An interdisciplinary look at legumes and their bacterial symbionts: some thoughts from Big Sky


16th International Congress on Nitrogen Fixation (ICNF), Big Sky, MT, USA, June 2009

The biennial Congress on Nitrogen Fixation aims to cover the whole range of biological N2 fixation research, from chemistry of the nitrogenase enzyme to the functioning of whole organisms and symbiotic systems. This year’s meeting brought together over 200 delegates from around the world. Over the last 10 yr the number of attendees has fallen, possibly owing to a growing number of specialist meetings, as well as regional conferences, that are more affordable for many people, especially students; however the smaller group at the 16th International Congress on Nitrogen Fixation (ICNF) allowed for more cross-disciplinary discussion and exchange of ideas than has often been the case. This report highlights just some of the talks and posters on legumes and their nitrogen-fixing symbionts. The latter include contributions on the important topics of model legumes and inoculant technology.

‘…after more than a century of research, we still have little idea of how rhizobia grow and metabolise in soil in the absence of their hosts.’

The bacterial partners

Many new nodulating bacteria have been described over the last 10 yr, most notably from the β as well as the α subgroups of the Proteobacteria (Sprent, 2009). Before this meeting, nine genomes of the classical α-rhizobia had been sequenced, and at the meeting two β-rhizobia, Cupriavidus taiwanensis (C. Boivin-Masson, INRA-CNRA, France) and Burkholderia tuberum (A. Hirsch, UCLA, CA, USA) were added. In addition, a new genus of nodulating α-rhizobia, yet to be named, in the family Methylobacterieaceae and nodulating Lotononis angolensis (and possibly also species from unrelated genera, such as Lupinus, Phaseolus and Indigofera) was described by J. Ardley, Murdoch University, Australia. The factors controlling use of different symbionts by legume hosts are complex. J. Ardley, M. Gerding, Murdoch University, Australia and F. Phallane, Plant Protection Research Institute, Pretoria, South Africa explored the host genotype, highlighting how some interactions are highly specific, while other are not. Another factor discussed was soil conditions and how low pH (J. Howieson, Murdoch University, Australia; P. Gyaneshwar, University of Wisconsin, USA; Garau et al., 2009) and low soil fertility (Garau et al., 2009), including N (Elliott et al., 2009) tend to favour nodulation by β-rhizobia. Despite our understanding of these factors, after more than a century of research, we still have little idea of how rhizobia grow and metabolise in soil in the absence of their hosts. It is now clear that β-rhizobia can nodulate a wide variety of legumes, from all three subfamilies (Sprent, 2009) and with both root hair (Mimosa spp. Cyclopia spp.; Elliott et al., 2007a,b) and nonhair infections (almost certainly true of Lotononis, inferred from nodule structure, which is similar to that of lupin; J. Ardley; Fig. 1). Comparison of the newly sequenced β-rhizobia with other sequences from both nodulating and related nonnodulating bacteria will help greatly in understanding the processes of infection and the constraints on nodulation in some harsh environments. The methods by which the host plant keeps its symbionts under control featured in a number of presentations. W. Broughton (LBMPS, University of Geneva, Switzerland) discussed how symbionts have adapted pathogenic strategies in terms of rhizobial secretion systems and P. Mergaert (CNRS, Gif-sur-Yvette, France) suggested that nodules in which the nitrogen-fixing bacteroids are terminally differentiated (i.e. have lost the ability to reproduce, as in peas, clovers and relatives) allow the plant to have greater control than in those where the bacteroids are not so differentiated (for example in soybeans, Vigna spp.). R. Oona & R. Denison (University of Minnesota, USA) put another slant on this by suggesting that the terminally differentiated bacteroids, which are also enlarged, fix more nitrogen per unit of bacterial mass than those that are not enlarged. All of this begs the question as to why legume nodules with bacteroids not terminally differentiated are so successful. Another way of separating symbiotic and pathogenic strategies was suggested by R. Guerts (University of Wageningen, The Netherlands) who noted that gene duplication about 60 Ma allowed legumes to separate these two processes.

Figure 1.

 Nodules on Lotononis bainesii, which form a collar around the subtending root. Photo courtesy of Ron Yates (Murdoch University, Australia).

Legumes from harsh environments: an important resource for the future

Harsh environments were discussed by J. Sprent (University of Dundee at SCRI, Scotland), who pointed out that nodulated legumes are now thought to have evolved in seasonally dry environments (Schrire et al., 2005) from where they spread (and are still spreading) into arid areas; therefore, it is not surprising that there are many legumes that are capable of growing and nodulating in conditions not normally regarded as favourable by agronomists. Some of these are now being targeted for use in the expanding areas of dry land, such as the wheat belts of Australia. J. Howieson (Murdoch University, Australia) explained how this is leading to a major change in the approach to use of nodulated legumes in Australian agriculture, from one based largely on annual species such as subterranean clover (Trifolium subterraneum) to one where the advantages of perennial growth (in particular woody rootstocks that can resprout after adverse conditions (Fig. 2), and can remain green in the dry season are exploited. Coincidentally, this search for new pasture species has highlighted the value of nodule morphology and/or endophyte composition as tools in legume taxonomy. Two examples were given in this meeting. First, the genus Lebeckia, in which F. Phallane (Plant Protection Research Institute, Pretoria, South Africa) found that trifoliate species nodulated with various α-rhizobia (Bradyrhizobium, Mesorhizobium, Sinorhizobium), but species with needle-like leaves nodulated with rhizobia related to Burkholderia. During the course of the 16th ICNF a paper by Boatwright et al. (2009) was published in which the old genus Calobota was restored for species with trifoliate leaves with Lebeckia being retained for those with needle-like leaves. Second, J. Ardley (Murdoch University, Australia) pointed out that Lotononis species from section Listia have collar nodules like those of lupin (Fig. 1), whereas other members of the genus have indeterminate nodules; they also nodulate with different bacteria. Legume nodule morphology is a generic character in all other cases known. The taxonomy of genus Lotononis is in urgent need of revision and is likely to be separated into two or more genera, one of which will be section Listia, thus removing this apparent anomaly (B.-E. van Wyk, pers. comm.).

Figure 2.

 Woody root on a species of Lebeckia ambigua. Photo courtesy of John Howieson (Murdoch University, Australia).

Enlarging the range of legumes to be exploited

The previous meeting in Cape Town, in 2007 aimed to highlight the possibilities for nitrogen fixation to alleviate poverty. The 16th ICNF had many contributions that described a series of legumes with great potential for improvement for food, forage and other uses. Forage legumes being developed for Australia have already been mentioned, and J. Howieson (Murdoch University, Australia) gave a clear account of the many aspects of taking a native plant and developing it into an agriculturally usable (and marketable) product (see also Howieson et al., 2008). F. Dakora (Tshwane University of Technology, Pretoria, S. Africa) and his colleagues in several contributions, championed the cowpea (Vigna unguiculata) as a multipurpose crop, pointing out the need for better plant and rhizobial germplasm and the interesting possibility that the crop could be developed as a source of micronutrients for humans. Traditional crops such as gum arabic, an important food additive, obtained from largely wild provenances of Acacia senegal are now being developed, using molecular markers, to give larger and more reliable yields and hence better incomes for poor farmers in very dry areas of subSaharan Africa (D. Odee, Kenyan Forestry Research Institute). Legume teas (rooibos, Aspalathus linearis and honeybush, Cyclopia spp.), being caffeine-free are widely sought. F. Dakora also noted they also grow on poor soils, with very low nutrient content and are being further developed in S. Africa. One of the nutrients that frequently limits nitrogen fixation in legumes is phosphorus (P). J.-J. Drevon (INRA-Supagro, France). showed that in dry beans (Phaseolus vulgaris), a staple food in large parts of South America and Africa, there are possibilities for using germplasm with improved phosphorus P use efficiency and P. Maistry, University of Cape Town, South Africa, showed that legume species differ greatly in this respect, some being very efficient at using P.

Wider benefits of legumes

In addition to the direct benefits from nitrogen fixation, legumes have long been known to have a longer-term effect on soils, via such means as reduction of pathogen load. Highly relevant to modern worries about greenhouse gases, B. Boddey (EMBRAPA Agrobiology, Brazil) showed that legumes can have a very positive effect, not only by reducing emission of nitrous oxide from excess fertilizer nitrogen, but also by sequestering carbon in soils. The overall benefit of nitrogen fixation by legumes was quantified by M. Peoples (CSIRO, Canberra, Australia), who, as in many other talks, stressed the importance of good root systems. M. Peoples also participated in a discussion workshop on the applied aspects of nitrogen fixation and in particular how to measure it at the field scale. This workshop was greatly appreciated by younger researchers from developing countries.

For research on nitrogen fixation by legumes to fulfil the important aims of feeding people in developing countries and using land sustainably, a spectrum of expertise from the molecular to field studies is necessary. Such a cross-disciplinary approach emerged from this conference and it is anticipated that the next meeting, to be held in Western Australia in 2011 will capitalize on this.


The author would like to thank Stephen Boatwright (Botanic Gardens, Kirstenbosch, Cape Town, South Africa) for sending her an advance copy of his paper on Lebeckia taxonomy, Ron Yates and John Howieson (Murdoch University, Australia) for the figures and the Royal Society of Edinburgh for financial assistance. The author would also like to apologize to all those who gave fascinating accounts of their work at the meeting, but for which there is no space here to discuss.