KRANNER, Ilse; Royal Botanic Gardens Kew, Seed Conservation Department, Wakehurst Place: The lichen symbiosis – antioxidants confer desiccation tolerance

Historically, and still today, the lichen symbiosis is often regarded as controlled parasitism to the benefit of the fungus. However, lichens have evolved a structure that neither fungus or alga can form by itself. This allows them to live above ground in habitats previously unavailable, rather than in soil and small crevices of rocks, but they are then exposed to irradiation and extremes of desiccation. Most higher plants die when desiccated, but lichens, mosses, some micro-organisms, a few ferns and ‘resurrection plants’ have mechanisms to survive. Reactive oxygen species (ROS) are a major cause of damage during desiccation, especially in photosynthetic organisms. Here we show changes in major ROS-scavenging and photoprotective biochemical pathways during desiccation and rehydration in the lichen Cladonia vulcani and its isolated partners. When alone, both alga and fungus can survive desiccation, but suffer oxidative damage, but in the lichen each induces up-regulation of protective systems in the other. Without the fungal contact the alga only tolerates very dim light and its photoprotective system is only partially effective; and without the alga, the fungus�s glutathione-based antioxidant system is slow and ineffective. In turn, this suggests that formation of the Cladonia vulcani symbiosis involves mutual signals not only for creating an aboveground structure but also to ensure, as a necessary consequence, greater speed and effectiveness of defense against oxidative damage. For both alga and fungus, the mutually enhanced antioxidative and photoprotective capacity is of the essence for life above ground which increases the chance of dispersal of reproductive propagules and evolutionary success.