EARLY SALT STRESS EFFECTS ON THE DIFFERENTIAL EXPRESSION OF VACUOLAR H-ATPASE GENES IN ROOTS AND LEAVES OF MESEMBRYANTHEMUM-CRYSTALLINUM()

In Mesembryanthemum crystallinum, the salt stress-induced metabolic sw itch from C-3 photosynthesis to Crassulacean acid metabolism is accomp anied by major changes in gene expression. However, early effects of s alt exposure (i.e. prior to Crassulacean acid metabolism induction) on genes coding for vacuolar transport functions have not yet been studi ed. Therefore, the expression of vacuolar Hf-ATPase genes was analyzed in different organs of 4-week-old plants stressed with 400 mM NaCl fo r 3, 8, or 24 h. Partial cDNAs for the subunits A, B, and c were clone d and used as homologous probes for northern blot analysis. In control plants, the mRNA levels for the different subunits showed organ-speci fic differences. In fully expanded leaves, subunit c mRNA was very low but increased transiently during the light period. Plant organs also differed in their salt-stress response. In roots and young leaves, mRN A levels for all three subunits increased about 2-fold compared to con trol plants, whereas in fully expanded leaves only subunit c mRNA resp onded to salt. The results indicate that the expression of vacuolar H-ATPase genes does not always involve a fixed stoichiometry of mRNAs f or the different subunits and that the mRNA level for subunit c is par ticularly sensitive to developmental and environmental changes.