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 ...

Bacteriocins are a large and functionally diverse family of toxins found in all major lineages of Bacteria. Colicins, those bacteriocins produced by Escherichia coli, serve as a model system for investigations of bacteriocin structure-function relationships, genetic organization, and their ecological role and evolutionary history. Colicin expression is often dependent on host regulatory pathways (such as the SOS system), is usually confined to times of stress, and results in death of the producing cells. This study investigates the role of the SOS system in mediating this unique form of toxin expression. A comparison of all the sequenced enteric bacteriocin promoters reveals that over 75 % are regulated by dual, overlapping SOS boxes, which serve to bind two LexA repressor proteins. Furthermore, a highly conserved poly-A motif is present in both of the binding sites examined, indicating enhanced affinity of the LexA protein for the binding site. The use of gene expression analysis and deletion mutations further demonstrates that these unique LexA cooperative binding regions result in a fine tuning of bacteriocin production, limiting it to times of stress. These results suggest that the evolution of dual SOS boxes elegantly accomplishes the task of increasing the amount of toxin produced by a cell while decreasing the rate of uninduced production, effectively reducing the cost of colicin production. This hypothesis may explain why such a promoter motif is present at such high frequencies in natural populations of bacteriocin-producing enteric bacteria.

Aims:  To identify physical and physiological conditions that affect the survival of Sinorhizobium meliloti USDA 1021 during desiccation. Methods and Results:  An assay was developed to study desiccation response of S. meliloti USDA 1021 over a range of environmental conditions. We determined the survival during desiccation in relation to (i) matrices and media, (ii) growth phase, (iii) temperature, and (iv) chloride and sulfate availability. Conclusions:  This study indicates that survival of S. meliloti USDA 1021 during desiccation is enhanced: (i) when cells were dried in the stationary phase, (ii) with increasing drying temperature at an optimum of 37°C, and (iii) during an increase of chloride and sulfate, but not sodium or potassium availability. In addition, we resolved that the best matrix to test survival was nitrocellulose filters. Significance and Impact of the Study:  The identification of physical and physiological factors that determine the survival during desiccation of S. meliloti USDA 1021 may aid in (i) the strategic development of improved seed inocula, (ii) the isolation, and (iii) the development of rhizobial strains with improved ability to survive desiccation. Furthermore, this work may provide insights into the survival of rhizobia under drought conditions.

Burkholderia phymatum is a soil bacterium able to develop a nitrogen-fixing symbiosis with species of the legume genus Mimosa, and is frequently found associated specifically with Mimosa pudica. The type strain of the species, STM 815T, was isolated from a root nodule in French Guiana in 2000. The strain is an aerobic, motile, non-spore forming, Gram-negative rod, and is a highly competitive strain for nodulation compared to other Mimosa symbionts, as it also nodulates a broad range of other legume genera and species. The 8,676,562 bp genome is composed of two chromosomes (3,479,187 and 2,697,374 bp), a megaplasmid (1,904,893 bp) and a plasmid hosting the symbiotic functions (595,108 bp).