Actinorhizal plant
As Aktinorrhiza root nodules are called in certain angiosperms, which are a symbiosis with nitrogen-fixing bacteria of the plants of this type Frankia alni.
Aktinorrhiza plants
With the exception of the herbaceous genus Datisca Aktinorrhiza all plants are trees or shrubs. Many academics are pioneer species in early successional stages. They are often the first plants recolonize the sites after a fault. Some representatives specialize in extreme locations such as flood plains, warm arid or semi-arid areas or rocky sites. The nitrogen fixation can be 240 up to 350 kg per hectare per year.
Aktinorrhiza is known from 24 species, spread over eight families in three orders:
- Fagales Betulaceae Alnus
- Myrica
- Comptonia
- Coriariaceae coriaria
- Datisca
- Casuarinaceae Gymnostoma
- Casuarina
- Allocasuarina
- Ceuthostoma
- Colletia
- Discaria
- Kenthrothammus
- Retanilla
- Telguenea
- Trevoa
- Ceanothus
- Dryas
- Purshia
- Cowaniana
- Cercocarpus
- Chamaebatia
The plants are widely used by the people: for soil restoration, plant and firewood, to the coast and rehabilitation to combat desertification.
Root nodules
Arising in the course of symbiosis - forming root nodules that are anatomically clearly from the externally similar root nodules of legumes. Aktinorrhiza nodules have a central vascular bundle. This is the infected tissue with the bacteria. This in turn is surrounded by a cortical parenchyma. The nodules are modified lateral roots. They are not determined, so have a potentially unlimited growth. There is an apical meristem, from which the Knöllchenparenchym developed at the nodule tip. At the meristem, a zone of uninfected cells closes, followed by freshly infected cells, which have the typical vesicles lacking ( infection zone). In the central fixation zone there are two cell types: on the one mature infected cells with vesicles. In these, the nitrogen fixation takes place. On the other uninfected cells, probably serve the assimilation of fixed nitrogen and the exchange of carbon. The spatial distribution of infected and uninfected cells is different depending on the plant species.
The infection of roots with the bacteria takes place depending on the plant species in one of two methods of infection:
- Intracellular or root hair infection is typical of the so-called primitive Aktinorrhiza plants. Here, a root hair inside out by indentation on a Frankia - filament. The infection strand grows in the direction of inner root bark. The root forms immediately after infection, a new meristem in the outer bark, the newly formed tissue is occupied by Frankia. Frankia is the typical vesicle and begins to form the requisite nitrogen for the fixing enzyme nitrogenase. This only temporarily existing tissue is referred to in English as prenodule ( " Vorknöllchen " ) and is characteristic of the intracellular infection. Later, the cells in the pericycle of the root to divide and form the primordium for the real nodules. This grows and then merges with the Vorknöllchen.
- In the intercellular infection, the Frankia filaments pass through the intercellular space in the root tissue. The middle lamella of the cell wall is dissolved, the filament passes through the epidermis into the outer cortex area. In response to the penetration of the plant deposited at the site of infection from extracellular electron-dense material.
In both cases, cell division, and the formation of nodule organs in the pericycle begins. The nodules formed analogously to a lateral root.
Little is known about the mechanisms of this symbiosis. The equivalent of NOD factors of rhizobia has not yet been discovered. In the nodules Actinorrhiza - like genes are expressed as the legumes. The lack of genetic regulators and the fact that the majority of Actinorrhiza plants are trees or shrubs, thus far hindered their studies. Recently three Frankia genomes were sequenced and RNAi developed, which should facilitate a better understanding of this symbiosis in the coming years.
Documents
- L. G. Wall: The Actinorhizal Symbiosis. Journal of Plant Growth Regulation, 2000, Volume 19, pp. 167-182. doi: 10.1007/s003440000027