Phosphoenolpyruvate carboxylase (PEPCase), the key
enzyme of Crassulacean acid metabolism, is induced by
water stress in leaves of Mesembryanthemum crystallinum. In water-stressed plants or excised leaves, exogenous
cytokinin suppresses PEPCase transcript accumulation in the leaves.
Cytokinin (6-benzylaminopurine) used in concentrations from 5 to 500 micromolar (a) inhibits the upregulation of PEPCase transcripts,
enzyme activity, and Crassulacean acid metabolism induction in
salt-stressed intact plants when sprayed once daily during the stress period, (b) inhibits the accumulation of PEPCase
mRNA in leaves from well-watered plants, (c) down-regulates PEPCase transcripts within 8 hours in prestressed, intact plants after a single spraying of an individual leaf, (d) inhibits accumulation of PEPCase transcripts in excised, wilting leaves, and (e) accelerates the net decrease of PEPCase transcripts in excised leaves from prestressed plants under
rehydration conditions. When roots, the main site of
cytokinin biosynthesis, are excised, PEPCase induction under drought stress is intensified. We propose that roots, acting as sensors of soil water status, may regulate PEPCase gene expression in the leaves with
cytokinin as a signal transducer.