Avalilação PREreview de The Magnaporthe oryzae Effector AvrPii Attenuates Rice Immunity by Targeting the Calcium-Associated ANK Protein AVIN8

Figure 1A: Yeast Two-Hybrid

Figure 1A is presented as the first evidence that AvrPii interacts with AVIN8. The evidence is not convincing. The yeast two-hybrid assay lacks the controls required to distinguish a specific interaction from a false-positive signal caused by a sticky bait or prey, and this issue is especially serious for a protein dominated by ankyrin repeats.

The negative controls are inadequate. Empty vector is not a meaningful biological control here. It tests autoactivation in the absence of an expressed partner, not whether AVIN8 interacts nonspecifically with unrelated or related proteins. A rigorous assay would include, at minimum, unrelated effectors and preferably closely related effectors or better AvrPii variants. Without such controls, the observed growth on selective medium cannot be assigned specifically to the AvrPii-AVIN8 pair.

This is not a minor technical quibble. Ankyrin-repeat proteins are well known to be problematic in protein-protein interaction assays because they can behave as sticky interaction modules. That property raises the bar for the quality of the assay and data. Figure 1A does not rule out that AVIN8 is generally sticky in yeast, or that AvrPii is selective for AVIN8 under the assay conditions used.

The omission of positive controls is also an issue. AvrPii has previously reported interactors in the form of Exo70 proteins. A known AvrPii interactor should have been included as a benchmark for the assay. This would have allowed to judge whether the AVIN8 signal is comparable to an established interaction that has been validated by the structure of the complex (De la Concepcion, et al. https://doi.org/10.1073/pnas.2210559119). As shown, Figure 1A establishes only that one yeast combination grows under one set of selective conditions. It does not establish a specific effector-target interaction.

Figure 1B: BiFC

Figure 1B does not rescue the problem. The BiFC experiment is also insufficiently controlled and cannot be treated as independent validation of the interaction. BiFC, particularly split-YFP BiFC, is notoriously prone to false positives. The assay is useful only when paired with rigorous controls for expression, protein stability, localization, and specificity. Those controls are missing here.

The central problem is inherent to the method. YFP fragments are unstable on their own and can be artificially stabilized when fused to proteins, even when the proteins do not form a biologically meaningful complex. Reconstitution of fluorescence is therefore not, by itself, evidence of interaction. It can report forced proximity, fusion-protein stabilization, overexpression artefacts, or nonspecific association. Figure 1B does not address these alternatives.

The absence of western blots is a major flaw. The figure gives no evidence that the fusion proteins are expressed at comparable levels, stable, intact, or absent from the relevant negative controls. A fluorescent signal in one combination and no signal in others could simply reflect differences in expression or stability of the fusion proteins. Without immunoblots for the BiFC constructs, the experiment is inconclusive.

The choice of controls is again too weak. The ideal negative control would be a related effector that does not bind AVIN8, or better still, an AvrPii mutant defective in AVIN8 binding. The latter requires knowing or testing the interface, but that is precisely the point: a convincing interaction claim must generate a falsifiable specificity control. Empty-vector-style controls do not test specificity; they test whether fluorescence appears when one half of the system is absent.

Independently of the lack of controls, fluorescence signal must be quantified across a statistically meaningful number of randomly selected fields of view, scored independently of fluorescence intensity; presenting representative images alone is insufficient.

Based on the BiFC result, the authors claim a direct protein-protein interaction between AvrPii and AVIN8 that occurs specifically at the plasma membrane. However, BiFC is inherently inadequate to substantiate these claims. First, BiFC cannot establish that two proteins interact directly: fluorescence reconstitution occurs whenever the two fluorescent protein fragments are brought into sufficient proximity, including through a shared binding partner or scaffolding complex. Second, the location of the BiFC signal alone cannot identify the precise site of interaction.

The authors also conclude that the complex is specifically localized to the plasma membrane without employing any established markers to verify this claim. However, even if plasma membrane markers had been used, BiFC remains fundamentally unsuitable for determining the true subcellular site of an interaction. Because YFP reconstitution is irreversible, the reassembled fluorophore forms a thermodynamically stable complex that cannot dissociate. The detected signal therefore merely reflects where the trapped complex accumulates at the time of imaging, rather than where the initial molecular contact occurred. Consequently, while colocalization with a marker could confirm where the complex ultimately resides, it cannot demosntrate that the interaction was initiated there. Supporting such a conclusion requires a reversible method like FRET-FLIM, which captures molecular proximity in real time at defined subcellular positions.

On Orthogonality

A possible defence is that yeast two-hybrid and BiFC are orthogonal assays and that concordance between them validates the AvrPii-AVIN8 interaction. This is not an acceptable argument. Orthogonality is valuable only when each experiment is itself interpretable. A poorly controlled experiment does not become informative because another poorly controlled experiment points in the same direction.

Here, both assays share the same core weakness: they do not test specificity with adequate controls, and they do not exclude artefacts caused by sticky domains, overexpression, fusion-protein behaviour, or protein stability. Two inconclusive experiments do not add up to one convincing result. Inconclusive is inconclusive.

Structural Context and Missing Controls

The evidentiary gap is especially striking given the existing structural context for AvrPii. AvrPii has previously been shown to bind Exo70, and that interaction was validated by a crystal structure of the complex. Against that precedent, the complete absence of structural modelling or interface analysis for the proposed AvrPii-AVIN8 complex is conspicuous.

The paper does not provide a model of how AvrPii could engage AVIN8. It does not map the interface on AvrPii. It does not map the interface on AVIN8. It does not use mutagenesis on either side to disrupt the interaction. It therefore does not generate the most informative controls for both Figure 1A and Figure 1B: interaction-defective mutants. This is the critical missing experiment. If an AvrPii mutant loses AVIN8 binding while retaining expression and folding, and if that mutant also fails in the relevant downstream biological assays, the interaction claim becomes testable.

The broader context also argues for caution. AvrPii has accumulated multiple proposed host targets across different pathways, but only one oif these has been validated by structural biology. That may be biologically true; effectors can have multiple host targets. But proliferation of claimed interactors for a single effector demands higher evidentiary standards, not looser ones. Each new target must be supported by specificity controls, interface validation, and a clear mechanistic link to function. Figure 1 does not meet that standard.

Conclusion

Figure 1 is the foundation for the central claim that AVIN8 is a host target of AvrPii. That foundation is not convincing. The yeast two-hybrid experiment lacks appropriate specificity controls for a sticky ankyrin-repeat protein. The BiFC experiment lacks the controls needed to rule out false positives, fusion artefacts, and protein-stability effects. The two assays are not mutually reinforcing because neither is adequately controlled. The absence of structural modelling, interface mutagenesis, and interaction-defective controls is particularly problematic given the established structural precedent for AvrPii binding to Exo70.

Because Figure 1 does not convincingly establish the AvrPii-AVIN8 interaction, the downstream logic of the paper is already compromised. On that basis, reviewing the remainder of the paper is warranted.

Competing interests

The authors declare that they have no competing interests.

Use of Artificial Intelligence (AI)

The authors declare that they did not use generative AI to come up with new ideas for their review.