ULTRASTRUCTURAL AND BIOCHEMICAL-BASES OF RESURRECTION IN THE DROUGHT-TOLERANT VASCULAR PLANT, SELAGINELLA-LEPIDOPHYLLA

Ultrastructural and enzymatic changes occurring during hydration of the "resurrection" plant, S. lepidophylla, were examined in an effort to understand the cellular bases of drought tolerance in this organism. Whole plants or isolated branches (fronds) were hydrated for up to 24 h under conditions of constant irradiance, temperature and relative humidity. Changes in palisade cell ultrastructure, recovery of photosynthetic ability and specific enzyme (ribulose bisphosphate carboxylase) activity are correlated and divisible into 3 phases. In phase 1 (0-6 h of hydration), electron micrographs show condensation of polyphenolics in cytoplasmic vacuoles and a reduction in density of chloroplast plastoglobuli. In phase 2 (6-12 h), polysomes form from disperse ribosomes in both the cytoplasm and chloroplast stroma. Elongation of chloroplasts is accompanied by the proliferation of thylakoid membrane correlated with a reduction in the number of plastoglobuli. These changes are in turn correlated with increases in ribulose bisphosphate carboxylase activity and the rate of CO2 fixation (net photosynthesis). The 3rd phase of hydration (12-24 h) is marked by the formation of grana, significant starch deposition, and continued increases in ribulose bisphosphate carboxylase activity and photosynthetic rates. Increases in ribulose bisphosphate carboxylase activity during hydration can be accounted for by increases in holoenzyme detectable by polyacrylamide gel electrophoresis (PAGE) of native proteins. The holoenzyme is shown to be composed of large and small subunits on denaturing (SDS) PAGE. The increase in enzyme activity is sensitive to chloramphenicol, but resistant to cycloheximide, suggesting that newly synthesized chloroplast protein (large subunits) combine with a pool of preexisting small subunits to form active holoenzyme. The results reveal conservation of cellular and basal enzymatic integrity in desiccated S. lepidophylla, and suggest a role for specific protein synthesis in photosynthetic recovery during hydration.