Scientists from the Riken Center for Sustainable Resource Science (CSRS) in Japan have identified an ancient protein that, with modifications, could confer broad-spectrum protection for plants against infection from bacteria and other pathogens.
The so-called Selective Cold Shock Protein (SCORE) receptor detects cold-shock proteins, variations of which are found in more than 85% of known bacteria as well as in fungi and insects. In the new paper, which is published in Science, the team presented data from experiments showing that swapping out key sections of SCORE can predictably change the type of cold-shock protein and pathogen it recognizes. It opens up the possibility of engineering SCORE to provide plants with a means of defending themselves against infections.
Full details of their work are reported in a paper titled “Systematic discovery and engineering of synthetic immune receptors in plants.” In it, they describe working with over 1,300 receptors from the genomes of 350 plant species. Typically, plant receptor proteins recognize molecules from pathogens and trigger an immune response. The challenge is that no immune receptor recognizes all current and future pathogens. Some studies show that immune receptors specific to plants from one branch of a family tree can be transferred to another lineage to help boost their defense systems.
However, there are hundreds of thousands of possible receptor-microbe pairs that exist in nature. In this study, which comes out of the lab of Ken Shirasu, PhD, group director of the plant immunity research group at CSRS, the scientists focused on developing an efficient strategy for identifying receptor-microbe pairs.
From their initial dataset, the researchers identified an unknown immune receptor in the common citrus fruit pomelo, which they named SCORE. A series of experiments revealed that this receptor reacts to a subset of cold-shock proteins. Specifically, it recognizes a small section of these proteins called csp15. The team found that swapping out some of csp15’s amino acids with others changed which cold-shock proteins SCORE could recognize.
Next, a detailed genome analysis showed that most pathogens, except viruses, produce at least one type of cold-shock protein. Comparing species, the team found that although most of the 15 amino acids in csp15 are conserved across species, locations 6, 7, 14, and 15 vary widely. Their analysis also revealed over 60 variations in SCORE that were specific to different plant lineages and could be traced to the last common ancestor of all flowering plants.
“This was a particularly unexpected finding,” said Yasuhiro Kodata, PhD, a research scientist in the Shirasu lab and a co-author on the study. “The extensive natural variation in CSP recognition across SCORE orthologs from different plant lineages suggests that this kind of immune receptor has repeatedly evolved to fine-tune pathogen detection through specific amino acid substitutions.”
Further analysis of the different SCOREs highlighted specific locations in their amino acid sequences that vary from one lineage to the next. Based on the charge associated with the location, scientists can predict which csp15s a given SCORE can recognize. Using the pomelo SCORE, they engineered new versions of the receptor capable of recognizing different pathogens by replacing some sections of the sequence with alternatives. For example, the natural pomelo SCORE does not recognize cold-shock protein from Ralstonia, Erwinia, or Xanthomonas species of bacteria. Using their approach, the scientists synthesized a new SCORE version that reacts to all of these.
“In the short term, this study provides a new framework for identifying and engineering immune receptors from non-model plants, especially long-lived perennials, for which genetic tools are limited,” Shirasu said. As part of their next steps, the researchers plan to introduce engineered SCORE variants into economically important crops to see if the change improves broad-spectrum resistance to pathogens and pests.