Phosphoenolpyruvate carboxylase kinase is controlled by a similar signaling cascade in CAM and C-4 plants

In Crassulacean acid metabolism (CAM) plants, phosphoenolpyruvate carboxyla se (PEPC) is subject to day-night regulatory phosphorylation of a conserved serine residue in the plant enzymes N-terminal domain. The dark increase i n PEPC-kinase (PEPC-k) activity is under control of a circadian oscillator, via the enhanced expression of the corresponding gene (1). The signaling c ascade leading to PEPC-k upregulation was investigated in leaves and mesoph yll cell protoplasts of the facultative; salt-inducible CAM species, Mesemb ryanthemum crystallinum Mesophyll cell protoplasts had the same PEPC-k acti vity as leaves from which they were prepared (i.e., high at night, low duri ng the day). However; unlike C-4 protoplasts (2), CAM protoplasts did not s how marked PEPC-k up-regulation when isolated during the day and treated wi th a weak base such as NH4CI. Investigations using various pharmacological reagents established the operation, in the darkened CAM leaf, of a PEPC-k c ascade including the following components: a phosphoinositide-dependent pho spholipase C (PI-PLC), inositol 1,4,5 P (IP3)-gated tonoplast calcium chann els, and a putative Ca2+/calmodulin protein kinase. These results suggest t hat a similar signaling machinery is involved in both C-4 (2,3) and CAM pla nts to regulate PEPC-k activity, the phosphorylation state of PEPC, and, th us, carbon flux through this enzyme during CAM photosynthesis. (C) 2001 Aca demic Press.