Actinorhizal plant

Actinorhizal plants are a group of "Angiosperms" characterized by their ability to form a symbiosis with the nitrogen fixing actinomycete "Frankia". This association leads to the formation of nitrogen-fixing root nodules.

Classification

Actinorhizal plants are dicots distributed among 3 angiosperm orders (Fagales, Cucurbitales and Rosales), 8 families and 24 genus (Betulaceae : "Alnus"; Casuarinaceae : "Gymnostoma", "Casuarina", "Allocasuarina", "Ceuthostoma"; Coriariaceae : "Coriaria"; Datiscaceae : "Datisca", Elaeagnaceae : "Eleagnus", "Hippophae", "Shepherdia"; Myricaceae: "Myrica", "Comptonia"; Rhamnaceae: "Colletia", "Discaria", "Kenthrothammus", "Retanilla", "Telguenea", "Trevoa", "Ceanothus"; Rosaceae: "Dryas", "Purschia", "Cowaniana", "Cercocarpus", "Chamaebatia") [Harvnb|Wall|2000] . Actinorhizal species are trees or shrubs, except for the genus "Datisca".

Many are common plants in temperate regions like alder, bayberry, sweetfern, "Dryas", "Chamaebatia" and "Coriaria". Some "Elaeagnus" species and Sea-buckthorns produce edible fruit. In tropical regions, "Casuarinas" are widely cultivated, becoming invasive species in some areas like "Casuarina glauca" in Florida.

Distribution & Ecology

Actinorhizal plants are found on all continents except for Antarctica. 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 like moraines, volcanic flows or sand dunes [Harvnb|Schwintzer|Tjepkema|1990] . 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 [Harvnb|Schwintzer|Tjepkema|1990] [Harvnb|Wall|2000] . Actinorhizal plants like alders are also common in the riparian forest [Harvnb|Schwintzer|Tjepkema|1990] .

Actinorhizal plants are the major contributors to nitrogen fixation in broad areas of the world, and are particularly important in temperate forests [Harvnb|Wall|2000] . 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 [Harvnb|Zavitovski|Newton|1968] .

Evolutionary origin

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 [Harvnb|Benson|Clawson|2000] . Nevertheless, actinorhizal plants and Legumes, the two major nitrogen-fixing groups of plants share a relatively close ancestor, as they all belong to the Rosid I clade. This ancestor may have developed a "predisposition" to enter in symbiosis with nitrogen fixing bacteria and this lead to the independent acquisition of symbiotic abilities by ancestors of the actinorhizal and Legume species. The genetic program used to establish the symbiosis has probably recruited elements of the arbuscular mycorrhizal symbioses, a much older and widely distributed symbiotic association between plants and fungi [Harvnb|Kistner|Parniske|2002] .

The symbiotic nodules

As in legumes, nodulation is favored by nitrogen deprivation and is inhibited by a 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 infection of a root hair and is followed by the formation of a primitive symbiotic organ lacking any particular organization, a prenodule [Harvnb|Laplaze|2000] . The second mechanism of infection is called crack entry and is well described in "Discaria" species. In this case bacteria penetrate the root extracellularly, growing between cracks formed as a consequence of lateral root emergence. 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.

Few information is available concerning the mechanisms leading to nodulation. 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 heamoglobin and other nodulins) are also found in actinorhizal plants where they are supposed to play similar roles [Harvnb|Vessey|2003] . 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 [Harvnb|Normand|2006] and the development of RNAi and genomic tools in actinorhizal species [Harvnb|Hocher|2006] [Harvnb|Gherbi|2008] should help to a far better understanding in the following years.

Notes

References

*citation
last=Wall
first= Luis
title=The actinorhizal symbiosis
journal=J. Plant Growth Regul.
volume=19
issue=
pages=167-182
year=2000
issn=0721-7595
doi=10.1007/s003440000027
url=http://www.springerlink.com/content/aygm4ljhjr6qdalj/

*citation
last1=Schwintzer
first1= Christa
last2=Tjepkema
first2=John
title=The Biology of Frankia and Actinorhizal Plants
publisher=Academic Press
location=
year=1990
isbn=:012633210X

*citation
last1=Benson
first1= D
last2=Clawson
first2=M
title=Evolution of the actinorhizal plant nitrogen-fixing symbiosis "in" Ecology: individuals, populations and communities
publisher=Horizon Scientific Press
location=Norfolk, UK
year=2000
isbn=1-898486-19-0

*citation
last1=Zavitovski
first1= J
last2=Newton
first2=M
title=Ecological importance of snowbrush Ceanothus velutinusin the Oregon Cascade
journal=Ecology
volume=49
issue=
pages=1134-1145
year=1968

*citation
last1=Kistner
first1= C
last2=Parniske
first2 =M
title=Evolution of signal transduction in intracellular symbiosis
journal=Trends in Plant Science
volume=7
issue=11
pages=511-518
year=2002
doi=10.1016/S1360-1385(02)02356-7
url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TD1-473MP9V-9&_user=4296857&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000012518&_version=1&_urlVersion=0&_userid=4296857&md5=147bf541d50d19d5017613c92f45d389

*citation
last1=Laplaze
first1= L
last2=et al.
first2 =
title= Casuarina glauca Prenodule Cells Display the Same Differentiation as the Corresponding Nodule Cells
journal=Mol Plant Microbe Interact.
volume=1
issue=13
pages=107-12
year=2000
pmid=10656591
url=http://www.apsnet.org/mpmi/pdfs/1999/1108-02R.pdf
*citation
last1=Vessey
first1= JK
last2=et al.
first2 =
title= Root-based N2-fixing symbioses: Legumes, actinorhizal plants, Parasponia sp. and cycads
journal=Plant and soil
volume=266
issue=1
pages=205-230
year=2004
issn=0032-079X
*citation
last1=Gherbi
first1= H
last2=et al.
first2 =
title= SymRK defines a common genetic basis for plant root endosymbioses with arbuscular mycorrhiza fungi, rhizobia, and Frankia bacteria.
journal=PNAS
volume=105
issue=12
pages=4928-32
year=2008
pmid=18316735
url=http://www.pnas.org/cgi/content/abstract/105/12/4928
*citation
last1=Hocher
first1= V
last2=et al.
first2 =
title= Expressed sequence-tag analysis in Casuarina glauca actinorhizal nodule and root.
journal= New Phytologist
volume=169
issue=4
pages=681-8
year=2006
pmid=16441749
url=http://www.blackwell-synergy.com/doi/abs/10.1111/j.1469-8137.2006.01644.x
*citation
last1=Normand
first1= P
last2=et al.
first2 =
title= Genome characteristics of facultatively symbiotic Frankia sp. strains reflect host range and host plant biogeography
journal=Genome Research
volume=17(1):7-15
issue=1
pages=7-15
year=2007
pmid=17151343
url=http://www.genome.org/cgi/content/abstract/17/1/7

External links

* [http://web.uconn.edu/mcbstaff/benson/Frankia/FrankiaHome.htm Frankia and Actinorhizal plant Website]