Franka Dicotyledon Symbiosis

The aerobic Gram-positive actinomycetes belonging to the genus Frankia are diazotrophic bacteria that are capable of inducing formation of N2-fixing nodule lobes in roots of many dicotyledonous angiosperms. The plants nodulated by Frankia strains are known as actinorhizal plants and include 8 families, 24 genera, and over 200 species, most of which are perennial woody shrubs or trees distributed in all landmasses except Antarctica. The actinorhizal plants share a predilection for marginally fertile soils and the majority are pioneers on nitrogen-poor sites. In addition, many actinorhizal species are able to tolerate environmental stresses such as heavy metals, high salinity, drought, cold, and extreme pH. They inhabit a variety of ecosystems, including coastal dunes, riparian zones, alpine communities, arctic tundra,

Table 3 Association between Frankia and actinorhizal plants

Frankia phylogenetic groups

Plant families

Plant genera

Group I strains

Coriariaceae

Coriaria

Datiscaceae

Datisca

Rosaceae

Cercocarpus, Chamaebatia, Dryas, Cowania, Purshia

Rhamnaceae

Ceanothus

Group II strains

Betulaceae

Alnus

Casuarinaceae

Casuarina, Allocasuarina, Ceuthostoma, Gymnostoma

Myriaceae

Myirica, Comptonia

Group III strainsĀ®

Elaeagnaceae

Elaeagnus, Hippophae, Shepherdia

Rhamnaceae

Calletia, Discaria, Kentrothamnus, Retanilla, Talguenea, Trevoa

aGroup III strains are more promiscuous and can occasionally inhabit root nodules of Rosaceae, Coriariaceae, and Ceanothus.

aGroup III strains are more promiscuous and can occasionally inhabit root nodules of Rosaceae, Coriariaceae, and Ceanothus.

glacial tills, and forests. Actinorhizal plants are especially important in high latitude regions, such as Scandinavia, Canada, Alaska, and New Zealand where Leguminosae are absent or rare while actinorhizal plants are abundant and capable of vigorous growth. Much of the new nitrogen entering these ecosystems comes from the actinorhizal symbioses that, on the whole, account for over 15% of the biologically fixed nitrogen worldwide.

The filamentous frankiae, besides in symbiotic association with actinorhizal plants, can also occur as free-living diazotrophic organisms. In pure culture, Frankia strains produce extensive hyphae and sporangia. In response to nitrogen deprivation, they also differentiate vesicles, named diazovesicles, which contain nitrogenase and are the site of N2 fixation. The diazovesicles are encapsulated by a series of laminated lipid layers that are rich in neutral lipids, glycolipids, and hopanoids. This envelope, whose thickness depends on the environmental O2 concentration, works as an oxygen-diffusion barrier, providing an anaerobic environment for nitrogenase to function inside vesicles.

The Frankia strains that nodulated actinorhizal plants can be phylogenetically distinct in three groups (groups I, II, and III) that infect specific dicotyledon families (Table 3).

In these actinorhizal symbioses, root nodule formation begins with the host-symbiont recognition through the exchange of molecular signals, the knowledge of which is still limited. However, some findings suggest that the signaling mechanisms of Frankia-actinorhizal plants might be similar to those of rhizobia-legumes.

Frankia strains can infect the host root by intracellular or intercellular mechanisms. Intracellular infection, such as that occurring in Myrica, Comptonia, Alnus, and Casuarina, starts with penetration of bacterial hyphae in a curled root hair. Afterward the hyphae move in cortical cells encapsulated with a layer of plant cell wall material surrounded by host plasmalemma. In intercellular infection, common in Elaeagnus, Ceanotus, and Cercocarpus, the bacterial hyphae penetrate between two adjacent rhizo-derm cells and progress apoplastically through cortical cells encapsulated in a pectic matrix. Concomitantly, cell divisions induced in the root pericycle give rise to the nodule lobe primordium to which the hyphae move. The mature actinorhizal nodule lobe resembles a modified lateral root with an apical meristem but without a root cap. It shows a central stele with vascular tissues and has Frankia hyphae restricted to the cortical cells (Figure 5).

In most actinorhizal symbioses, the N2-fixing activity of Frankia in infected cells is associated with differentiation of diazovesicles whose morphology is strictly controlled by the host plant. As in the free-living frankiae, these vesicles are surrounded by the multilayered lipid envelope and contain nitrogenase. However, in some symbioses (with Myrica, Coriaria, Comptonia, and Casuarina), the Frankia hyphae proliferate without forming vesicles.

In infected cells ofmature nodule lobes, some mechanisms take place to lower the oxygen tension near the site of the oxygen-intolerant nitrogenase. The first diffusion resistance to oxygen is provided in diazovesicles by the multilayered envelope and a further reduction of the pO2 is obtained through their high respiration rate. In many nodule lobes devoid of diazovesicles, the infected cells

Figure 5 Schematic drawing of a mature actinorhizal nodule lobe.

contain high levels ofhemoglobins that have homologous sequences to leghemoglobins and are believed to play the same role. In these nodules, moreover, a low pO2 may be maintained by lignification of the host cell walls. Finally, the activity of uptake hydrogenases can also help to protect the nitrogenase against O2 in both hyphae and diazovesicles of the symbiotic frankiae.

In free-living Frankia strains, as in the other free-living diazotyrophs, the ammonia produced by N2 fixation is assimilated by the organism via the GS-GOGAT pathway. On the contrary, these enzymes are differently regulated in the symbiotic frankiae. In diazovesicles of root nodule lobes GS activity is very low and ammonia remains unassimilated. As in rhizobia-legume symbiosis, NH 3 is released into the host cell where its assimilation gives rise to amino acids and other organic nitrogen compounds. Some are furnished to the bacterium, but most of them are transferred to the plant shoot.

The scarcity or lack of GS activity in the diazotrophic symbiont also characterizes the rhizobia legumes as well as some cyanobacterial symbioses such as Anabaena-Azolla, showing a remarkable convergence of physiological strategies in the N2-fixing associations.

The actinorhizal plant must provide photosynthates to the symbiotic bacterium. As in the rhizobia-legume symbiosis C4 dicarboxylates derived from sucrose metabolism occurring in the host cell are likely to be the carbon sources for Frankia strains in actinorhizal symbiosis.

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