ALL EXTRACELLULAR PROTEINS OF A FUNGAL PATHOGEN ARE POTENTIALLY RECOGNISED AS AVIRULENCE FACTORS BY HOST AND NON-HOST PLANTS
R LAUGE, R LUDERER, P VOSSEN, MHAJ JOOSTEN and PJGM DE WIT
Department of Phytopathology, Wageningen Agricultural University, Binnenhaven 9, 6709 PD Wageningen, The Netherlands
Background and objectives
In mammals, the immune system has the capacity to develop recognition specificities for every non-self compound that is present inside the organism. In plants, the numerous naturally occurring genotype-specific resistance responses to pathogens suggest that they also have a recognition system for non-self compounds . Lycopersicon spp. contain many different resistance genes to Cladosporium fulvum, of which the resistance provided by two, CfA and Cf-9, has been proven to be based on the recognition of proteinaceous elicitors of the fungus, AVR4 and AVR9, respectively . Since C. fulvum develops in the intercellular space without forming specialized feeding structures that enter the host cells, the fungal elicitors should be secreted to be recognized by the tomato cells, as has been demonstrated for AVR4 and AVR9. Based on the hypothesis that plants are able to recognize non-self compounds, we wanted to investigate whether a genetically diverse population of tomato plants is able to recognize any protein of C. fulvum that is present in the intercellular space.
Results and conclusions
We have purified five extracellular proteins (ECPs) from intercellular fluids isolated from tomato plants that were successfully colonized by C. fulvum (ECP1 to ECP5). These proteins were injected into leaves of 23 breeding lines of tomato, that were selected to carry different resistance genes to C. fulvum. Individual lines responded to injection of ECP2, ECP3 and ECP5. Since the results for these ECPs supported our hypothesis, we decided to search for plants responding to ECP1 and ECP4 in a selection of tomato plants with a more diverse genetic background. Accessions of wild Lycopersicon species were inoculated (40 accessions, four plants each) with recombinant potato virus X (PVX), expressing the Ecp1 or the Ecp4 gene, and indeed individual plants responding to ECP1 and ECP4 were identified. So far, for each ECP tested, responding plant(s) have been identified. This indicates that plants have the capacity to recognize a broad range of non-self proteins, including proteins derived from pathogens. To test whether this recognition extends beyond the host-pathogen level, we tested PVX constructs containing Avr4, Avr9, Ecp1, Ecp2 and Ecp4 on 46 accessions (eight plants each per PVX construct) of wild Nicotiana species. Interestingly, several accessions showed a response to ECP2. However, no responding plants were identified for the other proteins tested. Our results on tomato with the ECPs of C. fulvum fully support the existence of a versatile recognition system in plants for non-self proteins in plant-pathogen interactions. Moreover, our results with tobacco and ECP2 suggest that this recognition system might even extend beyond the pathogen range. Additional experiments with proteins of C. fulvum and proteins of non-pathogens with additional tomato accessions and other non-host plants might provide more insight into the versatility of the plant recognition system.
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