Taylor, Rosalind M.
The role of the interaction between PiAvr3a and CMPG1 in disease and plant defence.
PhD thesis, University of Glasgow.
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Investigating the individual proteins involved in plant defence and the pathways these proteins are part of is important to gain an understanding of infection and disease resistance processes. It is hoped that this knowledge will help develop solutions to prevent crop infection and increase crop yield. More specifically, CMPG1 was first found to be important in pathogen responses by Kirsch et al. (2001) in parsley. It has further been found to have homology to NbACRE74 and AtPUB20 and AtPUB21 implying it is common to many plant species. CMPG1 is an U-Box E3 Ubiquitin ligase and involved in a number of different defence responses (INF1, Cf-9 & Pto/AvrPto). PiAvr3a is an RxLR cytoplasmic effector protein from potato late blight pathogen Phytophthora infestans. Its virulence function was shown by its ability to suppress INF1 cell death by Bos et al. (2006). This P. infestans protein and the plant protein CMPG1 were found to interact in Yeast-two-hybrid studies performed by Dr M Armstrong (Bos et al., 2010).
The aims of this project were to determine the nature of this interaction, functionally and biochemically. Does the interaction found in the Yeast-two-hybrid take place in the plant? What are the biological implications of PiAvr3a and CMPG1 interaction? What is the biochemical nature of the interaction? This project used in planta studies to investigate the CMPG1-PiAvr3a interaction. The biological reason for why this interaction occurred was studied using virus induced gene silencing (VIGS) and hypersensitive response/cell death suppression assays. The biochemical nature of this interaction was investigated using ubiquitination assays, and purified proteins, in vitro. This thesis aimed to provided data to increase the overall understanding of the function of CMPG1 and PiAvr3a during infection.
Firstly, this project found evidence for the stabilisation of StCMPG1 by PiAvr3a as well as possible direct interaction inside plant cells. The stronger interaction with, and stabilisation of StCMPG1 by PiAvr3aKI, is in accordance with the stronger suppression of INF1 cell death by this form of the effector (Bos et al., 2006; 2009). Secondly, evidence for the role of CMPG1 in disease resistance to multiple plant pathogens, including oomycetes, bacteria and fungi was found. PiAvr3a suppresses all of these cell death responses. It is likely that CMPG1 is a target for other effectors from other pathogens. Cell death suppression by PiAvr3a is caused by stabilisation, and thus altered function, of CMPG1. Thirdly, promising and surprising data revealed that PiAvr3a may act as an E2 conjugating enzyme. Moreover, it appeared to act via both lysines 48 and 63, perhaps suggesting that it forms a mixed chain on CMPG1 substrates.
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