MLA10 is one of the many allelic intracellular immune receptors encoded at the barley Mildew locus a (Mla). It is a 951-residue coiled-coil, nucleotide-binding (NB-ARC), leucine-rich repeat protein (CC-NLR) that confers isolate-specific resistance to the powdery mildew fungus Blumeria graminis f. sp. hordei (Blumeria hordei) carrying the matching avirulence effector AVRA10, a small secreted protein with a predicted RNase-like fold encoded by the CSEP0141 locus. MLA10 associates with AVRA10 in yeast and in plant extracts, consistent with direct effector recognition, and the C-terminal LRR region largely determines its specificity for AVRA10 over the allelic effector AVRA22 recognised by MLA22. The NB-ARC domain acts as a nucleotide-dependent switch: the P-loop (K207R) mutation inactivates the receptor, whereas MHD-motif substitutions (e.g. D502V) make it autoactive. The N-terminal CC domain self-associates and is sufficient to trigger cell death. MLA10 is distributed between the cytoplasm and the nucleus. A nuclear pool is required and sufficient for resistance; there, after AVRA10 recognition, MLA10 associates with WRKY1/WRKY2 transcriptional repressors of basal defence and, via its CC domain, releases the MYB6 transcriptional activator from WRKY1 repression, linking effector recognition to defence gene reprogramming. The cytoplasmic pool drives a separable hypersensitive cell-death output that requires SGT1.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0002758 innate immune response-activating signaling pathway | IEA GO_REF:0000117 | ACCEPT | Summary: ARBA machine-learning transfer to a CC-NLR. MLA10 is the receptor that starts this pathway: recognition of AVRA10 activates MLA10 [PMID:30777147, PMID:33534797], which in the nucleus associates with WRKY repressors and releases MYB6-dependent defence gene expression [PMID:17185563, PMID:23532068]. Reason: The term describes a signalling pathway initiated by a ligand engaging its receptor and leading to activation of innate immunity. MLA10 does the receptor step itself (effector recognition) and its CC domain directly engages the downstream WRKY1/MYB6 transcriptional module, so the gene product executes steps of this pathway rather than merely being required for it. This is the best existing process term for the core signalling role. Supporting Evidence: PMID:17185563 Recognition of the fungal avirulence A10 effector by MLA10 induces nuclear associations between receptor and WRKY transcription factors. PMID:23532068 The activated form of full-length MLA10 receptor is needed to release MYB6 activator from WRKY1 repression and to stimulate MYB6-dependent gene expression. PMID:30777147 co-expression experiments with matching Mla-AVRa pairs indicate direct detection of the sequence-unrelated fungal effectors by MLA receptors. |
| GO:0006952 defense response | IEA GO_REF:0000002 | MODIFY | Summary: InterPro2GO mapping from the plant disease-resistance protein family (IPR044974). Correct but very general: the defence that MLA10 mediates is isolate-specific resistance to the powdery mildew fungus Blumeria graminis f. sp. hordei carrying AVRA10 [PMID:22685408, PMID:30777147]. Reason: MLA10 is an immune receptor whose only documented biological target is a fungus; nuclear-localised MLA10 is sufficient to restrict haustorium formation by an AVRA10-carrying Bgh isolate in barley epidermal cells. As the receptor that initiates this response, MLA10 participates in the defence process, so the more specific "defense response to fungus" (GO:0050832) is appropriate. Proposed replacements: defense response to fungus Supporting Evidence: PMID:22685408 nuclear localized MLA10 is sufficient to mediate disease resistance against powdery mildew fungus. PMID:22685408 As expected the MLA10-YFP fusion conferred resistance against Bgh infection, resulting in an haustorium index around 29%, similar to our previous study [44]. The K207R P-loop mutation reduced MLA10 resistance activity as indicated by a haustorium index as high as 65%. |
| GO:0009626 plant-type hypersensitive response | IEA GO_REF:0000117 | KEEP AS NON CORE | Summary: ARBA transfer. MLA10 does trigger cell death: the CC domain alone is sufficient to induce cell death, matching MLA10-AVRA10 pairs trigger cell death in barley protoplasts and N. benthamiana, and cytoplasmic MLA10 drives this output [PMID:21402358, PMID:22685408, PMID:30777147]. However, cell death and resistance are separable: nuclear-targeted MLA10 confers resistance without inducing cell death, and the CC L18E variant reduces cell death while retaining resistance [PMID:22685408]. Reason: The hypersensitive cell death is a genuine, receptor-executed output (the MLA10 CC domain is the minimal cell-death-triggering module, and its activity requires SGT1), so the annotation is correct. It is kept as non-core because the experimental work on MLA10 shows that disease resistance against powdery mildew is compartment-dependent and can be uncoupled from cell death, so cell death is not the core resistance mechanism of this receptor. Supporting Evidence: PMID:22685408 Remarkably, manipulation of cytoplasmic or nuclear MLA10 levels reveals that the cytoplasmic pool of MLA10 alone is sufficient to induce cell-death, whereas nuclear-localized MLA10 is unable to induce cell-death, but is capable of conferring disease resistance. PMID:21402358 MLA CC mutants that fail to self-interact impair in planta cell death activity triggered by the CC domain alone and by an autoactive full-length MLA receptor that mimics its ATP-bound state. file:HORVU/MLA10/MLA10-deep-research-falcon.md Importantly, cell death is not synonymous with the resistance readout: a CC-domain |
| GO:0042742 defense response to bacterium | IEA GO_REF:0000117 | REMOVE | Summary: ARBA transfer, presumably learned from Arabidopsis CC-NLRs that recognise bacterial effectors (e.g. RPS2, RPM1). There is no evidence that MLA10, or any MLA allele, acts against bacteria; MLA10 recognises the fungal effector AVRA10, and MLA-family receptors are documented for fungal pathogens (powdery mildew, and for homologues, rusts and blast). Reason: The pathogen specificity of an NLR is determined by the effector it recognises, which cannot be inferred from family membership. MLA10's recognised effector is a Blumeria protein and its documented immune output is resistance to powdery mildew. Transferring "defense response to bacterium" from bacterial-effector NLRs is a family-level over-propagation with no support for this receptor. Supporting Evidence: PMID:33534797 The Mildew locus a (Mla) NLR gene has been shown to confer resistance against diverse fungal pathogens in cereal crops. PMID:30777147 We report here the isolation of Bgh AVRa7, AVRa9, AVRa10, and AVRa22, which encode small secreted proteins recognized by allelic MLA7, MLA9, MLA10, and MLA22 receptors, respectively. |
| GO:0043531 ADP binding | IEA GO_REF:0000002 | KEEP AS NON CORE | Summary: InterPro2GO mapping from the NB-ARC domain (IPR002182). MLA10 has an intact NB-ARC whose nucleotide-binding P-loop is functionally essential (K207R inactivates the receptor) and whose MHD motif controls autoactivation (D502V, H501 variants), consistent with the ADP-bound resting / ATP-bound active switch of STAND NTPases [PMID:22685408, PMID:21402358]. Nucleotide binding has not been measured directly for MLA10. Reason: The NB-ARC nucleotide switch is conserved and functionally validated for MLA10 by P-loop and MHD mutagenesis, so ADP binding (the resting state of the switch) is a reasonable domain-based inference and is retained. It is a regulatory property of the receptor rather than its core activity, which is effector recognition and immune signalling. Supporting Evidence: PMID:22685408 As controls we generated CC(F83E)-YFP and FL(K207R)-YFP fusions, the F83E and the P-loop K207R mutation rendering MLA10 inactive PMID:21402358 an autoactive full-length MLA receptor that mimics its ATP-bound state |
| GO:0098542 defense response to other organism | IEA GO_REF:0000118 | MODIFY | Summary: TreeGrafter transfer from a PANTHER disease-resistance (PTHR23155) node. Correct at the family level, but for MLA10 the target organism is known: the powdery mildew fungus carrying AVRA10 [PMID:22685408, PMID:30777147]. Reason: The family-level term is not wrong, but MLA10's experimentally established defence role is against a fungus, so the specific child "defense response to fungus" (GO:0050832) is the informative term for this receptor. Proposed replacements: defense response to fungus Supporting Evidence: PMID:22685408 nuclear localized MLA10 is sufficient to mediate disease resistance against powdery mildew fungus. |
| GO:0140376 innate immune receptor activity | IDA PMID:30777147 Multiple pairs of allelic MLA immune receptor-powdery mildew... | NEW | Summary: MLA10 recognises the Blumeria effector AVRA10: matching MLA10-AVRA10 pairs trigger cell death in barley protoplasts and N. benthamiana, MLA10 and AVRA10 associate specifically in split-luciferase assays in plant extracts and in yeast two-hybrid assays [PMID:30777147], and the MLA10 LRR largely determines AVRA10 specificity [PMID:33534797]. Recognition then transmits a signal that initiates immune responses [PMID:17185563, PMID:23532068]. Reason: This is the molecular activity of MLA10 itself (receiving a pathogen-derived signal and transmitting it to initiate an innate immune response), and it is not represented in GOA, which carries only ADP binding as an MF. The comparator check supports this term: other plant NLRs that recognise pathogen effectors carry GO:0140376 by experimental evidence, including tomato I2, rice RGA5 and Arabidopsis RPP1, RPP4 and RPP13. "protein binding" is avoided in favour of this informative receptor term. Supporting Evidence: PMID:30777147 Consistent with a direct interaction of matching MLA and AVRA pairs, co-expression of BD-Mla10 with AD-AVRa10 in yeast resulted in leucine reporter gene activation and this was undetectable when Mla10 was replaced by either Mla7 or Mla22 PMID:30777147 Co-expression of AVRa10-nLUC with Mla10-cLUC resulted in the detection of a 3.5-fold higher LUC activity when compared to expression of its virulent variants AVRa10-V/AVRa22-V-nLUC and AVRa22-nLUC (Figure 5C). PMID:33534797 the results suggest that the 58 amino acid differences between the LRRs of Mla10 and Mla22 are major determinants of respective recognition specificities for the allelic AVRA10 and AVRA22 effectors. |
| GO:0005634 nucleus | IDA PMID:22685408 Structure-function analysis of barley NLR immune receptor ML... | NEW | Summary: Fluorescently tagged MLA10 is detected in the nucleus and cytoplasm of barley leaf epidermal cells; the nuclear pool is required for, and nuclear-targeted MLA10 is sufficient for, resistance, and MLA10 associates with WRKY and MYB6 transcription factors in the nucleus [PMID:17185563, PMID:22685408, PMID:23532068]. Reason: GOA has no cellular-component annotation for MLA10. Nuclear localisation is shown by imaging and is functionally relevant (the resistance output is nuclear). Supporting Evidence: PMID:22685408 We have previously shown that MLA10 locates to both the cytoplasm and the nucleus, and importantly, the nuclear pool is required for its disease resistance [44]. PMID:17185563 We show that in barley, intracellular mildew A (MLA) R proteins function in the nucleus to confer resistance against the powdery mildew fungus. |
| GO:0005737 cytoplasm | IDA PMID:22685408 Structure-function analysis of barley NLR immune receptor ML... | NEW | Summary: MLA10 also resides in the cytoplasm, and enforced cytoplasmic retention or nuclear export enhances MLA10-triggered cell death [PMID:22685408]. Reason: Cytoplasmic localisation is shown by imaging and is functionally relevant to the cell-death output. The finer compartment (cytosol versus membrane association) is not resolved for MLA10, so the general term is used. Supporting Evidence: PMID:22685408 Enforced MLA10 subcellular localization, by tagging with a nuclear localization sequence (NLS) or a nuclear export sequence (NES), shows that MLA10 activity in cell death signaling is suppressed in the nucleus but enhanced in the cytoplasm. |
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Download this section (compressed HTML)Q: Where is MLA10 activated by AVRA10 in native infections, and is the nuclear resistance model for MLA10 compatible with the cytosolic resistance shown for the wheat homologues Sr33 and Sr50?
Q: Does activated MLA10 form a CC-NLR resistosome with Ca2+-permeable channel activity, as proposed for other CC-NLRs, and if so how does such an assembly relate to the nuclear WRKY/MYB6 signalling output?
Q: Given that both MLA10 and AVRA10 are known, should the recognition step be captured in GO-CAM as MLA10 innate immune receptor activity with AVRA10 as input, rather than through a generic binding term?
Experiment: Purify MLA10 (or an MLA10 LRR fragment) with AVRA10 and determine binding affinity and a cryo-EM structure of the complex, testing whether the MLA13-AVRA13 heterodimer architecture is conserved for MLA10.
Experiment: Measure nucleotide occupancy (ADP versus ATP) of wild-type, K207R and D502V MLA10 proteins, and test whether AVRA10 triggers oligomerisation and Ca2+ influx in plant cells.
Experiment: Use AVRA10-inducible, compartment-restricted MLA10 variants in stable barley lines to determine whether nuclear MLA10 activity is required for resistance under native infection, in comparison with cytosolic Sr33/Sr50.
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