The Azolla–Anabaena symbiosis: endophyte continuity in the Azolla life‐cycle is facilitated by epidermal trichomes †

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Abstract
SUMMARY: Light, scanning electron, and transmission electron microscopy were used to document events tending to the re‐establishment of the Azolla mexicana–Anabaena azollae symbiosis during sexual reproduction. In mature megasporocarps. Anabaena akinetes occupy the space between the indusium and apical membrane, termed the inoculation chamber. Formation of the megagametophyte within the megaspore apparatus extends the floats but does not trigger akinete germination. The tatter is contingent upon embryogenesis. The embryo develops a cotyledonary leaf primordium bearing epidermal trichomes while within the archegonial chamber. Incipient akinete germination is discernible before the cotyledonary leaf with its four seriate trichomes, the cells of which exhibit TCU (transfer cull ultrastructure), and the shoot apex penetrate the apical membrane to enter the inoculation Chamber. Rapid development of the vase‐like cotyledonary leaf maintains and fosters the initial association of the Anabaena population, which includes akinetes, germinating akinetes and germlings, with the cotyledonary hairs and the shoot apex. It engulfs the Anabaena in the inoculation chamber as it surrounds the four trichomes and the shoot apes with its developing true leaves. The latter soon arch over the apes to further confine the Anabaena. The first four or five true leaves are unlobed. The first differentiates two multibranched epidermal trichomes while each of the others differentiate one. The cells of these distinctive trichomes exhibit TCU. They occur in a cavity‐like structure which forms in the unlobed leaves and are associated with Anabaena cells which appear to be degenerating. Oilier Anabaena cells entrapped in the region of the apical meristem yield vegetative filaments which provide the inoculum for the developing cavities of the normal, bilobed leaves. The possibility that akinete germination is initially triggered by diffusion of embryo‐produced oxygen and water vapour into the inoculation chamber is discussed as is the possibility that those Anabaena cells entrapped in the unlobed leaves arc cannabalized by the fern.