User:As2460/Actinorhizal plant

Article Draft
Summary

Actinorhizal plants are distributed within three clades, and are characterized by nitrogen fixation. They are distributed globally, and are pioneer species in nitrogen-poor environments. Their symbiotic relationships with Frankia evolved independently over time, and the symbiosis occurs in the root nodule infection site.

Lead
Actinorhizal plants are dicotyledons distributed within 3 orders, 8 families and 26 genera, of the angiosperm clade.

Article body
All nitrogen fixing plants are classified under the "Nitrogen-Fixing Clade", which consists of the three actinorhizal plant orders, and the order fabales. The most well-known nitrogen fixing plants are legumes, but are not classified as actinorhizal plants. The actinorhizal species are either trees or shrubs, except for those in the genus Datisca which are herbs. Other species of actinorhizal plants are common in temperate regions like alder, bayberry, sweetfern, avens, mountain misery and coriaria. Some Elaeagnus species, such as sea-buckthorns produce edible fruit. What characterizes an actinorhizal plant is the symbiotic relationship it forms with the bacteria Frankia, in which they infect the roots of the plant. This relationship is what is responsible for the nitrogen-fixation qualities of the plants, and what makes them important to nitrogen-poor environments

The symbiotic nodules[edit]
As in legumes, nodulation is favored by nitrogen deprivation and is inhibited by high nitrogen concentrations. Depending on the plant species, two mechanisms of infection have been described: The first is observed in casuarinas or alders and is called root hair infection. In this case the infection begins with an intracellular penetration of a Frankia hyphae root hair, and is followed by the formation of a primitive symbiotic organ known as a prenodule. The second mechanism of infection is called intercellular entry and is well described in Discaria species. In this case bacteria penetrate the root extracellularly, growing between epidermal cells then between cortical cells. Later on Frankia becomes intracellular but no prenodule is formed. In both cases the infection leads to cell divisions in the pericycle and the formation of a new organ consisting of several lobes anatomically similar to a lateral root. Cortical cells of the nodule are invaded by Frankia filaments coming from the site of infection/the prenodule. Actinorhizal nodules have generally an indeterminate growth, new cells are therefore continually produced at the apex and successively become infected. Mature cells of the nodule are filled with bacterial filaments that actively fix nitrogen. No equivalent of the rhizobial nod factors have been found, but several genes known to participate in the formation and functioning of Legume nodules (coding for haemoglobin and other nodulins) are also found in actinorhizal plants where they are supposed to play similar roles. The lack of genetic tools in Frankia and in actinorhizal species was the main factor explaining such a poor understating of this symbiosis, but the recent sequencing of 3 Frankia genomes and the development of RNAi and genomic tools in actinorhizal species should help to develop a far better understanding in the following years.

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