Factors influencing the distribution, growth and infectivity of Casuarina-infective Frankia

Soil dose to nitrogen-fixing actinorhizal plants have greater Frankia infectious capacity. In contrast to most frankiae, Casuarina-infective Frankia are usually not found outside the native range of Casuarina. The lack of Casuarina-infective Frankia beyond a zone of host influence led me to hypothesize that Casuarina may affect the growth and infectivity of its symbiont in soils. In a model system representing possible host influences on the soil environment I used a series of baiting studies with Casuarina seedlings as Frankia traps, cultured isolates of Frankia and other soil microorganisms, DNA analysis of microsymbionts, and analysis of soil chemical properties, to find supporting evidence for this hypothesis. An initial baiting study with Casuarina and Myrica seedlings revealed Casuarina-infective Frankia was detectable only in soils collected less than 20 m from Casuarina trees. Soils near Casuarina trees also had higher levels of micro- and macro-nutrients and organic matter. Frankia in Casuarina nodules was in the Casuarina “type 1” group, whereas Frankia in the Myrica nodules was not in the Casuarina group.; The growth of diverse Frankia isolates was stimulated by aqueous extracts of various Casuarina cunninghamiana tissue types. Green cladodes stimulated the growth of Casuarina-derived Frankia strains more than the other Frankia strains tested. Cladode extracts stimulated the growth of cultures of the actinomycete, Streptomyces albus, and the fungus, Rhizopus homothallicus . The growth of five other common isolates of soil microorganisms was either inhibited or unaffected by experimental addition of cladode extracts.; Further studies revealed that Casuarina cladodes increased the infectivity of a soil inoculum by a factor of 3.2, whereas leaves of a sympatric check plant, Terminalia catappa (Combretaceae), barely increased the same soil's infectivity. Charcoal and clay added to the seedling growth substrate to bind organic material significantly inhibited nodulation of C. cunninghamiana by a Frankia isolate, suggesting the host may release compounds necessary to the nodulation process. In a final study, the infectivity of soils inoculated with Frankia isolate CjI82001 decreased over time, whereas infectivity of soils without inoculum either increased or remained unaffected. A positive synergism was observed between the Frankia isolate and the soil biotic community, resulting in an increased infectivity of the Frankia isolate.