Rhizobiasymbiotic nitrogenfixing bacteria

The group of bacteria collectively referred to as rhizobia, with one known exception, use leguminous plant roots as the host. The taxonomy of the Gram-negative rhizobia is not resolved; however, for the purpose of illustration a list of some of these bacteria along with their plant hosts are given in Table 9-1 (Van Berkum and Eardly, 1998). In some cases the bacteria listed in Table 9-1 may nodulate additional species and the reader is referred to the publication of Hadri et al. (1998) concerning alternate hosts.

The Fe nutrition of root nodule bacteria has attracted the attention of many scientists and results indicate that considerable variation exists in the response of different strains to Fe deficiency. Rhizobia require Fe and phosphorous to grow in the rhizosphere and to nodulate roots of legumes (O'Hara, 2001). Tang et al. (1992) have reviewed investigations on the response of the legume symbiosis to Fe deficiency. Additionally, Fe, calcium, phosphorus, cobalt, copper, potassium, nickel, selenium, zinc, boron, and molybdenum are important for nodule development and nitrogen fixation. With respect to Fe utilization, rhizobia have been found to utilize FeCl3, Fe-citrate, synthetic Fe-chelates, siderophores, and even heme. A listing of some of the various biochelates utilized by rhizospheric bacteria including rhizobia is given in Table 9-2 and additional siderophores for rhizobia are described in an earlier review (Barton et al., 1994).

Table 9-1. Examples of plants with nodules attributed to rhizobia.

Bacterial symbiont

Plant host

Common name or description

Azorhizobium caulinodan

Sesbania rostrata

aquatic tropical legume

Bradyrhizobium sp.

Parasponia (formerly Trema)

a non-legume

Bradyrhizobium japonicum

Glycine max (L.) Merrill

soybean

Bradyrhizobium sp.*

Aeschynomene indica

aquatic plant with adventitious roots

Mesorhizobium ciceri

Cicer arietinum (L.)

chick pea

Mesorhizobium loti

Lotus corniculatus

lotus

Rhizobium etli

Phaseolus vulgaris (L.)

bean

Rhizobium tropicli

Phaseolus vulgaris (L.)

bean

Rhizobium leguminosarum

bv

Phaseolus vulgaris (L.)

bean

phaseoli

Rhizobium leguminosarum trifoli

bv

Trifolium repens (L.)

clover

Rhizobium leguminosarum viciae

bv

Pisum sativum (L.)

pea

Sinorhizobium meliloti

Medicago sativa

alfalfa

*It has been proposed that this pigmented strain be Photorhizobium, a new genus.

*It has been proposed that this pigmented strain be Photorhizobium, a new genus.

Relatively few strains of Bradyrhizobium have been demonstrated to produce siderophores when using traditional chemical assays. In a survey of several strains Carson et al. (2000) did not find any that produced siderophores and Guerinot et al., (1990) found that citrate was the only siderophore produced by one of 20 strains of Bradyrhizobium japonicum. The examination of 18 strains of Bradyrhizobium nodulating groundnut using the chrome azurol S (CAS) reagent (Schwyn and Neilands, 1987) revealed only small halos around colonies on the blue agar plates. In a liquid CAS assay, it appeared that either only a very low level of siderophore was produced by these strains when under Fe stress conditions or that the Fe chelators had a relatively low affinity for Fe3+ (Van Rossum et al., 1994). In a separate study, Abd-Alla (1999) found that only two out of six strains of Bradyrhizobium spp. effective in nodulating lupin produced a hydroxamate-type siderophore when grown under Fe-stress conditions and that mannitol proved to be the best carbon source for this siderophore production.

In addition to Fe-citrate utilization, strains of Bradyrhizobium japonicum have been reported to use ferrichrome, rhodotorulate, and pseudobactin which are siderophores produced by microorganisms other than rhizobia (Plessner et al., 1993). Both Bradyrhizobium japonicum and Rhizobium leguminosarum bv viciae have an Fe uptake system that uses heme supplied as hemoglobin or leghemoglobin; however, for these organisms, heme is an alternative Fe supply system for free-living bacteria and is not required for nitrogen-fixation (Nienaber et al., 2001; Wexler et al., 2001). The heme uptake system is under the regulation of Fur and consists of 9 genes including a heme receptor in the outer membrane, a periplasmic binding protein and an ATP driven ABC transporter in the plasma membrane. To energize the uptake of heme, a TonB homologue protein extends from the plasma membrane to the outer membrane (Wexler et al., 2001). Fur does not control the production of this TonB homologue even though TonB-like protein is reduced by Fe starvation. LeVier and Guerinot (1996) have reported an outer membrane protein, FegA, of 80 kDa with a sequence similar to Ton-B dependent proteins from other bacteria. Production of FegA is under the influence of Fe starvation and this protein appears structurally similar to protein receptors for the hydroxamate class of siderophores.

Table 9-2. A selection of siderophores used by rhizospheric bacteria.

Bacteria

Iron uptake system

Siderophore

Reference

Azotobacter vinelandii

protochelin

tricatecholate

1

azotocholate

dicatecholate

1

aminochelin

monocatecholate

1

Azospirillum brasilense

spirillobactin

hydroxamate

2

Azospirillum lipoferum

dihydroxybenzoic acid

hydroxamate

3

Bradyrhizobium japonicum

heme

not applicable

4

Frankia sp.

frankobactin

hydroxamate

5

Rhizobium leguminosarum

not specified

hydroxamate

6

bv. phaseoli

Rhizobium leguminosarum

vicibactin

trihydroxamate

7

bv. viciae

Rhizobium leguminosarum

vicibactin

trihydroxamate

8

bv. trifolii

Sinorhizobium meliloti

rhizobactin 1021

citrate derivative

9

1. Cornish and Page, 1998. 2. Bachhawat and Ghosh, 1987. 3. Saxena et al., 1986. 4. Nienaber et al., 2001. 5. Boyer et al., 1999. 6. Carrillo-Castaneda and Cano, 2000. 7. Dilworth et al., 1998. 8. Lynch et al., 2001, 9. Persmark et al., 1993.

1. Cornish and Page, 1998. 2. Bachhawat and Ghosh, 1987. 3. Saxena et al., 1986. 4. Nienaber et al., 2001. 5. Boyer et al., 1999. 6. Carrillo-Castaneda and Cano, 2000. 7. Dilworth et al., 1998. 8. Lynch et al., 2001, 9. Persmark et al., 1993.

The structure of the siderophore produced by Sinorhizobium meliloti strain 1021 was established by Persmark et al. (1993). It is a citrate derivative containing two diaminopropane moieties and this siderophore has been designated rhizobactin 1021. Other strains known to produce rhizobactin include strains 2011 and SU47. The production of rhizobactin 1021 by Sinorhizobium meliloti strain 2011 results from a cluster of 8 genes found on a megaplasmid and these genes function for regulation, biosynthesis, and transport (Lynch et al., 2001). From analysis of the 6 genes (rhbABCDEF) required for biosynthesis of rhizobactin 1021, it appears that initially the 1, 3-diaminopropane structures are produced and subsequently, the diamino-propane moieties are attached to citrate.

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