FUNCTIONING OF THE TONOPLAST IN VACUOLAR C-STORAGE AND REMOBILIZATIONIN CRASSULACEAN ACID METABOLISM

Nocturnal fixation of CO2 and storage of carbon in the form of malic a cid in vacuoles of photosynthetically active cells are key processes o f crassulacean acid metabolism (CAM), A prerequisite for malate transp ort into the vacuole is the concerted action of primary-active H+-pump s, i.e. the tonoplast V0V1-H+-ATPase and H+-PP(i)ase, and a secondary- active carboxylate transporter, During the transition from C-3-photosy nthesis to CAM the H+-ATPase of the C-3/CAM-intermediate plant Mesembr yanthemum crystallinum undergoes particular structural changes indicat ing an adaptation of the enzyme to the requirements for an increased c apacity of tonoplast transport systems, The role of the H+-PP(i)ase in energization of carbon transport in CAM remains unclear, While in Kal anchoe blossfeldiana cv, Tom Thumb, where CAM is induced by shortening of daylength, there is a higher expression of the H+-PP(i)ase in the CAM state as compared to the C-3-state, the enzyme disappears in M. cr ystallinum when CAM is induced by salt stress. However, the H+-PPase c ould also be involved in the regulation of the H+-ATPase as shown by t he PPi-stimulated ATP-dependent H+-transport activity found in K. daig remontiana and K. blossfeldiana cv, Tom Thumb, Although the tonoplast carboxylate transporter has not yet been identified, the kinetic param eters of the enzyme were characterized after partial purification and functional reconstitution into proteoliposomes, While the influx of ma late into the vacuole is mediated by the co-operation of different enz ymes, the regulation of the passive efflux of malic acid from the tono plast during the day may depend on biophysical membrane properties, i, e, the degree of order of the vacuolar membrane, which seems to be mod ulated by Ca2+.