Functional Role of Bradyrhizobium japonicum Trehalose Biosynthesis and Metabolism Genes during Physiological Stress and Nodulation

Abstract

Trehalose, a disaccharide accumulated by many microorganisms, acts as a protectant during periods of physiological stress, such as salinity and desiccation. Previous studies reported that the trehalose biosynthetic genes (otsA, treS, and treY) in Bradyrhizobium japonicum were induced by salinity and desiccation stresses. Functional mutational analyses indicated that disruption of otsA decreased trehalose accumulation in cells and that an otsA treY double mutant accumulated an extremely low level of trehalose. In contrast, trehalose accumulated to a greater extent in a treS mutant, and maltose levels decreased relative to that seen with the wild-type strain. Mutant strains lacking the OtsA pathway, including the single, double, and triple ⌬otsA, ⌬otsA ⌬treS and ⌬otsA ⌬treY, and ⌬otsA ⌬treS ⌬treY mutants, were inhibited for growth on 60 mM NaCl. While mutants lacking functional OtsAB and TreYZ pathways failed to grow on complex medium containing 60 mM NaCl, there was no difference in the viability of the double mutant strain when cells were grown under conditions of desiccation stress. In contrast, mutants lacking a functional TreS pathway were less tolerant of desiccation stress than the wild-type strain. Soybean plants inoculated with mutants lacking the OtsAB and TreYZ pathways produced fewer mature nodules and a greater number of immature nodules relative to those produced by the wild-type strain. Taken together, results of these studies indicate that stress-induced trehalose biosynthesis in B. japonicum is due mainly to the OtsAB pathway and that the TreS pathway is likely involved in the degradation of trehalose to maltose. Trehalose accumulation in B. japonicum enhances survival under conditions of salinity stress and plays a role in the development of symbiotic nitrogen-fixing root nodules on soybean plants.Rhizobia induce the formation of nodules on the roots of legume plants, in which atmospheric nitrogen is fixed and supplied to the host plant, thereby enhancing growth under nitrogen-limiting conditions. The symbiotic interaction between rhizobia and their cognate leguminous plants is important for agricultural productivity, especially in less developed countries. However, physiological stresses, such as desiccation and salinity, negatively affect these symbiotic interactions by limiting nitrogen fixation (44). The osmotic environment within the rhizosphere may affect root colonization, infection thread development, nodule development, and the formation of effective N 2 -fixing nodules (21). Moreover, when legume seeds are inoculated with appropriate rhizobial strains prior to planting in the field, the vast majority of nodules produced are often not formed by the inoculant bacteria but rather by indigenous strains in the soil (36). This is in part due to the death of inoculant strains from rapid seed coat-mediated desiccation. Therefore, improvement of the survival of rhizobia under conditions of physiological stresses may promote biological nitrogen fixation and enhance plant growth.Rhizobia synthesiz...

Trehalose and Abiotic Stress in Biological Systems

“…Bradyrhizobium japonicum, the root nodule symbiont of soybeans, accumulates trehalose using three independent trehalose biosynthesis pathways (Streeter, 2006). Interestingly, mutants of the cells lacking the TreS degradation pathway showed a low survival under desiccation stress (Sugawara et al, 2010). This happened presumably because the high concentrations of trehalose affected the refolding and reactivation of denatured proteins by molecular chaperones and explains the reason why trehalose is quickly degraded after stress has ceased (Singer & Lindquist, 1998).…”

“…In the first step, maltooligosyltrehalose synthase (TreY) catalyzes the convertion of maltopentaose into maltooligosyl trehalose by intramolecular transglycosylation, and in the second step, maltooligosyltrehalose trehalohydrolase (TreZ) hydrolizes the maltooligosyl trehalose, releasing free trehalose (Maruta et al, 1995). This pathway is also found in other bacterial species such as Rhizobium (Maruta et al, 1996a), Bradyrhizobium japonicum (Sugawara et al, 2010) and Corynebacterium (Tzvetkov et al, 2003), but is missing in other major bacterial groups including E. coli and Bacillus subtilis. Archaea Sulfolobus also uses this pathway for trehalose synthesis (Maruta et al, 1996b).…”

“…Trehalose accumulates in rhizobium Bradyrhizobium japonicum in response to salt treatment. Mutant strains lacking the trehalose biosynthetic genes failed to grow on salt containing medium, indicating that trehalose plays a role as a osmoprotectant for growth under salt-induced osmotic stress (Sugawara et al, 2010). In a different study investigating four rhizobial strains isolated from nodules of Phaseolus vulgaris under salt-stress conditions revealed that all strains under study accumulated trehalose (Fernandez-Aunion et al, 2010).…”

“…R48 (Nishimoto et al, 1995) and so far it has been detected only in bacteria (Paul et al, 2008). Due to the reversible nature of the enzyme, TreS contributes trehalose synthesis during osmotic stress in Pseudomonas syringae (Freeman et al, 2010), while TreS functions in trehalose catabolism in B. japonicum (Sugawara et al, 2010).…”

“…Furthermore, E. coli, S. meliloti , B. japonicum mutants lacking trehalose biosynthesis genes are sensitive to osmotic stress. (Strom et al, 1993;Dominguez-Ferreras et al, 2009;Sugawara et al, 2010). Most of cyanobacteria are living in waters of different or changing salinities.…”