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... In addition to scattered contributions at conferences on, for example, N2 fixation, there has been a series of international meetings on Frankia and actinorhizal plants (Table 1). A broad range of aspects of actinorhiza has recently been covered in review articles and in book chapters by Dawson (1986), Gardner (1986), Tjepkema, Schwintzer & Benson (1986), Newcomb & Wood (1987), Torrey (1988), Benoit & Berry (1990), Berry & Sunell (1990), Dawson (1990), Diem & Dommergues (1990), Hibbs & Cromack (1990), Huss-Danell (1990), Lechevalier & Lechevalier (1990), Quispel (1990), Schwintzer (1990), Silvester, Harris & Tjepkema (1990), Torrey (1990), Wheeler & Miller (1990), Winship & Tjepkema (1990), Reddell, , Benson & Silvester (1993), Berry (1994), , Molina, Myrold & Li (1994) and Myrold (1994). ...
... Enumeration of Frankia in soil is restricted to infective units when inoculation procedures are used. However, the nature of the host species and grow ing conditions influence nodulation (Huss-Danell & Myrold, 1994). An alternative method is to study Frankia DNA extracted from soil and amplified with PCR (reviewed by Myrold et al., 1994). ...
More than 200 angiosperms, distributed in 25 genera, develop root nodule symbioses (actinorhizas) with soil bacteria of the actinomycetous genus Frankia. Although most soils studied contain infective Frankia , cultured strains are available only after isolation from root nodules. Frankia infects roots via root hairs in some hosts or via intercellular penetration in others. The nodule originates in the pericycle. The number of nodules in Alnus is determined by the plant in an autoregulated process that, in turn, is modulated by nutrients such as nitrogen and phosphate. Except in the genera Allocausarina and Casuarina, Frankia in nodules develops so‐called vesicles where nitrogenase is localized. Sporulation of Frankia occurs in some symbioses. As a group, actinorhizal plants show a large range of anatomical and biochemical adaptations in order to balance the oxygen tension near nitrogenase. In symbioses with well aerated nodule tissue like Alnus , the vesicles have a multilayered envelope composed mainly of lipids, bacterio‐hopanetetrol and their derivatives. This envelope is assumed to retard the diffusion of oxygen into the nitrogenase‐containing vesicle. In symbioses like Casuarina , the infected plant cells themselves, rather than Frankia , appear to retard oxygen diffusion, and high concentrations of haemoglobin indicate an infected region with a low oxygen tension. At least in Alnus spp., ammonia resulting from N 2 fixation is assimilated by glutamine synthetase in the plant. The carbon compound(s) used by Frankia in nodules is not yet known. Nitrogenase activity decreases in response to a number of environmental factors but recovers upon return to normal conditions. This dynamism in nitrogenase activity is often explained by loss and recovery of active nitrogenase and has been traced to loss and recovery of the nitrogenase proteins themselves. Recovery is partly due to growth of Frankia and to development of new vesicles in the Alnus nodules. In the field, varying conditions continuously affect the plants and the measured rate of N 2 fixation is a result not only of the conditions prevailing at the moment but also of the conditions experienced over preceding days. N 2 fixed by actinorhizal plants is substantial and actinorhizal plants have great potential in soil reclamation and in various types of forestry. Several species are also useful in horticulture.
CONTENTS
Summary
375
I.
Introduction
376
II.
The partners of actinorhizal symbioses
377
III.
Root nodules
380
IV.
Nitrogen fixation and related processes
385
V.
Environmental effects on nitrogen fixation
389
VI.
Ecological role
397
VII.
Concluding remarks
398
Acknowledgements
398
References
398
... Fresh and active nodules (light color) from a single tree were harvested at each location. After collection, the nodules in a container with moist filter paper in the ice boxes were transferred to the laboratory [13].In the laboratory, soils adhering to the nodules washed thoroughly under running water. The nodules were surface sterilized with sodium Hypochlorite (1% active chlorine) and hydrogen peroxide (30%) for one minute, then rinsed several times with sterile distilled water [14].The nodules were stored in a refrigerator for about 3 weeks before they used for inoculation. ...
The growth and nitrogen fixation of Caucasian alder (Alnus subcordata C.A.Mey) that inoculated with Frankia were investigated in pots filled with sterilized sand. The Frankia suspensions prepared from root nodules of A. subcordata that isolated from 25 different sites of Hyrcanian forest, northern Iran. The seedling growth, N2-fixation and nodulation were measured 10 weeks after inoculation. Dry weight of shoots, roots and nodules, and N content ofthe seedlings inoculated with Frankia were higher than those without inoculation. The greatest impact on the nitrogen xing ability, microbial inoculation effects and plant biomass was observed in seedlings inoculated with AS50 Frankia crushed nodules compared with other treatments. The results of these experiments showed that the Frankia crushed nodules could improve the growth and nitrogen xation of A. subcordata. Thus, selection true sources of inoculums that have a considerable influence to A. subcordata and optimizing the sustainable production of these inoculums is needed.
Key message
We investigated a
Frankia
–
Alnus
sieboldiana
symbiosis, including the minimum inoculum dose for constant nodulation, the period of time to nodulation after inoculation, and the effects of N on nodulation.
Abstract
Frankia is a nitrogen-fixing actinomycete that forms root nodules in some dicotyledonous plants, which are called actinorhizal. We studied nodule formation in Alnus sieboldiana, an actinorhizal plant, after inoculation with a Frankia isolate to establish techniques for Frankia inoculation and the cultivation of inoculated plants. Root nodules formed on seedlings of A. sieboldiana by 2 weeks after inoculation, and N2 fixation measured by acetylene reduction activity started 3 weeks after inoculation. Nodulation was observed after inoculation with a Frankia isolate at 0.001 μL packed cell volume (pcv). The number of nodules formed on the seedlings inoculated with Frankia at more than 0.05 μL pcv was not significantly different. Nodule development and N2 fixation were reduced when inoculated seedlings were treated weekly with 15 mM NH4NO3-N. In contrast, treatment with 3.75 or 0.9375 mM NH4NO3-N did not inhibit nodule development or N2 fixation of inoculated seedlings by 15 weeks of N treatment.
The phylogenetic position of two Frankia strains, isolated from two different host plants, was analyzed by reverse transcriptase sequencing and/or oligonucleotide cataloguing of their 16S ribosomal RNA. The two strains are highly related, showing a distinct relationship to Geodermatophilus obscurus and a strain of “Blastococcus”. These organisms constitute a main subline of descent within the phylogenetic radiation of the order Actinomycetales. In contrast to the present classification the genera Geodermatophilus and Dermatophilus cannot be considered members of the same family. Geodermatophilus is transferred into the family Frankiaceae for which an emended description is given.