Rhizobial strain‐dependent restriction of nitrogen fixation in a legume‐ Rhizobium symbiosis
2017; Wiley; Volume: 93; Issue: 1 Linguagem: Inglês
10.1111/tpj.13791
ISSN1365-313X
Autores Tópico(s)Nematode management and characterization studies
ResumoThe Plant JournalVolume 93, Issue 1 p. 3-4 Research HighlightFree Access Rhizobial strain-dependent restriction of nitrogen fixation in a legume-Rhizobium symbiosis Sheila McCormick, Sheila McCormick Research Highlights EditorSearch for more papers by this author Sheila McCormick, Sheila McCormick Research Highlights EditorSearch for more papers by this author First published: 15 December 2017 https://doi.org/10.1111/tpj.13791Citations: 7 Linked article: This is a Research Highlight about Hiroko Yamaya-Ito et al. To view this article visit https://doi.org/10.1111/tpj.13759. AboutSectionsPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinkedInRedditWechat In the past 20 years, there has been a lot of progress in understanding the molecular genetics of plant-microbe symbiosis. In addition to studying the economically important soybean, two model plants and their corresponding bacterial symbionts, i.e. Lotus japonicus with Mesorhizobium loti, and Medicago truncatula with Sinorhizobium meliloti, are widely used. In both models, the early stages are similar: in the vicinity of plant roots, the rhizobacteria produce signal compounds, so-called Nod factors, and when the roots perceive these signals, they form an infection thread and induce cell divisions to form nodule primordia: the bacteria then enter the primordia, differentiate into bacteroids and establish nitrogen fixation. Most research efforts have focused on the early stages of infection and the subsequent signaling pathways (reviewed in Oldroyd, 2013). Why do researchers bother to use two model organisms to study symbiosis? In Medicago, the nodules are indeterminate, with a persistent meristem, and an important component of final differentiation into bacteroids is mediated by so-called NCR (nodule-specific cysteine-rich) peptides (reviewed in Maroti et al., 2015). In Lotus (and in soybean), the nodules are determinate, in that meristematic activity is limited to the early stages. In determinate nodules, bacteroids can revert to a free-living state, but the mechanisms and host genes involved in bacteroid differentiation are largely unknown – as Lotus and soybean do not have NCR genes in their genomes. Rhizobia don't fix nitrogen when they are free-living in the soil, but only in nodules – meaning that nitrogen fixation must be strictly regulated by the host legumes. The so-called Fix- mutants form apparently normal nodules that contain intracellular bacteria, but nonetheless have defects in nitrogen fixation. Identifying the underlying genes that are disrupted in Fix- mutants thus provides a way to understand the mechanisms that control nitrogen fixation activity. Numerous Fix- genes have been characterized, in both Lotus and Medicago, encoding metabolic enzymes (e.g. Hakoyama et al., 2009), transporters (e.g. Hakoyama et al., 2012), and, in Medicago, the NCR peptides (Maroti et al., 2015). How did the findings in the highlighted paper (Yamaya-Ito et al., 2018) come about? For over 10 years several Japanese groups had been isolating and characterizing Fix- mutants of L. japonicus (e.g. Kumagai et al., 2007; Hakoyama et al., 2009). For about 20 years, Yosuke Umehara has been a senior researcher at the National Institute of Agrobiological Sciences, in Tsukuba. Hiroko Yamaya-Ito, the first author, was a postdoc there when this work was underway, although she also did her Ph.D. studies in this field, at Tokyo University of Agriculture and Technology, where she studied the mechanisms controlling soybean nodulation. APN1 (ASPARTIC PEPTIDASE NODULE-INDUCED 1), the gene underlying the Fix- phenotype characterized in the highlighted paper, is particularly interesting, because its Fix- phenotype is rhizobial-strain specific. As shown in Figure 1, the apn1 mutant does not form effective nodules with the TONO strain of M. loti, but is able to form effective nodules with the MAFF303099 strain of M. loti. The TONO and MAFF303099 strains have sequenced genomes and are frequently used in the Lotus research community in Japan. It was only by chance that the strain specificity for the Fix- phenotype of apn1 was discovered. In their lab, they typically used TONO for inoculation, but to confirm complementation of apn1 by the APN1 gene, they used a DsRED-labelled MAFF303099, since it was convenient to have the DsRED marker. To their surprise they found that MAFF303099 formed effective nodules on apn1. After a broader survey, they found that this ability of MAFF303099 was not unique – four other strains also formed effective nodules on apn1, while three other strains. like TONO, formed ineffective nodules. Figure 1Open in figure viewerPowerPoint Differential nodulation responses. In each photo, WT is on the left and apn1 is on the right. Left panels, inoculation with Mesorhizobium loti strain TONO, right panels, inoculation with M. loti strain MAFF303099 Image credit: Hiroko Yamaya-Ito. The discovery of rhizobial strain dependence for nitrogen fixation is important, because it highlights that strain-dependent fine tuning of the host cell-symbiont interactions are essential even at late stages of nodule development, i.e. at the nitrogen fixation level. Yamaya-Ito et al. (2018) speculate that APN1 might be required to proteolytically degrade (still unidentified) effectors from strains such as TONO. In this scenario, these effectors would not be degraded in apn1 plants, and their continued presence would then stimulate plant immunity responses, and so symbiosis could not be sustained. To address this hypothesis, they are using a tagged mutant library prepared from TONO and screening for mutants that can form effective nodules on apn1 plants. They further speculate that strains such as MAFF303099 might not encode such factors at all, or might express them less than in TONO. Genome alignments and expression analyses of the various M. loti strains that can or cannot form effective nodules on apn1 will further help in testing their hypotheses. Lastly, unlike the NCR genes, there are APN1 homologs in Medicago and other legumes – so understanding the role of APN1 might generally help in understanding how nitrogen-fixing symbioses are established. References Hakoyama, T., Niimi, K., Watanabe, H. et al. (2009) Host plant genome overcomes the lack of a bacterial gene for symbiotic nitrogen fixation. Nature, 462, 514– 517. Hakoyama, T., Niimi, K., Yamamoto, T. et al. (2012) The integral membrane protein SEN1 is required for symbiotic nitrogen fixation in Lotus japonicus nodules. Plant Cell Physiol. 53, 225– 236. Kumagai, H., Hakoyama, T., Umehara, Y., Sato, S., Kaneko, T., Tabata, S. and Kouchi, H. (2007) A novel ankyrin-repeat membrane protein, IGN1, is required for persistence of nitrogen-fixing symbiosis in root nodules of Lotus japonicus. Plant Physiol. 143, 1293– 1305. Maroti, G., Downie, J.A. and Kondorosi, E. (2015) Plant cysteine-rich peptides that inhibit pathogen growth and control rhizobial differentiation in legume nodules. Curr. Opin. Plant Biol. 26, 57– 63. Oldroyd, G.E.D. (2013) Speak, friend, and enter: signalling systems that promote beneficial symbiotic associations in plants. Nat. Rev. Microbiol. 11, 252– 263. Yamaya-Ito, H., Shimoda, Y., Hakoyama, T. et al. (2018) Loss-of function of ASPARTIC PEPTIDASE NODULE-INDUCED 1 (APN1) in Lotus japonicus restricts efficient nitrogen-fixing symbiosis with specific Mesorhizobium loti strains. Plant J. 93, 5– 16. Citing Literature Volume93, Issue1January 2018Pages 3-4 FiguresReferencesRelatedInformation
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