AVR-Pia is a small (85 aa) secreted effector of the rice blast fungus Pyricularia (Magnaporthe) oryzae. After an N-terminal signal peptide, the mature protein folds into a six-stranded beta-sandwich stabilised by a single disulfide bond, placing it in the sequence-diverse but structurally conserved MAX (Magnaporthe Avrs and ToxB-like) effector family. The gene is expressed only during infection, from the onset of appressorium differentiation; the protein accumulates in the biotrophic interfacial complex and is translocated into the cytoplasm of invaded rice cells. Inside the host cell AVR-Pia binds heavy metal-associated (HMA) domains. In rice carrying the Pia locus it binds the integrated HMA (RATX1) decoy domain of the sensor NLR RGA5 (apparent Kd about 8 uM), relieving RGA5-mediated repression of the executor NLR RGA4 and triggering a hypersensitive cell death and race-specific resistance; the naturally occurring F24S/T46N allele (AVR-Pia-H3) no longer binds the HMA domain and escapes recognition. AVR-Pia also binds, with higher affinity, the HMA domains of the rice HMA plant proteins OsHPP09, OsHPP10, OsHPP11 and the isoprenylated HMA protein OsHIPP21, which are candidate virulence targets. AVR-Pia contributes to virulence, but how binding these host HMA proteins promotes susceptibility is unknown. The gene shows presence/absence polymorphism among field isolates.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0140404 effector-mediated perturbation of host innate immune response by symbiont | EXP PMID:23548743 The rice resistance protein pair RGA4/RGA5 recognizes the Ma... | MODIFY | Summary: Cesari et al. (2013; abstract only in the cache) showed that the rice NLR pair RGA4/RGA5 recognises AVR-Pia by direct binding of the effector to RGA5-A, that a non-binding AVR-Pia allele is not recognised, and that this recognition confers resistance. The annotation records that AVR-Pia modulates (here, activates) rice innate immunity. Reason: The broad term (modulation, either activation or suppression, of host innate immunity) is correct for an avirulence effector whose recognition activates RGA4/RGA5-dependent resistance, and it is backed by experiments on AVR-Pia itself (binding of wild-type but not the inactive allele to RGA5-A). The full text is not cached, so the assay is not second-guessed; only the term is refined. The immune output of AVR-Pia recognition by RGA4/RGA5 is a hypersensitive cell death (shown directly in PMID:25024433 and PMID:28087830), so the informative term is GO:0080185, a descendant of GO:0140404. "Perturbation" is easily read as an immunosuppressive virulence activity, which has not been shown for AVR-Pia. Proposed replacements: effector-mediated activation of plant hypersensitive response by symbiont Supporting Evidence: PMID:23548743 RGA4 and RGA5 also mediate recognition of the unrelated M. oryzae effector AVR-Pia PMID:23548743 Direct binding seems to be required for resistance as an inactive AVR-Pia allele did not bind RGA5-A. PMID:25024433 Upon recognition of the pathogen effector AVR-Pia by direct binding to RGA5, repression is relieved and cell death occurs. file:PYROR/AVR-Pia/AVR-Pia-deep-research-falcon.md This is a **host defence pathway activated by detection of AVR-Pia** |
| GO:0140404 effector-mediated perturbation of host innate immune response by symbiont | EXP PMID:28087830 Recognition of the Magnaporthe oryzae Effector AVR-Pia by th... | MODIFY | Summary: Ortiz et al. (2017) mapped the AVR-Pia surface that binds the RGA5 RATX1/HMA domain (ITC, NMR titration, Y2H, co-IP) and tested structure-informed AVR-Pia mutants in transgenic M. oryzae on Pia rice. Isolates expressing wild-type or binding-competent AVR-Pia were avirulent and gave HR lesions on Kitaake (Pia), whereas the non-binding F24S mutant was fully virulent; untagged binding-competent mutants also induced RGA4/RGA5-dependent cell death in N. benthamiana. Reason: The experiments show that AVR-Pia, through binding the RGA5 HMA domain, activates an R-gene-dependent hypersensitive response and resistance in the host. This is exactly the scope of GO:0080185 (effector-mediated activation of plant hypersensitive response by symbiont), a descendant of GO:0140404 whose definition includes direct recognition of the effector by an R protein. The refinement drops the ambiguity of "perturbation" (activation or suppression), since no suppression of host immunity by AVR-Pia has been shown, and matches the treatment of the Cladosporium avirulence effector AVR9 (genes/FULFL/AVR9). AVR-Pia does part of the work of the process: it is the ligand whose binding to the RGA5 decoy domain releases RGA4. Proposed replacements: effector-mediated activation of plant hypersensitive response by symbiont Supporting Evidence: PMID:28087830 were completely avirulent and produced either no symptoms or small HR lesions characteristic of resistance PMID:28087830 Isolates expressing AVR-Pia F24S were highly virulent on Kitaake and produced large numbers of disease lesions PMID:28087830 binding to the RATX1 domain is necessary for pathogen recognition but can be of moderate affinity PMID:28087830 AVR-Pia mutants with wild-type binding to RGA5 RATX1 induced cell death, indicating that they are recognized by RGA5/RGA4 |
| GO:0140404 effector-mediated perturbation of host innate immune response by symbiont | IDA PMID:25024433 The NB-LRR proteins RGA4 and RGA5 interact functionally and ... | MODIFY | Summary: Cesari et al. (2014) showed that co-expression of AVR-Pia with RGA4 and RGA5 triggers rapid, strong cell death in N. benthamiana and HR-like cell death (luciferase-reporter loss) in rice protoplasts, whereas the non-binding AVR-Pia-H3 allele does not. AVR-Pia binding to RGA5 relieves RGA5 repression of the autoactive cell-death executor RGA4. Reason: The direct assay is AVR-Pia-triggered, R-gene-dependent HR-like cell death, which is precisely GO:0080185. The general "perturbation" parent is correct but uninformative and does not distinguish activation from suppression of immunity. The executor of cell death is RGA4, and AVR-Pia's contribution is the recognised ligand step (binding RGA5 to de-repress RGA4), which is what GO:0080185 describes on the symbiont side. Proposed replacements: effector-mediated activation of plant hypersensitive response by symbiont Supporting Evidence: PMID:25024433 Co-expression of AVR-Pia, RGA4, and RGA5 induced rapid and strong cell death indistinguishable in timing and appearance from the response induced by RGA4 alone PMID:25024433 Upon recognition of the pathogen effector AVR-Pia by direct binding to RGA5, repression is relieved and cell death occurs. PMID:25024433 The lack of activity of AVR-Pia_H3 indicates that binding of AVR-Pia to RGA5 is crucial to relieve RGA5 repressor activity. |
| GO:0030430 host cell cytoplasm | IDA PMID:27528510 Fluorescent reporter analysis revealed the timing and locali... | NEW | Summary: Live-cell imaging of fluorescently labelled AVR-Pia during rice infection showed accumulation in biotrophic interfacial complexes together with the cytoplasmic effector PWL2, followed by delivery into the host cytoplasm. Reason: Host cell cytoplasm is where AVR-Pia meets both its recognising receptor RGA5 (a cytosolic NLR) and its candidate HMA-protein targets, and recognition inside rice cells is independently supported by the faster Pia-dependent cell death when the protein is expressed without its signal peptide in rice. No location annotation exists for AVR-Pia in GOA. This is a cellular component term and does not raise the process-participation concerns that apply to NEW process terms. Supporting Evidence: PMID:27528510 AVR-Pia accumulates in biotrophic interfacial complexes before being delivered to the plant cytosol PMID:19454732 supporting the idea that pex22 is recognized by Pia inside rice cells |
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Download this section (compressed HTML)Q: Does binding of AVR-Pia to OsHPP09, OsHPP10, OsHPP11 or OsHIPP21 alter the stability, localisation or metal-binding activity of these HMA proteins, and is that the mechanism by which AVR-Pia promotes virulence?
Q: Should a symbiont-side molecular function term exist for an effector's ligand role towards a host immune receptor (here, AVR-Pia binding the RGA5 integrated decoy domain)? Signaling receptor binding (GO:0005102) was considered and not asserted: it is defined for hormone/neurotransmitter-type receptors, and no fungal or oomycete gene product carries it with experimental evidence in GOA, so effector-NLR binding is currently captured only by the effector-side process term.
Experiment: Generate AVR-Pia deletion strains in a Pia-negative background and compare virulence on wild-type rice versus OsHPP09 E16 (non-binding) knock-in or OsH(I)PP knockout lines, to test whether the HMA-protein interactions account for AVR-Pia's virulence contribution.
Type: genetic interaction / infection assay
Experiment: Determine the co-crystal structure of AVR-Pia with the RGA5 HMA domain and compare it with the AVR-Pia/OsHPP09-HMA complex to explain the ~70-fold affinity difference and guide engineering of RGA5 for stronger recognition.
Type: structural biology
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