Actinorhizal plants form N2-fixing root nodules with the actinomycete Frankia. There are some 200 species of actinorhizal plants belonging to 25 genera in eight families (Huss-Danell 1997). Nearly all of them are woody plants. They are useful in soil reclamation, bioenergy production, as wind-breaks and as ornamentals. Some species serve as browse, and HippophaƩ rhamnoides has edible and highly nutritious fruits and can provide raw material for phytochemical products. Representatives of actinorhizal plants are native in most regions of the world and several species, in particular Casuarina spp., are planted in many areas.

Strains of Frankia have been isolated from nodules of nearly all host genera (Benson, Silvester 1993). The majority of Frankia strains are from macerated nodules and only few originate from a single spore. Frankia grows by extension and branching of its septate, ca 0.5 to 1.5 pm wide filaments (sometimes called hyphae). In N-free culture and in nodules (except for genera Casuarina, Allocasuarina) tips of branched filaments differentiate into so-called vesicles, the site of nitrogenase. In symbiosis vesicles vary in size (up to 6 pm in diameter), shape and septation as determined by the host (Huss-Danell 1997). In culture and in some root nodules Frankia filaments also differentiate into sporangia with spores (Benson, Silvester 1993).

Actinorhizal nodules show a large variation in their morphology and anatomy. There is also a large variation in their physiological and biochemical solutions on how to cope with the dilemma of aerobic nodule metabolism coupled to oxygen sensitive nitrogenase (Huss-Danell 1997). All these characters are determined by the host. Actinorhizal nodules thus offer a range of fascinating interactions between plants and microbes for further investigation.

Depending on the host there are two different modes of infection by Frankia (Wall 2000). Intracellular penetration via root hairs occurs in the families Betulaceae, Casuarinaceae and Myricaceae. Cell divisions in root cortex give rise to a so-called prenodule but the actual root nodule originates in the pericycle and appears like a modified lateral root. Intercellular penetration occurs in the families Elaeagnaceae, Rhamnaceae and Rosaceae. Root hairs are not affected and prenodules are not formed. The root nodule originates in pericycle. In families Coriariaceae and Datiscaceae, the infection pathway is not yet described.

In intracellularly infected plants (Berry, Sunell 1990; Berg 1999), already from the very first ingrowth into a root hair, Frankia is encapsulated in a plant derived "capsule", a modified primary cell wall, and a membrane continuous with the plasmalemma. This encapsulation persists throughout the nodule and surrounds filaments as well as vesicles. In intercellularly infected plants (Miller, Baker 1985) Frankia grows between cells and becomes encapsulated once it grows into a cell of the emerging root nodule.

Actinorhizal nodules, or clusters of nodule lobes, are not evenly distributed on the root system. Rather, they occur singly or in groups with some spacing in between. This indicates that nodulation is regulated. Indeed, regulatory processes resembling "autoregulation" in legumes are seen in Alnus (Wall, Huss-Danell 1997) and in Discaria (Valverde, Wall 1999). Nodulation is also influenced by environmental factors such as nutrients. Like in legumes, N inhibits nodulation as well as N2-fixation (Huss-Danell 1997). In Alnus a multivariate study comprising all six macronutrients showed that N2 fixation per plant and the proportion of N in plants derived from N2 fixation (%Ndfa) was inhibited by N but stimulated by P. The highest %Ndfa was obtained at low N and high P (Ekblad, Huss-Danell 1995). Also in legumes a stimulating effect of P is seen and has been ascribed to either a generally stimulated growth (e.g. Robson et al. 1981) or a specific effect on nodules (Israel 1987). In our work on nodulation in actinorhizal plants we address the following questions. Can P stimulation counteract N inhibition? Does P act via stimulated plant growth or does P act specifically on nodulation? Does P act locally or systemically? Are there differences among host genera or infection pathways?

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