Desiccation tolerance is switched on and off in the resurrection fern, Morhia caffrorum
Résumé
Resurrection plants are unique in that the vegetative tissues have the ability to dry to 5% relative water content (RWC) and recover full metabolism in existing tissues on rehydration. The mechanisms whereby this is achieved varies among the orders. In bryophytes, where drying and rehydration is rapid, tissues suffer damage during drying but is repaired upon rehydration. In the angiosperms, extensive protection is laid down during protracted drying and little by way of repair is required. The pteridophytes include several resurrection species, but it is not known whether survival is by protection, repair or a combination of both. The present study was undertaken to determine the mechanism of tolerance in the resurrection fern Morhia caffrorum. Plants were collected from Table Mountain in summer (the dry season) and maintained in a glass house before and during experimentation. Morphological (frond folding), anatomical and ultrastructural (SEM and TEM), physiological (relative water content [RWC] and electrolyte leakage) and biochemical (quantification of sugars, LEAs and antioxidants) were assessed during drying and rehydration of plants. Plants dried in summer were desiccation tolerant. Fronds dried to 5% RWC had minimal electrolyte leakage and recovered full turgor on rehydration. They curled inwards and chlorophyll shading occurred facilitated by a dense layer of adaxial scales. Sucrose levels increased and a number of heat stable LEA-like proteins were produced de novo during drying. These declined during rehydration to levels present in pre-dried fronds. The enzymic antioxidants, ascorbate peroxidase, catalase, glutathione reductase and superoxide dismutase, remained active during the desiccation and rehydration. Subcellular organisation was retained without evidence of damage. During the winter months, when rain is prevalent (albeit that they did not experience the rain), the plants lost the ability to recover from desiccation stress. Fronds did not curl during drying and full rehydration did not occur. Sucrose levels did not increase and no new heat stable proteins appeared upon dehydration. Antioxidant enzymes became denatured and lost activity upon dehydration. During winter, the plants produces spores which became desiccation tolerant and have all the same characteristics of desiccation tolerant “summer” fronds. Upon germination of these spores, an event which occurs in spring and early summer, the new ferns were once again desiccation tolerant. To our knowledge, this is the first report of a species in which desiccation tolerance is seasonal.