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Index >> Actinorhizal Plants (Frankia Induced Nodulation) >> Nitrogen Fixation and Assimilation

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Actinorhizal Plants Frankia Induced Nodulation Root Nodules Caused by Frankia Actinorhizal Symbiosis Cross Inoculation Experiments Isolation and Characteristics of Frankia Cultural Characteristics of Frankia Taxonomy of Frankia Entry of Frankia to the Host Plant Structure of Actinorhizal Nodule

Nitrogen Fixation and Assimilation

	Nitrogen Fixation and Assimilation Unequivocal evidence has been presented using 15N enriched gas to demonstrate that Casuarina nodules in intact plants are capable of fixing nitrogen. The nitrogenase enzyme in Alnus glutinosa has properties similar to legume nodule nitrogenase, except the fact that sources of ATP and reductant have not been identified. The nodules have an efficient uptake hyrogenase system in Alnus and Casuarina. Nitrogenase activity is host as well as Frankia strain dependent, especially on the morphological state of Frankia whether in the form of spores or hyphae. Nitrogenase has been detected in vesicles as well as hyphae but abundance of vesicles coincides with high nitrogenase activity.
In 9 out of 21 genera of actinorhizal plants nodules have been encountered with many sporangia having plenty of spores designated as spore⁺ nodules. In other plants sporangia with less abundant spores have been seen, referred to as spore^- nodules. In general, Sp^- nodules are far superior to Sp⁺ nodules in nitrogen fixation and high plant biomass production.Two hypotheses have been suggested to indicate the possible mechanism of ammonia assimilation on lines similar to the ones operating in cyanobacterial heterocysts involving GS and GOGAT enzymes.
One of these assumes that vesicles have the potential to produce glutamine, which could be transferred to vegetative hyphae through the constricted stem cell of the vesicles. In the hyphae, glutamine would be converted by GOGAT to glutamate with one of the resulting glutamates going back to the vesicles to act as an ammonia acceptor for repeating the reaction. In the second hypothesis, it is assumed that the GS is not active in ammonia assimilation in vesicles which leads to accumulation of the fixed product in the hyphae and surroundings where it would be assimilated by the GS-GOGA T system, presumably aided by the high affinity ammonia permease present in nitrogen starved hyphae which helps in mopping up all free ammonia.
Evidences for the fixation of nitrogen in non-leguminous root nodules have come from two angles: (1) Long-term experiments with nodulated and nodule-free plants grown on nitrogen-free rooting media: Nodulating Alnus glutinosa can accumulate as much as 300 mg nitrogen per plant in contrast to nodule-free plants which exhausted the nitrogen in the cotyledons and made little further growth.
	When plants were raised in nitrogen-free mineral medium, total nitrogen content of nodulated (N) and non-nodulated (NN) plants at the end of 17 to 23 weeks were as follows: (1) Myrica cerifera-84.5mg (N)/1.0 mg (NN) per plant; Myrica gale158.0mg (N)/1.0 mg (NN) per plant; Alnus glutinosa-492.0mg (N)/0.5mg (NN) per plant. (2) By the use of ¹⁵N: In experiments with nine species of Myrica, nodules and apparently nodule-free roots of the same plants were exposed to ¹⁵N labelled gas. Samples were analysed for enrichment in nodules and roots. Convincing evidence was obtained for nitorgen fixation in nodules which invariably showed higher accumulation of ¹⁵N in nodulating species of Alnus, Hippophae, Casuarina, Shepherdia and Ceanothus.

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Nitrogen Fixation and Assimilation Field Experiments on the Benefit of Frankia Inoculation Mycorrhizae and Actinorhizal Plants Proteoid Roots Genetics of Frankia Rhizobium Induced Root Nodulation of Parasponia Leaf Nodules Gymnosperm Nodules

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