Abstract

Expression of hundreds of S. meliloti genes changed more than two-fold in response to either nitrogen or phosphate limitation. When these two stresses were applied together, stress responsive gene expression shifted dramatically. In particular, the nitrogen stress response in the presence of phosphate stress had only 30 of about 350 genes in common with the 280 genes that responded to nitrogen stress with adequate phosphate. Expression of sRNAs was also altered in response to these stresses. 82% of genes that responded to nitrogen stress also responded to phosphate stress, including 20 sRNAs. A subset of these sRNAs is known to be chaperoned by the RNA binding protein, Hfq. Hfq had previously been shown to influence about a third of the genes that responded to both nitrogen and phosphate stresses. Phosphate limitation influenced changes in gene expression more than nitrogen limitation and, when both stresses were present, phosphate stress sometimes reversed the direction of some of the changes induced by nitrogen stress. These nutrient stress responses are therefore context dependent.

Highlights

  • Soils vary substantially in characteristics that influence bacterial growth and survival, such as nutrient or water availability, salt content, pH, and the abundance of competing and predatory organisms (Zahran, 1999; Vitousek et al, 2010; Fanin et al, 2016)

  • The appearance of previously identified genes in this study indicated that the bacteria responded to the nutrient stress in a consistent manner, while the inability to identify all the genes from previous studies most likely was due to experimental differences and a more stringent statistical limit

  • We analyzed the glnB−glnK− strain, BK pstC+, in addition to the 1021 pstC+ strain for gene expression changes that resulted from nitrogen and/or phosphate stress

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Summary

Introduction

Soils vary substantially in characteristics that influence bacterial growth and survival, such as nutrient or water availability, salt content, pH, and the abundance of competing and predatory organisms (Zahran, 1999; Vitousek et al, 2010; Fanin et al, 2016). Low levels of usable nitrogen and phosphate are the most commonly limiting nutrients (Vitousek et al, 2010; Fanin et al, 2016) and the ability to cope with nitrogen or phosphate starvation is important. Rhizobia are best known for their nitrogen-fixing symbiotic relationships with legumes in which the bacteria convert atmospheric dinitrogen into ammonium, which can be assimilated by the legumes and support plant growth. This chemically available nitrogen is an important input in natural and agricultural ecosystems (Bohlool et al, 1992; Mylona et al, 1995). Since several stresses can occur simultaneously in Sinorhizobium Nitrogen and Phosphate Stress natural ecosystems, understanding how stress response mechanisms affect each other is important in understanding bacterial stress adaptation

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