Sinorhizobium meliloti lsrB is involved in alfalfa root nodule development and nitrogen-fixing bacteroid differentiation

Abstract

Rhizobia interact with host legumes to induce the formation of nitrogen-fixing nodules, which is very important in agriculture and ecology. The development of nitrogen-fixing nodules is stringently regulated by host plants and rhizobial symbionts. In our previous work, a new Sinorhizobium meliloti LysR regulator gene (lsrB) was identified to be essential for alfalfa nodulation. However, how this gene is involved in alfalfa nodulation was not yet understood. Here, we found that this gene was associated with prevention of premature nodule senescence and abortive bacteroid formation. Heterogeneous deficient alfalfa root nodules were induced by the in-frame deletion mutant of lsrB (lsrB1-2), which was similar to the plasmid-insertion mutant, lsrB1. Irregular senescence zones earlier appeared in these nodules where bacteroid differentiation was blocked at different stages from microscopy observations. Interestingly, oxidative bursts were observed in these nodules by DAB staining. The decreased expression of lipopolysaccharide core genes (lpsCDE) was correspondingly determined in these nodules. S. meliloti lipopolysaccharide is required for suppression of oxidative bursts or host cell defense. These findings demonstrate that the S. meliloti lsrB gene is involved in alfalfa root nodule development and bacteroid differentiation by suppressing oxidative bursts or defense responses in host cells. Root nodules begin senescence when a host developmental program is activated at the beginning of pod filling or under several different kinds of environmental stresses including darkness, drought, and temperature stress [4]. Leguminous root nodules exhibit differential senescence processes. Senescence of determinant root nodules (round root nodules lacking persistent meristems) on Glycine max (soybean) initiates from the inside and spreads towards the exterior. Senescence of indeterminate root nodules (com- Sinorhizobium meliloti,

Transcriptional Regulator LsrB of Sinorhizobium meliloti Positively Regulates the Expression of Genes Involved in Lipopolysaccharide Biosynthesis

“…In our previous work, the lsrB1 -2 and lsrB1 (plasmid insertion) mutants induced heterogeneous, ineffective nodules on alfalfa (19,33). Premature senescence appeared in these nodules, and abnormal bacteroid differentiation was found (33).…”

“…Premature senescence appeared in these nodules, and abnormal bacteroid differentiation was found (33). Correspondingly, oxidative bursts were observed in most defective nodules (33). Therefore, it is possible that LsrB is involved in suppression of the host defense response.…”

The <italic>Sinorhizobium meliloti</italic> LysR family transcriptional factor LsrB is involved in regulation of glutathione biosynthesis

“…meliloti lsrB1-2 and oxyRsm mutants produces less GSH Both S. meliloti lsrB 1-2 (lsrB in-frame deletion, described in [11]) and gshB1 minus mutants elicited similar, deficient root nodules on alfalfa plants [6], which prompted us to examine the GSH level in the lsrB mutant. Q-TOF LC/MS analysis showed that the level of GSH decreased by 14.28%-18.58% in the lsrB1-2 mutant compared with the wild-type (Fig.…”

“…Bacterial strains and growth conditions Sinorhizobium meliloti 1021, lsrB1-2 [11] and oxyRsm [10] were grown in Luria-Bertani (LB) medium supplemented with 2.5 mM MgSO 4 and 2.5 mM CaCl 2 (LB/MC) at 288C [12]. The following antibiotics were used at the concentrations indicated: chloramphenicol, 10 mg/ml; neomycin, 200 mg/ml; spectinomycin, 100 mg/ml; tetracycline, 10 mg/ ml; and streptomycin, 500 mg/ml.…”

Regulation of cysteine residues in LsrB proteins from Sinorhizobium meliloti under free‐living and symbiotic oxidative stress

“…In a previous study, we found that a new Sinorhizobium LysR family transcriptional regulator gene, lsrB , is required for efficient alfalfa nodulation (Luo et al ., ). The nodules induced by a S. meliloti lsrB deletion mutant showed premature senescence, indicating impaired bacteroid differentiation (Tang et al ., ). LsrB directly regulates the expression of genes involved in lipopolysaccharide biosynthesis (Tang et al ., ); this regulator is associated with glutathione production and with induction of katA in an OxyR‐dependent manner (Lu et al ., ).…”