australian journal of plant physiology

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AUSTRALIAN JOURNAL OF PLANT PHYSIOLOGY Volume 27, 2000 © CSIRO 2000

An international journal of plant function

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Aust. J. Plant Physiol., 2000, 27, 885–892

Review:

Commonality of root nodulation signals and nitrogen assimilation in tropical grain legumes belonging to the tribe Phaseoleae Felix D. Dakora Botany Department, University of Cape Town, Private Bag, Rondebosch 7701, South Africa. Email: [email protected] Abstract. The tribe Phaseoleae (family Leguminosae) is home to many of the annual food legumes cultivated in the tropics. Cowpea (Vigna unguiculata (L.) Walp.), Bambara groundnut (Vigna subterranea (L.) Verdc.), Kersting’s bean (Macrotyloma geocarpum L.), mung bean (Vigna radiata (L.) Wilczek) and common bean (Phaseolus vulgaris L.), all belonging to subtribe Phaseolinae, and together with soybean (Glycine max (L.) Merr., subtribe Glycininae) and pigeon pea (Cajanus cajan L., subtribe Cajaninae), are important members of the tribe Phaseoleae. These legumes are unique in their use of identical root chemical molecules to induce the expression of nodulation genes in their respective homologous microsymbionts during nodule formation. Of those studied so far, common bean, soybean, Bambara groundnut, Kersting’s bean and cowpea all use the isoflavones daidzein, genistein and coumestrol as root exudate signals to induce the expression of nod genes in their rhizobial partners. Additionally, members of the Phaseoleae tribe are easily recognised on the basis of their tropical biogeographic origin, broad host nodulation habit, route of Rhizobium entry into roots, chemotaxonomy and use of a common isoflavone biosynthetic pathway, determinate nodulation phenotype and internal nodule anatomy, xylem composition and transportable solutes of fixed N, site of NO3– reduction and metabolic response of N2-fed plants to NO3– supply. These shared traits and their potential application for agriculture are discussed in this review. Keywords: Phaseoleae, isoflavone nodulation signals, N assimilation, ureides, NO3– reduction. Introduction Historically, legumes have been divided into tropical and temperate species. However, their choice of bacterial symbionts for inciting nodule formation has also been used as a basis for further functional separation of members of the Leguminosae. Thus, tropical legumes were believed to nodulate strictly with slow-growing bradyrhizobia, and temperate legumes with only fast-growing rhizobia (Fred et al. 1932). However, various studies (Van Rensburg and Strijdom 1971, 1972a, b; Dakora and Vincent 1984) have found fastgrowing rhizobia-like bacteria to co-exist with the true nodule-forming bradyrhizobia in cowpea and soybean nodules. Recently, a number of studies have, in fact, established the ability of many tropical grain legumes in the tribe Phaseoleae (for examples, see Fig. 1) to nodulate with both fast- and slow-growing root nodule bacteria (Keyser et al. 1982a; Mpepereki et al. 1996; Martins et al. 1997; Neves and Rumjanek 1997). Soybean is, for example, a compatible host to strains of the fast-growing Rhizobium fredii (Keyser et al. 1982a; Scholla and Elkan 1984) and the slow-growing Bradyrhizobium japonicum (Jordan 1982), as well as Bradyrhizobium elkani (Kuykendall et al. 1992). Also, mix-

tures of fast- and slow-growing root nodule bacteria have been isolated from cowpea plants in Brazil (Martins et al. 1997; Neves and Rumjanek 1997) and Zimbabwe (Mpepereki et al. 1996). More recently, both fast- and slow-growing colony types have been obtained from root nodules of Bambara groundnut and shown to be the authentic nodule-forming organisms (F. D. Dakora and R. Nyemba, unpublished results). So, although nodulation of temperate legumes (e.g. Vicia, Pisum, Melilotus, Trifolium, Medicago; see Gualtieri and Bisseling 2000, fig. 3IV, tribes 2 and 3) may still be strictly caused by fast-growing rhizobia, many more tropical grain legumes belonging to the tribe Phaseoleae are now reported to nodulate with both fast- and slow-growing bacterial symbionts, a direct contradiction to the old dogma. This exhibition of broad host range by tropical, but not temperate, species has implications for host–strain interaction, especially in relation to the signals regulating nodule development. The initial steps leading to root hair infection and nodule formation have been described by Vincent (1980) and reviewed by several other researchers (Sprent 1989; Brewin

Abbreviations used: LCO, lipochito-oligosaccharide; NIN protein, a nodule inception gene product. © CSIRO 2000

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1991; Franssen et al. 1992; Hirsch 1992). Although rhizobial entry into legume roots can occur through root hair infection, cracks/wounds at points of lateral root emergence, or through undamaged epidermal tissue (Sprent 1989; Hirsch 1992), cases exist where a single rhizobial strain can infect different leguminous hosts via one or more entry points (Sen and Weaver 1984). Within the tribe Phaseoleae, nodulation is mainly via root hair invasion, followed by infection thread formation and mitotic cell division in the hypodermis. These early events are co-ordinated in the plant by the activity of a NIN protein (a nodule inception gene product), whose absence arrests the formation of infection thread and nodule primordia (Schauser et al. 1999). This finding confirms earlier suggestions that the infection process and nodule morphogenesis are controlled by the host plant (Sprent 1989; Hadri et al. 1998). Although root hair infection and nodule organogenesis follow a strict pattern of molecular control in all legumes (Hirsch 1992; Hadri et al. 1998), the morphology of the organ so formed (i.e. the nodule) and its internal differentiation can differ significantly. Compared to temperate legumes, tropical species form fairly round determinate or desmodioid nodules (see Fig. 2), which lack vascular transfer cells, persistent meristem, and vacuoles in infected cells (Corby et al. 1983), but have calcium oxalate crystals in the outer cortex (Sutherland and Sprent 1984) and diploid cells as opposed to tetraploids in indeterminate nodules (Corby et al. 1983). This distinct nodulation phenotype and internal anatomy of tropical grain legumes offer an opportunity for their use as a taxonomic tool in studies of the Leguminosae.

F. D. Dakora

nodulated by other species of Rhizobium and Bradyrhizobium (Mpepereki et al. 1996; Martins et al. 1997; Neves and Rumjanek 1997). Various studies (Doku 1969; Keyser et al. 1982b; Sadowsky et al. 1988; Dakora and Muofhe 1996; Mpepereki et al. 1996) have, however, shown that B. japonicum and R. fredii strains, which normally nodulate soybean, can also effectively nodulate cowpea, common bean, Bambara groundnut, siratro and Kersting’s bean. Similarly, root nodule bacteria from Bambara groundnut can successfully nodulate soybean, lima bean and cowpea, and vice versa (Doku 1969; Neves and Rumjanek 1997). A study by Mpepereki et al. (1996) has, in fact, shown that both slow- and fast-growing isolates from cowpea nodules can infect and effectively nodulate a wide range of tropical and temperate leguminous hosts. Rhizobium sp. NGR234 and Rhizobium fredii USDA257, which have been isolated from the tribe Phaseoleae (Lablab purpureus and Glycine soja, respectively), exhibit the highest level of promiscuity ever recorded for any rhizobial strain. Rhizobium NGR234 nodulates 112 genera of legumes, and the closely related R. fredii USDA257 nodulates about 77 of those genera (Broughton and Perret 1999; Pueppke and

Nodulation signals from root exudates of the Phaseoleae Nodulating legumes have been divided into cross-inoculation groups on the basis of the bacteria that infect their roots and form effective symbioses (Fred et al. 1932). So, even the members of the tribe Phaseoleae (shown in Fig. 1 and Table 1) belong to different cross-nodulation groups. Soybean, for example, is nodulated by strains of B. japonicum, B. elkani, and R. fredii, while cowpea is

Fig. 1. Selected members of the tribe Phaseoleae (L to R): cowpea, Bambara groundnut, soybean, common bean.

Fig. 2. Nodulated cowpea (A) and determinate nodules on cowpea (B).

Common nodulation signals and N assimilation in Phaseoleae

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Table 1.

Mode of Rhizobium infection, nodulation phenotype and export products of N2 fixation in some tropical grain legumes belonging to the tribe Phaseoleae Data obtained from Pate et al. (1980), Sprent (1981, 1989), Corby et al. (1983) and Dakora et al. (1992)

Tribe

Species

Mode of infection

Nodulation phenotype

N solute formed

Cowpea Soybean Bambara groundnut Kersting’s bean Lima bean Pigeon pea Mung bean Winged bean

Root hair Root hair Root hair Root hair Root hair Root hair Root hair Root hair

Determinate Determinate Determinate Determinate Determinate Determinate Determinate Determinate

Ureides Ureides Ureides Ureides Ureides Ureides Ureides Ureides

Aeschynomeneae

Groundnut

Crack

Determinate

Asparagine

Cicereae

Chickpea

?

Determinate

Asparagine

Phaseoleae

Broughton 1999). Some members of the Phaseoleae (e.g. common bean and siratro) are so promiscuous that they even nodulate with Sinorhizobium meliloti, the known symbiont of alfalfa (Bromfield and Barran 1990). Conversely, R. etli, a microsymbiont of common bean, also effectively nodulates alfalfa (van Berkum and Eardly 1998). So, as hosts, members of the Phaseoleae are not only highly promiscuous, but they also occur in tropical environments where there is widespread existence of broad host range rhizobia. The ability of 77 genera of legumes (which include members of the Phaseoleae) to simultaneously nodulate with both Rhizobium NGR234 and R. fredii USDA257 suggests that the plants involved probably produce similar signal molecules for communicating with these microsymbionts, and vice versa. Nodulation in symbiotic legumes is based on a two-way molecular interaction between plant and bacterial partner. Analysis of the signals involved in legume nodulation have identified flavonoids, aldonic acids and betaines as the molecules used by legumes to communicate with their rhizobial partners (Phillips 1999). These compounds induce the expression of nodulation (nod/nol/noe) genes in symbiotic members of the Rhizobiaceae. Consequently, different signal molecules are released by different legumes within the crossnodulation group. While alfalfa uses four root-secreted compounds [namely, 4′,4-dihydroxy-2′-methoxychalcone, 4′,7-dihydroxyflavanone, 4′,7-dihydroxyflavone and formononetin-7-O-(6″-O-malonylglucoside)] to induce nod genes in Sinorhizobium meliloti (Phillips et al. 1994), soybean uses three isoflavones (daidzein, genistein and coumestrol) in its root exudates (Fig. 3) to induce the transcription of nod genes in B. japonicum (Kosslak et al. 1987). This difference in the profile of nod gene inducers produced by legumes is a major determinant of host–strain specificity (Dakora 1994).

Fig. 3. Nodulation gene-inducing molecules isolated from root exudates of members of the Phaseoleae tribe.

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F. D. Dakora

Interestingly, all members of the Phaseoleae studied so far use the same chemical molecules in root exudates as signals to mutualistic symbionts in the rhizosphere (Table 2). Soybean roots release the isoflavones daidzein, genistein and coumestrol (Fig. 3), which serve as signals to B. japonicum (Kosslak et al. 1987). Bambara groundnut, Kersting’s bean and cowpea similarly employ the same isoflavones (daidzein, genistein and coumestrol) as signals to their microsymbionts during nodule formation (Table 2; Dakora and Le Roux 1995). Although known for its nodulation with R. leguminosarum bv. phaseoli, R. tropici (Martinez-Romero et al. 1991) and R. etli (van Berkum and Eardly 1998), the common bean can also nodulate with root nodule bacteria from Bambara groundnut, lima bean, and cowpea (Doku 1969). This is due not only to the great diversity of rhizobia that nodulate this species, but also to the fact that common bean uses daidzein, genistein and coumestrol as root exudate signals to its bacterial partner during symbiotic establishment (Hungria et al. 1991; Dakora et al. 1993). The commonality of root-secreted isoflavones by different genera in the tribe Phaseoleae (Table 2) closely parallels the situation with rhizobia, where some phylogenetically unrelated bacteria in the genera Rhizobium, Sinorhizobium and Mesorhizobium produce lipochito-oligosaccharide (LCO) molecules with a similar structure, which enables them to nodulate different leguminous hosts in the tribes Trifolieae, Vicieae and Galegeae (Gualtieri and Bisseling 2000). Apparently they produce similar LCOs with polyunsaturated fatty acids (Yang et al. 1998). Taken together, these reports indicate that many members of the Phaseoleae from the tropical environments of Africa, Table 2.

Asia and South America synthesise and release similar root compounds that serve as communication molecules during interaction with their symbiotic partners, and thus provide the biological basis for the observed cross-infectivity of species such as soybean, cowpea, Bambara groundnut, lima bean, and other tropical legumes by each other’s strains (Doku 1969; Keyser et al. 1982b; Sadowsky et al. 1988; Dakora and Muofhe 1996; Mpepereki et al. 1996; Martins et al. 1997; Neves and Rumjanek 1997). From the profile of root signals released, these legumes share a common isoflavone biosynthetic pathway, which also has been the basis for the chemotaxonomy of the Phaseoleae (Ingham 1990). Members of the Phaseoleae share a tribal distribution of both simple and complex isoflavonoid compounds (Ingham 1981), and produce similar isoflavone phytoalexins and phytoanticipins in response to pathogen attack (Ingham 1990; Dakora and Phillips 1996) as observed for plant nodulation signals during Rhizobium infection (Table 2). This reinforces the concept of a common isoflavone biosynthetic pathway that is activated during infection of this group of legumes by mutualists and pathogens. These shared events during symbiotic establishment and nodule formation in the tribe Phaseoleae may well suggest some similarity in nodule functioning. Nodule function and export products in the Phaseoleae Analysis of the organic products of N2 fixation in the xylem of symbiotic legumes has permitted the division of nodulated legumes into ureide (allantoin and allantoic acid) or amide (asparagine and glutamine) producers, depending on which solute is dominant in the xylem stream (Fig. 4). As shown in

Nodulation gene-inducing molecules from root exudates of some tropical grain legumes belonging to the tribe Phaseoleae

Legume species

nod-Gene inducer

Bacterial strain

Reference

Bambara groundnut

Daidzein Genistein Coumestrol

Rhizobium NGR234

Dakora & Muofhe 1996

Bambara groundnut


B. japonicum USDA123

Dakora & Muofhe 1996

Common bean


R.l. bv. phaseoli 4292

Dakora et al. 1993 Hungria et al. 1991 Dakora et al. 1993

Soybean


B. japonicum USDA123

Kosslak et al. 1987

Cowpea


Rhizobium NGR234

F. D. Dakora, unpublished results

Kersting’s bean


Rhizobium NGR234

F. D. Dakora, unpublished results


889

Table 1, members of the Phaseoleae export their fixed N as ureides (Pate et al. 1980; Sprent 1981; Atkins 1982; Corby et al. 1983; Dakora et al. 1992; Dakora 1998), except for groundnut and chickpea, which export their N in the form of asparagine, an amide (Fig. 4). Up to 98% of the xylemborne N entering the shoots of nodulated cowpea plants is in the form of a ureide, while in the amide-producing clovers, peas and medics, up to 81% of xylem N is asparagine (Atkins 1987). Therefore, members of the Phaseoleae share not only a common route of bacterial invasion, signal molecules for inducing transcription of nod genes, and a determinate nodulation phenotype, but also a common export product of fixed N, which together make them a biologically unique group of legumes. Although the absence of a metabolic intermediate in any biological system could suggest the absence of the relevant catalytic enzyme, this is not the case with ureide or amide biosynthesis. Major enzymes such as 5′-nucleotidase, purine nucleosidase, xanthine dehydrogenase, uricase and allantoinase, which are involved in ureide biosynthesis, are present and active in root nodules of both ureide- and amideproducing legumes such as soybean and pea (Reynolds et al. 1982; Christensen and Jochimsen 1983). Furthermore, these enzymes have also been detected in bacteroids and freeliving forms of both pea and soybean rhizobia (Christensen and Jochimsen 1983). The activity of asparagine synthetase, the major enzyme involved in amide biosynthesis, was similarly recorded in nodules of ureide-producing symbioses (Reynolds et al. 1982). So, even though amide-producing legumes consistently show lower activities for enzymes of ureide biosynthesis and vice versa, these observed activities nevertheless indicate that the genes controlling synthesis of these proteins are present and functionally expressed in both ureide and amide-producing symbioses. This clearly demonstrates that there is no enzymic basis for the differences in translocation of nitrogenous solutes of fixed N by ureide- or amideproducing legumes. However, as indicated by Pate (1973) Ureides NH2

Amides

CO

NH

NH2

NH2

CO CO

NH

CH

NH

CO

Allantoin (C:N=1)

NH2 CO

COOH

NH

CH

NH

Allantoic acid (C:N=1)

CH2 CH COOH Asparagine (C:N=2)

NH2

NH2

CO

CO

NH2

CH2 CH2 CH Glutamine (C:N=2.5)

COOH

Fig. 4. Transportable forms of fixed N in tropical legumes (ureides) and temperate legumes (amides).

and Corby et al. (1983), the marked expression of ureide biosynthesis in nodules of tropical legumes, but not in temperate species, may well relate to their environments of origin. Compared to amides, ureides are less soluble in water, and show decreasing solubility with decreasing temperature (Sprent 1981). As transport molecules, ureides are therefore better suited to legumes of the warmer tropics, where the relatively higher temperatures would promote both increased water uptake and greater solubility of these compounds for internal transport. This uptake of water along with solutes such as NO3– could be the basis for a common pattern of NO3– assimilation in these species. Nitrate reduction in symbiotic and NO3–-grown legumes in the tribe Phaseoleae Tropical grain legumes differ from other legumes in the proportion of free NO3– transported in the xylem stream. Nonnodulated lupin plants fed 5 mM NO3– transport 6–10% of NO3– in xylem (Pate et al. 1979) compared to 30% in cowpea supplied with similar NO3– concentrations (Pate et al. 1980). This marked difference in the xylem load of NO3– is related to the site of NO3– reduction. In temperate legumes, up to 90% of the NO3– reduction occurs in the root (Pate et al. 1979), as opposed to 90% or more in the shoot of tropical legumes belonging to the tribe Phaseoleae (Atkins et al. 1980). A survey of 11 legume species belonging to the tribes Phaseoleae, Vicieae, Genisteae and Trifolieae shows that a higher proportion of NO3– accumulation and reduction occurs in the shoots of the Phaseoleae compared to the other tribes (Wallace 1986). On average, members of the Phaseoleae showed more than 87% nitrate reductase activity in the shoot, while for temperate pea and vetch, nitrate reductase activity was 50% in the shoot (Wallace 1986). As an extreme example, the common bean, which belongs to the tribe Phaseoleae, reduces 99% of its NO3– in the shoot (Wallace 1986). This preferential NO3– reduction in the shoot (Atkins et al. 1980; Sprent 1980; Atkins 1982; Wallace 1986) is another major distinguishing feature of the Phaseoleae tribe. Also, the proportion of nitrate reductase activity in the shoot and root of the Phaseoleae stays relatively constant irrespective of NO3– concentration. This is in sharp contrast to temperate species, where shoot NO3– reduction increases with NO3– supply (Andrews et al. 1984; Wallace 1986). As indicated previously, the xylem exports of ureide-producing legumes differ significantly from those of amideforming symbioses. Amide-producers transport fixed N mainly as asparagine and glutamine in the xylem stream, so the xylem composition does not change appreciably with N source, be it N2, NO3–, NH4 or urea (Atkins et al. 1979). However, the xylem sap of N2-fed ureide-producers is characterised by a high concentration of ureides and glutamine (Pate et al. 1980); with NO3– supply, nitrogenase activity declines, and the NO3–/ureide and Asn/Gln ratios both rise with increasing NO3– concentration (Atkins et al. 1980; Pate

890

et al. 1980; Dakora et al. 1992). The xylem exports of ureideand amide-producing legumes are therefore very similar if dependent solely on combined N such as NO3–, but different if feeding symbiotically on atmospheric N2. Recent evidence shows that some landraces of tropical African legumes belonging to the Phaseoleae exhibit an unusually gradual decrease in nitrogenase activity with NO3– supply to the roots, resulting in only a minimal change to the composition of the xylem stream (Dakora 1998). So, on the basis of their tropical biogeographic origin, broad host nodulation habit, route of Rhizobium entry into roots, profile of root nodulation signals, shared isoflavone biosynthetic pathway and chemotaxonomy, nodule morphology and internal anatomy, xylem composition and transportable solutes of fixed N, site of NO3– reduction and metabolic response of N2-fed plants to NO3– supply, members of the tribe Phaseoleae are biologically unique, especially in their acquisition and assimilation of atmospheric N2 and mineral N. Genetic manipulation of these common traits could lead to increased yields of agronomic species. Future research The observed commonality of root nodulation signals (Fig. 3) in tropical grain legumes belonging to the Phaseoleae tribe (Table 1) offers an opportunity for developing simple, usable techniques and bioassays for field measurements of these signal molecules in order to enhance N2 fixation for increased grain yields of these legumes. The breeding approach used by Australian scientists (Beck 1964; Francis and Millington 1965a, b) to overcome the oestrogenic effects of isoflavones (genistein, coumestrol, biochanin A and formononetin), which were richly present in pasture legumes (clovers and alfalfa) grazed by sheep, could be used as a model for increasing the concentrations of nodgene-inducing isoflavones for higher N2 fixation in tropical grain legumes. Although undesired in those studies (Francis and Mullington 1965a, b), some breeding lines were found to have very high concentrations of daidzein and genistein. This would have been extremely beneficial to symbiotic establishment if the breeding program were based on the Phaseoleae, as high concentrations of root-secreted and soilavailable daidzein and genistein would enhance nodulation and N2 fixation (Zhang and Smith 1997). Such a breeding program could also improve legume yields in cooler environments, where tissue concentrations of nod-gene-inducing molecules can be low in some members of the Phaseoleae (Zhang and Smith 1996). The ability of Rhizobium NGR234 and R. fredii USDA257 to simultaneously nodulate 77 genera of legumes (including members of the Phaseoleae) also offers a chance to develop a ‘universal inoculant’ from one or both of these highly promiscuous rhizobia for use on food grain legumes after field testing for competitiveness. Alternatively, because

F. D. Dakora

nodulation promiscuity is a heritable trait in legumes (Kueneman et al. 1984), the host range of economic varieties of tropical legumes could be broadened through transfer of promiscuity genes to enhance N2 fixation and yields. Also, characterising the genes controlling NO3– tolerance of nodule function in those members of the Phaseoleae that exhibit this trait could increase our understanding of NO3– tolerance in symbiotic legumes and facilitate the development of more species with the ability to fix large amounts of atmospheric N2 in high-N soils. That way, soil N would be spared for non-leguminous crops. The preferential reduction of NO3– in shoots of the tribe Phaseoleae can also be genetically manipulated to increase plant yield. Shoot NO3– reduction is in close proximity to the photosynthetic machinery of the plant and thus saves transport cost of ATP for plant growth and productivity. Identifying the genes controlling shoot NO3– reduction and manipulation of gene products has potential for increasing yields of these agronomically important legumes. The common physiological traits outlined here for members of the tribe Phaseoleae are important tools that can be used by legume taxonomists for systematic analysis of the Leguminosae, and thus serve as a useful complement to the phylogenetic traits described by Doyle (1998) and Gualtieri and Bisseling (2000) for the evolution of nodulation by plants and their microsymbionts. Doyle (1998) has shown that the tribes Psoraleae, Desmodieae and Millettieae are related to the Phaseoleae. It would therefore be interesting to know whether members of these tribes also exhibit the same internal uniqueness in terms of the signals produced for interaction with their microsymbionts as well as determine their molecular relatedness to the Phaseoleae. Acknowledgments This study was supported with funds from the National Research Foundation (NRF) Pretoria, and the University Research Committee of the University of Cape Town. References Andrews M, Sutherland JM, Thomas RJ, Sprent JI (1984) Distribution of nitrate reductase activity in six legumes: the importance of the stem. New Phytologist 98, 301–310. Atkins CA (1982) Ureide metabolism and the significance of ureides in legumes. In ‘Advances in agricultural microbiology’ (Ed. NS Subba Rao) pp. 53–88. (Oxford and IBH Publications: New Delhi) Atkins CA (1987) Metabolism and translocation of fixed nitrogen in the nodulated legume. Plant and Soil 100, 157–169. Atkins CA, Pate JS, Layzell DB (1979) Assimilation and transport of N in non-nodulated (NO3–-grown) Lupinus albus L. Plant Physiology 64, 1078–1082. Atkins CA, Pate JS, Griffiths GJ, White ST (1980) Economy of carbon and nitrogen in nodulated and non-nodulated (NO3–-grown) cowpea (Vigna unguiculata (L.) Walp.). Plant Physiology 66, 978–983. Beck AB (1964) The oestrogenic isoflavones of subterranean clover. Australian Journal of Agricultural Research 15, 223–230. Brewin NJ (1991) Development of the legume root nodule. Annual Review of Cell Biology 7, 191–226.


Bromfield ESP, Barran LR (1990) Promiscuous nodulation of Phaseolus vulgaris, Macroptilium atropurpureum and Leucaena leucocephala by indigenous Rhizobium meliloti. Canadian Journal of Microbiology 36, 369–372. Broughton WJ, Perret X (1999) Genealogy of legume-Rhizobium symbioses. Current Opinion in Plant Biology 2, 305–311. Christensen TMIE, Jochimsen BU (1983) Enzymes of ureide synthesis in pea and soybean. Plant Physiology 72, 56–59. Corby HDL, RM Polhill, Sprent JI (1983) Taxonomy. In ‘Nitrogen fixation, Volume 3: Legumes’. (Ed. WJ Broughton) pp. 1–35. (Clarendon Press: Oxford) Dakora FD (1994) Nodulation gene induction and genetic control in the legume–Rhizobium symbiosis. South African Journal of Science 90, 596–599. Dakora FD (1998) Nodule function in symbiotic Bambara groundnut (Vigna subterranea L.) and Kersting’s bean (Macrotyloma geocarpum L.) is tolerant of nitrate in the root medium. Annals of Botany 82, 687–690. Dakora FD, Le Roux RJ (1995) Phosphorus nutrition alters root flavonoid content, N2 fixation and phosphorus partitioning in cowpea. In ‘Nitrogen fixation: fundamentals and applications’. (Eds IA Tikhonovich, NA Provorov, VI Romanov and WE Newton) p. 324. (Kluwer: Dordrecht) Dakora FD, Muofhe ML (1996) Molecular signals involved in nodulation of the African Bambara groundnut (Vigna subterranea L.). In ‘Proceedings of the international Bambara groundnut symposium, 23–25 July 1996’. pp. 171–179. (University of Nottingham: Nottingham, UK) Dakora FD, Phillips DA (1996) Diverse functions of isoflavonoids in legumes transcend anti-microbial definitions of phytoalexins. Physiological and Molecular Plant Pathology 49, 1–20. Dakora FD, Vincent JM (1984) Fast-growing bacteria from nodules of cowpea (Vigna unguiculata (L.) Walp.). Journal of Applied Bacteriology 56, 327–330. Dakora FD, Atkins CA, Pate JS (1992) Effect of NO3 on N2 fixation and nitrogenous solutes of xylem in two nodulated West African geocarpic legumes, Kersting’s bean (Macrotyloma geocarpum L.) and Bambara groundnut (Vigna unguiculata (L.) Walp.). Plant and Soil 140, 255–262. Dakora FD, Joseph CM, Phillips DA (1993) Common bean root exudates contain elevated levels of daidzein and coumestrol in response to Rhizobium inoculation. Molecular Plant–Microbe Interactions 6, 665–668. Doku EV (1969) Host specificity among five species in the cowpea cross-inoculation group. Plant and Soil 30, 126–128. Doyle JJ (1998) Phylogenetic perspectives on nodulation: evolving views of plants and symbiotic bacteria. Trends in Plant Science 3, 473–478. Francis CM, Millington AJ (1965a) Isoflavone mutations in subterranean clover. I. Their production, characteristics and inheritance. Australian Journal of Agricultural Research 16, 565–573. Francis CM, Millington AJ (1965b) Varietal variation in the isoflavone content of subterranean clover: its estimation by a microtechnique. Australian Journal of Agricultural Research 16, 557–564. Franssen HJ, Vijn I, Yang WC, Bisseling T (1992) Developmental aspects of the Rhizobium–legume symbiosis. Plant Molecular Biology 19, 89–107. Fred EB, IL Baldwin, McCoy E (1932) ‘Root nodule bacteria and leguminous plants.’ (University of Wisconsin: Madison) Gualtieri G, Bisseling T (2000) The evolution of nodulation. Plant Molecular Biology 42, 181–194. Hadri A-E, Spaink HP, Bisseling T, Brewin NJ (1998) Diversity of root nodulation and rhizobial infection process. In ‘The Rhizobiaceae’. (Eds HP Spaink, A Kondorosi and PJJ Hooykaas) pp. 347–360. (Kluwer Academic Press: Dordrecht)

891

Hirsch AM (1992) Developmental biology of legume nodulation. New Phytologist 122, 211–227. Hungria M, Joseph CM, Phillips DA (1991) Rhizobium nod gene inducers exuded naturally from roots of common bean (Phaseolus vulgaris L.). Plant Physiology 97, 751–758. Ingham JL (1981) Phytoalexin induction and its taxonomic significance in the Leguminosae (subfamily Papilionoideae). In ‘Advances in legume systematics, Volume 2’. (Eds RM Polhill and PH Raven) pp. 599–626. (Her Majesty’s Stationery Office: London) Ingham JL (1990) Systematic aspects of phytoalexin formation within tribe Phaseoleae of the Leguminosae (subfamily Papilionoideae). Biochemical Systematics and Ecology 18, 329–343. Jordan DC (1982) Transfer of Rhizobium japonicum Buchanan 1980 to Bradyrhizobium gen. nov., a genus of slow-growing, root nodule bacteria from leguminous plants. International Journal of Systematic Bacteriology 32, 136–139. Keyser HH, van Berkum P, Weber DF (1982a) A comparative study of the physiology of symbioses formed by Rhizobium japonicum with Glycine max, Vigna unguiculata and Macroptilium atropurpureum. Plant Physiology 70, 1626–1630. Keyser HH, Bohlool BB, Hu TS, Weber DF (1982b) Fast-growing rhizobia isolated from root nodules of soybean. Science 215, 1631–1632. Kosslak RM, Bookland R, Barkei J, Paaren HE, Appelbaum ER (1987) Induction of Bradyrhizobium japonicum common nod genes by isoflavones isolated from Glycine max. Proceedings of the National Academy of Sciences USA 84, 7428–7432. Kueneman EA, Root WR, Dashiell KE, Hohenberg J (1984) Breeding soybeans for the tropics capable of nodulating effectively with indigenous Rhizobium spp. Plant and Soil 82, 387–396. Kuykendall LD, Saxena B, Devine TE, Udell SE (1992) Genetic diversity in Bradyrhizobium japonicum Jordan 1982 and a proposal for Bradyrhizobium elkanii sp. nov. Canadian Journal of Microbiology 38, 501–505. Martinez-Romero E, Segovia L, Mercante FM, Franco AA, Graham P, Pardo MA (1991) Rhizobium tropici, a novel species nodulating Phaseolus vulgaris L. beans and Leucaena sp. trees. International Journal of Systematic Bacteriology 41, 417–426. Martins LMV, Neves MCP, Rumjanek NG (1997) Growth characteristics and symbiotic efficiency of rhizobia isolated from cowpea nodules of the North-East region of Brazil. Soil Biology and Biochemistry 29, 1005–1010. Mpepereki S, Wollum AG II, Makonese F (1996) Diversity in symbiotic specificity of cowpea rhizobia indigenous to Zimbabwean soils. Plant and Soil 186, 167–171. Neves MCP, Rumjanek NG (1997) Diversity and adaptability of soybean and cowpea rhizobia in tropical soils. Soil Biology and Biochemistry 29, 889–895. Pate JS (1973) Uptake, assimilation and transport of nitrogen compounds by plants. Soil Biology and Biochemistry 5, 109–119. Pate JS, Layzell DB, Atkins CA (1979) Economy of carbon and nitrogen in a nodulated and non-nodulated (NO3–-grown) legume. Plant Physiology 64, 1083–1088. Pate JS, Atkins CA, White ST, Rainbird RM, Woo KC (1980) Nitrogen nutrition and xylem transport of nitrogen in ureide-producing grain legumes. Plant Physiology 65, 961–965. Phillips DA (1999) Biosynthesis and release of rhizobial nodulation gene inducers by legumes. In ‘Prokaryotic nitrogen fixation: a model system for the analysis of a biological process’. (Ed. EW Triplett) pp. 349–364. (Horizon Scientific Press: Wymondham, UK) Phillips DA, Dakora FD, Sande E, Joseph CM, Zon J (1994) Synthesis, release and transmission of alfalfa signals to rhizobial symbionts. Plant and Soil 161, 69–80.

892

F. D. Dakora

Pueppke SG, Broughton WJ (1999) Rhizobium sp. strain NGR234 and Rhizobium fredii USDA257 share exceptionally broad, nested host ranges. Molecular Plant–Microbe Interactions 12, 293–318. Reynolds PHS, Blevins DG, Boland MJ, Schubert KR, Randall DD (1982) Enzymes of ammonia assimilation in legume nodules: a comparison between ureide- and amide-transporting plants. Physiologia Plantarum 55, 255–260. Sadowsky MJ, Cregan PB, Keyser HH (1988) Nodulation and nitrogen fixation efficacy of Rhizobium fredii with Phaseolus vulgaris genotypes. Applied and Environmental Microbiology 54, 1907–1910. Schauser L, Roussis A, Stiller J, Stougaard J (1999) A plant regulator controlling development of symbiotic root nodules. Nature 402, 191–195. Scholla MH, Elkan GH (1984) Rhizobium fredii sp. nov., a fast-growing species that effectively nodulates soybeans. International Journal of Systematic Bacteriology 34, 484–486. Sen D, Weaver DW (1984) A basis for different rates of N2 fixation by the same strain of Rhizobium in peanut and cowpea root nodules. Plant Science Letters 34, 239–246. Sprent JI (1980) Root nodule anatomy, type of export product and evolutionary origin in some Leguminosae. Plant, Cell and Environment 3, 35–43. Sprent JI (1981) Functional evolution in some Papilionoid root nodules. In ‘Advances in legume systematics, Vol. 2’. (Eds RM Polhill and P Raven) pp. 671–676. (Her Majesty’s Stationery Office: London) Sprent JI (1989) Which steps are essential for the nodulation of functional legume nodules? New Phytologist 111, 129–153. Sutherland JM, Sprent JI (1984) Calcium oxalate crystals and crystal cells in determinate root nodules of legumes. Planta 161, 193–200. van Berkum P, Eardly BD (1998) Molecular evolutionary systematics of the Rhizobiaceae. In ‘The Rhizobiaceae’. (Eds HP Spaink, A Kondorosi and PJJ Hooykaas) pp. 1–24. (Kluwer Academic Publishers: Dordrecht)

Van Rensburg HJ, Strijdom BW (1971) Stability of infectivity in strains of Rhizobium japonicum. Phytophylactica 3, 125–130. Van Rensburg HJ, Strijdom BW (1972a) A bacterial contaminant in nodules of leguminous plants. Phytophylactica 4, 1–8. Van Rensburg HJ, Strijdom BW (1972b) Information on the mode of entry of a bacterial contaminant into nodules of some leguminous plants. Phytophylactica 4, 73–78. Vincent JM (1980) Factors controlling the legume–Rhizobium symbiosis. In ‘Nitrogen fixation, Vol 2’. (Eds WE Newton and WH OrmeJohnson) pp. 103–127. (University Park Press: Baltimore) Wallace W (1986) Distribution of nitrate assimilation between the root and shoot of legumes and a comparison with wheat. Physiologia Plantarum 66, 630–636. Yang GP, Debelle F, Ferro M, Maillet F, Schiltz O, Vialas C, Savagnac A, Prome JC, Dénarie J (1998) Rhizobium nod factor structure and the phylogeny of temperate legumes. In ‘Biological nitrogen fixation for the 21st century’. (Eds C Elmerich, A Kondorosi and WE Newton) pp. 185–188. (Kluwer Academic Publisher: Dordrecht) Zhang F, Smith DL (1996) Genistein accumulation in soybean (Glycine max (L.) Merr.) root systems under suboptimal root zone temperatures. Journal of Experimental Botany 47, 755–792. Zhang F, Smith DL (1997) Application of genistein to inocula and soil to overcome low spring soil temperature inhibition of soybean nodulation and nitrogen fixation. Plant and Soil 192, 141–151.

Manuscript received 7 February 2000, accepted 22 May 2000

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