Characterization of Five Abscisic Acid-Responsive cDNA Clones Isolated from the Desiccation-Tolerant Plant Craterostigma plantagineum and Their Relationship to Other Water-Stress Genes.

Leaves of resurrection plants tolerate desiccation as do embryos of many higher plants. From the resurrection plant Craterostigma plantagineum a number of desiccation-related transcripts have recently been cloned; they are abundantly expressed in dried leaves and abscisic acid-treated dried callus (D Bartels, K Schneider, G Terstappen, D Piatkowski, F Salamini [1990] Planta 18: 27-34). Five distinct cDNA clones representing low copy number genes were selected for further characterization. Their nucleotide sequences were determined and proteins were predicted with a molecular mass between 16 and 34 kilodaltons. Three of these proteins have unusual amino acid compositions and extreme hydrophilic characters. Two of them contain a cluster of contiguous serine residues and lysine-rich repeats. These sequence motifs display homologies to desiccation-related genes expressed in embryos or dehydrated seedlings of several plants. A third cDNA clone contains tracts of sequences which are related to a cotton Lea (late embryogenesis abundant) gene (JC Baker, C Steele, L Dure III [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 wound-stressed 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.