Abstract
Lichens are symbiotic associations (holobionts) established between fungi (mycobionts) and certain groups of cyanobacteria or unicellular green algae (photobionts). This symbiotic association has been essential in the colonization of terrestrial dry habitats. Lichens possess key mechanisms involved in desiccation tolerance (DT) that are constitutively present such as high amounts of polyols, LEA proteins, HSPs, a powerful antioxidant system, thylakoidal oligogalactolipids, etc. This strategy allows them to be always ready to survive drastic changes in their water content. However, several studies indicate that at least some protective mechanisms require a minimal time to be induced, such as the induction of the antioxidant system, the activation of non-photochemical quenching including the de-epoxidation of violaxanthin to zeaxanthin, lipid membrane remodeling, changes in the proportions of polyols, ultrastructural changes, marked polysaccharide remodeling of the cell wall, etc. Although DT in lichens is achieved mainly through constitutive mechanisms, the induction of protection mechanisms might allow them to face desiccation stress in a better condition. The proportion and relevance of constitutive and inducible DT mechanisms seem to be related to the ecology at which lichens are adapted to.
Highlights
Water is the most abundant compound in any living being; water is the universal solvent where inorganic salts and biomolecules are dissolved in; water is essential for many important chemical reactions in organisms; water maintains the structure of cell macromolecules and membranes
The aim of this review is to present an overview of our current knowledge about DT in lichens with a particular emphasis on the most recent progress
To results of Centeno et al [84] and Bertuzzi et al [89] comparing the responses of two lichens with different ecology (Parmotrema perlatum, hygrophilous, and Xanthoria parietina, meso-xerophylous) exposed to oxidative stress induced by ozone and to wet or dry conditions, showed that both species differed in their antioxidant profile, and in the activity of reactive oxygen species (ROS)-scavenging enzymes, more intense in the hygrophilous P. perlatum than in the meso-xerophilous X. parietina
Summary
Water is the most abundant compound in any living being; water is the universal solvent where inorganic salts and biomolecules are dissolved in; water is essential for many important chemical reactions in organisms; water maintains the structure of cell macromolecules and membranes. Lichens occupy habitats with low water retention capacity such as rock surfaces, sand, gypsum soils or tree bark, where land plants cannot survive This adaptive strategy is mainly based on the tolerance to desiccation. This strategy had a great evolutionary success and earliest lichens diversified on land even after the rise and spread of multicellular plants. Many kinds of physiological measurements and observational approaches were used to determine the ability of lichens to survive to extreme environments One of these methods was to observe whether the symbionts isolated from a stressed lichen thallus grew in culture [14,15]. The aim of this review is to present an overview of our current knowledge about DT in lichens with a particular emphasis on the most recent progress
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