To evade recognition and to increase their virulence, pathogens deploy effectors into host cells to modify or remove host proteins. Plants can detect these effector mediated changes via Resistance proteins (NLRs). A model thus proposes that NLRs ‘guard’ host ‘guardees’. A corollary to this model is that forms of plant autoimmunity are due to inappropriate NLR activation. We show here that NLR triggered immunity in pat1 can be prevented by the expression of a dominant negative (DN) allele of the NLR SUMM2. We further find that this DN approach is faster than crossing in knock-outs, and has the advantage of ‘poisoning’ redundant genes in cases of NLR homo- and hetero-dimerization. To exploit this, we did a large-scale screen for suppressors of autoimmunity in Arabidopsis and found that camta3 autoimmunity is suppressed by the expression of DN versions of two different NLRs, Dominant Suppressor of CAMTA3 (DSC) 1 & 2. EDS1 levels are elevated in camta3 and CAMTA3 was claimed to negatively regulate immunity through transcriptional regulation of EDS1. In contrast, recent studies provide evidence for CAMTA3 being involved in positive regulation of an early stress response. We report here that EDS1 mRNA levels are no longer elevated in camta3 expressing DSC1-DN or DSC2-DN. This indicates that the camta3 autoimmune phenotype is likely due to inappropriate activation of the NLRs. Interestingly, we found that dsc1 or dsc2 null mutants do not suppress camta3, but camta3/dsc1/dsc2 triple mutants no longer exhibit autoimmunity. We furthermore provided evidence showing that DSC1 interacts with both CAMTA3 and DSC2 suggesting a degree of cooperation between the two NLRs in guarding of CAMTA3. Taken together our data indicate that the increased levels of immune-related transcripts in camta3 are due to NLR activation and not to the loss of CAMTA3 as a transcriptional repressor of immunity.
This thesis comprises an introduction to plant innate immunity with focus on NLRs. The results are presented in three manuscripts. The first, “The mRNA decay factor PAT1 functions in a pathway including MAP kinase 4 and immune receptor SUMM2” published in EMBO journal, link MPK4 to mRNA decapping and further establishes that the autoimmune phenotype of pat1 is NLRdependent. The second, “Matching NLR immune receptors to autoimmunity in camta3 mutants using antimorphic NLR alleles”, describing the main focus of my PhD research, is accepted for publication in Cell Host & Microbe. This article establishes camta3 autoimmunity to be NLR dependent and describes a robust systematic method for uncovering the roots of autoimmune mutants. Finally, a draft manuscript entitled “Inhibition of autophagy prior to FAPP2 removal restores cargo transfer to the plasma membrane”, links mammalian FAPP2 to the molecular machinery of autophagy.