Actinorhizal nodules, referred to as actinorhiza, consist of multiple lobes, each of which resembles a modified lateral root without a cap (Fig. 2c). Nodule lobes have a central vascular bundle, a superficial periderm and both infected and non-infected cortical cells. The non-infected cells are thought to play an important role in the metabolism and transport of the primary product of N2 fixation and assimilation. In the genera Casuarina, Datisca, Myrica, Comptonia and Gymnostoma, a so-called root nodule is also observed at the apex of each lobe (Fig. 2d) (Duhoux et al. 1996). This root nodule lacks root hairs, has a reduced root cap and displays negative geotropism. It might be involved in the diffusion of gas, especially oxygen, within and outside the nodule lobe.
Due to activity of the apical meristem, actinorhizal nodule lobes show an indeterminate growth pattern and have a developmental zonation in which specific patterns of gene expression are observed (Fig. 2e) (Duhoux et al. 1996). In zone 1, the apical meristem is free of Frankia. Adjacent to the meristem is an infection zone where some of the young cortical cells resulting from the meristem activity are infected by Frankia. The subsequent fixation zone contains both infected and uninfected cortical cells. Hypertrophied-infected cells are filled with Frankia hyphae that differentiate N2 fixing vesicles. In some species like Casuarina, infected cells have a lignified cell wall and no vesicles are observed (Fig. 2f) (Berg and McDowell 1987). Finally, a basal senescence zone is observed in old nodules; plant cells and bacteria degenerate and nitrogen fixation is switched off. Although all nodule lobes have an apical meristem, the growth of individual lobes is limited. Additional branch lobes are formed as lateral primordia in the pericycle of the preceding lobe. In plantations of Casuarina, nodules of an average size of 25 cm have been observed.
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