Eugenia Chaia

Life in soil by the actinorhizal root nodule endophyte Frankia. A review

Frankia is a genus of soil actinomycetes famous for its ability to form N2-fixing root nodule symbioses with actinorhizal plants. Although Frankia strains display a high diversity in terms of ecological niches in soil, current knowledge about Frankia is dominated by its life as an endophyte in root nodules. Increased use of molecular methods has refined and expanded insights into endophyte-host specificities and Frankia phylogeny. This review has focus on Frankia as a soil organism, including its part of microbial consortia, and how to study Frankia in soil. We highlight the use of nodulation tests and molecular methods to reveal population size and genetic diversity of Frankia in soil and discuss how autoregulation of nodulation and interactions with other soil microorganisms may influence the results. A comprehensive record of published interactions between Frankia and other soil microbes is summarized.

Isolation of an effective strain of Frankia from nodules of Discaria trinervis (Rhamnaceae)

Frankia strain BCU110501 was isolated from root nodules of the native Patagonian actinorhizal plant Discaria trinervis. The strain was grown on BAP medium with sodium propionate or glucose as carbon sources. Colonies grown in nitrogen-free medium showed branched hyphae bearing polymorphic sporangia and vesicles, which were capable of nitrogen fixation. Old cultures produced a red pigment. The infectivity and effectivity of a Frankia strain isolated from Discaria on its own host, D. trinervis and also in D. chacaye, is reported for the first time. Frankia BCU110501 has physiological properties that are intermediate between categories proposed by Lechevalier et al. (1983) to classify Frankia.

Variation of 15N natural abundance in leaves and nodules of actinorhizal shrubs in Northwest Patagonia

This study was performed to assess the N2-fixing capability of the native actinorhizal species Ochetophila trinervis (sin. Discaria trinervis) and Discaria chacaye (Rhamnaceae) in Northwest Patagonia. We measured the N concentration and 15N natural abundance in leaves and nodules of O. trinervis and D. chacaye, in leaves of associated non-actinorhizal vegetation, and in the soils under each sampled plant. O. trinervis and D. chacaye had foliar N concentrations that were about twice that of non-actinorhizal shrubs growing at the same sites, even though soils varied four-fold in total N across the sites. Leaves of both actinorhizal plants had a similar δ15N at any site and were close to atmospheric values. The foliar δ15N of non-actinorhizal plants and soil δ15N were strongly correlated across the sites. Nodules were depleted in δ15N relative to the foliage of the respective actinorhizal species. In conjunction with the uniformly high foliage N concentration of these actinorhizal plants and the universal presence of vesicles observed in root nodules, these data strongly suggest that O. trinervis and D. chacaye obtain a significant amount of their N from N2 fixation. To calculate the proportion of N derived from atmosphere, theoretical B-values were estimated. In all cases where the δ15N of fixing and reference foliage were significantly different, O. trinervis and D. chacaye obtained almost all of their N from N2 fixation. These results are the first to demonstrate N2 fixation by O. trinervis and D. chacaye in the field and therefore suggest an important role for these actinorhizal plants in the N economy of ecosystems in northwest Patagonia as well as their potential use for restoration of degraded lands in this region.

Are Northwestern Patagonian “mallín” wetland meadows reservoirs of Ochetophila trinervis infective Frankia?

Abstract“Mallín” (plural mallines) is a particular kind of wetland occurring in Patagonian steppe and forests. In Northwest Patagonia, mallines are humid meadows with high net primary production. It was previously found that a mallín soil in the steppe devoid of actinorhizal plants had a higher Frankia nodulation capacity in Ochetophila trinervis (sin. Discaria trinervis) than other soils in the region. Under the hypothesis that mallín wetland meadows are reservoir of infective Frankia, we studied the Frankia nodulation capacity in O. trinervis of 12 mallín and their neighbouring steppe soils, by using plant bioassays. A qualitative plant bioassay showed that infective Frankia was present in most soils. The number of nodules per plant in seedlings inoculated with mallín soils was negatively correlated with soil water content while the opposite was true for plants inoculated with soils from neighbouring steppe. A quantitative bioassay was performed with eight representative soils, selected according to the number of nodules per plant produced in the qualitative assay and to the presence or not of different actinorhizal plants at the sites. Frankia nodulation units per cm3 of soil (NU) in mallín soils were higher than those in steppe. Water and organic matter content of soils were correlated with the higher nodulation capacity of mallines, which may account for the saprotrophic growth of Frankia in soils. The symbiosis was effective in plants inoculated with all soil samples. These results suggest that Northwestern Patagonian mallín wetland meadows are reservoirs of infective and effective Frankia propagules in O. trinervis.

Specificity in Discaria - Frankia Symbioses

For some years we have been studying symbioses in native actinorhizal plants belonging to the Family Rhamnaceae, namely Discaria trinervis, Discaria chacaye and Discaria articulata. All these species are distributed in the south west of South America, at the Patagonia. We are interested in the mechanisms of regulation of the nodulation and the meaning of specificity and recognition in these symbioses, in order to look for the unknown symbiotic signals involved in the Frankia actinorhizal association.

Vertebrate faeces as sources of nodulating Frankia in Patagonia

Frankia strains nodulate the native actinorhizal plant Ochetophila trinervis (sin. Discaria trinervis), which grows in stream margins and nearby areas in northwest Patagonia (Argentina). Infective Frankia are found in soils with presence of host plants but also may be found in areas lacking them. This may be partly explained by water transport of Frankia propagules but there are other possible sources. The aim of this study was to discover whether the faeces of introduced mammalian herbivores, including cows (Bos taurus, adult and calf), horses (Equus caballus), sheep (Ovis aries), red and/or fallow deer (Cervus elaphus and Dama dama, respectively), wild boar (Sus scrofa), European hare (Lepus capensis), or the native upland goose (Chloephaga picta), could be a source of infective Frankia, and enhance its dispersal. Faecal material and soil samples were aseptically sampled in different plant communities, and tested via plant bioassays using O. trinervis. The faeces of all animals contained infective Frankia and led to an effective symbiosis with this plant. Faeces of large introduced herbivores gave rise to higher nodulation (number of nodulated plants with respect to the total number of inoculated plants) than faeces of hare and upland goose. Soils from the sites where the cow (two sites), sheep, wild boar and deer faeces were collected did not contain infective Frankia. This suggests that the animals may have ingested Frankia from plant material and that the Frankia propagules passed through the digestive tracts of the animals without losing its infectivity. We conclude that the faeces of large introduced herbivores contribute to the dispersal of infective Frankia in Northwest Patagonia.

Special Volume devoted to the 15th International Frankia and Actinorhizal Plant Meeting

The 15th International Frankia and Actinorhizal Plant Meeting was held from October 19th to 23rd, 2008, in Bariloche, Argentina. Frankia is an actinomycete that nodulates roots of a group of angiospermous plants. Frankia fixes atmospheric nitrogen symbiotically with its specific host plants in 24 genera within eight families. This was the first of the biennial meetings to be held in South America and only the second, after New Zealand, to be held outside of North America and Europe. Thirty four scientists and students from nine different countries in South America, North America, Europe and Asia attended. The site of the meeting, Bariloche, is in the Patagonian region of the southern Andean Mountains, where a distinct group of actinorhizal plants and their Frankia symbionts occur. These plants are in the tribe Colletiae (family Rhamnaceae) and the meeting participants had the opportunity to explore their natural environs from steppe to humid forest in the Nahuel Huapi National Park near Bariloche and to hear reports of the most recent research on this actinorhizal group. Much research progress has been made on Frankia and on actinorhizal plants since the first meeting was held at Harvard Forest in Petersham, Massachusetts in 1978. At that meeting, hosted by Dr. John Torrey, an icon in the field of Frankia research, the common interests of a small group of researchers had evolved from developments in three areas. The first area of advancement was the effort, sponsored by the International Biological Program, to establish a worldwide accounting for actinorhizal plants, then known to be comprised of 15 genera in seven “diverse” families. These morphologically “diverse” groups are now known to cluster together with other plant taxa, including legumes, in the subclade rosidae, apparently a taxonomic unit with plants predisposed to diazotrophic symbioses. The second impetus came from the increased recognition by foresters and other natural resource managers that actinorhizal plants possessed nitrogen fixation capacities equal to those of legumes and that actinorhizal plants conferred the dual benefits of superior host-plant productivity on poor soils and enhancement of soil nitrogen fertility. The third development was the first successful isolation of a Frankia strain (CpI1 from Comptonia peregrina) in pure culture and reinfection of a host plant by this isolate. This feat opened the door to a detailed understanding of this nitrogen-fixing actinomycete, its symbiosis with angiospermous host plants, and its potential for genetic improvement through biotechnology. Since the first conference, and even the most recent one held at the University of Umea in Sweden in 2006, substantial changes in emphasis and developments have occurred and were featured at this 15th meeting in Argentina. These included (1) notable advancements in E. Chaia (*) Universidad Nacional del Comahue, Centro Regional Universitario Bariloche, Quintral 1250, Bariloche 8400, Argentina e-mail: echaia@crub.uncoma.edu.ar

NITROGEN FIXATION IN AGRICULTURE: FORAGE LEGUMES IN SWEDEN AS AN EXAMPLE

Perennial grasslands, managed for forage production and leys, represent the majority of cultivated land in Sweden. In northern Sweden, the leys are typically comprised of the grasses timothy, Phleum pratense, and meadow fescue, Festuca pratensis, together with red clover, Trifolium pratense. It is common to establish the leys as an undersowing in barley and then keep them for about 3 years in a crop rotation. Leys are normally harvested twice per year for silage, but some leys are also grazed.