Identification and Characterization of a NaCl-Responsive Genetic Locus Involved in Survival during Desiccation in Sinorhizobium meliloti

Abstract

The Rhizobiaceae are a bacterial family of enormous agricultural importance due to the ability of its members to fix atmospheric nitrogen in an intimate relationship with plants. Their survival as naturally occurring soil bacteria in agricultural soils as well as popular seed inocula is affected directly by drought and salinity. Survival after desiccation in the presence of NaCl is enabled by underlying genetic mechanisms in the model organism Sinorhizobium meliloti 1021. Since salt stress parallels a loss in water activity, the identification of NaCl-responsive loci may identify loci involved in survival during desiccation. This approach enabled identification of the loci asnO and ngg by their reduced ability to grow on increased NaCl concentrations, likely due to their inability to produce the osmoprotectant N-acetylglutaminylglutamine (NAGGN). In addition, the mutant harboring ngg:: Tn5luxAB was affected in its ability to survive desiccation and responded to osmotic stress. The desiccation sensitivity may have been due to secondary functions of Ngg (N-acetylglutaminylglutamine synthetase)-like cell wall metabolism as suggested by the presence of a D-alanine-D-alanine ligase (dAla-dAla) domain and by sensitivity of the mutant to ␤-lactam antibiotics. asnO:: Tn5luxAB is expressed during the stationary phase under normal growth conditions. Amino acid sequence similarity to enzymes producing ␤-lactam inhibitors and increased resistance to ␤-lactam antibiotics may indicate that asnO is involved in the production of a ␤-lactam inhibitor. The Rhizobiaceae are a bacterial family of enormous agricultural importance due to their ability to fix atmospheric nitrogen in an intimate relationship with plants (1). They occur naturally in most agricultural soils, and their survival is affected directly by both drought and salinity (2). Unfortunately, changes in climate patterns are occurring and, as a direct consequence, salinification and desertification are some of the major threats to agricultural land use. It is estimated that over 40% of arable land will be affected by desiccation and salinity by 2025 (2). Furthermore, production of seed inocula often includes a drying phase negatively affecting CFU of added rhizobia (3, 4), potentially resulting in desiccation-induced viable but nonculturable cells (5).A multitude of conditions affecting survival during desiccation of agriculturally important Rhizobiaceae have been studied (6,7,8,9,10,11,12,13,14,15,16,17,18,19,20). These conditions include the intrageneric differences to cope with desiccation stress which affect survival (5,6,9,10,13,18,21,22). These data suggest that no single gene affects the ability of rhizobia to survive desiccation but that several mechanisms are likely responsible.One of the conditions affecting the ability of Sinorhizobium meliloti to survive desiccation is NaCl availability (10, 13); hence, we studied the response of S. meliloti 1021 to drought in conjunction with different salt stresses (13). We showed that survival of the strain in the presen...

Characterization of the l-arabinofuranose-specific GafABCD ABC transporter essential for l-arabinose-dependent growth of the lignocellulose-degrading bacterium Shewanella sp. ANA-3

“…viciae 3841, namely RL2376, was found to bind d -galactose and its use as a biosensor was proposed [ 55 ], although it is highly likely to also bind l -arabinose and might not be particularly selective. Another study found that the GafA Sm -encoding operon was expresed in response to desiccation induced by high NaCl in Sinorhizobium meliloti 1021 [ 56 ], and a known response to this is the production of galactose-containing oligosaccharides [ 57 ], consistent with a function of scavenging d -galactose to provide precursors for these glycans.…”

Appearance of Membrane Compromised, Viable but Not Culturable and Culturable Rhizobial Cells as a Consequence of Desiccation

“…However, how likely is to have a molecular junction of a NaCl inducible gene that, when knocked out, leads to temperature dependent desiccation sensitivity? In addition to some NaCl induced loci described earlier, locus smb01590, found by Vriezen et al [2013], also affects survival during desiccation (Figure 4). Interestingly, the ability of the mutant carrying a Tn5luxAB insertion in ORF smc01590 (Sce-1) to survive desiccation is better, albeit not significant (P=0.22), than that of the reference strain at In mutant Sce-1, it appears that the inability to correctly adjust the membrane midpoint temperature at increased temperature leads to the opposite effect.…”

The two-component regulatory system CenK–CenR regulates expression of a previously uncharacterized protein required for salinity and oxidative stress tolerance in Sinorhizobium meliloti

“…These cellular lifestyles occur in very different environments and the soil environment particularly can involve widely varying physicochemical conditions. Accordingly, S. meliloti has been the subject of attempts to identify genes that when disrupted conditionally impair growth under stressful conditions that include high salinity (Wei et al, 2004;Domínguez-Ferreras et al, 2006;Miller-Williams et al, 2006;Vriezen et al, 2013;Calatrava-Morales et al, 2017), low pH (Reeve et al, 1998;Lagares et al, 2016;Albicoro et al, 2021), oxidative stress (Davies and Walker, 2007;Flechard et al, 2009;Lehman and Long, 2018) and other physical or chemical conditions.…”