Investigating the role of downy mildew effectors in host resistance and susceptibility

Piquerez, Sophie (2011) Investigating the role of downy mildew effectors in host resistance and susceptibility. Doctoral thesis, University of East Anglia.

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Through specialised structures called haustoria, filamentous eukaryotic plant pathogens
such as rusts and mildews can deliver “effector” proteins directly into plant cells in order to
manipulate specific defence responses and processes. Hyaloperonospora arabidopsidis (Hpa)
is a haustorium-producing biotrophic oomycete pathogen that causes downy mildew on
Arabidopsis. Over 100 candidate effectors (called “HaRxLs”), carrying a characteristic RxLR
protein motif, have been predicted from the Hpa genome sequence. This PhD work focused
on the characterisation of some of these effectors and their interaction with plant defences.
First, the Pseudomonas syringae pv. tomato (Pst) strain DC3000 type III secretion system
was used to examine the effect of single oomycete effectors in planta. One of these, ATR13
from Hpa isolate Emco5 (ATR13Emco5) has been shown to be recognised in some Arabidopsis
accessions by the RPP13 protein, preventing Hpa Emco5 from completing its life cycle on
such resistant plants. Hpa Emco5 can however complete its life cycle on Ws-0 but
paradoxically, ATR13Emco5 is also recognised in this accession. Here, I show that ATR13Emco5
is weakly recognised in Ws-0 by a single RPP13-independent, EDS1-independent dominant
gene called RHA13, shown by rough mapping to position on Arabidopsis chromosome 4.
Second, I contributed to two independent functional screens performed in our laboratory to
experimentally characterise the Hpa effectorome. The first screen focused on HaRxLs effects
on bacteria virulence using the EDV system. The second screen was based on HaRxLs
subcellular localisation using Agrobacterium-mediated transient expression. As presented in
this work, I showed that both screens identified putative interesting effectors which increased
plant susceptibility to Pst and Hpa and localised to various plant subcellular compartments.
One particular effector candidate, HaRxL79, localised to microtubules in planta and
interacted with two microtubule-associated proteins in a yeast-two-hybrid assay. HaRxL79
was also found to interact with Arabidopsis histone chaperones (AtNAP1s) in the plant
cytoplasm, which were observed to play a role in plant susceptibility to Hpa and partially to
the necrotroph Botrytis cinerea, but not to the hemibiotrophs Pst and Phytophthora
parasitica. AtNAP1s also interact with HaRxL67, a vacuole-associated Hpa effector. From
this, I propose that AtNAP1s are susceptibility factors for Hpa, specifically targeted by Hpa
effectors HaRxL67 and HaRxL79 in order to promote susceptibility and maintain biotrophy.

Item Type: Thesis (Doctoral)
Faculty \ School: Faculty of Science > School of Biological Sciences
Depositing User: Users 2259 not found.
Date Deposited: 20 Dec 2012 10:28
Last Modified: 20 Dec 2012 10:28


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