Two types of nitrogen-fixing root nodules have been identified in actinorhizal plants based on the extent of spore formation by Frankia within nodules. They are called spore (-) for nodules where spores are absent or few and spore (+) for those containing many spores.
Frankia grows slowly and often takes 2 months to show up in culture. They are known to exhibit polymorphism of colonies ranging from starfish, diffuse or compact shapes. The formation of round, cylindrical or highly irregular compartmentalized sporangia intrahyphally or terminally, filled with spores in submerged culture is unique to the genus Frankia. The hyphae are poorly branched, may be colourless or pigmented depending upon the nature of the medium. Round, cylindrical, stipitate vesicles are formed in nitrogen-free media. These swollen tips of hyphae (vesicles) assume various shapes ranging from pear, club or filamentous types and are regarded as the sites of nitrogen fixation (like heterocysts of blue-green algae) both in culture media and within nodules. The exception to this rule is Casuarina nodules which lack vesicles and where other mechanisms operate for nitrogen fixation. Vesicles in general possess an intrinsic oxygen protection mechanism to sustain continued nitrogenase activity.
Morphological Features of Frankia in Culture Medium
| A.Open mesh Microcolony From an Exponential Phase |
B. Septate Hyphae with Intercalary Sporangia Same and the strain in the Rhizophere |
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| C. Root Hairs of C.Equisetifolia with Infection threads |
D.3 Week old Nodules (N) of C.Equisetifolia with Nodular Roots |
E-Enlarged version of frantia Showing Bulbous Vesicles and large Sporangia Indicated by Black Arrows |
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