Inoculant made of encapsulated Frankia: assessment of Frankia growth within alginate beads

Combining alders, frankiae, and mycorrhizae for the revegetation and remediation of contaminated ecosystems

Alder shrubs and trees that are capable of forming symbioses with mycorrhizal fungi and the nitrogen-fixing actinomycete Frankia sp. are particularly hardy species found worldwide in harsh and nutrient-deficient ecosystems. The mycorrhizal symbiosis may assist alders in nutrient and water uptake, while the actinorhizal symbiosis provides assimilable nitrogen. It is through these highly efficient symbioses, in which microsymbionts benefit from plant photosynthates, that actinorhizal plants such as alders colonize poor substrates, enrich soil, and initiate plant succession. These natural capabilities, combined with careful screening of microsymbionts and host plants, may prove useful for the rehabilitation of disturbed ecosystems. Although alders have been used extensively at industrial scales in forestry, nurse planting, and contaminated land revegetation, relatively little research has focussed on their actinorhizal and mycorrhizal plant–microbe interactions in contaminated environments. To study such a topic is, however, critical to the successful development of phytotechnologies, and to understand the impact of anthropogenic stress on these organisms. In this review, we discuss two alder-based phytotechnologies that hold promise: the stimulation of organic contaminant biodegradation (rhizodegradation) by soil microflora in the presence of alders, and the phytostabilization of inorganic contaminants. We also summarize the plant–microbe interactions that characterize alders, and discuss important issues related to the study of actinorhizal and (or) mycorrhizal alders for the rehabilitation of disturbed soils.

Effects of storage time and temperature on the infectivity and effectiveness of Frankia entrapped in polyacrylamide gel

Four Frankia-Casuarina endosymbiont strains were tested for their infectivity and effectiveness on C. equisetifolia plants after 1 d, 3 and 6 months of Frankia storage at 7, 28 and 40 degrees C as liquid-cultures and polyacrylamide gel (PAG)-immobilized inocula. At lower temperature the number of nodules was the same or higher than control for liquid inocula except after 6 months of storage. For the PAG-entrapped Frankia lower numbers of nodules were recorded due to the use of high Frankia titers. In general, the results showed comparable plant dry mass, total nitrogen, root to shoot and nodules to plant ratios at lower temperature treatments. Increasing time and temperature was accompanied with reduced infectivity and effectiveness on inoculated plants. No nodulation was scored on plants inoculated with liquid and PAG-entrapped inocula stored at 40 degrees C for 6 months; subsequently, plant growth was inhibited. The variations in results obtained for different strains and treatments lead to variations in plant nitrogen-fixing ability. The use of PAG as a carrier for different Frankia strains is suggested; the recommended storage temperature for PAG-immobilized Frankia in 7-28 degrees C for up to 3 months.

Improved delivery of biocontrol Pseudomonas and their antifungal metabolites using alginate polymers

Alginate polymer was evaluated as a carrier for seed inoculation with a genetically modified strain Pseudomonas fluorescens F113LacZY, which protects sugar-beet against Pythium-mediated damping-off. F113LacZY survived in alginate beads at 5 log10 CFU/bead or higher counts for 8 weeks of storage, regardless of the conditions of incubation. In plant inoculation experiments, colonisation of the growing area of the root by F113LacZY, derived from alginate beads placed in the soil next to the seed or from an alginate coating around the seeds, was improved compared with application of just free cells of the strain. F113LacZY trapped in alginate beads was an effective producer of antifungal phloroglucinols as indicated by direct HPLC quantification of phloroglucinols and in vitro inhibition of both the indicator bacterium Bacillus subtilis A1 and the pathogenic fungus Pythium ultimum. Alginate polymer represents a promising carrier for the delivery of biocontrol inoculants for root colonisation and production of antifungal metabolites.