New Sugar Underpins Plant-Microbe Symbiosis

It takes never-before-seen sugar chemistry for a symbiotic soil bacterium to make nice with its plant host. That discovery sheds light on an alternative to the best-known symbiosis setup. Usually, symbiotic bacteria produce carbohydrate signaling molecules called Nod factors in response to metabolites secreted by the plant. That’s not the case for some symbiotic relationships, including the one the legume plant Aeschynomene indica forms with the microbe Bradyrhizobium sp. BTAi1 to convert nitrogen in soil to ammonia. With NMR and tests of plant immune responses, Antonio Molinaro of the University of Naples and colleagues learned that a lipopolysaccharide—a bacterial membrane molecule that’s involved in plant-microbe interactions—plays a role in Bradyrhizobium’s symbiosis (Angew. Chem. Int. Ed., DOI: 10.1002/anie.201106548). One component of the lipopolysaccharide turned out to be a sugar with no analogs in nature, so the team dubbed it bradyrhizose. They think bradyrhizose evolved as a way for the microbe to get around plant immune defenses. Computational experiments suggest bradyrhizose forms a helical polymer in its biological context, and Molinaro’s team plans to find out why it does so.