Casuarina Root Exudates Alter the Physiology, Surface Properties, and Plant Infectivity of Frankia sp. Strain CcI3

Abstract

The actinomycete genus Frankia forms nitrogen-fixing symbioses with 8 different families of actinorhizal plants, representing more than 200 different species. Very little is known about the initial molecular interactions between Frankia and host plants in the rhizosphere. Root exudates are important in Rhizobium-legume symbiosis, especially for initiating Nod factor synthesis. We measured differences in Frankia physiology after exposure to host aqueous root exudates to assess their effects on actinorhizal symbioses. Casuarina cunninghamiana root exudates were collected from plants under nitrogen-sufficient and -deficient conditions and tested on Frankia sp. strain CcI3. Root exudates increased the growth yield of Frankia in the presence of a carbon source, but Frankia was unable to use the root exudates as a sole carbon or energy source. Exposure to root exudates caused hyphal "curling" in Frankia cells, suggesting a chemotrophic response or surface property change. Exposure to root exudates altered Congo red dye binding, which indicated changes in the bacterial surface properties at the fatty acid level. Fourier transform infrared spectroscopy (FTIR) confirmed fatty acid changes and revealed further carbohydrate changes. Frankia cells preexposed to C. cunninghamiana root exudates for 6 days formed nodules on the host plant significantly earlier than control cells. These data support the hypothesis of early chemical signaling between actinorhizal host plants and Frankia in the rhizosphere.A ctinorhizal symbioses are mutualistic interactions that occur between actinorhizal plants and the actinomycete genus Frankia (27,38). Frankia exists either in a free-living state in the soil or in symbiosis with actinorhizal plants (2, 4, 31). Actinorhizal plants consist of 8 different plant families, including over 200 different species of woody dicotyledonous trees and shrubs (31,38,42). The globally distributed actinorhizal plants are found on every continent except Antarctica and are able to grow in a diverse set of natural habitats, including arid lands, plains, tundra, and temperate forests. Actinorhizal plants are pioneer plant species that are able to grow in extremely nutrient-poor soil conditions and, with the aid of Frankia, are able to reclaim surrounding soil in disrupted environments. Actinorhizal plants are important in agroforestry, in soil reclamation, and as a fuel source.While actinorhizal symbiosis has been well studied at a morphological level since the 1970s, very little is known about the molecular interactions that occur between the plant hosts and Frankia during the establishment of the association. The establishment of the symbiosis encompasses the infection and nodulation processes (23,27,38). In general, the bacterium needs to recognize a host plant and the host needs to identify the bacterium as a friend, not a foe. The bacteria enter the plant and establish the association after several steps. The infection ultimately leads to the formation of the mature nodule.There is a paucity of information o...

Diversity of Bacterial Microbiota of Coastal Halophyte Limonium sinense and Amelioration of Salinity Stress Damage by Symbiotic Plant Growth-Promoting Actinobacterium Glutamicibacter halophytocola KLBMP 5180

“…This finding is in accordance with the reports of the bacterial microbiota of the coastal halophytes Limonium vulgare (39) and Messerschmidia sibirica (40). This is probably due to the exudates secreted by plant roots, which could be used as carbon and nitrogen sources for bacterial growth in the rhizosphere (41).…”

Aspects of nitrogen-fixingActinobacteria, in particular free-living and symbioticFrankia

“…Root exudates play an important role in Rhizobium –legume interactions, particularly for initiating synthesis of proteins required for nodulation (Nod factors). Root exudates were collected from C. cunninghamiana plants grown under nitrogen‐sufficient and nitrogen‐deficient conditions and tested on Frankia strain CcI3 (Beauchemin et al ., ). The presence of root exudates increased the growth of Frankia CcI3, but the strain could not use the root extract as the sole carbon source.…”

“…These two results suggest that the presence of root extracts caused a chemotrophic response and/or changes in the surface properties of the bacteria at the fatty acid level. In addition, plants inoculated with Frankia previously exposed to root extracts nodulated after 15 days, while plants inoculated with unexposed Frankia nodulated only after 19.6 days (Beauchemin et al ., ). Physiological changes to Frankia from host root exudates were therefore beneficial to the infection and nodulation process.…”

The role of flavonoids in the establishment of plant roots endosymbioses with arbuscular mycorrhiza fungi, rhizobia and Frankia bacteria

“…23 In the case of actinorhizal symbiosis, Frankia signals are unknown. 24,25 However, it has been previously shown that Frankia alni (ACN14a) produces a root hair deforming factor (RHDF) in culture supernatant that reacts with Alnus glutinosa root hair cells inducing branching and curling of root hair cells. 26 This factor was shown to have a molecular weight below 3,000 da, to be heat-stable (similar to Rhizobium Nod factor) but also to be hydrophilic and to resist to chitinases (contrary to Rhizobium Nod factor).…”