| Mesembryanthemum crystallinum, common ice plant, is a halophyte and an inducible Crassulacean acid metabolism. A light-grown cell culture established from hypocotyl of M. crystallinum seedlings maintained substantial growth rate under mild salt stress but exhibited limited capacity to perform CAM. This cell culture was used to examine salt tolerance in this halophyte without the complication of CAM. A 24 kD salt-responsive cell wall glycoprotein was isolated and characterized. Its N-terminal amino acid sequence has high homology to osmotin, a known salt-responsive protein classified as group 5 pathogenesis-related (PR-5) protein, and has a unique carbohydrate structure which resists endoglycosidase H digestion. Southern analysis using a cDNA clone containing partial SRgp24 sequence showed that only one copy of the gene existed per haploid genome. Immunoblotting and immunocytochemical evidence indicated that these cultured halophytic cells continued to secrete SRgp24 into the intercellular space when maintained in a medium without NaCl. The accumulation of SRgp24 ceased under high salinity or water-deficit environment and this process occurred rapidly and reversibly. In the whole plant, organ-specific accumulation was observed in leaves and stems. Furthermore, this protein was found predominantly in the leaves of sodium-deficient plants, to a lesser extent in sodium-stressed plants, and was undectable in plants grown in low salt. Considering the halophytic nature of common ice plant, the sodium requirement for optimal growth may be one factor that causes the accumulation of this stress protein. In conclusion, these data indicated that SRgp24 is the first PR-5 protein found in M. crystallinum where it appears to be involved in a broad stress response and is not involved only in osmotic stress. |
