Purine Biosynthesis, Biofilm Formation, and Persistence of an Insect-Microbe Gut Symbiosis

The Riptortus-Burkholderia symbiotic system is an experimental model system for studying the molecular mechanisms of an insect-microbe gut symbiosis. When the symbiotic midgut of Riptortus pedestris was investigated by light and transmission electron microscopy, the lumens of the midgut crypts that harbor colonizing Burkholderia symbionts were occupied by an extracellular matrix consisting of polysaccharides. This observation prompted us to search for symbiont genes involved in the induction of biofilm formation and to examine whether the biofilms are necessary for the symbiont to establish a successful symbiotic association with the host. To answer these questions, we focused on purN and purT, which independently catalyze the same step of bacterial purine biosynthesis. When we disrupted purN and purT in the Burkholderia symbiont, the Delta purN and Delta purT mutants grew normally, and only the Delta purT mutant failed to form biofilms. Notably, the Delta purT mutant exhibited a significantly lower level of cyclic-di-GMP (c-di-GMP) than the wild type and the Delta purN mutant, suggesting involvement of the secondary messenger c-di-GMP in the defect of biofilm formation in the Delta purT mutant, which might operate via impaired purine biosynthesis. The host insects infected with the Delta purT mutant exhibited a lower infection density, slower growth, and lighter body weight than the host insects infected with the wild type and the Delta purN mutant. These results show that the function of purT of the gut symbiont is important for the persistence of the insect gut symbiont, suggesting the intricate biological relevance of purine biosynthesis, biofilm formation, and symbiosis.