How a Virus Stops Plant Defenses to Boost Immune Response

Greg Howard
18th July, 2024

How a Virus Stops Plant Defenses to Boost Immune Response

The Tomato spotted wilt virus effector NSm triggers robust plant immunity by competitively blocking the host E3 ligase SBP1 from ubiquitinating and degrading the active state of the NLR immune receptor Sw-5b, leading to its stabilization and accumulation.

Image adapted from: Wang et al. / CC BY (Source)

Key Findings

  • The study from Nanjing Agricultural University focuses on the tomato NLR protein Sw-5b and its role in plant immunity
  • In the absence of pathogens, Sw-5b is tagged for degradation by the E3 ligase SBP1, keeping its levels low
  • During infection, the viral effector NSm disrupts this tagging, stabilizing Sw-5b and triggering a strong immune response
Plant intracellular nucleotide-binding and leucine-rich repeat immune receptors (NLRs) are crucial for activating robust pathogen defense responses. Despite their importance, NLR proteins accumulate at very low levels in the absence of pathogen effectors, raising questions about how such low levels can trigger strong immune responses upon pathogen recognition. Recent research from Nanjing Agricultural University sheds light on this mechanism, focusing on the tomato NLR Sw-5b[1]. The study reveals that in the absence of pathogen effectors, the inactive form of Sw-5b is targeted for ubiquitination by an E3 ligase known as SBP1. Ubiquitination is a process where proteins are tagged for degradation. SBP1 interacts with Sw-5b via its N-terminal domain, leading to a slow turnover of the inactive NLR. However, when Sw-5b is in its auto-active state, SBP1 is upregulated and interacts with both the N-terminal and NB-LRR domains, resulting in a rapid turnover of the active NLR. This dynamic regulation ensures that Sw-5b levels remain low under normal conditions but can quickly ramp up in response to pathogen attack. During infection with the tomato spotted wilt virus, the viral effector NSm interacts with Sw-5b. This interaction disrupts the binding of Sw-5b with SBP1, stabilizing the active form of Sw-5b and allowing it to induce a robust immune response. This finding provides a detailed understanding of how low levels of NLR proteins can still mount a strong defense response when needed. This study builds on earlier research into plant immune responses and NLR regulation. For instance, previous work has shown that NLRs exist in an equilibrium between ON and OFF states, with effector binding stabilizing the ON state to trigger defense signaling[2]. This new research supports the notion of such equilibrium and adds a layer of understanding by explaining how ubiquitination and protein turnover play a role in maintaining this balance. Additionally, the study aligns with findings on the role of pathogen effectors in modulating plant immune responses. For example, the effector AVR3a from the potato blight pathogen Phytophthora infestans has been shown to interact with and stabilize a host E3 ligase, thereby modifying its normal activity to prevent host cell death during infection[3]. Similarly, the NSm effector in the present study disrupts the interaction between Sw-5b and SBP1, stabilizing the active NLR to promote a strong immune response. Moreover, the study provides insights into the broader concept of PAMP-triggered immunity (PTI), where general microbial elicitors trigger plant defense responses[4]. While PTI is often masked by pathogen virulence effectors that suppress it, the findings on Sw-5b and SBP1 illustrate how specific interactions at the molecular level can fine-tune immune responses, even when overall protein levels are low. In summary, the research from Nanjing Agricultural University elucidates a critical mechanism by which low levels of NLR proteins can still induce robust immune responses upon pathogen recognition. By detailing the role of the E3 ligase SBP1 in regulating the turnover of the tomato NLR Sw-5b, the study provides valuable insights into the dynamic regulation of plant immune receptors and their interaction with pathogen effectors. This work not only enhances our understanding of plant immunity but also opens up potential avenues for developing more resilient crops through targeted manipulation of NLR pathways.

GeneticsBiochemPlant Science

References

Main Study

1) A viral effector blocks the turnover of a plant NLR receptor to trigger a robust immune response.

Published 17th July, 2024

https://doi.org/10.1038/s44318-024-00174-6

Related Studies

2) Comparative Analysis of the Flax Immune Receptors L6 and L7 Suggests an Equilibrium-Based Switch Activation Model.

https://doi.org/10.1105/tpc.15.00303

3) Phytophthora infestans effector AVR3a is essential for virulence and manipulates plant immunity by stabilizing host E3 ligase CMPG1.

https://doi.org/10.1073/pnas.0914408107

4) Innate immunity in plants: an arms race between pattern recognition receptors in plants and effectors in microbial pathogens.

https://doi.org/10.1126/science.1171647


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