Erysiphe necator is an obligate biotrophic fungal pathogen that causes powdery mildew in grapevine, Vitis vinifera. This study focuses on a family of effectors named RNase-like
effector proteins expressed in haustoria (RALPH) effectors secreted by E. necator, and
chitin-triggered immunity of V. vinifera. RALPH effectors have high structural similarity to
fungal ribonucleases but are lack of catalytic activity. The first aim of this study was to
confirm the role that presumptive E. necator RALPH effectors (EnCSEPs) play in promoting
virulence using a pathogenicity assay in a heterologous pathosystem. The results of these assays showed that EnCSEP56, EnCSEP65 and EnCSEP115 expression in Nicotiana
benthamiana increased its susceptibility to Peronospora tabacina. The second aim of this
study was to identify components involved in chitin triggered immunity. Chitin is the major
component of fungal cell wall. It acts as a microbe associated molecular pattern (MAMP) to trigger the activation of mitogen activated protein kinase (MAPK) cascades, which induce
robust immune responses. VvLYK1-1 is a V. vinifera pattern recognition receptor (PRR)
responsible for chitin perception and also required for resistance against non-adapted
powdery mildew species. However, the downstream components by which VvLYK1-1
transmits immune signals to the MAPK cascade are not known. The grapevine family of
RLCKs and MAPKKKs were annotated in order to select potential candidates which act
downstream of VvLYK1-1. In this study, I found that VvLYK1-1 interacted with three V. vinifera receptor like cytoplasmic kinases (VvPBL27, VvPBS7 and VvPBL7-1) in two in planta assays, co-immunoprecipitation and bimolecular fluorescence complementation.
Yeast two-hybrid and co-immunoprecipitation assay indicated that VvPBL27 interacted with
VvMAPKKK7/16. On the basis of my results, I propose that a chitin-triggered signal transduction pathway in V. vinifera involving of VvLYK1-1-VvPBL27/VvPBS1/VvPBL7-1 and
VvMAPKKK6/17. To further investigate the roles of RLCKs and MAPKKK proteins in nonadapted
powdery mildew resistance, the Arabidopsis AtPBL27 and AtMAPKKK5 mutant lines
were challenged with grapevine and barley powdery mildew. Both mutants showed a
significant increase in penetration of these two non-adapted powdery mildew species.
Overall, the results of this study emphasise the importance of RALPH effectors in the
modulation of plant immunity and identify components of chitin-mediated MAPK activation pathway, which also play an important role in resistance against non-adapted powdery mildew pathogens.