Investigating the variation in susceptibility between Arabidopsis thaliana ecotypes in response to Botrytis cinerea infection

Master Thesis


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In order to combat the various pathogens that are constantly exposed to, plants have developed a complex multi-layered innate immune system. Arabidopsis thaliana is a widely used model organism for studying the molecular basis of the host defence response to pathogen attack. One reason for this is the phenotypic variation in resistance found across the many Arabidopsis ecotypes, which provides researchers with a rich resource to dissect the molecular basis of resistance (or susceptibility) to a given pathogen. In this study, the resistance of three Arabidopsis ecotypes (Col-0, RLD and No-0) to the necrotrophic fungal pathogen Botrytis cinerea was investigated using detached leaf infection assays, expression profiling of genes known to play a role in defence against B. cinerea, and by generating F1 hybrids from the three ecotypes. The infection assays demonstrated that Col-0 and RLD develop similar sized secondary (spreading) lesions when unable to contain the initial B. cinerea infection, but that RLD was better able to contain the primary lesion, thus preventing a secondary lesion from developing. In contrast the No-0 ecotype was highly susceptible, displaying both larger secondary lesions and no ability to prevent the initial lesion from spreading. These experiments confirmed previously published data on the susceptibility of these three ecotypes. The variation in susceptibility between the three ecotypes is not due to any modulation of circadian clock regulation ofthe host defence response, asincreased resistance to B. cinerea was observed in all three ecotypes when infected at subjective morning versus subjective night. As a first step towards understanding the molecular basis of this phenotypic variation, quantitative real-time PCR analysis of five defence genes was performed on plants infected with B. cinerea. While the expression profile of these five genes wassimilar in Col-0 and RLD, No-0 displayed a markedly different pattern of expression; no induction of ORA59 (a transcription factor which integrates jasmonic acid and ethylene signalling) or PDF1.1 (a defensin with anti-fungal activity) was observed in response to B. cinerea infection in No-0, which correlated with high constitutive expression ofthe JAZ-ZIM repressor JAZ1. Finally, to determine whether the differential susceptibility to B. cinerea exhibited by the three ecotypes falls under the control of a single genetic locus or not, the ecotypes were crossed and infection assays performed on the F1 hybrids. In all three cases the susceptibility displayed by the hybrids was intermediate between that displayed by the parental ecotypes. This pattern is not consistent with control residing in a single locus and so provides further evidence that resistance to B. cinerea is governed by multiple interacting genes in Arabidopsis.