Symbiotic nitrogen fixation by rhizobia — the roots of a success story

Current Opinion in Plant Biology

Volumn 44, Issue , 2018, Pages 7-15

  Masson Boivin, Catherine ^a   Sachs, Joel L ^b  

^a UNIVERSITÉ DE TOULOUSE   (France)

^b UNIVERSITY OF CALIFORNIA   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

GENETICS; IMMUNOLOGY; MICROBIOLOGY; NITROGEN FIXATION; NODULATION; PHYSIOLOGY; PLANT ROOT; RHIZOBIUM; SYMBIOSIS;

NITROGEN FIXATION; PLANT ROOTS; RHIZOBIUM; ROOT NODULES, PLANT; SYMBIOSIS;

EID: 85039742524     PISSN: 13695266     EISSN: None     Source Type: Journal    
DOI: 10.1016/j.pbi.2017.12.001     Document Type: Review

References (87)

1. Kneip, C., Lockhart, P., Voss, C., Maier, U.G., Nitrogen fixation in eukaryotes — new models for symbiosis. BMC Evolut Biol, 7, 2007.
2. 2-fixation in angiosperms. Nat Commun, 5, 2014.
3. Van Velzen, R., Holmer, R., Bu, F., Rutten, L., van Zeijl, A., Liu, W., Santuari, L., Cao, Q., Sharma, T., Shen, D., et al. Parallel loss of symbiosis genes in relatives of nitrogen-fixing non-legume Parasponia. bioRxiv, 2017, 10.1101/169706.
4. Remigi, P., Zhu, J., Young, J.P.W., Masson-Boivin, C., Symbiosis within symbiosis: evolving nitrogen-fixing legume symbionts. Trends Microbiol, 24, 2016 63–75.54.
5. Delaux, P.-M., Radhakrishnan, G., Oldroyd, G., Tracing the evolutionary path to nitrogen-fixing crops. Curr Opin Plant Biol 26 (2015), 95–99.
6. Raymond, J., Siefert, J.L., Staples, C.R., Blankenship, R.E., The natural history of nitrogen fixation. Mol Biol Evol 21 (2004), 541–554.
7. Oldroyd, G.E.D., Speak, friend, and enter: signalling systems that promote beneficial symbiotic associations in plants. Nat Rev Microbiol 11 (2013), 252–263.
8. Perret, X., Staehelin, C., Broughton, W.J., Molecular basis of symbiotic promiscuity. Microbiol Mol Biol Rev 64 (2000), 180–201.
9. Jones, K.M., Kobayashi, H., Davies, B.W., Taga, M.E., Walker, G.C., How rhizobial symbionts invade plants: the Sinorhizobium–Medicago model. Nat Rev Microbiol 5 (2007), 619–633.
10. Mathis, R., Van Gijsegem, F., De Rycke, R., D'Haeze, W., Van Maelsaeke, E., Anthonio, E., Van Montagu, M., Holsters, M., Vereecke, D., Lipopolysaccharides as a communication signal for progression of legume endosymbiosis. Proc Natl Acad Sci U S A 102 (2005), 2655–2660.
11. Kawaharada, Y., Kelly, S., Nielsen, M.W., Hjuler, C.T., Gysel, K., Muszynski, A., Carlson, R.W., Thygesen, M.B., Sandal, N., Asmussen, M.H., et al. Receptor-mediated exopolysaccharide perception controls bacterial infection. Nature 523 (2015), 308–312.
12. Mergaert, P., Uchiumi, T., Alunni, B., Evanno, G., Cheron, A., Catrice, O., Mausset, A., Barloy-Hubler, F., Galibert, F., Kondorosi, A., et al. Eukaryotic control on bacterial cell cycle and differentiation in the rhizobium-legume symbiosis. Proc Natl Acad Sci U S A 103 (2006), 5230–5235.
13. Guefrachi, I., Pierre, O., Timchenko, T., Alunni, B., Barriere, Q., Czernic, P., Villaecija-Aguilar, J.A., Verly, C., Bourge, M., Fardoux, J., et al. Bradyrhizobium BclA is a peptide transporter required for bacterial differentiation in symbiosis with Aeschynomene legumes. Mol Plant Microbe Interact 28 (2015), 1155–1166.
14. Haag, A.F., Baloban, M., Sani, M., Kerscher, B., Pierre, O., Farkas, A., Longhi, R., Boncompagni, E., Herouart, D., Dall'Angelo, S., et al. Protection of Sinorhizobium against host cysteine-rich antimicrobial peptides is critical for symbiosis. PLoS Biol, 9, 2011.
15. Hubber, A., Vergunst, A.C., Sullivan, J.T., Hooykaas, P.J.J., Ronson, C.W., Symbiotic phenotypes and translocated effector proteins of the Mesorhizobium loti strain R7A VirB/D4 type IV secretion system. Mol Microbiol 54 (2004), 561–574.
16. Bartsev, A.V., Deakin, W.J., Boukli, N.M., McAlvin, C.B., Stacey, G., Malnoe, P., Broughton, W.J., Staehelin, C., NopL, an effector protein of Rhizobium sp. NGR234, thwarts activation of plant defense reactions. Plant Physiol 134 (2004), 871–879.
17. Price, P.A., Tanner, H.R., Dillon, B.A., Shabab, M., Walker, G.C., Griffitts, J.S., Rhizobial peptidase HrrP cleaves host-encoded signaling peptides and mediates symbiotic compatibility. Proc Natl Acad Sci U S A 112 (2015), 15244–15249.
18. Maillet, F., Poinsot, V., Andre, O., Puech-Pages, V., Haouy, A., Gueunier, M., Cromer, L., Giraudet, D., Formey, D., Niebel, A., et al. Fungal lipochitooligosaccharide symbiotic signals in arbuscular mycorrhiza. Nature, 469, 2011 58-U1501.
19. Zhang, X.W., Dong, W.T., Sun, J.H., Feng, F., Deng, Y.W., He, Z.H., Oldroyd, G.E.D., Wang, E.T., The receptor kinase CERK1 has dual functions in symbiosis and immunity signalling. Plant J 81 (2015), 258–267.
20. Tisserant, E., Malbreil, M., Kuo, A., Kohler, A., Symeonidi, A., Balestrini, R., Charron, P., Duensing, N., Frey, N.F.D., Gianinazzi-Pearson, V., et al. Genome of an arbuscular mycorrhizal fungus provides insight into the oldest plant symbiosis. Proc Natl Acad Sci U S A 110 (2013), 20117–20122.
21. Liang, Y., Cao, Y.R., Tanaka, K., Thibivilliers, S., Wan, J.R., Choi, J., Kang, C.H., Qiu, J., Stacey, G., Nonlegumes respond to rhizobial Nod factors by suppressing the innate immune response. Science 341 (2013), 1384–1387 Shows that NFs can suppress MAMP-associated immunity in legumes.
22. Lee, K.B., De Backer, P., Aono, T., Liu, C.T., Suzuki, S., Suzuki, T., Kaneko, T., Yamada, M., Tabata, S., Kupfer, D.M., et al. The genome of the versatile nitrogen fixer Azorhizobium caulinodans ORS571. BMC Genomics, 9, 2008.
23. Nandasena, K.G., O'Hara, G.W., Tiwari, R.P., Sezmis, E., Howieson, J.G., In situ lateral transfer of symbiosis islands results in rapid evolution of diverse competitive strains of mesorhizobia suboptimal in symbiotic nitrogen fixation on the pasture legume Biserrula pelecinus L. Environ Microbiol 9 (2007), 2496–2511.
24. Sullivan, J.T., Patrick, H.N., Lowther, W.L., Scott, D.B., Ronson, C.W., Nodulating strains of Rhizobium loti arise through chromosomal symbiotic gene transfer in the environment. Proc Natl Acad Sci U S A 92 (1995), 8985–8989 First demonstration of transfer of MGE to diverse recipient genomes.
25. Edwards, J., Johnson, C., Santos-Medellin, C., Lurie, E., Podishetty, N.K., Bhatnagar, S., Eisen, J.A., Sundaresan, V., Structure, variation, and assembly of the root-associated microbiomes of rice. Proc Natl Acad Sci U S A 112 (2015), E911–E920.
26. Ling, J., Wang, H., Wu, P., Li, T., Tang, Y., Naseer, N., Zheng, H.M., Masson-Boivin, C., Zhong, Z.T., Zhu, J., Plant nodulation inducers enhance horizontal gene transfer of Azorhizobium caulinodans symbiosis island. Proc Natl Acad Sci U S A 113 (2016), 13875–13880.
27. Sachs, J.L., Skophammer, R.G., Regus, J.U., Evolutionary transitions in bacterial symbiosis. Proc Natl Acad Sci U S A 108 (2011), 10800–10807.
28. Shapiro, B.J., Levade, I., Kovacikova, G., Taylor, R.K., Almagro-Moreno, S., Origins of pandemic Vibrio cholerae from environmental gene pools. Nat Microbiol, 2, 2017.
29. Pobigaylo, N., Szymczak, S., Nattkemper, T.W., Becker, A., Identification of genes relevant to symbiosis and competitiveness in Sinorhizobium meliloti using signature-tagged mutants. Mol Plant Microbe Interact 21 (2008), 219–231.
30. Capela, D., Marchetti, M., Clerissi, C., Perrier, A., Guetta, D., Gris, C., Valls, M., Jauneau, A., Cruveiller, S., Rocha, E.P.C., et al. Recruitment of a lineage-specific virulence regulatory pathway promotes intracellular infection by a plant pathogen experimentally evolved into a legume symbiont. Mol Biol Evol 34 (2017), 2503–2521 Shows that the rewiring of a genetic network regulating virulence allows a radically different type of symbiotic interaction during experimental evolution of a plant pathogen into legume symbionts.
31. Remigi, P., Capela, D., Clerissi, C., Tasse, L., Torchet, R., Bouchez, O., Batut, J., Cruveiller, S., Rocha, E.P., Masson-Boivin, C., Transient hypermutagenesis accelerates the evolution of legume endosymbionts following horizontal gene transfer. PLoS Biol, 12, 2014, e1001942 Experimentally shows that the co-transfer of nod genes with error-prone polymerases accelerates the evolution of rhizobia.
32. Zhao, R., Liu, L.X., Zhang, Y.Z., Jiao, J., Cui, W.J., Zhang, B., Wang, X.L., Li, M.L., Chen, Y., Xiong, et al. Adaptive evolution of rhizobial symbiotic compatibility mediated by co-evolved insertion sequences. ISME J, 2017, 10.1038/ismej.2017.136.
33. Heath, K.D., Tiffin, P., Stabilizing mechanisms in a legume-rhizobium mutualism. Evolution 63 (2009), 652–662.
34. Sachs, J.L., Russell, J.E., Lii, Y.E., Black, K.C., Lopez, G., Patil, A.S., Host control over infection and proliferation of a cheater symbiont. J Evol Biol 23 (2010), 1919–1927.
35. Tang, F., Yang, S.M., Liu, J.G., Zhu, H.Y., Rj4, a gene controlling nodulation specificity in soybeans, encodes a thaumatin-like protein but not the one previously reported. Plant Physiol 170 (2016), 26–32.
36. − Bradyrhizobium japonicum mutant against the wild-type strain. FEMS Microbiol Lett, 1986, 143–148.
37. Amarger, N., Competition for nodule formation between effective and ineffective strains of Rhizobium meliloti. Soil Biol Biochem 13 (1981), 475–480.
38. Kiers, E.T., Rousseau, R.A., West, S.A., Denison, R.F., Host sanctions and the legume-rhizobium mutualism. Nature 425 (2003), 78–81.
39. Oono, R., Anderson, C.G., Denison, R.F., Failure to fix nitrogen by non-reproductive symbiotic rhizobia triggers host sanctions that reduce fitness of their reproductive clonemates. Proc R Soc B Biol Sci 278 (2011), 2698–2703.
40. Regus, J.U., Gano, K.A., Hollowell, A.C., Sachs, J.L., Efficiency of partner choice and sanctions in Lotus is not altered by nitrogen fertilization. Proc R Soc B Biol Sci, 281, 2014.
41. Simms, E.L., Taylor, D.L., Povich, J., Shefferson, R.P., Sachs, J.L., Urbina, M., Tausczik, Y., An empirical test of partner choice mechanisms in a wild legume-rhizobium interaction. Proc R Soc B Biol Sci 273 (2006), 77–81.
42. Regus, J.U., Gano, K.A., Hollowell, A.C., Sofish, V., Sachs, J.L., Lotus hosts delimit the mutualism-parasitism continuum of Bradyrhizobium. J Evol Biol 28 (2015), 447–456.
43. West, S.A., Kiers, E.T., Pen, I., Denison, R.F., Sanctions and mutualism stability: when should less beneficial mutualists be tolerated?. J Evol Biol 15 (2002), 830–837.
44. West, S.A., Kiers, E.T., Simms, E.L., Denison, R.F., Sanctions and mutualism stability: why do rhizobia fix nitrogen?. Proc R Soc B Biol Sci 269 (2002), 685–694.
45. Regus, J.U., Quides, K.J., O'Neill, M.R., Suzuki, R., Savory, E.A., Chang, J.H., Sachs, J., Cell autonomous sanctions in legumes target ineffective rhizobia in nodules with mixed infections. Am J Bot 104 (2017), 1299–1312 Discovery of cell-autonomy of legume sanctions in determinate nodules.
46. Daubech, B., Remigi, P., Doin de Moura, G., Marchetti, M., Pouzet, C., Auriac, M.C., Gokhale, C.S., Masson-Boivin, C., Capela, D., Spatio-temporal control of mutualism in legumes helps spread symbiotic nitrogen fixation. Elife, 2017, 10.7554/eLife.28683 Evidence of cell-autonomy of legume sanctions in indeterminate nodules. Shows that rare nitrogen fixing variants invade a non-fixing population in appropriate conditions.
47. VanInsberghe, D., Maas, K.R., Cardenas, E., Strachan, C.R., Hallam, S.J., Mohn, W.W., Non-symbiotic Bradyrhizobium ecotypes dominate North American forest soils. ISME J 9 (2015), 2435–2441.
48. Parker, M.A., The spread of Bradyrhizobium lineages across host legume clades: from Abarema to Zygia. Microbial Ecol 69 (2015), 630–640.
49. Denison, R.F., Kiers, E.T., Lifestyle alternatives for rhizobia: mutualism, parasitism, and forgoing symbiosis. FEMS Microbiol Lett 237 (2004), 187–193.
50. Sachs, J.L., Russell, J.E., Hollowell, A.C., Evolutionary instability of symbiotic function in Bradyrhizobium japonicum. PLoS One, 6, 2011, 10.
51. Thies, J.E., Woomer, P.L., Singleton, P.W., Enrichment of Bradyrhizobium spp. populations in soil due to cropping of the homologous host legume. Soil Biol Biochem 27 (1995), 633–636.
52. Nandasena, K.G., O'Hara, G.W., Tiwari, R.P., Howieson, J.G., Rapid in situ evolution of nodulating strains for Biserrula pelecinus L. through lateral transfer of a symbiosis island from the original mesorhizobial inoculant. Appl Environ Microbiol 72 (2006), 7365–7367.
53. Parker, M.A., Legumes select symbiosis island sequence variants in Bradyrhizobium. Mol Ecol 21 (2012), 1769–1778.
54. Sachs, J.L., Simms, E.L., The origins of uncooperative rhizobia. Oikos 117 (2008), 961–966.
55. Porter, S.S., Simms, E.L., Selection for cheating across disparate environments in the legume-rhizobium mutualism. Ecol Lett 17 (2014), 1121–1129.
56. Kiers, E.T., Hutton, M.G., Denison, R.F., Human selection and the relaxation of legume defences against ineffective rhizobia. Proc R Soc B Biol Sci 274 (2007), 3119–3126.
57. Weese, D.J., Heath, K.D., Dentinger, B.T.M., Lau, J.A., Long-term nitrogen addition causes the evolution of less-cooperative mutualists. Evolution 69 (2015), 631–642.
58. Gano-Cohen, K.A., Stokes, P.J., Blanton, M.A., Wendlandt, C.E., Hollowell, A.C., Regus, J.U., Kim, D., Patel, S., Pahua, V.J., Sachs, J.L., Nonnodulating Bradyrhizobium spp. modulate the benefits of legume-rhizobium mutualism. Appl Environ Microbiol 82 (2016), 5259–5268.
59. Hollowell, A.C., Regus, J.U., Gano, K.A., Bantay, R., Centeno, D., Pham, J., Lyu, J., Moore, D., Bernardo, A., Lopez, G., et al. Epidemic spread of symbiotic and non-symbiotic Bradyrhizobium genotypes across California. Microbial Ecol 71 (2016), 700–710.
60. Hollowell, A.C., Regus, J.U., Turissini, D., Gano-Cohen, K.A., Bantay, R., Bernardo, A., Moore, D., Pham, J., Sachs, J.L., Metapopulation dominance and genomic-island acquisition of Bradyrhizobium with superior catabolic capabilities. Proc R Soc B Biol Sci, 283, 2016.
61. Sachs, J.L., Skophammer, R.G., Bansal, N., Stajich, J.E., Evolutionary origins and diversification of proteobacterial mutualists. Proc R Soc B Biol Sci, 281, 2014, 9.
62. Ferguson, B.J., Indrasumunar, A., Hayashi, S., Lin, M.H., Lin, Y.H., Reid, D.E., Gresshoff, P.M., Molecular analysis of legume nodule development and autoregulation. J Integr Plant Biol 52 (2010), 61–76.
63. Yates, R.J., Howieson, J.G., Reeve, W.G., O'Hara, G.W., A re-appraisal of the biology and terminology describing rhizobial strain success in nodule occupancy of legumes in agriculture. Plant Soil 348 (2011), 255–267.
64. Marchetti, M., Capela, D., Glew, M., Cruveiller, S., Chane-Woon-Ming, B., Gris, C., Timmers, T., Poinsot, V., Gilbert, L.B., Heeb, P., et al. Experimental evolution of a plant pathogen into a legume symbiont. PLoS Biol, 8, 2010.
65. Guan, S.H., Gris, C., Cruveiller, S., Pouzet, C., Tasse, L., Leru, A., Maillard, A., Medigue, C., Batut, J., Masson-Boivin, C., et al. Experimental evolution of nodule intracellular infection in legume symbionts. ISME J 7 (2013), 1367–1377.
66. Mueller, U.G., Sachs, J.L., Engineering microbiomes to improve plant and animal health. Trends Microbiol 23 (2015), 606–617.
67. Yang, J.G., Xie, X.Q., Yang, M.X., Dixon, R., Wang, Y.P., Modular electron-transport chains from eukaryotic organelles function to support nitrogenase activity. Proc Natl Acad Sci U S A 114 (2017), E2460–E2465 Role of some NCR peptides in selecting specific bacterial genotypes.
68. 2+ spiking and NIN gene expression in the actinorhizal plant Casuarina glauca. New Phytol 209 (2016), 86–93.
69. D'Haeze, W., Holsters, M., Nod factor structures, responses, and perception during initiation of nodule development. Glycobiology 12 (2002), 79R–105R.
70. Okazaki, S., Kaneko, T., Sato, S., Saeki, K., Hijacking of leguminous nodulation signaling by the rhizobial type III secretion system. Proc Natl Acad Sci U S A 110 (2013), 17131–17136 Shows that Bradyrhizobium T3SS can activate nodulation in legumes.
71. Okazaki, S., Tittabutr, P., Teulet, A., Thouin, J., Fardoux, J., Chaintreuil, C., Gully, D., Arrighi, J.F., Furuta, N., Miwa, H., et al. Rhizobium-legume symbiosis in the absence of Nod factors: two possible scenarios with or without the T3SS. ISME J 10 (2016), 64–74 Evidence of two distinct Nod-alternative strategies, the T3SS and non-Nod/non T3SS.
72. Giraud, E., Moulin, L., Vallenet, D., Barbe, V., Cytryn, E., Avarre, J.C., Jaubert, M., Simon, D., Cartieaux, F., Prin, Y., et al. Legumes symbioses: absence of Nod genes in photosynthetic bradyrhizobia. Science 316 (2007), 1307–1312 First evidence of an alternative strategy to the Nod strategy.
73. Bonaldi, K., Gourion, B., Fardoux, J., Hannibal, L., Cartieaux, F., Boursot, M., Vallenet, D., Chaintreuil, C., Prin, Y., Nouwen, N., et al. Large-scale transposon mutagenesis of photosynthetic Bradyrhizobium sp. strain ORS278 reveals new genetic loci putatively important for nod-independent symbiosis with Aeschynomene indica. Mol Plant Microbe Interact 23 (2010), 760–770.
74. Chaintreuil, C., Rivallan, R., Bertioli, D.J., Klopp, C., Gouzy, J., Courtois, B., Leleux, P., Martin, G., Rami, J.F., Gully, D., et al. A gene-based map of the Nod factor-independent Aeschynomene evenia sheds new light on the evolution of nodulation and legume genomes. DNA Res 23 (2016), 365–376.
75. Fabre, S., Gully, D., Poitout, A., Patrel, D., Arrighi, J.F., Giraud, E., Czernic, P., Cartieaux, F., Nod factor-independent nodulation in Aeschynomene evenia required the common plant-microbe symbiotic toolkit. Plant Physiol 169 (2015), 2654–2664.
76. Sugawara, M., Epstein, B., Badgley, B.D., Unno, T., Xu, L., Reese, J., Gyaneshwar, P., Denny, R., Mudge, J., Bharti, A.K., et al. Comparative genomics of the core and accessory genomes of 48 Sinorhizobium strains comprising five genospecies. Genome Biol, 14, 2013.
77. Mazurier, S., Lemunier, M., Hartmann, A., Siblot, S., Lemanceau, P., Conservation of type III secretion system genes in Bradyrhizobium isolated from soybean. FEMS Microbiol Lett 259 (2006), 317–325.
78. Deakin, W.J., Broughton, W.J., Symbiotic use of pathogenic strategies: rhizobial protein secretion systems. Nat Rev Microbiol 7 (2009), 312–320.
79. Chaintreuil, C., Arrighi, J.F., Giraud, E., Miche, L., Moulin, L., Dreyfus, B., Munive-Hernandez, J.A., Villegas-Hernandez, M.D., Bena, G., Evolution of symbiosis in the legume genus Aeschynomene. New Phytol 200 (2013), 1247–1259.
80. Okubo, T., Fukushima, S., Minamisawa, K., Evolution of Bradyrhizobium-Aeschynomene mutualism: living testimony of the ancient world or highly evolved state?. Plant Cell Physiol 53 (2012), 2000–2007.
81. Masson-Boivin, C., Giraud, E., Perret, X., Batut, J., Establishing nitrogen-fixing symbiosis with legumes: how many rhizobium recipes?. Trends Microbiol 17 (2009), 458–466.
82. Marchetti, M., Jauneau, A., Capela, D., Remigi, P., Gris, C., Batut, J., Masson-Boivin, C., Shaping bacterial symbiosis with legumes by experimental evolution. Mol Plant Microbe Interact 27 (2014), 956–964.
83. Czernic, P., Gully, D., Cartieaux, F., Moulin, L., Guefrachi, I., Patrel, D., Pierre, O., Fardoux, J., Chaintreuil, C., Nguyen, P., et al. Convergent evolution of endosymbiont differentiation in dalbergioid and inverted repeat-lacking clade legumes mediated by nodule-specific cysteine-rich peptides. Plant Physiol 169 (2015), 1254–1265.
84. Van de Velde, W., Zehirov, G., Szatmari, A., Debreczeny, M., Ishihara, H., Kevei, Z., Farkas, A., Mikulass, K., Nagy, A., Tiricz, H., et al. Plant peptides govern terminal differentiation of bacteria in symbiosis. Science 327 (2010), 1122–1126.
85. Wang, Q., Yang, S.M., Liu, J.G., Terecskei, K., Abraham, E., Gombar, A., Domonkos, A., Szucs, A., Kormoczi, P., Wang, T., et al. Host-secreted antimicrobial peptide enforces symbiotic selectivity in Medicago truncatula. Proc Natl Acad Sci U S A 114 (2017), 6854–6859 Role of some NCR peptides in selecting specific bacterial genotypes.
86. Yang, S.M., Wang, Q., Fedorova, E., Liu, J.G., Qin, Q.L., Zheng, Q.L., Price, P.A., Pan, H.R., Wang, D., Griffitts, J.S., et al. Microsymbiont discrimination mediated by a host-secreted peptide in Medicago truncatula. Proc Natl Acad Sci U S A 114 (2017), 6848–6853 Role of some NCR peptides in selecting specific bacterial genotypes.
87. Marlow, V.L., Haag, A.F., Kobayashi, H., Fletcher, V., Scocchi, M., Walker, G.C., Ferguson, G.P., Essential role for the BacA protein in the uptake of a truncated eukaryotic peptide in Sinorhizobium meliloti. J Bacteriol 191 (2009), 1519–1527.