Desiccation-tolerance in bryophytes: a review

Desiccation-tolerance (DT), the ability to lose virtually all free intracellular water and then recover normal function upon rehydration, is one of the most remarkable features of bryophytes. The physiology of bryophytes differs in major respects from that of vascular plants by virtue of their smaller size; unlike vascular plants, the leafy shoots of bryophytes equilibrate rapidly with the water potential in their surroundings and tend to be either fully hydrated or desiccated and metabolically inactive. The time required to recover from desiccation increases and degree of recovery decreases with length of desiccation; both also depend upon temperature and intensity of desiccation. Tolerance in at least some species shows phenotypic plasticity. Recovery of respiration, photosynthesis and protein synthesis takes place within minutes or an hour or two; recovery of the cell cycle, food transport and the cytoskeleton may take 24 hours or more. Positive carbon balance is essential to survival of repeated cycles of drying and wetting; significant growth requires continuously wet periods of a few days or more. Male and female gametophytes, and gametophyte and sporophyte, may differ in tolerance. Desiccation-tolerance is essential to dispersal and establishment of spores and vegetative propagules. The mechanisms of DT in bryophytes, including expression of LEA proteins, high content of non-reducing sugars and effective antioxidant and photo-protection, are at least partly constitutive, allowing survival of rapid drying, but changes in gene expression resulting from mRNA sequestration and alterations in translational controls elicited upon rehydration are also important to repair processes following re-wetting. Phylogenetic and ecological considerations suggest that DT is a primitive character of land plants, lost in the course of evolution of the homoiohydric vascular-plant shoot system, but retained in spores, pollen and seeds, and re-evolved in the vegetative tissues of vascular “resurrection plants.” Bryophytes have retained the poikilohydry and DT that are probably the optimal pattern of adaptation at their scale, but modern bryophytes are specialized and diverse, and are removed by the same span of evolutionary time as the flowering plants from their primitive origins.