User:Slave Knight Gael/Actinorhizal plant

= Actinorhizal plant = From Wikipedia, the free encyclopedia Jump to navigationJump to search Actinorhizal plants are a group of perennial angiosperms characterized by their ability to form a symbiosis with the nitrogen fixing actinomycetota Frankia. This association leads to the formation of nitrogen-fixing root nodules. All actinorhizal plants are trees and shrubs with the exception of one genus which is an herbaceous plant. Along with forming associations with Frankia many actinorhizal plants also can form associations with mycorrhizal fungi which can create unique three-way plant-microbe interactions.

Contents

 * 1Classification
 * 2Distribution and ecology
 * 3Evolutionary origin
 * 4The symbiotic nodules
 * 5Microbial Associations
 * 6Notes
 * 7References
 * 8External links

Classification[edit]
Actinorhizal plants are dicotyledons distributed among three angiosperm orders, 8 families and 25 genera: These three orders form a single clade within the Rosids, which is a sister taxon to the other major nitrogen-fixing order, the Fabales. All actinorhizal species are trees or shrubs, except for the genus Datisca. Many are common plants in temperate regions like alder, bayberry, sweetfern, Avens, mountain misery and Coriaria. Some Elaeagnus species and Sea-buckthorns produce edible fruit. In tropical regions, Casuarinas are widely cultivated.[citation needed]

Distribution and ecology[edit]
The distribution of actinorhizal plants. Actinorhizal plants represent approximately 200 species of woody shrubs and trees predominantly in temperate climates with the exception of the genus Discaria which is an herbaceous plant. Actinorhizal plants are found on all continents except for Antarctica. Their distribution reflects both natural distributions and where these plants have been introduced. Many genera have been introduced by humans to non-native areas for multiple reasons including land reclamation and forestry. Their ability to form nitrogen-fixing nodules confers a selective advantage in poor soils. Most actinorhizal plants are therefore pioneer species that colonize young soils where available nitrogen is scarce, such as moraines, volcanic flows or sand dunes. Being among the first species to colonize these disturbed environments, actinorhizal shrubs and trees play a critical role, enriching the soil and enabling the establishment of other species in an ecological succession. Historically during and after glacial times Actinorhizal plants were more abundant in Europe and North America as the soils were poor in growth limiting matter such as nitrogen and organic matter as a whole. As Actinorhizal plants deposited nitrogen and organic matter back into these soils they began to be displaced as more nitrogen and organic matter allowed the growth of non-nitrogen fixing plants to grow. Actinorhizal plants like alders are also common in the riparian forest. Actinorhizal plants are the major contributors to nitrogen fixation in broad areas of the world, and are particularly important in temperate forest. The nitrogen fixation rate measured for some alder species is as high as 300 kg of N2/ha/year, close to the highest rate reported in legumes.

Evolutionary origin[edit]
Evolutionary origin of nitrogen-fixing nodulation No fossil records are available concerning nodules, but fossil pollen of plants similar to modern actinorhizal species has been found in sediments deposited 87 million years ago. The origin of the symbiotic association remains uncertain. The ability to associate with Frankia is a polyphyletic character and has probably evolved independently in different clades. Morphological and anatomical data gathered from actinorhizal nodules suggests that different genera have different symbiotic origins. Differences in nodules which suggest different origins are those such as the presence or lack of a specialized oxygen diffusion-limiting cell layer and higher levels of hemoglobin which protect nitrogenase. Similarly, some genera of actinorhizal plants possess nodule roots or nodule lenticels which many other genera simply lack. Current evidence based off morphological, anatomical, and genetic data suggests that the actinorhizal symbiosis is likely to have occurred from multiple combinations of independent gains and losses and has evolved at least four times and could have possibly evolved up to six times. With a nitrogen-fixing symbiosis being very advantageous for plants it is not exactly known why certain lineages would lose the ability to form nitrogen-fixing symbioses however the high cost of maintaining nodules is thought to be the driving factor. In certain cases such as high nitrogen content in the soil the cost of creating and maintaining nodules to house the Frankia might have been to high for the plant to reasonably sustain.

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 infection mechanism is root-hair infection. Root hair infection has been observed to occur in the genera Myrica, Casuarina, Comptonia, and Alnus. In this case the infection begins with an intracellular infection of a root hair and is followed by the formation of a primitive symbiotic organ lacking any particular organization, a prenodule. The second mechanism of infection is called intercellular penetration and is well described in Discaria and in Elaeagnaceae 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. This organ is the actinorhizal nodule also called actinorhizae. Cortical cells of the nodule are invaded by Frankia filaments coming from the site of infection or 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. Little information is available concerning the mechanisms leading to nodulation but several genes known to participate in the formation and functioning of Legume nodules (coding for heamoglobin and other nodulins) are also found in actinorhizal plants where they are supposed to play similar roles. More recently however new studies have examined the genomes of Frankia strains and have discovered that some Frankia genomes contain operons of nod genes similar to that of canonical nodABC genes such as those found in Rhizobium . Nod genes are the genes that encode for proteins which are involved in the synthesis and secretion of nodulation factors otherwise known as nod factors. Nod factors are lipochitooligosaccharide molecules which are typically expelled by rhizobia in response to flavonoid compounds secreted by host plants. Nod factors are then taken in by neighboring plant cells and initiate the developmental process of forming root nodules to house bacteria. The presence of these nod operons in some Frankia strains could suggest that Frankia initiate nodulation similar to rhizobia however that remains to be determined as nod factors have not been directly observed in Frankia strains. It is also know that some actinorhizal plant species like the Alnus glutinosa can secrete compounds called flavonols which can enhance the level of nodulation but the exact reason nodulation is enhanced is still unknown. Data within the last few years has suggested that auxin (a plant hormone which is responsible for the elongation of plant cells) can actually regulate the Frankia infection process and the production and development of plant nodules. Similarly some species of actinorhizal plants such as Alnus rubra can secrete flavonoids that can enhance or actually inhibit nodulation by Frankia however the reasoning behind this is unknown as well . The lack of genetic tools in Frankia and in actinorhizal species is 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.

Microbial Associations - Frankia
The microsymbiont of actinorhizal plants was first identified as Frankia in 1888 by Brunchorst and was later classified as an actinomycete after studies conducted by Krebber in 1932. Frankia are a genus of gram-positive and gram-variable actinomycetes. Frankia grows as filamentous colonies under normal conditions however when grown with limited nitrogen they can grow as Filamentous colonies, vesicles, or multilocular sporangia. Vesicles are particularly important in nitrogen limited environments as the vesicles contain the nitrogenase enzymes capable of fixing atmospheric nitrogen into usable forms. Vesicles in actinorhizal nodules are generally spherical, elliptical, or club shaped. Multilocular sporangia in Frankia are also particularly important. These Multilocular sporangia structures are filled with Frankia spores. These spores can remain dormant for many years even in unfavorable conditions. Frankia spores are also very important as they are suspected to be one of the main ways Frankia can disseminate in nature along with bacterial hypha spread . It is important to note that not all Frankia strains are capable of infecting every actinorhizal plant and they can be grouped into four groups. HSG1 Frankia strains that are only capable of infecting Alnus and ''Myrica. HSG2 Frankia are only capable of infecting Casurina, Gymnostoma, and some species of Allocasuarina. HSG3 Frankia strains are capable of infecting Elaeagnus, Hippophae, Shepherdia, and Myrica''. There is also a single strain of Frankia that are capable of infecting only the Elaeagnaceae.

Uses of Actinorhizal plants
Actinorhizal plants have many uses for humans as a result of their association with nitrogen fixing bacteria. Nitrogen is an essential plant nutrient and is most commonly the element soils lack. Soils can lose nitrogen very easily as a result of erosion, removal by crops, leaching, and denitrification. Nitrogen must be restored to these deficient soils and to do so humans can utilize the nitrogen fixing capabilities of actinorhizal plants in combination with Frankia to achieve this. The actinorhizal symbiosis captures nitrogen gas from our atmosphere and converts it to usable forms which will end up in the soils replenishing soils of nitrogenous substances. The increased nitrogen in soils is not only good for actinorhizal plants but also for future plants to take hold in soils. For this reason actinorhizal plants are often some of the first plants to show up after environments are disturbed. Actinorhizal plants have more use than just replenishing soil nitrogen. Actinorhizal plants have become very important trees in areas infected with conifer root disease as they are resistant to this disease and they can be grown for valuable timber and pulpwood. Actinorhizal plants also possess the ability to adapt to a variety of environments and can be used in sustainable agricultural practices such as creating windbreaks and preventing soil erosion.

Notes[edit]

 * 1) ^ Jump up to:a b c Wall 2000
 * 2) ^
 * 3) ^ Jump up to:a b c Schwintzer & Tjepkema 1990
 * 4) ^ Zavitovski & Newton 1968
 * 5) ^ Benson & Clawson 2000
 * 6) ^ Kistner & Parniske 2002
 * 7) ^ Laplaze et al. 2000
 * 8) ^ Vessey, Pawlowski & Bergman 2005
 * 9) ^ Normand et al.
 * 10) ^ Hocher et al.
 * 11) ^ Gherbi et al. 2008

References[edit]

 * Wall, L. (2000), "The actinorhizal symbiosis", Journal of Plant Growth and Regulation, 19 (2): 167–182,


 * Schwintzer, C. R.; Tjepkema, J. (1990), The Biology of Frankia and Actinorhizal Plants, Academic Press,


 * Benson, D. R.; Clawson, M. L. (2000), "Evolution of the actinorhizal plant nitrogen-fixing symbiosis", in Triplett, E. (ed.), Prokaryotic Nitrogen Fixation: A Model System for the Analysis of a Biological Process, Norfolk, UK: Horizon Scientific Press, pp. 207–224,


 * Zavitovski, J.; Newton, M. (1968), "Ecological importance of snowbrush Ceanothus velutinus in the Oregon cascade", Ecology, 49 (6): 1134–1145,


 * Kistner, C.; Parniske, M. (2002), "Evolution of signal transduction in intracellular symbiosis", Trends in Plant Science, 7 (11): 511–518


 * Laplaze, L.; Duhoux, E.; Franche, C.; Frutz, T.; Svistoonoff, S.; Bisseling, T.; Bogusz, D.; Pawlowski, K. (2000), "Casuarina glauca prenodule cells display the same differentiation as the corresponding nodule cells", Molecular Plant-Microbe Interactions, 13 (1): 107–112,


 * Vessey, J. K.; Pawlowski, K.; Bergman, B. (2005), "Root-based N2-fixing symbioses: Legumes, actinorhizal plants, Parasponia sp. and cycads", Plant and Soil, 266 (1): 205–230,


 * Gherbi, H.; Markmann, K.; Svistoonoff, S.; Estevan, J.; Autran, D.; Giczey, G.; Auguy, F.; Péret, B.; Laplaze, L.; Franche, C.; Parniske, M.; Bogusz, D. (2008), "SymRK defines a common genetic basis for plant root endosymbioses with arbuscular mycorrhiza fungi, rhizobia, and Frankia bacteria", Proceedings of the National Academy of Sciences, 105 (12): 4928–4932,


 * Hocher, V.; Auguy, F.; Argout, X.; Laplaze, L.; Franche, C.; Bogusz, D. (2006), "Expressed sequence-tag analysis in Casuarina glauca actinorhizal nodule and root", New Phytologist, 169 (4): 681–688,


 * Normand, P.; Lapierre, P.; Tisa, L. S.; Gogarten, J. P.; Alloisio, N.; Bagnarol, E.; Bassi, C. A.; Berry, A. M.; Bickhart, D. M.; Choisne, N.; Couloux, A.; Cournoyer, B.; Cruveiller, S.; Daubin, V.; Demange, N.; Francino, M. P.; Goltsman, E.; Huang, Y.; Kopp, O. R.; Labarre, L.; Lapidus, A.; Lavire, C.; Marechal, J.; Martinez, M.; Mastronunzio, J. E.; Mullin, B. C.; Niemann, J.; Pujic, P.; Rawnsley, T.; Rouy, Z. (2006). "Genome characteristics of facultatively symbiotic Frankia sp. Strains reflect host range and host plant biogeography". Genome Research. 17 (1): 7–15.
 * Benson, David (1993). "Biology of Frankia Strains, Actinomycete Symbionts of Actinorhizal Plants". American Society for Microbiology. 57: 294–312.
 * Persson, Tomas (2015). "Candidatus Frankia Datiscae Dg1, the Actinobacterial Microsymbiont of Diatisca glomerata, Expresses the Canonical nod Genes nodABC in Symbiosis with its Host Plant"
 * Swensen, Susan (1996). "The Evolution of Actinorhizal Symbiosis: Evidence for multiple origins of the Symbiotic Association". American Journal of Botany: 1503–1512.
 * Froussart, Emilie (2016). "Recent Advances in actinorhizal symbiosis signaling". Plant Molecular Biology: 613–622.
 * Benoit, Larry (1997). "Flavonoid-like compounds from seeds of red alder (Alnus rubra) influence host nodulation by Frankia (Actinomycetales)". Physiologia Plantarum: 588–593.
 * Boonkerd, Nantakorn. "Symbiotic association between Frankia and actinorhizal plants". Developments in Plant and Soil Sciences. 79: 327–328.
 * Schwencke, Jamie (2001). "Advances in Actinorhizal Symbiosis: Host Plant-Frankia Interactions, Biology, and Applications in Arid LAnd Reclamation. A Review". Arid Land Research and Management: 285–327.