Rhizobitoxine Production by Bradyrhizobium elkanii Enhances Nodulation and Competitiveness on Macroptilium atropurpureum

Abstract

Application of 1-aminoocyclopropane-1-carboxylic acid, an ethylene precursor, decreased nodulation of Macroptilium atropurpureum by Bradyrhizobium elkanii. B. elkanii produces rhizobitoxine, an ethylene synthesis inhibitor. Elimination of rhizobitoxine production in B. elkanii increased ethylene evolution and decreased nodulation and competitiveness on M. atropurpureum. These results suggest that rhizobitoxine enhances nodulation and competitiveness of B. elkanii on M. atropurpureum.The symbiotic interactions between a legume and (brady) rhizobia result in a unique, nitrogen-fixing plant organ, the nodule. Recent studies have shown that the phytohormone ethylene inhibits nodule formation in some legumes (8,9,16,24,25). Application of 1-aminoocyclopropane-1-carboxylic acid (ACC), a precursor of ethylene, inhibits nodulation in Medicago truncatula (24).Rhizobitoxine [2-amino-4-(2-amino-3-hydropropoxy)-transbut-3-enoic acid] is an ethylene synthesis inhibitor that is produced by the legume symbiont Bradyrhizobium elkanii (15, 17-19, 22, 39). It is thought that production of this compound enhances nodulation of the host legume because of its inhibitory effect on ethylene synthesis. However, some reports have shown that there is not a significant difference in nodule number between plants inoculated with B. elkanii USDA61 and plants inoculated with rhizobitoxine-deficient mutants during nodulation of Glycine max, Glycine soja, Vigna unguiculata, and Macroptilium atropurpureum (26,39). Recently, Duodu et al. observed a significant difference in nodule number between plants inoculated with isogenic variants of USDA61 during nodulation of Vigna radiata (7). Although these findings do not seem to be consistent with the hypothesis that rhizobitoxine has a positive effect on nodulation, the inconsistency can be explained by differences in the ethylene sensitivity of nodulation among leguminous species; nodulation of G. max is generally not sensitive to ethylene (10, 31, 38), while nodulation of V. radiata is sensitive (7). The inconsistency could also result from differences in the abilities of the strains used in the experiments to produce rhizobitoxine; strain USDA61 is a weak producer of rhizobitoxine (39).In addition to G. max, the leguminous plant M. atropurpureum is a nodulating host for B. elkanii and Bradyrhizobium japonicum (12,15). Although the effect of ethylene on nodulation has been studied in many leguminous host plants so far, the effect of ethylene in M. atropurpureum is not known. B. elkanii was found to be more competitive than B. japonicum for nodulation of M. atropurpureum in a multistrain environment when a field soil was inoculated with a mixture of several strains isolated from the field soil (21). In general, B. elkanii accumulates rhizobitoxine in cultures and in nodules, while B. japonicum does not (5,15,18,19). These results led us to investigate the role of rhizobitoxine production on the nodulation and competitiveness of B. elkanii on M. atropurpureum by using a B. elkanii strain that produces...

Rhizobitoxine producers gain more poly-3-hydroxybutyrate in symbiosis than do competing rhizobia, but reduce plant growth

“…However, in contrast to other work (Yuhashi et al, 2000;Okazaki et al, 2003), we found that Rtx( þ ) rhizobia were no more competitive than were Rtx(À), founding only 42% of nodules (t ¼ 0.84, P ¼ 0.42, d.f. ¼ 10, two-sided t-test) during co-inoculation.…”

“…Some rhizobia make the ethylene inhibitor, rhizobitoxine (Rtx), which increases nodules per plant and competitiveness for nodule formation (Duodu et al, 1999;Yuhashi et al, 2000;Okazaki et al, 2003). Increasing the number of nodules formed by a plant will not increase an Rtx strain's relative fitness (proportional representation in the population) unless Rtx rhizobia are disproportionately represented in the additional nodules, but other fitness benefits are possible.…”

Population genomics of Sinorhizobium medicae based on low-coverage sequencing of sympatric isolates

“…PCR results indicate that this gene cluster is present in all 39 sampled S. medicae strains. In B. elkanii, rhizobitoxine has been shown to enhance nodulation capabilities of the bacteria by inhibiting ethylene synthesis in plant tissues (Yuhashi et al, 2000). If the rtx homologues of S. medicae encode the same function, the strains we collected might have an increased capability to induce nodules on the roots of their host when compared with S. medicae WSM 419.…”

Expression Islands Clustered on the Symbiosis Island of the Mesorhizobium loti Genome

“…4C) and competitively (Fig. 4D), indicating that the induced ACC deaminase gene enhances nodulation and competitiveness in a manner similar to rhizobitoxine, a bradyrhizobial inhibitor of ACC synthase (26,44,45).…”