Responses of Rhizobia to Desiccation in Relation to Osmotic Stress, Oxygen, and Temperature

Abstract

3One of the most severe and widespread problems facing the agricultural industry is the degradation of soil quality due to desiccation and salinity. In fact, almost 40% of the world's land surface is affected by salinity-related problems (131). These two harsh environmental conditions can have a dramatic impact on the endogenous soil bacteria (38, 48). Of particular importance to the agricultural industry is the impact of these harsh environmental conditions on the endogenous group of proteobacteria, the rhizobia. These bacteria induce formation of nodules on legumes, in which atmospheric nitrogen is fixed and supplied to the plant, enhancing growth under nitrogenlimiting conditions. Desiccation and salinity negatively affect such interactions by limiting nitrogen fixation (131).The importance of nitrogen fixation for agriculture cannot be understated and is illustrated by the numerous studies of the impact of soil management on rhizobial populations in arid regions (52), as well as the isolation and characterization of desiccation-and salt-resistant strains (28, 56,128). Furthermore, to enhance nodulation and nitrogen fixation efficiency, techniques that allow close contact between the bacteria and the host seed have been developed. Despite such methods, there has been a decline in the number of viable bacteria on plant seeds, soil, and carrier material, in part because of the stresses caused by fertilizer and chemical applications (110), desiccation (100, 101), temperature changes (63, 74, 116), salinity changes (63), light (68), and growth media employed (23,35,63). These factors are encountered during the manufacture, storage, and use of the coated seeds, with desiccation as the principal cause of reduced bacterial survival on the seed (33, 119).Although improvement of long-term survival and seed inoculum storage time has been the focus of desiccation research (25, 33), relatively little work has focused primarily on the bacterial cell. Many questions remain regarding the physiological response of rhizobia to desiccation. In this review, we evaluate studies of the physiological responses of rhizobia to environmental stresses (osmotic, salt, temperature, and oxygen) that affect desiccation survival. Our discussion will focus on one species of rhizobia in particular, Sinorhizobium meliloti. DESICCATION RESPONSES OF RHIZOBIAEarly rhizobial desiccation research. Desiccation produces many stress responses in the bacterial cell. In 1932, Fred and coworkers reported loss of viability in rhizobia used as seed inocula (41) and suggested that the nature of the suspending medium, pH, and temperature are important factors in the survival of the inoculum in the dry state. This led to the recommendation for farmers to refrain from using rhizobia in dry form. Vincent et al. (119) showed that the decline in abundance of Rhizobium trifolii during drying on glass beads correlates with the extraction of water, which suggests that part of the decline in viable counts is caused by both "seed factors" and the drying itself. The ...

Role of the extracytoplasmic function sigma factor CarQ in oxidative response of Bradyrhizobium japonicum

“…Previous research also indicated that desiccation can interfere with the interaction between plant and their symbiotic microorganisms by limiting nitrogen fixation (52). In addition, desiccating conditions could induce ROS generation in rhizobial species (51). Interestingly, our study also showed that cell viability of the carQ mutant significantly decreased when the mutant was exposed to a severe desiccating condition (i.e., 27% RH) compared to the wild type.…”

The Physiological Mechanisms of Desiccation Tolerance in Rhizobia

“…Improved survival may also be related to a higher osmotic pressure in the concentrated extract, since survival decreased after dilution or removal of large molecular fractions. This has been suggested previously in studies where growth rate of rhizobia was restricted in low water activity medium and enhanced desiccation tolerance was attributed to adaptation to osmotic and other stresses during growth (Lloret et al, 1995;Vriezen et al, 2007).…”

“…Rhizobia accumulate intracellular compounds referred to as compatible solutes as a response to osmotic and desiccation stress (Vriezen et al, 2007). Compatible solutes can be either synthesised de novo by rhizobia or transported into the cell from the environment via active uptake systems (Streeter, 1985).…”

RpoE2 of Sinorhizobium meliloti is necessary for trehalose synthesis and growth in hyperosmotic media

“…Trehalose accumulation is involved in desiccation resistance of Rhizobium leguminosarum; mutants that do not accumulate trehalose are sensitive to drying (McIntyre et al, 2007). Desiccation tolerance is improved when cells are collected during the stationary growth phase (Vriezen et al, 2007). This corresponds to maximal induction of rpoE2 and trehalose accumulation by S. meliloti.…”