Year

2013

Degree Name

Doctor of Philosophy

Department

School of Biological Sciences

Abstract

Plants defend their genomes by using RNA–directed DNA methylation (RdDM), an epigenetic mechanism driven by small interfering RNAs (siRNAs) to repress parasitic invaders. In Arabidopsis thaliana, DNA glycosylases of the DEMETER (DME) family prune cytosine methylation from DNA. Demethylation by DME in Arabidopsis is necessary for gene imprinting while DNA demethylation by the DEMETER-Like (DML) enzymes and REPRESOR OF SILENCING1 (ROS1) removes silencing directed by RNA silencing pathways. Prior to this work, the role of demethylases in plant defense was not known. Moreover, the role of the RdDM pathway in plant defense against non viral pathogens is poorly understood.

Here in a genetic screen designed to characterize RNA silencing factors regulating plant defense in Arabidopsis thaliana, against the root infecting fungal pathogen Fusarium oxysporum, we identified downstream factors of the RdDM pathway, RNA Polymerases V (PolV) and ARGONAUTE 4 (AGO4) that are critical for plant defense. In addition, the novel role of DNA demethylases in plant defense was also revealed.

The mutant plants deficient in DNA methylation and demethylation factors showed increases in disease susceptibility to F. oxysporum. By employing microarray techniques, we found misexpression of a large number of genes in mutant plants deficient in methylation and demethylation factors relative to the wild-type plants. A large proportion of these genes, especially in the demethylase mutant, are associated with biotic stress. These results suggest that RdDM and DNA demethylation play a critical role in plant disease resistance. Furthermore, my results show that the RdDM pathway factor PolV negatively regulates the expression of PATHOGENESIS RELATED 1 (PR1) gene, a molecular marker of the salicylic acid defense pathway (SA), suggesting the involvement of epigenetic mechanisms in modulating defense signaling pathways.

Taken together my results highlight the importance of two antagonistic mechanisms: DNA methylation and demethylation in the regulation of plant immunity against the fungal pathogen F. oxysporum and perhaps against other biotic and abiotic stresses.

FoR codes (2008)

0607 PLANT BIOLOGY

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.