Leaves of the resurrection plant Craterostigma plantagineum Hochst, can be desiccated up to 1% relative water content and are still viable after rehydration. To clone genes related to this extreme desiccation tolerance, an in-vitro system was first developed which allows the induction of the same resurrection response in callus tissue upon treatment with abscisic acid (ABA). Several proteins and in-vitro-synthesized polypeptides were then identified which can be induced both in desiccation-tolerant, naturally dried leaves and in ABA-treated calli surviving after rehydration. Complementary-DNA clones corresponding to mRNAs expressed only in desiccation-tolerant tissues were obtained and classified into several gene families. In hybrid-selected translation experiments, representative cDNA clones were associated with water stress and ABA-inducible polypeptides abundantly expressed in dried leaves and ABA-treated calli. The expression pattern of several of these abundant transcripts was analyzed in RNA-hybridization experiments. Upon stress or ABA treatment the transcription levels increased rapidly, but they declined after relief from the stress state. This, together with data on genomic copy numbers indicated that a set of abundantly expressed genes are involved in the desiccation process of resurrection plants. Data on endogenous ABA contents before and after stress applications and on the physiological effects of exogenous ABA treatments indicate that in Craterostigma plantagineum the induction of an extreme desiccation tolerance is mediated by this plant hormone.

[1988] Plant Mol Biol II: 277-291). Secondary structure predictions are discussed and suggest that the deduced proteins could play a role in protecting core cell structures in a dehydrated cell. It is concluded that at least in part the gene products involved in the desiccation-induced pathways are common to leaves of resurrection plants and embryos. Two cDNA clones appear to code for Craterostigma-specific mRNAs. The expression patterns of all five transcripts were studied in comparison to desiccated leaves in dehydrated roots, in woundstressed leaves and in salt-stressed callus. The data obtained point to the possibility that not only specificity of induction but also the expression level of specific gene products may be of importance for osmoprotection.

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