CC_R‐NB‐LRR proteins MdRNL2 and MdRNL6 interact physically to confer broad‐spectrum fungal resistance in apple (Malus × domestica)

Abstract

Apple leaf spot, a disease caused by Alternaria alternata f. sp. mali and other fungal species, leads to severe defoliation and results in tremendous losses to the apple (Malus 3 domestica) industry in China. We previously identified three RPW8, nucleotide-binding, and leucine-rich repeat domain CC R -NB-LRR proteins (RNLs), named MdRNL1, MdRNL2, and MdRNL3, that contribute to Alternaria leaf spot (ALT1) resistance in apple. However, the role of NB-LRR proteins in resistance to fungal diseases in apple remains poorly understood. We therefore used MdRNL1/2/3 as baits to screen ALT1-inoculated leaves for interacting proteins and identified only MdRNL6 (another RNL) as an interactor of MdRNL2. Protein interaction assays demonstrated that MdRNL2 and MdRNL6 interact through their NB-ARC domains. Transient expression assays in apple indicated that complexes containing both MdRNL2 and MdRNL6 are necessary for resistance to Alternaria leaf spot. Intriguingly, the same complexes were also required to confer resistance to Glomerella leaf spot and Marssonina leaf spot in transient expression assays. Furthermore, stable transgenic apple plants with suppressed expression of MdRNL6 showed hypersensitivity to Alternaria leaf spot, Glomerella leaf spot, and Marssonina leaf spot; these effects were similar to the effects of suppressing MdRNL2 expression in transgenic apple plantlets. The identification of these novel broad-spectrum fungal resistance genes will facilitate breeding for fungal disease resistance in apple.

Genome-wide identification and characterization of NBLRR genes in finger millet (Eleusine coracana L.) and their expression in response to Magnaporthe grisea infection

“…Yr10 [ 39 ] and RPM1 [ 40 ] for stripe rust and Pm21 for powdery mildew resistances [ 41 ] in wheat, and Pb1 [ 42 ] for panicle blast resiatnce in rice. The NBLRRs with a RPW8-like CC domain are referred to as CCR-NBLRRs or RNLs [ 43 , 44 ]. The five EcNBLRRs, viz., EcNBLRR12, EcNBLRR65, EcNBLRR77, EcNBLRR102 and EcNBLRR105 showed RPW8 domins with CC structures at N-Terminal.…”

Ectopic Expression of HbRPW8-a from Hevea brasiliensis Improves Arabidopsis thaliana Resistance to Powdery Mildew Fungi (Erysiphe cichoracearum UCSC1)

“…Homogeneously, the VpRPW8s could also enhance plant resistance to P. viticola , P. capsici , and P. parasitica [ 41 ]. Additionally, MdRNL2 and MdRNL6 (encode RPW8-NBS-LRR protein) were considered to be spectrum fungal resistance genes [ 66 ]. By transient expression in tobacco leaves and overexpression in Col-WT Arabidopsis , we showed that HbRPW8-a is a broad-spectrum resistance gene with resistance to E. cichoracearum UCSC1 and Pst DC3000.…”

Pathogenic Factors and Mechanisms of the Alternaria Leaf Spot Pathogen in Apple

“…The plants were transferred to fresh medium every 4 weeks. Approximately 4-week-old apple seedlings were used in the fungal infection and toxin injection experiments when the fifth leaf of the seedling had expanded (leaf length: 2 cm) [25].…”

“…Plants have evolved a primary defense system that responds to pathogen-associated molecular patterns (PAMPs) via pattern recognition receptors, a signaling cascade that is termed PAMP-triggered immunity (PTI) [24,25]. PTI causes rapid defense reactions, such as the production of ROS, ion fluxes, and cell wall reinforcement.…”

“…However, successful pathogens interfere with PTI by delivering effectors, evading recognition, or suppressing signaling pathways. In turn, the host uses resistance proteins to recognize the effectors, leading to the secondary defense system of effector-triggered immunity (ETI) [24,25]. In addition to AM-toxin, ALT also releases a set of effectors into apple cells that modify the host defense reaction.…”