OLIVER, Melvin J.; USDA-ARS: Desiccation tolerance in bryophytes; is tolerance the primitive condition in plants?

The majority of desiccation-tolerant plants are found in the less complex clades that constitute the algae, lichens and bryophytes. However, within the larger and more complex groups of vascular land plants there are some 120-130 species that exhibit some degree of vegetative desiccation tolerance. By considering the evidence for the mechanisms of desiccation tolerance in different plants, including differences in cellular protection and repair, and coupling this evidence with a phylogenetic framework, a working hypothesis as to the evolution of desiccation tolerance in land plants can be generated. We hypothesize that the primitive mechanism of tolerance probably involved constitutive cellular protection coupled with active cellular repair when rehydration occurs. Such a mechanism could accommodate the requirements necessary for the invasion of land by early plants from a freshwater origin that derive from their structural simplicity and inability to prevent water loss. Evidence for such a mechanism stems from cellular and molecular studies of modern day desiccation tolerant bryophytes. Ultrastructural and physiological data for the moss Tortula ruralis support the notion that desiccation-tolerant bryophytes maintain a certain level of cellular protection components in preparation for drying. The type of gene expression level response this moss experiences during a wet/dry/wet cycle also indicates such constitutive protection. Gene expression studies also indicate a reliance on a rehydration induced cellular repair strategy. More recent bioinformatics, genomic and biochemical level studies support these conclusions and offer insights into the nature of this particular mechanism of tolerance. Is tolerance the primitive condition in plants, we do have good evidence that it is.