Helper NLRs Nrc2 and Nrc3 act co-dependently with Prf/Pto and activate MAPK signaling to induce immunity in tomato
Plant intracellular immune receptors, primarily nucleotide-binding, leucine-rich repeat proteins (NLRs), can detect virulence proteins (effectors) from pathogens and activate NLR-triggered immunity (NTI). Recently, sensor NLRs have been reported to function with helper NLRs to activate immune responses. We investigated the role of two helper NLRs, Nrc2 (NLR required for cell death 2) and Nrc3, on immunity in tomato to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) mediated by the sensor NLR Prf and the Pto kinase. Loss-of-function mutations in both Nrc2 and Nrc3 completely compromised Prf/Pto-mediated NTI to Pst containing the cognate effectors AvrPto and AvrPtoB. An nrc3 mutant showed intermediate susceptibility between wild-type tomato plants and a Prf mutant, while an nrc2 mutant developed only mild disease symptoms. These observations indicate that Nrc2 and Nrc3 act additively to contribute to Prf/Pto-mediated immunity. We also examined at what point Nrc2 and Nrc3 act in the Prf/Pto-mediated immune response. In the nrc2/3 mutant, programmed cell death (PCD) normally induced by constitutively-active variants of AvrPtoB, Pto or Prf was abolished, but that induced by M3K or Mkk2 was not. PCD induced by a constitutively active variant of Nrc3 was also abolished in a Nicotiana benthamiana line with reduced expression of Prf. MAPK activation triggered by expression of AvrPto in the wild-type Pto-expressing tomato plants was completely abolished in the nrc2/3 mutant. These results indicate that Nrc2 and Nrc3 act in concert with Prf/Pto and upstream of MAPK signaling. Nrc2 and Nrc3 were not required for the HR triggered by Ptr1, another sensor NLR mediating Pst resistance, although these helper NLRs do appear to be involved in resistance to certain Pst race 1 strains.