen, Hyaloperonospora parasitica, infects the model plant Arabidopsis in the wild. Analysis of the interaction between these organisms has revealed an extensive set of host resistance genes complemented by a range of avirulence genes in the pathogen. The resistance genes and matching avirulence genes reveal high levels of polymorphism suggesting that a co-evolutionary 'arms race' is occurring between plant and pathogen. Previously we have cloned the host resistance gene, RPP1-Nd, and we have recently cloned the matching pathogen avirulence gene, ATR1. In other interactions avirulence proteins have been shown to interact with host proteins that may play a role in basal immunity and that resistance genes have evolved to detect such interactions. We have shown that RPP1 genes from different Arabidopsis accessions have different recognition profiles and that RPP1-WsB is also capable of recognising different alleles of ATR1. We now wish to use this genetic diversity to understand the role of ATR1 in establishing a successful infection of Arabidopsis. To achieve this we will identify the host target of various ATR1 alleles and confirm these interactions in planta. By using mutations in and over expression of the target genes we will assess their role in host immunity. We will also identify the amino acids required to trigger RPP1 mediated recognition of ATR1 and whether or how RPP1 interacts with the host targets of ATR1. In this way, we will use ATR1 to reveal part of the plant host defence mechanisms and contribute to the growing picture of plant immune mechanisms.
Organisations
| Description | Plants are under constant attack by pathogens that wish to grow and reproduce using plant tissues as a source of food. When this occurs on crop plants devastating epidemics can result. Serious crop losses are caused by a group of pathogens called the oomycetes, which include potato blight (Phytophthora infestans, the cause of the Irish Potato Famine) and downy mildew of brassicas (Hyaloperonospora parasitica). Plants have developed two levels of resistance to such diseases: a general or innate immunity that provides a broad level of resistance and disease resistance genes which provide protection against specific isolates of the pathogens. These resistance genes have been shown to produce proteins that vary greatly in sequence between different plants suggesting that they are fighting a battle with pathogen proteins that are adapting to avoid detection. This "arms race" implies active selection on both host and pathogen in a battle for supremacy during an attempt to infect. The plant resistance genes detect pathogen proteins (avirulence proteins) and initiate a defence response. Such avirulence proteins are likely to be proteins produced by the pathogen to suppress host defence or access nutrients from the plant cell. Therefore, we aimed to clone two such avirulence proteins, one from potato blight and one from a downy mildew. We successfully cloned the Avr3a gene from P. infestans and ATR1from H.parasitica. The AVR3a protein essentially was only present in two forms and all isolates that were recognised by the potato R3 resistance gene contained the same form of the Avr3a gene. In contrast the ATR1gene, that is recognised by the Arabidopsis RRP1 resistance gene, was highly variable between most isolates of the pathogen suggesting a strong evolutionary selective pressure for change. This difference in variability between the pathogen genes could be because the P.infestans isolates have been under selection from a single R3 resistance gene introduced from Mexico into European potato breeding populations. Hence, only one changed form has been selected in the pathogen Avr3a gene to overcome this resistance gene. In contrast the Arabidopsis RPP1 resistance gene is present in different forms in different accessions of the plant and no human selection has occurred to reduce variability at the RPP1 locus hence matching variability is seen at the ATR1 locus. We showed that different RPP1 proteins were capable of recognising different combinations of ATR1 proteins and that some RPP1 proteins recognise proteins other than ATR1, demonstrating a diversity of detection capability. Furthermore, our work suggested that some downy mildew isolates produce proteins that suppress the function of the RPP1 resistance protein, hinting at a complex balance between resistance and suppression of resistance. Finally, we have identified two candidate plant proteins that are the target of the ATR1 protein. Future analysis of these proteins will reveal their role in host plant defence. Detailed comparisons of the regions of DNA containing Avr3a and ATR1 reveals that they lie in the same place in the two genomes and yet they are completely different in protein sequence. This may imply that although genes involved in pathogenicity are conserved at the same loci diversifying selection has changed the proteins so much over time they are no longer recognisable as the same gene. However, more detailed comparison of AVR3a and ATR1 proteins and other oomycete proteins reveals a conserved sequence of amino acids (RXLR) towards the beginning of the gene. This motif is remarkably similar to the sequence RXL discovered in pathogenicity factors from the malarial parasite Plasmodium which has been shown to be involved in getting the pathogen protein into the host blood cells. We suggest, therefore, that the RXLR motif is important to the transfer of oomycete proteins into host plant cells. Hence, comparison of these related but diverse pathogens has revealed some remarkable features of how oomycetes invade their hosts and will allow us to recognise genes involved in invading their host plants. |
| Exploitation Route | Definition of the RXLR motif has allowed effector proteins to be identified form many oomycete pathogens allowing extensive research into the function of effectors in the suppressing host plant immune systems |
| Sectors | Agriculture, Food and Drink |
| Description | BBSRC quota |
| Amount | £60,000 (GBP) |
| Organisation | Biotechnology and Biological Sciences Research Council (BBSRC) |
| Sector | Public |
| Country | United Kingdom |
| Start | 03/2003 |
| End | 03/2006 |
| Description | DEFRA studentship |
| Amount | £45,000 (GBP) |
| Organisation | Department For Environment, Food And Rural Affairs (DEFRA) |
| Sector | Public |
| Country | United Kingdom |
| Start | 10/2005 |
| End | 09/2008 |