Molecular dissection of Arabidopsis RAR1 and SGT1 functions in plant immunity

Plants possess several layers of defence against pathogens. RAR1 (required for Ml-a12 conditioned resistance) and SGT1 (suppressor of G2 allele of skp1) are regulators of disease resistance conditioned by Resistance (R) proteins that recognise specific pathogen effectors. The model plant, Arabidopsis thaliana, has one copy of RAR1 (AtRAR1) and two recently duplicated copies of SGT1 (AtSGT1a and AtSGT1b). Despite their high sequence homology (78% identity at the amino acid level), AtSGT1b, but not AtSGT1a, is genetically recruited for resistance mediated by a subset of R proteins and for phytohormone signalling controlled by at least two plant SCF E3 ligases (SCF TIR1 and SCF COI1). AtRAR1, but not AtSGT1a or AtSGT1b, was also shown to contribute to plant basal defence against virulent pathogens, in which Arabidopsis EDS1 (Enhanced Disease Susceptibility 1) is an essential regulator. Recent studies revealed roles of RAR1 as co-chaperones of HSP90 to promote accumulation of pre-activated R proteins. SGT1 also shares molecular features of known cochaperones. SGT1 from plant, yeast and human interact with HSP90 and, in human and yeast, is an assembly factor in kinetocore complex formation. The precise role of SGT1 in plant defence was unclear. Recent biochemical experiments showed that SGT1 is required for Bs2 R protein folding that implies SGT1 activity in R protein complex assembly. However, recent genetic data in Arabidopsis suggested that SGT1 acts antagonistically with RAR1 in R protein accumulation, suggesting of a role of SGT1 in R protein degradation. The presence of an additional copy of SGT1 in Arabidopsis and lethality of the sgt1a/sgt1b double mutant complicates genetic interpretation using this system. This study aimed to characterize further the activities of RAR1 and SGT1 in plant immunity using various approaches. Several pieces of key data on the activities of RAR1 and SGT1 in plant immunity were generated in this study. AtRAR1, AtSGT1a and AtSGT1b proteins were expressed in all tissue tested and, although direct interaction between these proteins was not found, Hsc70 was identified as a potential interacting partner of AtRAR1. AtRAR1 regulates AtSGT1b accumulation in the nucleus. I established that both AtSGT1b and AtSGT1a are capable of functioning in R protein-mediated defence and phytohormone signalling in a dose-dependent manner. Lower levels of AtSGT1a in plant cells are likely insufficient to show a genetic effect on sgt1a mutants due to the presence of the more abundant AtSGT1b. The finding of AtSGT1a activity prompts us to reconsider the current model of RAR1/SGT1 antagonism in defence based on purely genetic data using Arabidopsis. I found that AtRAR1 and AtSGT1b contribute to basal defence. Intriguingly, the rar1 and sgt1b mutants lower EDS1 protein accumulation and change the molecular character of EDS1. The activities of AtRAR1 and AtSGT1b in basal defence may be through EDS1. EDS1 is an indispensable regulator of resistance conditioned by the TIR (Toll-Interleukin-1 Receptor) class of nucleotide-binding/leucine-rich-repeat (NB-LRR) R protein. These data therefore suggest a potential molecular link between EDS1 and TIR-NB-LRR via RAR and SGT1. My results highlight the need for further analysis to dissect mechanisms of TIR-NBLRR protein assembly and activation and their molecular connection with EDS1 and the chaperone/cochaperone machinery.