Structure of the Nod factor receptor LYK3 in blue and the chitin receptor CERK6 in green. Credit: Kasper Røjkjær Andersen
How do plants distinguish between harmful and beneficial microbes? An international collaboration of researchers has found motifs present on lysine receptors (LysM) drive symbiotic and immunity response in legumes through recognition of chitin and nodulation factors released by fungi and bacteria, respectively. The knowledge enables the potential to engineer a ‘switch’ for these activities in agriculture or other applications in plant science.
The mechanisms plants use to initiate symbiosis with beneficial bacteria known as rhizobia, or protections from invading microbes are unresolved in plant biology. Scientists seek to understand the ligands signalling specificity of the receptors. The study is part of the global ENSA (Engineering Nitrogen Symbiosis for Africa) project dedicated to developing sustainable food crops for Africa.
“To really understand at the molecular level how plants distinguish microbes, we needed structural insight,” said Kasper Røjkjær Andersen, Research Group Leader and Assistant Professor in the Department of Molecular Biology and Genetics at Aarhus University. “A big breakthrough came when we solved the crystal structure of the symbiotic receptor LYK3 and could compare it to the chitin receptor CERK6.”
Plant receptors recognize different carbohydrate signals. Chitin fragments released by fungi can triggers an immune response that essentially blocks infection. Other receptors detect symbiotic carbohydrates called Nod factors that initiate nitrogen-fixing symbiosis.
Engineering plant response
The team of researchers led by Aarhus University in Denmark sought to elucidate how LysM receptors establish signalling for Nod factors and chitin. Plant experiments and crystallographic structural analysis found that two motifs in NFR1 and LYK3, in the LysM1 domain elicit the specific signalling response in plants in the presence of microbes. The crystalline data collection was carried out at the I911-3 beamline at MAX lab in Lund, Sweden.
Taking this a step further, scientist altered the binding specificity in a LysM receptor from pathogenic to symbolic, by swapping for Nod factor recognition and signalling in a chitin receptor. This type of ‘receptor’ engineering holds promise to produce a symbiotic, nitrogen-fixing response in cereal plants potentially on demand.
“We can now begin to engineer receptors and program them for different functions, which is an important step towards our goal of engineering the nitrogen-fixing symbiosis into crops for a more sustainable agriculture that we hope can benefit small holder farmers in Africa,” added Røjkjær Andersen.
Zoltan Bozsoki, Kira Gysel, Simon B. Hansen, Damiano Lironi, Christina Krönauer, Feng Feng, Noor de Jong, Maria Vinther, Manoj Kamble, Mikkel B. Thygesen, Ebbe Engholm, Christian Kofoed, Sébastien Fort, John T. Sullivan, Clive W. Ronson, Knud J. Jensen, Mickaël Blaise, Giles Oldroyd, Jens Stougaard, Kasper R. Andersen, Simona Radutoiu. (7 August 2020) Ligand-recognizing motifs in plant LysM receptors are major determinants of specificity. Science 663-670. DOI: 10.1126/science.abb3377