Receptor structures at the plant-microbe interface

The independent research group of Dr. Alexander Förderer focuses on the protein structure determination of plant receptors that monitor and mediate responses to environmental microbes. The group uses various expression systems (e.g., insect cells, bacteria, tobacco) for recombinant protein expression and uses cryo-EM technology for protein structure determination. The knowledge gain is viewed in light of protein structure evolution in the green lineage and applied for structure-guided crop improvement. Partly a protein structure laboratory, partly a plant genetics/functional biology laboratory, the group includes a variety of plant models (e.g., rice, wheat, potato, soy bean) for their research to understand molecular mechanisms in the biological context.

Addressing Food Security Challenges

Food security hinges on the ability to maximize crop performance in the face of climate change, which necessitates improved resistance to rising pathogen pressure and increased yields while conserving resources. Genetic manipulation of microbial detection and subsequent cellular responses has the potential to reduce pesticide use for disease control and decrease reliance on artificial fertilizers containing nitrogen and phosphate for plant nutrition. With advancements in cryo-electron microscopy (cryo-EM) and modernized purification methods, higher order protein complexes involving microbial components and host receptors have become accessible for investigation. Our team is at the frontline of these breakthroughs and paves the way for the rational design of plant responses to microbes by receptor engineering and gene editing to better-equip crops for the changes in agriculture affected by climate change.

The Power of Cryo-EM

Cryo-EM has achieved remarkable breakthroughs in recent years, providing atomic-level insights into the structures of challenging biological targets such as plant immune receptors. This so-called ‘resolution revolution’ of cryo-EM was driven by the development of direct electron detectors, single-particle analysis, increased computational power and targeted redesign of microscope technology. It has expanded the scope of structural biology and opened up new avenues for understanding complex biological processes and developing targeted therapeutics in humans. For example, new drugs and vaccines were developed based on protein structure knowledge of the SARS-CoV-2 spike protein and its natural variation. Drawing inspiration from the role of structural biology in medicine, the team combines gene-editing of crops, functional biology (genetics), analysis of natural variation and understanding of evolutionary history, to unleash the full potential of structural biology in plants for crop improvement.

Unlocking the Potential

The diversity of plant receptors that monitor the presence of beneficial or detrimental microbes in their environment is breathtaking. Evolutionary processes have shaped a repertoire of receptors that is highly diversified and expanded in the green lineage. Plant immune receptors, particularly intracellular nucleotide-binding leucine-rich repeat (NLR) immune receptors and pattern recognition receptors (PRRs) at the cell surface, play critical roles in defending crops against harmful pathogens (e.g., fungi, bacteria, viruses) and in mediating symbiotic interactions with microbes (e.g., root nodule, arbuscular mycorrhiza symbiosis).

The ongoing coevolutionary arms race between plants and their pathogens has resulted in NLRs emerging as one of the largest gene families in the green lineage that contains some of the most polymorphic genes in plant genomes. Much of this significant sequence diversity and copy number variation is contained in allelic series in both the NLRome of plants and the effector repertoire of pathogens. Yet, without structural information of NLRs in complex with their cognate ligand, the molecular basis of this allelic variation remains unexplained and inaccessible to deliberate receptor engineering for crop improvement. The group aims to harness the allelic variation of NLR-effector pairs, particularly of crops such as wheat, rice, potato and pepper.

The NLR Sr35 from wheat confers resistance to the highly pathogenic race Ug99 of wheat stem rust. Structure determination by cryo-EM revealed that Sr35 directly binds and recognizes the effector protein AvrSr35 from the pathogenic fungus. Detailed knowledge of the protein atomic structure is used by the group for crop improvement. A. negative staining image using transmission electron microscopy (TEM) of Sr35-AvrSr35 protein complex purified from insect cells. B. 2D classification of the Sr35-AvrSr35 protein complex as part of cryo-EM single particle analysis (SPA). C. cryo-EM density map with 3 Angstrom and structural model. Protein domains colored according to in-figure legend.Figure adapted from: Förderer, A., Li, E., Lawson, A. W., Deng, Y. N., Sun, Y., Logemann, E., ... & Chai, J. (2022). A wheat resistosome defines common principles of immune receptor channels. Nature, 610(7932), 532-539 — The NLR Sr35 from wheat confers resistance to the highly pathogenic race Ug99 of wheat stem rust. Structure determination by cryo-EM revealed that Sr35 directly binds and recognizes the effector protein AvrSr35 from the pathogenic fungus. Detailed knowledge of the protein atomic structure is used by the group for crop improvement. A. negative staining image using transmission electron microscopy (TEM) of Sr35-AvrSr35 protein complex purified from insect cells. B. 2D classification of the Sr35-AvrSr35 protein complex as part of cryo-EM single particle analysis (SPA). C. cryo-EM density map with 3 Angstrom and structural model. Protein domains colored according to in-figure legend. Figure adapted from: **Förderer, A.**, Li, E., Lawson, A. W., Deng, Y. N., Sun, Y., Logemann, E., ... & Chai, J. (2022). A wheat resistosome defines common principles of immune receptor channels. Nature, 610(7932), 532-539

licensed: CC BY

The NLR Sr35 from wheat confers resistance to the highly pathogenic race Ug99 of wheat stem rust. Structure determination by cryo-EM revealed that Sr35 directly binds and recognizes the effector protein AvrSr35 from the pathogenic fungus. Detailed knowledge of the protein atomic structure is used by the group for crop improvement. A. negative staining image using transmission electron microscopy (TEM) of Sr35-AvrSr35 protein complex purified from insect cells. B. 2D classification of the Sr35-AvrSr35 protein complex as part of cryo-EM single particle analysis (SPA). C. cryo-EM density map with 3 Angstrom and structural model. Protein domains colored according to in-figure legend.

Figure adapted from: **Förderer, A.**, Li, E., Lawson, A. W., Deng, Y. N., Sun, Y., Logemann, E., ... & Chai, J. (2022). A wheat resistosome defines common principles of immune receptor channels. Nature, 610(7932), 532-539

licensed: CC BY

The ancient evolutionary trajectory of the receptors and signaling modules that participate in root nodule and arbuscular mycorrhiza symbiosis (RNS and AMS) stands in contrast to the evolutionary trajectory of NLRs. Development of arbuscular mycorrhiza is an early evolutionary innovation of land plants and the involved receptors and signaling modules are typically conserved in copy number and have accumulated relatively few amino acid polymorphisms. It appears that several plant components mediating AMS were repurposed during the evolution of RNS, which happened only at the basis of four plant clades, most famously the Fabales. Yet, the molecular basis of this switch from AMS to RNS is unclear and is likely encoded in clade-specific adaptation of common symbiosis proteins. The Förderer group aims to shed light on the molecular basis of symbiotic plant-microbe interactions in an evolutionary context. Understanding at atomic resolution how plant recognize and respond to microbes is crucial in rational design of these responses in crops such as symbiotic efficiency in rice, soy bean and the model Lotus japonicus.

Selected Publications

Förderer, A., Li, E., Lawson, A. W., Deng, Y. N., Sun, Y., Logemann, E., ... & Chai, J. (2022). A wheat resistosome defines common principles of immune receptor channels. Nature, 610(7932), 532-539. Related secondary literature: Outram, M. A., & Dodds, P. N. (2022). Wheeling in a new era in plant immunity. Nature Plants, 1-2.
Förderer, A., & Kourelis, J. (2023). NLR immune receptors: structure and function in plant disease resistance. Biochemical Society Transactions.
Förderer, A., & Chai, J. (2023). Die another day: phytosulfokine at the molecular trade‐off between growth and defense in plants. The EMBO Journal, 42(6), e113540.
Förderer, A., Yu, D., Li, E., & Chai, J. (2022). Resistosomes at the interface of pathogens and plants. Current Opinion in Plant Biology, 67, 102212.
Song, W., Förderer, A., Yu, D., & Chai, J. (2021). Structural biology of plant defence. New Phytologist, 229(2), 692-711.