No abstract

Symbioses between the root nodule-forming, nitrogen-fixing actinomycete Frankia and its angiospermous host plants are important in the nitrogen economies of numerous terrestrial ecosystems. Molecular characterization of Frankia strains using polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) analyses of the 16S rRNA-ITS gene and of the nifD-nifK spacer was conducted directly on root nodules collected worldwide from Casuarina and Allocasuarina trees. In their native habitats in Australia, host species contained seven distinctive sets of Frankia in seven different molecular phylogenetic groups. Where Casuarina and Allocasuarina trees are newly planted outside Australia, they do not normally nodulate unless Frankia is introduced with the host seedling. Nodules from Casuarina trees introduced outside Australia over the last two centuries were found to contain Frankia from only one of the seven phylogenetic groups associated with the host genus Casuarina in Australia. The phylogenetic group of Frankia found in Casuarina and Allocasuarina trees introduced outside Australia is the only group that has yielded isolates in pure culture, suggesting a greater ability to survive independently of a host. Furthermore, the Frankia species in this group are able to nodulate a wider range of host species than those in the other six groups. In baiting studies, Casuarina spp. are compatible with more Frankia microsymbiont groups than Allocasuarina host spp. adapted to drier soil conditions, and C. equisetifolia has broader microsymbiont compatibility than other Casuarina spp. Some Frankia associated with the nodular rhizosphere and rhizoplan, but not with the nodular tissue, of Australian hosts were able to nodulate cosmopolitan Myrica plants that have broad microsymbiont compatibility and, hence, are a potential host of Casuarinaceae-infective Frankia outside the hosts' native range. The results are consistent with the idea that Frankia symbiotic promiscuity and ease of isolation on organic substrates, suggesting saprophytic potential, are associated with increased microsymbiont ability to disperse and adapt to diverse new environments, and that both genetics and environment determine a host's nodular microsymbiont.

Using the simple serial dilution technique, five strains of actinomycetes were isolated from nodules of Casuarina equisetifolia. In spite of the fact that these strains did not nodulate the host plant, they were assumed to belong to the genus Frankia because they possessed the morphological and cultural characteristics now admitted as specific to this genus.

SummaryField and cylinder experiments conducted in France and in Senegal showed that polyacrylamide, previously proposed as an entrapping gel for preparing Rhizobium inoculants, could be replaced by alginate (AER inoculant) or a mixture of xanthan and carob gum (XER inoculant). Semi-dried or dried AER and XER were used successfully provided that their storage time was less than 90 days. In soil inoculation trials, no marked differences were observed among semi-dried XER, dried AER, and dried XER. A number of seed inoculation experiments indicated that dried XER significantly outranked AER. Seeds preinoculated by up to 48 days with XER yielded plants which were comparable in nodulation and growth parameters to those derived from plant receiving peat inoculation at the time of planting.

No abstract

Thirteen Acacia species were classified into three groups according to effective nodulation response patterns with fastand slow-growing tropical strains of Rhizobium. The first group nodulated effectively with slow-growing, cowpea-type Rhizobium strains; the second, with fast-growing Rhizobium strains; and the third, with both fastand slow-growing Rhizobium strains. The Rhizobium requirements of the Acacia species of the second group were similar to those of Leucaena leucocephala. Shrubs and trees of the legume genus Acacia (Mimosaceae) are abundant in savannas and arid regions of Australia, Africa, South and North America, and India. In the Sahel region of Africa, Acacia is often the dominant tree species, where they grow in barren soils and dry sites unsuited for most crops. The Acacia species stabilize sandy and eroded soils and exploit deep underground water by virtue of their extensive root systems. They provide shade, forage for animals, firewood, charcoal, and gums. Most Acacia species nodulate with Rhizobium and fix N2 (1, 2, 4, 6), but little is known about the specificity and the characteristics of Rhizobium

Volume 45, no. 2, p. 711, column 1, paragraph 3, lines 5-7: ".. a nitrogen-free medium (LO medium), LO medium), and a complete medium (YL medium ..." should read "a nitrogen-free medium (LO medium), LO medium supplemented with 1 g of (NH4)2S04 per liter (LN medium), and a complete medium (YL medium....

Attempting to isolate Frankia from Casuarina, we found that this endophyte exhibited two unexpected characteristics: (i) an inability of most infected nodule cells to produce Frankia colonies; and (ii) the absence of specific nutritional requirements in the isolation medium. These findings led us to the formulation of some principles for isolating Frankia from Casuarina.