Upon pathogen invasion, each plant cell has the ability to mount an innate
immune response. Plants have evolved R genes, which typically encode
nucleotide-binding domain and leucine-rich repeat-containing immune
receptors (NLRs). The model plant species, Arabidopsis, harbors the paired
NLRs, RPS4 and RRS1, the products of which function cooperatively to
confer recognition of the Pseudomonas syringae effector, AvrRps4, and the
Ralstonia solanacearum effector, PopP2. The exact mechanism underlying
RPS4/RRS1-mediated effector recognition remains unclear; therefore, the
function of RPS4 and RRS1 was further elucidated.
Firstly, by investigating the avirulence activity of natural variants of PopP2
isolated from R. solanacearum strains from across the Republic of Korea,
popP2 was demonstrated to be well-conserved and RPS4/RRS1-mediated
recognition of PopP2 could tolerate multiple natural polymorphisms in the
popP2 sequence. Moreover, a conserved PopP2 EAR motif was identified
and characterized; the EAR motif was shown to be required for in planta
PopP2 stability and recognition.
Secondly, utilizing suppressor of slh1 immunity (sushi) mutants generated in a
forward genetic screen on slh1 mutant seeds, insight was gained into the
differential requirements for RRS1 auto-activity and effector perception. A
leucine-rich repeat (LRR) mutation, L816F, was identified, which affected
auto-activity but not effector recognition. Furthermore, a WRKY domain
mutation, C1243Y, was identified, which conferred auto-activity with distinct
features compared to other known auto-active RRS1 variants. Notably, a TIR
mutant harboring a C15Y mutation was identified that impaired RPS4/RRS1
TIR/TIR heterodimer formation and full-length RRS1 function.
Finally, an analagous self-association interface (DE) identified in the crystal
structure of the TNL, SNC1, was investigated for its role in RPS4 function. It
was demonstrated that the DE interface mutations, R116A and M150R,
disabled RPS4 TIR domain effector-independent cell death induction and
impaired full-length RPS4 signaling.