Effector-centred proximity-dependent labelling enables the discovery of cell-surface immune receptors in plants

This study establishes effector-centred proximity-dependent labelling using TurboID as a versatile and scalable method for identifying plant cell-surface immune receptors and their guarded targets, successfully validating known interactions and discovering new ones such as SlEix1 as the functional receptor for the effector XEG1.

The Gist

Imagine a high-stakes game of "Hide and Seek" happening inside a plant's body. The plant is trying to defend itself against invisible invaders (bacteria and fungi), while the invaders are trying to sneak in and cause damage.

For a long time, scientists have struggled to find the plant's "security guards" (immune receptors) because the invaders are very good at hiding. They often interact with the guards in fleeting, weak, or indirect ways, making them impossible to catch using traditional methods.

This paper introduces a brilliant new strategy to catch these guards in the act: The "TurboID Tagging" System.

Here is the story of how they did it, explained simply:

1. The Problem: The Invisible Invaders

Think of the plant's immune system as a castle. The invaders (pathogens) send out spies called effectors. These spies try to break the castle walls or trick the guards.

• Some spies talk directly to the guards.
• Others sneak up and tamper with a "decoy" (a fake target), and the real guard only notices the tampering.

Traditional methods are like trying to photograph a spy who moves too fast or only appears for a split second. You often miss them, or you can't tell who they are talking to.

2. The Solution: The "TurboID" Glue Gun

The scientists invented a clever trick using a tool called TurboID. Imagine TurboID as a super-fast, magical glue gun that shoots out sticky "biological stickers" (biotin).

They took the spy (the pathogen effector) and glued this TurboID glue gun onto its back.

• The Plan: When they release this "Spy-Gun" into the plant, the glue gun starts firing stickers at everything standing within arm's reach.
• The Result: If the spy is standing next to a plant immune receptor, that receptor gets covered in stickers. Even if they only touch for a second, the sticker stays.

3. The Hunt: Fishing with a Magnet

Once the stickers are on, the scientists act like fishermen. They use a special "magnet" (streptavidin) that only grabs onto the stickers.

• They wash away everything that doesn't have a sticker.
• What's left on the magnet? The immune receptors that were standing right next to the spy!

This allowed them to identify the guards even if they had never met the spy before.

4. The Big Discoveries

The team tested this on three different types of spies:

• Spy #1 & #2 (Avr4 and Avr2): They knew who the guards were for these spies. The TurboID method successfully found them, proving the system works. It even found the "decoy" targets that the spies were messing with.
• Spy #3 (XEG1): This was the mystery case. Scientists knew this spy made plants sick, but they didn't know which guard was supposed to stop it.
  • The Breakthrough: The TurboID method found a new guard in tomatoes called SlEix1.
  • The Proof: When they tested this new guard, it turned out to be a superhero. It recognized the spy and triggered a strong defense alarm (cell death) to stop the infection.

5. Why This Matters

Think of this like upgrading from a blurry security camera to a high-definition, motion-sensing drone.

• Before: Scientists had to guess which gene was the guard, often taking years of trial and error.
• Now: They can drop a "Spy-Gun" into a crop plant, tag the guards, and find the solution in weeks.

This is huge for farming. It means we can quickly find the "super-genes" in wild plants that make them resistant to diseases and move them into our food crops (like tomatoes and wheat) to protect our food supply without using so many chemicals.

In a nutshell: The scientists gave the plant's enemies a "sticky tag" to see who they were hugging. By seeing who got tagged, they finally found the plant's hidden immune heroes.

Technical Summary

1. Problem Statement

Identifying plant disease resistance (R) proteins, particularly cell-surface receptors (RLPs and RLKs) that perceive apoplastic pathogen effectors, remains a significant bottleneck in plant pathology.

• Limitations of Current Methods: Classical genetic mapping is time-consuming. Sequence-based approaches (RenSeq, MutRenSeq) operate at the DNA level and often fail to capture the protein complexes formed during recognition.
• The Interaction Challenge: Many effector-receptor interactions are transient, weak, or indirect (e.g., "guard" or "decoy" mechanisms where the receptor monitors a host target modified by the effector rather than binding the effector directly). Standard affinity purification (pull-down) methods often fail to capture these dynamic interactions because they require stable binding.
• Knowledge Gap: While proximity-dependent labelling (PL) using TurboID has been used to find host susceptibility factors, it has not been systematically applied to identify the specific immune receptors that recognize apoplastic effectors in crop species.

2. Methodology

The authors developed an effector-centred proximity-dependent labelling (PL) strategy using the engineered biotin ligase TurboID.

• Construct Design: Three apoplastic effectors were fused C-terminally to Yellow Fluorescent Protein (YFP) and TurboID:
  • Avr4 (from Fulvia fulva)
  • Avr2 (from Fulvia fulva)
  • XEG1 (from Phytophthora sojae)
• Experimental System:
  • Host Plants: Nicotiana benthamiana (model) and Tomato (Solanum lycopersicum, crop).
  • Expression: Transient expression via Agrobacterium tumefaciens agroinfiltration.
  • Labeling: Leaves were infiltrated with biotin (and ATP for tomato to boost enzymatic activity) to allow TurboID to biotinylate proteins within a ~10nm radius of the effector bait.
• Validation & Analysis:
  • Localization: Confirmed apoplastic accumulation via apoplastic fluid (AF) isolation and immunoblotting.
  • Functional Assays: Tested if fusion proteins retained elicitor activity (Hypersensitive Response/HR) in plants expressing known receptors (e.g., Cf-4, Cf-2).
  • Proteomics: Streptavidin-based enrichment of biotinylated proteins followed by LC-MS/MS (Mass Spectrometry).
  • Bioinformatics: Volcano plot analysis to identify significantly enriched proteins; AlphaFold 3 for structural modeling of receptor-effector complexes.
  • Functional Validation: Co-expression assays in N. benthamiana (WT and SOBIR1 KO) to test if candidate receptors enhance HR.

3. Key Contributions

1. Proof of Concept: Established that effector-TurboID fusions can serve as effective baits to capture both direct and indirect immune recognition complexes in the apoplast.
2. Methodological Optimization: Demonstrated that supplementing extracellular ATP significantly enhances biotinylation efficiency in tomato, likely by compensating for low apoplastic ATP levels and potentially priming immune signaling.
3. Discovery of a New Receptor: Identified SlEix1 as the functional ortholog of NbRXEG1 in tomato, resolving a long-standing question about XEG1 perception in this crop.
4. Locus Characterization: Defined the tomato EIX locus as a multi-effector immune hub encoding receptors with distinct specificities for different pathogen families (GH11 and GH12 glycoside hydrolases).

4. Key Results

• Validation of Known Systems:

  • Avr4-TurboID: Successfully biotinylated the receptor Cf-4 in both N. benthamiana and tomato. The fusion protein retained full elicitor activity, triggering a Cf-4-dependent HR.
  • Avr2-TurboID: Successfully biotinylated the guarded host target Rcr3 (the protease inhibited by Avr2) and the receptor Cf-2 (in a Cf-2 background). This confirmed the method's ability to capture indirect "guard" mechanisms.
  • Specificity: No cross-labeling occurred between non-matching pairs (e.g., Avr4-TurboID did not label Cf-2 or FLS2-Cf-9).

• Discovery of SlEix1:

  • In tomato, XEG1-TurboID did not yield a statistically significant hit in standard volcano plots due to low peptide counts in one replicate. However, manual inspection revealed that SlEix1 (Solyc07g008620) was the only RLP detected exclusively in XEG1-TurboID samples across all replicates.
  • Functional Validation: Co-expression of SlEix1 with XEG1-TurboID in N. benthamiana significantly enhanced the HR compared to controls. This enhancement was abolished in SOBIR1 knockout plants, confirming SlEix1 functions as a signaling-competent RLP.
  • Structural Modeling: AlphaFold 3 modeling showed that SlEix1 shares critical structural features (N-loopout and Island Domain) with the known receptor NbRXEG1 necessary for XEG1 binding, whereas its homolog SlEix2 lacks these features.

• The EIX Locus as a Hub:

  • The study positions the tomato EIX locus as a complex immune hub. It encodes:
    • SlEix2: Primary receptor for EIX (GH11 family).
    • SlEix1: Primary receptor for XEG1 and other GH12 proteins (and potentially acts as a decoy for EIX).
    • SlEil2: Receptor for Sclerotinia SsSCP.
  • This demonstrates a single genomic cluster evolving to recognize diverse pathogen effectors.

5. Significance

• Accelerated Gene Discovery: This approach provides a scalable, protein-level route to match effectors to receptors, bypassing the need for complex genetic mapping or large mutant populations. It is particularly valuable for polyploid crops or those with long generation times.
• Capturing Indirect Recognition: Unlike traditional pull-downs, this method successfully captures "guard" mechanisms (receptors monitoring host targets), which are common in plant immunity but difficult to detect.
• Broad Applicability: The strategy is applicable to both model plants and crops, and can be adapted for cytoplasmic effectors and intracellular NLRs.
• Future Breeding: By rapidly identifying functional R genes and their specific ligands, this method facilitates the deployment of durable resistance in crop breeding programs, addressing the urgent need for sustainable disease control in the face of climate change and reduced chemical inputs.