Plants utilise receptor proteins to sense invading pathogens and upregulate defence responses. One group of receptors, consisting predominantly of Nucleotide binding- leucine rich repeat receptors (NLRs), acts to recognise the intracellular presence of pathogen derived effector proteins. NLRs consist of a three-domain architecture, comprised of an N-terminal signalling domain, a central nucleotide binding domain that contains a P-loop motif that binds ADP/ATP and acts as an on/off switch for the receptor as well as a C-terminal Leucine rich repeat. The Arabidopsis thaliana accession (Col-0) is fully resistant to the biotrophic Oomycete pathogen Albugo candida, however other accessions e.g. Ws-2 are susceptible. Recombinant inbred lines (RILs) derived from a cross of these two accessions exhibited multiple different resistance phenotypes. We determined which genes were responsible for the resistance phenotypes and then investigated the mechanisms that the identified genes employ to confer resistance to A. candida. We identified three novel White Rust Resistance (WRR) genes that cause the resistance phenotypes to A. candida in Col-0. These include, WRR5A, WRR5B and WRR7, two of which (WRR5B and WRR7) encode integrated LIN-11, Isl1 and MEC-3 (LIM)- Zinc-metallopeptidase (Peptidase) domains (LIM-Peptidase). We report that WRR5A and WRR5B form a heterodimeric complex that localises to the plasma membrane in A. thaliana and operate by the sensor-helper model of NLR activation. However, unlike other sensor-helper NLR systems the intact P-loops of both proteins are required to stimulate an immune response. In addition, we found that WRR7 stimulates an immune response independent of other NLRs and that after A. candida infection this gene is activated by Calmodulin binding transcriptional activator 2 (CAMTA2), independently of CAMTA1 and CAMTA3 which have previously been shown to have functional redundancy with CAMTA2. Furthermore, we implicated Chromatin remodelling protein 4, a Mitogen activated protein kinase (MAP3Kδ4) and MOS 4 associated complex 7 as being involved with the WRR7 resistance mechanism. Therefore, we have shown that the Col-0 genome harbours multiple resistance genes that operate by three distinct mechanisms to cause immunity to one phytopathogen. Expanding our repertoire of distinct resistance gene systems will enable us to understand how NLRs cause immunity to a plethora of different plant pathogens. This knowledge will pave the way for NLR based engineering approaches to generate novel resistant crop lines in the future.
|Date of Award||16 Sep 2020|
|Supervisor||Volkan Cevik (Supervisor) & Roderick Scott (Supervisor)|