Activation of a helper NLR by plant and bacterial TIR immune signaling

TIR immune signalling is blocked by phosphorylation to maintain plant growth

Toll/interleukin-1 receptor (TIR) domain proteins are immune signalling components and function as NAD+-cleaving enzymes to activate defence responses. In plants, TIR activation triggers cell death and severely represses growth, especially under osmotic stress, while in animals, it promotes axon degeneration. However, the mechanisms regulating TIR suppression remain unclear. Here we show that TIR NADase activity requires a conserved serine residue spatially close to the catalytic glutamate. The osmotic-stress-activated plant Ca2+-dependent protein kinases (CPKs), the mammalian Ca2+/calmodulin-dependent protein kinase II delta (CAMK2D) and TANK-binding kinase 1 (TBK1) phosphorylate TIR domains at this conserved serine, which blocks TIR NADase activities and functions, thereby maintaining growth in plants and suppressing SARM1 TIR signalling in animals. Our findings define a fundamental molecular mechanism by which phosphorylation at a conserved serine residue inhibits TIR signalling in plants and animals and sustains plant growth.

The resistance awakens: Diversity at the DNA, RNA, and protein levels informs engineering of plant immune receptors from Arabidopsis to crops

Plants rely on germline-encoded, innate immune receptors to sense pathogens and initiate the defense response. The exponential increase in quality and quantity of genomes, RNA-seq datasets, and protein structures has underscored the incredible biodiversity of plant immunity. Arabidopsis continues to serve as a valuable model and theoretical foundation of our understanding of wild plant diversity of immune receptors, while expansion of study into agricultural crops has also revealed distinct evolutionary trajectories and challenges. Here, we provide the classical context for study of both intracellular nucleotide-binding, leucine-rich repeat receptors and surface-localized pattern recognition receptors at the levels of DNA sequences, transcriptional regulation, and protein structures. We then examine how recent technology has shaped our understanding of immune receptor evolution and informed our ability to efficiently engineer resistance. We summarize current literature and provide an outlook on how researchers take inspiration from natural diversity in bioengineering efforts for disease resistance from Arabidopsis and other model systems to crops.

H2O2-driven plant immunity requires post-translational modification as a switch

Hydrogen peroxide (H2O2) has an essential role in plant stress and immunity responses, but how H2O2 regulates these processes remain unclear. Recent findings showed that H2O2 enhances resistance in infected or distal tissues by fine-tuning the post-translational modifications (PTMs) of two key transcription factors, basic helix-loop-helix 25 (bHLH25) and CCA1 HIKING EXPEDITION (CHE), within plant immunity signaling pathways.

Activation and inhibition mechanisms of a plant helper NLR

Plant nucleotide-binding leucine-rich repeat (NLR) receptors sense pathogen effectors and form resistosomes to confer immunity1. Some sensor NLR resistosomes produce small molecules to induce formation of a heterotrimer complex with two lipase-like proteins, EDS1 and SAG101, and a helper NLR called NRG1 (refs. 2,3). Activation of sensor NLR resistosomes also triggers NRG1 oligomerization and resistosome formation at the plasma membrane4,5. We demonstrate that the Arabidopsis AtEDS1-AtSAG101-AtNRG1A heterotrimer formation is stabilized by the AtNRG1A loss-of-oligomerization mutant L134E5,6. We report structures of AtEDS1-AtSAG101-AtNRG1A L134E and AtEDS1-AtSAG101-AtNRG1C heterotrimers with similar assembly mechanisms. AtNRG1A signalling is activated by the interaction with the AtEDS1-AtSAG101 heterodimer in complex with their small-molecule ligand. The truncated AtNRG1C maintains core interacting domains of AtNRG1A but develops further interactions with AtEDS1-AtSAG101 to outcompete AtNRG1A. Moreover, AtNRG1C lacks an N-terminal signalling domain and shows nucleocytoplasmic localization, facilitating its sequestration of AtEDS1-AtSAG101, which is also nucleocytoplasmic. Our study shows the activation and inhibition mechanisms of a plant helper NLR.

Toll/interleukin-1 receptor-only genes contribute to immune responses in maize

Proteins with Toll/interleukin-1 receptor (TIR) domains are widely distributed in both prokaryotes and eukaryotes, serving as essential components of immune signaling. Although monocots lack the major TIR nucleotide-binding leucine-rich repeat-type (TNL) immune receptors, they possess a small number of TIR-only proteins, the function of which remains largely unknown. In the monocot maize (Zea mays), there are 3 conserved TIR-only genes in the reference genome, namely ZmTIR1 to ZmTIR3. A genome-wide scan for TIR genes and comparative analysis revealed that these genes exhibit low sequence diversity and do not show copy number variation among 26 diverse inbred lines. ZmTIR1 and ZmTIR3, but not ZmTIR2, specifically trigger cell death and defense gene expression when overexpressed in Nicotiana benthamiana leaves. These responses depend on the critical glutamic acid and cysteine residues predicted to be essential for TIR-mediated NADase and 2',3'-cAMP/cGMP synthetase activity, respectively, as well as the key TIR downstream regulator Enhanced Disease Susceptibility 1 (EDS1). Overexpression of ZmTIR3 in N. benthamiana produces signaling molecules, including 2'cADPR, 2',3'-cAMP, and 2',3'-cGMP, a process that requires the enzymatic glutamic acid and cysteine residues of ZmTIR3. ZmTIR expression in maize is barely detectable under normal conditions but is substantially induced by different pathogens. Importantly, the maize Zmtir3 knockout mutant exhibits enhanced susceptibility to the fungal pathogen Cochliobolus heterostrophus, highlighting the role of ZmTIR3 in maize immunity. Overall, our results unveil the function of the maize ZmTIRs. We propose that the pathogen-inducible ZmTIRs play an important role in maize immunity, likely through their enzymatic activity and via EDS1-mediated signaling.

TIR-mediated immune signaling through the EDS1:PAD4:ADR1 node is conserved in monocots and dicots

Plants defend themselves against pathogens by activating two interconnected layers of immunity: pattern- and effector-triggered immunity. Recent studies shed light on a number of unanswered questions in these pathways and demonstrate a conserved structural basis, in monocots and dicots, of signaling through the EDS1:PAD4:ADR1 module, downstream of Toll/interleukin-1 receptor (TIR) domain-containing immune receptors that cleave the dinucleotide nicotinamide adenine dinucleotide (NAD+). The studies define a new function for TIR domain proteins in monocots and suggest that the signaling molecule 5″-phosphoribosyl (pRib)-AMP is derived from the TIR product 2'-adenosine diphosphate ribose (2'cADPR). These findings have important implications for developing the strategies to breed resistant crops.

Transcellular regulation of ETI-induced cell death

To address the persistent challenge of cell death spread and limitation during effector-triggered immunity (ETI), we propose a 'concentric circle' model. This model outlines a regulatory framework, integrating multiple cells and diverse signaling molecules, including salicylic acid (SA), jasmonic acid (JA), and Ca2+. By accounting for the varying concentrations and spatiotemporal distributions of these molecules, our model aims for precision in immune defense and regulated cell death. To validate this model, a pathosystem-triggering ETI without pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) is required. Here, we review potential ETI elicitors, including victorin, thaxtomin A, and second messengers. We anticipate that future discovery of 'pure' ETI-triggering effectors will provide deeper insights into the transcellular regulation of immune response in plants.

A canonical protein complex controls immune homeostasis and multipathogen resistance

The calcium (Ca2+) sensor ROD1 (RESISTANCE OF RICE TO DISEASES1) is a master regulator of immunity in rice. By screening suppressors of rod1 mutants, we show that ROD1 governs immune homeostasis by surveilling the activation of a canonical immune pathway. Mutations in OsTIR (TIR-only protein), OsEDS1 (enhanced disease susceptibility 1), OsPAD4 (phytoalexin deficient 4), and OsADR1 (activated disease resistance 1) all abolish enhanced disease resistance of rod1 plants. OsTIR catalyzes the production of second messengers 2'-(5″-phosphoribosyl)-5'-adenosine monophosphate (pRib-AMP) and diphosphate (pRib-ADP), which trigger formation of an OsEDS1-OsPAD4-OsADR1 (EPA) immune complex. ROD1 interacts with OsTIR and inhibits its enzymatic activity, whereas mutation of ROD1 leads to constitutive activation of the EPA complex. Thus, we unveil an immune network that fine-tunes immune homeostasis and multipathogen resistance in rice.