The Sinorhizobium meliloti MsbA2 protein is essential for the legume symbiosis

Abstract

Sinorhizobium meliloti is a beneficial legume symbiont, closely related to Brucella species, which are chronic mammalian pathogens. We discovered that the S. meliloti MsbA2 protein is essential to ensure the symbiotic interaction with the host plant, alfalfa. S. meliloti invades plant cells via plant-derived structures known as infection threads. However, in the absence of MsbA2, S. meliloti remains trapped within abnormally thickened infection threads and induces a heightened plant defence response, characterized by a substantial thickening of the nodule endodermis layer and the accumulation of polyphenolic compounds. The S. meliloti MsbA2 protein is homologous to the Escherichia coli lipopolysaccharide/phospholipid trafficking protein MsbA. However, MsbA2 was not essential for the membrane transport of either lipopolysaccharide or phospholipids in S. meliloti. We determined that the msbA2 gene is transcribed in free-living S. meliloti and that in the absence of MsbA2 the polysaccharide content of S. meliloti is altered. Consequently, we propose a model whereby the altered polysaccharide content of the S. meliloti msbA2 mutant could be responsible for its symbiotic defect by inducing an inappropriate host response.

The Sinorhizobium meliloti LpxXL and AcpXL Proteins Play Important Roles in Bacteroid Development within Alfalfa

“…The results of this study also provide evidence that the BacA-mediated lipid A VLCFA modification (4) is unlikely to be solely responsible for the essential role of BacA in the S. meliloti-alfalfa symbiosis (8). We found recently that BacA plays an essential role in the uptake of a truncated eukaryotic peptide, Bac7 [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] , in S. meliloti (15). Since it is thought that hundreds of root nodulespecific cysteine-rich peptides are produced by Medicago truncatula (1,17,18), the BacA-mediated uptake of one or more of these peptides combined with the BacA-mediated lipid A VLCFA modification may account for the essential role of BacA in the legume symbiosis.…”

“…The precise function(s) of the BacA proteins has not been resolved, but free-living S. meliloti and B. abortus mutants lacking BacA have increased resistance to the glycopeptide bleomycin (9,12) and there are ϳ50% decreases in their lipid A very-long-chain fatty acid (VLCFA) contents (4,7). It has also been determined that the increased resistance of an S. meliloti bacA null mutant to bleomycin and a truncated eukaryotic peptide, Bac7 [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] , is independent of its lipid A VLCFA alteration (6,15). Together, these findings support a model in which BacA could have multiple nonoverlapping functions which lead to lipid A VLCFA modification and peptide uptake.…”

Partial Complementation of Sinorhizobium meliloti bacA Mutant Phenotypes by the Mycobacterium tuberculosis BacA Protein

“…meliloti-alfalfa interaction. To determine the ability of S. meliloti strains to form a successful symbiosis with alfalfa, 3-day-old alfalfa seedlings were inoculated with S. meliloti on Jensen's agar exactly as described previously (27). Plant growth and nodule morphology were monitored over a 4-week incubation period.…”

The Sinorhizobium meliloti SyrM Regulon: Effects on Global Gene Expression Are Mediated by syrA and nodD3

“…Moreover, within functional classes common to the three transcriptomes, there were some genes that showed no altered expression in the Ptrp-syrM strain; for example, 22 motility and chemotaxis genes (decreased) and six exo genes (increased) were changed only in the Ptrp-nodD3 or Ptrp-syrA strain (exoH, exoM, exoO, exoU, exoZ, and SMb20950). Likewise, of four genes in an operon predicted to be involved in the production of a novel glycan (76,77), two had increased expression in all three transcriptomes (SMb21190 and msbA2) and two in the Ptrp-syrA and Ptrp-nodD3 strains only (SMb21188 and SMb21189). We suggest nodD3 was overexpressed.…”