CYTOKININ AS A NEGATIVE EFFECTOR OF PHOSPHOENOLPYRUVATE CARBOXYLASE INDUCTION IN MESEMBRYANTHEMUM-CRYSTALLINUM

The possible role of cytokinin in the signal transduction pathway lead ing to phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) inductio n in the halophilic species Mesembryanthemum crystallinum L. was inves tigated. PEPCase is known to be induced by water stress in this specie s. When 6-benzylaminopurine (BA) was applied to M. crystallinum plants , water deficit developed as indicated by a decrease in leaf water con tent and by accumulation of proline. This effect was least pronounced when BA was applied to the shoots. Application of BA to the roots resu lted in stronger symptoms of water stress, and simultaneous applicatio n to the roots and the shoots was most effective, leading to a 40% dec rease in leaf water content and a 20-fold increase in proline within 3 days. PEPCase mRNA and enzyme activity were increased in the root-tre ated plants but an additional application of BA to the leaves could pr event PEPCase induction efficiently. We therefore investigated the pos sible role of endogenous cytokinins in CAM induction. Fourteen species of endogenous cytokinins could be identified and were measured quanti tatively. Endogenous cytokinin content was negatively correlated with mRNA levels specific for PEPCase but showed only weak correlation with PEPCase activity. In excised, wilting leaves, cytokinins decreased an d PEPCase mRNA strongly increased. In rehydrating excised leaves deriv ed from pre-stressed plants, PEPCase mRNA was down-regulated, but cyto kinins were essentially unchanged. Taken together, the data support a negative regulatory function of leaf cytokinins in PEPCase induction.