AmirAli Toghani

Small RNA-mediated gene silencing contributes to plant immunity. The secondary small interfering RNA (siRNA) pathway promotes defense by silencing target genes in invading fungal and oomycete pathogens. Many secondary siRNAs are derived from transcripts potentially encoding pentatricopeptide repeat (PPR) proteins. Here, we report that siRNA production is an ancient function of a conserved clade of PPR genes that undergo extensive within-species diversification. In Arabidopsis thaliana , siRNA-source PPR s are physically clustered on Chromosome 1. These sequences are diversified through gene duplication followed by sequence diversification and accumulation of high-impact variations including pseudogenization, leading to the accumulation of a diverse siRNA pool. These features are consistent with the engagement of PPR -siRNAs in a co-evolutionary arms race with the pathogens. This study defines siRNA-producing PPR s as a class of defense genes and highlights the potential of PPR -siRNA-based engineering as a strategy to enhance disease resistance.

Nucleotide-binding domain and leucine-rich repeat immune receptors (NLRs) are known for their rapid evolution, even at the intraspecific level, yet the rates of evolution differ significantly across NLRs. However, the degree to which evolutionary patterns reflect functional divergence remains poorly understood, notably in important crop species. Within the NRC (NLR Required for Cell Death) network in Asterids, sensor NLRs detect pathogen presence but require NRC helpers for signaling and to confer immunity. We conducted a comparative analysis of NLRs across 40 Solanales and 29 Asterales genomes to explore NRC network expansion and diversification within the less-studied Asterales order. Our findings reveal that the NRC network has expanded less in Asterales compared to Solanales. We functionally validated an Asterales NRC network with 2 helpers and 9 sensors in common lettuce ( Lactuca sativa ). Through selection analysis and structural modeling of NRC gene family in the Lactuca genus, we found distinct evolutionary trajectories between NRC helpers and sensors. Sensors reliant on the phylogenetically conserved helper NRC0 experience limited diversification, whereas sensors dependent on other NRC helpers show higher rates of positive selection and gene duplication. Our results highlight the lineage- and function-specific evolution of the NRC network, offering insights into the evolutionary pressures shaping plant immune receptor networks.

Plants activate pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) to combat pathogens. However, how these systems coordinate immune activation while preventing autoimmunity remains poorly understood. In this study, we uncovered a regulatory mechanism in which surface immune signaling unlocks nucleotide-binding leucine-rich repeat (NLR) immune receptor activation through mRNA splicing. We identified an N-terminal prodomain in the potato late blight resistance protein Rpi-vnt1.1 that inhibits resistosome formation, preventing potential autoactivation of this NLR. Upon pathogen perception, PTI signaling induced alternative splicing of Rpi-vnt1.1 mRNA, removing this inhibitory element. This primed Rpi-vnt1.1 for activation by the Phytophthora infestans effector AVRvnt1, enabling resistosome assembly and immune signaling. The widespread conservation of N-terminal extensions in coiled coil–type NLRs points to a common regulatory mechanism in preventing potential autoactivation while preserving pathogen sensitivity.

Pathogens counteract central nodes of NLR immune receptor networks to suppress immunity. However, the mechanisms by which pathogens hijack helper NLR pathways are poorly understood. We show that an effector from the late blight pathogen Phytophthora infestans interacts with the host protein NbTOL9a and a helper NLR to suppress immunity. We solved the crystal structure of the RXLR-LWY effector AVRcap1b in complex with the ENTH domain of NbTOL9a. The structure revealed that, unlike other RXLR-LWY effectors, AVRcap1b has a previously unidentified L-shaped fold that defines a distinct structural family of effectors in the genus Phytophthora . We defined the AVRcap1b/NbTOL9a binding interface and designed effector mutants that do not bind NbTOL9a, impairing immune suppression. This suggests that ENTH binding is required for full virulence activity. Last, we show that AVRcap1b associates specifically with activated NbNRC2 independently of NbTOL9a binding. We propose a model in which the effector interconnects NbNRC2 with the NbTOL9a pathway. Our results illustrate a previously uncharacterized pathogen mechanism to hijack NLR pathways and suppress immunity.

NLR immune receptor networks consist of expanded disease resistance proteins (sensor NLRs) that signal via core executors of immunity known as helper NLRs. Although some sensor NLRs are thought to activate their cognate helpers via an activation-and-release mechanism, the structural basis of sensor-helper communication remains poorly understood. Here, we identify and validate sensor-helper NLR interfaces that are critical for immune activation in the NRC network of coiled-coil NLR immune receptors. Using AlphaFold 3 we predicted a high confidence model between the virus resistance protein Rx and its helper NLR NRC2. We validated the interfaces by loss and gain-of-function mutagenesis, including reconstituting a critical salt bridge through reciprocal mutations. We showed that these interfaces are conserved across the NRC network of asterid plants despite over 120 million years of divergence and validated the sensor-NRC interfaces within the common lettuce network. Structure-guided bioengineering of a lettuce sensor NLR enabled expansion of its NRC helper compatibility profile. These results are consistent with the activation-and-release model and point to bioengineering sensor- helper specificity in economically important crop species.

Artificial intelligence (AI) systems such as AlphaFold have transformed structural biology by enabling accurate prediction of protein structures. However, their capacity to uncover new classes of macromolecular assemblies remains largely untapped. We developed the Structural Novelty Index (SNI), a quantitative framework for identifying protein complexes that diverge from canonical architectures. As one implementation of SNI, we developed SNI _NRC-Hexa , to identify unconventional resistosomes formed by nucleotide-binding, leucine-rich repeat immune receptors (NLRs). We used it to analyze AlphaFold 3 models of 637 non-redundant NRC proteins from 346 genomes representing 85 plant species. This analysis identified candidates with predicted architectures distinct from the canonical hexameric resistosomes of NRC proteins. Biochemical purification and negative-stain transmission electron microscopy of NRC7 orthologs from multiple species supported the SNI prediction and revealed an unexpected undecameric (11-mer) assembly. Our results establish SNI as a scalable approach for discovering atypical protein complexes.

Upon activation, plant nucleotide-binding leucine-rich repeat (NLR) immune receptors are known to assemble into oligomeric resistosomes that insert into the plasma membrane, forming calcium (Ca ²⁺ )–permeable channels and triggering immunity. Here, we found that the RPW8-like coiled-coil NLR (CC _R -NLR) N requirement gene 1 (NRG1) primarily targets organelles instead of the plasma membrane. Unlike canonical CC-NLRs, activated NRG1 accumulated at the chloroplast envelope and channeled stromal Ca ²⁺ into the cytosol. AlphaFold modeling of the NRG1 resistosome revealed an unusually long amino-terminal membrane-insertion structure that could span the double membrane of the chloroplast. Nanobody-mediated relocalization showed functional membrane specificity: Chloroplast trapping abolished activity of the canonical helper CC-NLR NRC4 but not NRG1. NRG1 orthologs, from nonflowering lineages to angiosperms, targeted chloroplasts, suggesting that organelle-centered defense dates back at least ~360 million years. We propose that CC-NLR diversification has enabled compartment-specific immune signaling to capture diverse Ca ²⁺ stores.

Agrobacterium -mediated transient expression has revolutionized plant research, enabling numerous landmark discoveries across diverse areas of plant biology. Yet this powerful approach remains largely confined to solanaceous species, leaving most economically important crop families without a comparable rapid assay platform. Here, we show that an engineered Rhizobium rhizogenes strain, AS109, mediates efficient transient expression across diverse dicot species spanning multiple taxonomic families, consistently outperforming commonly used laboratory agrobacterial strains. Leveraging the broad host range of AS109, we establish a suite of functional assays in faba bean ( Vicia faba ), including protein localisation, RNA interference-mediated gene silencing, cell-surface elicitor recognition screens, nucleotide-binding leucine-rich repeat receptor (NLR) activation, and infection cell biology at the host-pathogen interface. We further demonstrate that both singleton NLRs and sensor-helper NLR pairs from Solanaceae retain effector recognition and cell death activity when transferred into faba bean, establishing a rapid platform for evaluating cross-family transferability of disease-resistance genes. AS109 thus provides an accessible and versatile chassis for functional genomics in non-model crops, bridging the widening gap between hypothesis generation and experimental validation across diverse plant species.

Nucleotide-binding and leucine-rich repeat immune receptors of the NRC clade act in one direction, with autoactive helpers able to trigger hypersensitive cell death without their sensor partners.

Small RNA-mediated gene silencing contributes to plant immunity. The secondary small interfering RNA (siRNA) pathway promotes defense by silencing target genes in invading fungal and oomycete pathogens. Many secondary siRNAs derive from transcripts potentially encoding pentatricopeptide repeat (PPR) proteins. Here, we report that siRNA production is an ancient function of an evolutionarily conserved clade of PPR genes that undergo extensive within-species diversification. In Arabidopsis thaliana , siRNA-source PPR s are physically clustered in one locus on Chromosome 1. These sequences are diversified through gene duplication followed by sequence diversification as well as accumulation of high-impact variations including pseudogenization. This diversity leads to the accumulation of a diverse PPR -siRNA pool, consistent with an engagement in a co-evolutionary arms race with the pathogens. This study defines siRNA-producing PPRs as a family of defense genes and highlights the potential of PPR -siRNA-based engineering for enhancing broad-spectrum disease resistance.