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Index >> Actinorhizal Plants (Frankia Induced Nodulation) >> Entry of Frankia to the Host Plant

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Actinorhizal Plants Frankia Induced Nodulation Root Nodules Caused by Frankia Actinorhizal Symbiosis Cross Inoculation Experiments Isolation and Characteristics of Frankia Cultural Characteristics of Frankia Taxonomy of Frankia Entry of Frankia to the Host Plant Structure of Actinorhizal Nodule

Entry of Frankia to the Host Plant

	Entry of Frankia to the Host Plant Frankia propagules normally occur in soils of all types including sandy soils on the sea shore where casuarinas grow profusely as a sand binder and fuel tree. The events that have been shown to occur in the root zone of Casuarina after Frankia inoculation under axenic conditions can be summarized as follows: Frankia cells get embedded in a mucilage layer in the root region or the spores may get attached to root hairs, the root hairs get deformed or curled.
The actual entry of Frankia into root hairs has not been seen but hyphae are seen as simple or multiple threads often branching inside the deformed hair in a host derived cell wall material that is continuous with the root hair cell wall (encapsulation). The threads could be seen penetrating the cortex and in some root sections, prenodule formation can be seen within 10-14 days. This primordiun protrudes from the root. Mitotic activity is rapid at this time probably due to growth substances, auxins, cytokinins, gibberellins and many unidentified substances. The infected cells and the nucleus appear enlarged with a prominent nucleolus.
Sooner or later, lateral roots in the vicinity of the primary nodule primordium appear, their meristems undergo branching and progressively get infected with Frankia resulting in the formation of a typical adult nodular structure referred to as a rhizothamnion'. In a sense, actinorhizal root nodule is essentially a modified lateral root. The nodule consists of a central vascular bundle surrounded by the endodermis and a cortical parenchyma with pockets of infected cells scattered all over. Ineffective nodules have few infected cells with only hyphae and lack vesicles whereas effective nodules have both sporangia and vesicles with active cytoplasm.
Haemoglobin-like substances whose role in actinorhizal N₂ fixation is not clear have been encountered in Alnus, Hippophae, Myrica and Casuarina nodules. However, the red colour of nodules in' some other actinorhizal plants is due to 'pigments of the anthocyanin type. Under hot humid coastal areas in Senegal casuarinas are known to produce aerial nodules on stem which fix nitrogen.

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Nitrogen Fixation and Assimilation Field Experiments on the Benefit of Frankia Inoculation Mycorrhizae and Actinorhizal Plants Proteoid Roots Genetics of Frankia Rhizobium Induced Root Nodulation of Parasponia Leaf Nodules Gymnosperm Nodules

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