Frankiaactinorhizal root nodule symbiosis

Frankia is an actinorhizal bacterium that nodulates roots of woody shrubs or trees and a partial listing of plants with Frankia are given in Table 9-3. Nodules attributed to Frankia have been reported for eight families and 25 genera of dicotyledonous plants. All of these nodule bacteria are collectively referred to as Frankia with species designations to be established after further molecular analysis. An excellent summation of the activities of Frankia is given in the book edited by Schwintzer and Tjepkema (1990). The nitrogenase activity is found in thick walled vesicles that are found on short branches extending from the bacterial mycelium and the rate of nitrogen fixation by nodules containing Frankia is comparable to that of legume nodules with rhizobia. Unlike rhizobia cultures, some Frankia isolates can be adapted in culture to readily fix dinitrogen under aerobic conditions.

Relatively few strains of Frankia have been found to produce sidero-phores. Siderophores of the catechol class were produced by four isolates from roots of members of the Casuarinaceae (Arahou et al., 1998). However, Frankia strain 52065 which was isolated from Ceanothus americanus produces a hydroxamate siderophore when cultivated under Fe limited conditions of growth (Boyer et al., 1999). Siderophores from Frankia have been termed frankobactin and Frankia strain 52065 produces two siderophores: frankobactin and frankobactin A. The structural difference of these siderophores is that the phenyl oxazoline ring in frankobactin is closed while in frankobactin A the ring is open (Boyer et al., 1999). The status of the oxazoline ring accounting for two structural types of siderophores is not novel for frankobactin because similar dual structures of agrobactin and agrobactin A are reported for Agrobacterium tumefaciens (Ong et al., 1979), while parabactin and parabactin A have been reported for Paracoccus denitrificans (Peterson et al., 1980). Both frankobactin and frankobactin A are produced simultaneously by Frankia strain 52065 in rapidly growing cultures; however, these experiments are not for the impatient because the lag phase for an Fe-limited culture is about 30 days (Boyer et al., 1999).

When following the uptake of 55Fe, Boyer et al. (1999) observed that Frankia strain 52065 showed greater preference for ferric loaded desferal or frankobactin than for citrate, EDTA, or pseudobactin. Since desferal is a derivative of ferrioxamine B, the main siderophore of Streptomyces pilosus, ferrioxamine B was also tested and the rate of 55Fe uptake with both of these siderophores was comparable. The uptake of frankobactin by Frankia is by an energy-requiring mechanism (Boyer and Aronson, 1994).

Symbiotic bacteria that are not covered in this review include the cyanobacteria Anabaena and Nostoc that grow with Azolla and Gunnera, respectively. The reader is referred to a publication by Wilheim (1995) for physiological responses of cyanobacteria to Fe-limited growth.

Table 9-3. A selection of perennial dicots producing nodules with Frankia.

Family

Genus

Common name

Betulaceae

Alnus

Alder

Casuarinaceae

Allocasuarina

Black she-oak

Casuarina

Beef wood

Gymnostoma

Daintree

Coriariaceae

Coriaria

a low growing shrub

Datiscaceae

Datisca

Durango root

Elaeagnaceae

Elaeagnus

Russian olive, Autumn olive

Hippophae

Sea buckthorn

Shepherdia

Buffalo berry

Myricaceae

Comptonia

Sweet fern

Myrica

Bog myrtle

Rhamnaceae

Ceanothus

Buckbrush, Wild lilac

Colletia

Anchor plants

Discaria

Matagouri, Tumatakuru

Retanilla

No common name

Talguenea

Talguen

Trevoa

Trevo bush

Rosaceae

Cercocarpus

Mountain mahogany

Cowania

Cliffrose

Purshia

Desert bitterbrush

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