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= Actinorhizal Symbiosis =

Introduction
Actinorhizal symbiosis refers to nitrogen-fixing nodulating bacteria of the genus Frankia and their interactions with their host plants, more specifically referred to as actinorhizal plants. The actinobacterium, Frankia, infects only eight dicotyledonous families. These particular bacteria can invade their host plant both intercellularly and intracellularly depending on the species. The resulting nodules are multilobed coralloid structures of a modified root. Both free-living and host invaded nitrogen fixation occurs within the actinobacteria. Genomic approaches have been applied as a means to better understand the symbiosis that occurs and the dynamics such as nutrient transport between the bacteria and host plant. With three main clusters that constitute the genus Frankia, debate on the phylogeny and genetic relationships among the clusters exist.

Classification
Frankia was first isolated in 1956 and later classified in 1978 by Callahan from the plant Comptonia peregrina and later named by Becking.

Phylogeny
Actinobacteria belongs to the order Actinomycetales in the family Frankiaceae in the genus Frankia with three related clusters: Actinorhizal plants belong to the same clade as legumes and parasponia although they all infect different plants. The genus Frankia has not been described to the species level, only categorized into the four clusters (sometimes referred to as lineages). Actinobacteria encompasses non-actinorhizal taxa as well
 * Elaeagnus umbellata with Frankia.JPGer 1: Nodulate plants within the Fagales in Betulaceae and Myricaceae. Referred to as the "Alnus strains". This includes an exception in the case of Gymnostoma.
 * Cluster 2: Nodulate plants in Rosales and Cucurbitales composing four families. Referred to as the "Rosaceous strains".
 * Cluster 3: Nodulate plants in Fagales and Rosales composing five families. Referred to as the "Elaeagnus strains".
 * A fourth cluster is rarely recognized and is referred to as the "Atypical strains" due to not being able to fix nitrogen and having ineffective root nodules.

Frankia Genome
Considered to be nodulating bacteria having nod genes coupled with horizontal transfer among both alpha and beta proteobacteria. The 16S rRNA sequences show 97% identical relationship between the genus Frankia. The strains range from five to nine Mbp in terms of their genome sizes reflecting the diversity of plant infection. This indicates a general trend of the larger the genome, the more plants that are infected by that specific strain.

Evolution
The four Frankia lineages are believed to have appeared 100 million years ago with the arrival of major angiosperm groups. This appearance is hypothesized to have originated in the families Myricaceae and Betulaceae.

Physiology and Morphology
Frankia is distinguished from other bacteria by its specialized thick-walled organelle called a diazovesicle in the mycelium as well as its nonmotile spores contained in a multiocular sporangia. What sets Frankia apart from other bacteria symbionts is that it can fix nitrogen under aerobic and microaerobic conditions. The strains of Frankia can form three different cell types growing in hyphae form as well as the multicular sporangia form mentioned above. It has also been shown that Frankia can fix nitrogen as a free-living bacterium.

Root Nodules
The nodules created by actinobacteria allow the host plant to colonize harsh environments that it otherwise would not be able to. The root symbionts nodulate perennial woody dicotyledonous plants occupying more than 200 species but do not infect leguminous plants. The nodules created are multilobed structures of modified lateral roots with no root cap with the infected cells in the cortex along with a meristem at the apex. Auxin is an important component to the growth of Frankia in its host plant at the time of infection when an accumulation develops in the infected cells. The purpose of the nodules are to fix nitrogen and this occurs within the vesicles of Frankia where nitrogenase is formed and dinitrogen is fixed. It is important to note that the nodules of Casuarina sp. and Allocasuarina sp. have no vesicle formation. It is hypothesized that a symCgHb (hemoglobin) protein provides nitrogenase protection allowing for this absence of vesicles to occur. Vesicles are often observed when there are low levels of nitrogen with normal oxygen availability. Several layers of hopanoids compose the vesicles that restricts oxygen concentrations.

Intracellular infection
Frankia intracellularly infects Fagales by root hair penetration. This is seen in the genera Myrica, Comptonia, Alnus, and Casuarina by root hair deformation followed by penetration. The infection penetrates into the root cortex by the means of a tubular matrix called the infection thread creating the prenodule. The hyphae originate in the prenodule and expand from there. The hyphae later infect the primordial cells creating the nodule lobe. After successful nodulation, a multilobed nodule will develop

Intercellular infection
Frankia intercellularly infects Rosales and Cucurbitales. There is no observed root hair deformation in the genera Discaria, Eleagnus, Ceanothus and Cercocarpus. Instead, Frankia invades the middle lamella and travels between cortical cells via the apoplastic pathway in an electron-dense matrix. The hyphae extension to the prenodule is not observed intercellularly as it is seen intracellularly.

Atypical Strains
A strain by the name PtI1 is currently unclassified and found to have relatives that cannot reinfect the original host as well as lacking nif genes, meaning that they cannot fix nitrogen.

Herbivory
Frankia causes the down-regulation of carbon-based defenses which can increase the rates of herbivory on the host plant temporarily in the early leaf/seedling stages of the host plant. It has been found that this does not affect the net positive outcome of the plant symbiosis due to positive effects of Frankia on photosynthetic performance and growth.

Helper Bacteria
Pseudomonas spp. is not part of the genus Frankia but is hypothesized to act as a helper bacteria by preparing the root hair for penetration. It has not been proven that root hair deformation must occur in order for Frankia to successfully nodulate. The helper bacteria does speed the process up causing root hair deformation within 36 hours of inoculation. The four helper bacteria strains were found to be Gram-negative motile rods with rapid growth under aerobic conditions. Two strains were classified as Pseudomonas while one was classified as Chromobacteruym and the last left unclassified. It is to be noted that unspecified strains of Frankia do not modulate unless in the presence of the helper bacteria.

Geographic Distribution
Frankia symbiosis occurs over a broad range of distribution. Casuarinaceae are native to Australia and the western Pacific with Elaeagnaceae and Betulaceae occurring cross-continentally. Coriariaceae and Datiscaceae occur in the temperate zone on both hemispheres. Every continent has all three recognized clusters of Frankia except for Africa and Antartica.

Stressors
Some strains of Frankia have a metal resistance mechanism based on siderophores that allow them to cope with heavy metals, to an extent. This allows a tolerance to divalent cations. In alders growing on soil contaminated with metals, it was observed that Frankia helped alders increase biomass when present. When no nodulation occurs in alders with high metal concentrations, the Alders were observed to have less biomass in the year 2014. This has shown the possibility of Frankia having high metal tolerance though the bacteria can still be limited in its functionality in terms of symbiotic effects.