Formation of rhizobial symbiosis requires chemical chaperones
A recent "mBio" paper by the Fischer group (IMB) in collaboration with the Vorholt (IMB) and Zamboni groups (IMSB) demonstrates a critical role of chemical chaperones during infection of soybean host plants by the nitrogen-fixing soybean root nodule symbiont Bradyrhizobium diazoefficiens.
Symbiotic nitrogen fixation by rhizobia living in root nodules of legume host plants is initiated by a chemical dialogue between the symbiotic partners, which results in the controlled infection and colonization of roots by compatible rhizobia. The transition from free-living bacteria in the soil to intracellular endosymbionts is accompanied by host-imposed physiological adaptations including induction of a general stress response (GSR).
In the present study, it is demonstrated that GSR-controlled synthesis of the disaccharide trehalose is critical for efficient infection of soybean roots by B. diazoefficiens and synchronization of host colonization with nodule organogenesis. GSR- or trehalose biosynthesis-deficient mutants elicit empty pseudonodules and aberrant nodules with impaired nitrogen fixation activity. Symbiotic efficiency and free-living osmotolerance is restored in trehalose-deficient mutants engineered to synthesize the well-characterized chemical chaperones (hydroxy)ectoine. It is concluded that trehalose acts as chemical chaperone enabling rhizobia to cope with adverse osmotic stress conditions during early stages of host infection. Notably, GSR activity declines at later symbiotic stages indicating that rhizobia present experience more favorable conditions in cells of mature nodules.
Link to the paper in external page mBio