Desiccation tolerance in vegetative plant cells

Plants that can withstand desiccation of their vegetative tissues represent all but one class (the gymnosperms) within the kingdom Plantae. Phylogenetic evidence suggests that this trait has evolved a minimum of twelve separate occasions. Nevertheless desiccation-tolerant plants can be grouped into two categories: (1) the desiccation-tolerant plants whose internal water content rapidly equilibrates to the water potential of the environment and (2) the modified desiccation-tolerant plants that all employ mechanisms to retard and control the rate of water loss. Desiccation tolerance can be achieved by mechanisms that incorporate one of two alternatives, viz. cellular protection or cellular recovery (repair). The majority of plants probably utilize aspects of both. Studies to date indicate that modified desiccation-tolerant plants (angiosperms and pteridophytes) appear to rely more heavily upon drying-induced (abscisic acid controlled) cellular protection strategies for survival. Desiccation-tolerant species, in particular the moss Tortula ruralis, appear to utilize a tolerance strategy that combines a constitutive protection system and a rehydration-inducible recovery mechanism. The moss is capable of storing mRNAs during slow drying that are rapidly utilized upon rehydration. Rapidly dried moss has to rely on rapid synthesis of the same mRNAs upon rehydration. Analysis of protein profiles upon rehydration has facilitated the identification of several ''rehydrins,'' proteins whose synthesis is specific to the rehydration phase. Northern analysis of rehydrin cDNAs confirm the importance of translational controls in producing rehydrin proteins and an additional role of transcription and/or mRNA stability in the response of Tortula to desiccation. These studies indicate that there is more to desiccation tolerance mechanisms than protection of the cells from drying, but rather rehydration-induced events are important in the over all manifestation of desiccation tolerance.