Identification and receptor mechanism of TIR-catalyzed small molecules in plant immunity

Abstract

Plant nucleotide-binding leucine-rich-repeat receptors (NLRs) with an N-terminal toll/interleukin-1 receptor (TIR) domain sense pathogen effectors to enable TIR-encoded NADase activity for immune signaling. TIR-NLR (TNL) signaling requires conserved helper NLRs NRG1 and ADR1 and the lipase-like protein EDS1 that functions as a heterodimer with each of its paralogs PAD4 and SAG101. We show that TIR-containing proteins catalyze production of 2'-(5''-phosphoribosyl)-5'-adenosine mono-/di-phosphate (pRib-AMP/ADP) in vitro and in planta. Biochemical and structural data demonstrate that EDS1-PAD4 is a receptor complex for pRib-AMP/ADP. pRib-ADP binding triggers a conformational change in the PAD4 C-terminal domain to allosterically promote EDS1-PAD4 interaction with ADR1-L1 but not NRG1A. Our study identifies TIR-catalyzed pRib-AMP/ADP as a missing link in TIR signaling via EDS1-PAD4 and as likely second messengers for plant immunity.

TIR-catalyzed ADP-ribosylation reactions produce signaling molecules for plant immunity

“…Additionally, a trace amount of a product with molecular weight matching that of pRib-ADP was detected (fig. S1C), supporting weak pRib-ADP activity in promoting EDS1-SAG101 association with NRG1A ( 28 ).…”

“…This is consistent with the observation that overexpression of EDS1 with PAD4 leads to increased Arabidopsis pathogen resistance but not host cell death ( 24 ). The strong activity of pRib-AMP in promoting EDS1-PAD4 interaction with ADR1 but a very low capacity for promoting EDS1-SAG101 interaction with NRG1A ( 28 ) suggests that the TIR product might induce effective plant immunity without cell death. The identified TIR-catalyzed signaling molecules are produced with unequal molar ratios (Fig 5C and fig.…”

“…These structural analyses indicate that ADPr-ATP binding triggers conformational changes in the SAG101 EP domain which enable EDS1-SAG101 interaction with NRG1A. Thus, ADPr-ATP and pRib-ADP ( 28 ) employ a conserved allosteric mechanism to activate EDS1-SAG101 and EDS1-PAD4, respectively.…”

“…An R493A mutation only reduced induced EDS1-SAG101 interaction with NRG1A (Fig. 2C), in contrast with a loss of pRib-ADP-induced EDS1-PAD4 interaction with ADR1-L1 ( 28 ). This would explain why an EDS1 R493A variant disturbed mainly EDS1-PAD4 mediated pathogen resistance in vivo ( 16, 25 ).…”

Allelic variation in theArabidopsisTNL CHS3/CSA1 immune receptor pair reveals two functional regulatory modes

“…Upon effector activation, NLRs oligomerize and form “resistosome” complexes to activate calcium influx (CNLs) or to activate TNL NADase activity. The latter generates small molecules that activate an EDS1 anchored complex to initiate cell death and defense through the calcium channel function of a class of ‘helper’ NLRs called RNLs (Bi et al, 2021; Huang et al, 2022; Jacob et al, 2021; Martin et al, 2020; Ma et., 2020; Jubic et al, 2019; Wan et al, 2019; Wang et al, 2019a, 2019b; Yu et al, 2021). NLRs can function as singletons, in genetically linked pairs, or in immune networks (Adachi et al, 2019; Wu et al, 2018).…”