Helper NLRs Nrc2 and Nrc3 act codependently with Prf/Pto and activate MAPK signaling to induce immunity in tomato

Abstract

SUMMARYPlant intracellular immune receptors, primarily nucleotide‐binding, leucine‐rich repeat proteins (NLRs), detect pathogen effector proteins and activate NLR‐triggered immunity (NTI). Recently, ‘sensor’ NLRs have been reported to function with ‘helper’ NLRs to activate immunity. We investigated the role of two helper NLRs, Nrc2 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. An nrc2/nrc3 mutant no longer activated Prf/Pto‐mediated NTI to Pst containing the effectors AvrPto and AvrPtoB. An nrc3 mutant showed intermediate susceptibility between wild‐type plants and a Prf mutant, while an nrc2 mutant developed only mild disease. These observations indicate that Nrc2 and Nrc3 act additively in Prf‐/Pto‐mediated immunity. We 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 M3Kα or Mkk2 was not. PCD induced by a constitutively active 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 tomato plants was completely abolished in the nrc2/3 mutant. These results indicate that Nrc2 and Nrc3 act with Prf/Pto and upstream of MAPK signaling. Nrc2 and Nrc3 were not required for PCD 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

Bolstering the defence line: The vital role of helper NLRs in tomato disease resistance

“…To pinpoint where the two helper NLRs act in the Prf/Pto signalling cascade, Zhang et al. (2023) performed epistasis analyses between the nrc mutants and lines expressing autoactive variants of Prf/Pto signalling components. While autoactive versions of Prf and Pto triggered programmed cell death in a wild‐type background, this cell death phenotype was absent when the proteins were expressed in a nrc2 nrc3 background.…”

“…The findings by Zhang et al . (2023) highlight the important role Nrc2 and Nrc3 play in plant immunity. The authors believe that helper NLRs, such as Nrc2 and Nrc3, represent promising targets for enhancing disease resistance through breeding or gene editing approaches.…”

Related PP2C phosphatases Pic3 and Pic12 negatively regulate immunity in tomato toPseudomonas syringae

“…However, to trigger ETI, Pto interacts and forms a complex with the NLR protein Prf (Ntoukakis et al, 2013). A number of signaling components acting downstream of Pto/Prf have been identified, including NLR proteins Nrc2 and Nrc3, 14-3-3 proteins Tft1, Tft3 and Tft7, MAPKKKα-MKK1/2-WIPK/NTF6 cascade and protein kinase Mai1 (Ekengren et al, 2003;Mucyn et al, 2006;Oh et al, 2010;Oh and Martin, 2011;Roberts et al, 2019;Sheikh et al, 2023;Zhang et al, 2024).…”

PP2C phosphatase Pic14 negatively regulates tomato Pto/Prf-triggered immunity by inhibiting MAPK activation

“…Generally, CNLs and TNLs are considered to function as ‘sensor’ NLRs, which detect effectors, and RNLs are considered as ‘helper’ NLRs that act in downstream events (Chia & Carella, 2023; Huang et al, 2023). In tomato and other solanaceous plants, N LR required for c ell death (Nrc), i.e., Nrc2 and Nrc3 proteins, are helper NLRs that function redundantly to regulate Pto/Prf-mediated immune activation (Wu et al, 2017; Zhang et al, 2024). Upon their activation, some NLRs form large oligomeric complexes called resistosomes through their variable N-terminal domains (Chia & Carella, 2023).…”