Effective symbiosis between Rhizobium etli and Phaseolus vulgaris requires the alarmone ppGpp.
The symbiotic interaction between Rhizobium etli and Phaseolus vulgaris, the common bean plant, ultimately results in the formation of nitrogen-fixing nodules. Many aspects of the intermediate and late stages of this interaction are still poorly understood. The R. etli relA gene was identified through a genome-wide screening for R. etli symbiotic mutants. RelA has a pivotal role in cellular physiology, as it catalyzes the synthesis of (p)ppGpp, which mediates the stringent response in bacteria. The synthesis of ppGpp was abolished in an R. etli relA mutant strain under conditions of amino acid starvation. Plants nodulated by an R. etli relA mutant had a strongly reduced nitrogen fixation activity (75% reduction). Also, at the microscopic level, bacteroid morphology was altered, with the size of relA mutant bacteroids being increased compared to that of wild-type bacteroids. The expression of the sigma(N)-dependent nitrogen fixation genes rpoN2 and iscN was considerably reduced in the relA mutant. In addition, the expression of the relA gene was negatively regulated by RpoN2, the symbiosis-specific sigma(N) copy of R. etli. Therefore, an autoregulatory loop controlling the expression of relA and rpoN2 seems operative in bacteroids. The production of long- and short-chain acyl-homoserine-lactones by the cinIR and raiIR systems was decreased in an R. etli relA mutant. Our results suggest that relA may play an important role in the regulation of gene expression in R. etli bacteroids and in the adaptation of bacteroid physiology.
Moris
Martine
M
Centre of Microbial and Plant Genetics, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, B-3001 Heverlee, Belgium.
Braeken
Kristien
K
Schoeters
Eric
E
Verreth
Christel
C
Beullens
Serge
S
Vanderleyden
Jos
J
Michiels
Jan
J
D000222:Adaptation, Physiological; D004268:DNA-Binding Proteins / Q000235:genetics / Q000378:metabolism; D012321:DNA-Directed RNA Polymerases / Q000235:genetics / Q000378:metabolism; D015964:Gene Expression Regulation, Bacterial; D005798:Genes, Bacterial; D006159:Guanosine Tetraphosphate / Q000096:biosynthesis; D008025:Ligases / Q000235:genetics / Q000502:physiology*; D008969:Molecular Sequence Data; D009586:Nitrogen Fixation; D027805:Phaseolus / Q000382:microbiology / Q000502:physiology*; D051877:RNA Polymerase Sigma 54; D042163:Rhizobium etli / Q000235:genetics / Q000502:physiology*; D012808:Sigma Factor / Q000235:genetics / Q000378:metabolism; D013559:Symbiosis
D004268:DNA-Binding Proteins; D012808:Sigma Factor; D006159:Guanosine Tetraphosphate; D012321:DNA-Directed RNA Polymerases; D051877:RNA Polymerase Sigma 54; D008025:Ligases; C028812:guanosine 3',5'-polyphosphate synthetases