CAM Induction in Mesembryanthemum crystallinum: Protein Expression

Abstract

When Mesembryanthemum crystallinum is irrigated with nutrient solutions containing high amounts of sodium chloride, the plants shift from C3 to CAM photosynthesis. We have measured the amounts of three CAM enzymes, phosphoenolpyruvate carboxylase (PEP-Case), pyruvate, orthophosphate dikinase (PPDK) and NADP-malic enzyme (NADP-ME) over the induction period using quantitative immunoblot analysis. The antiserum against PPDK was raised using a fusion protein of PPDK with the carboxyterminal part of bacterial s galactosidase (expressed in E. coli) as antigen. The antiserum was specific for PPDK with no indication of cross reactivity with plant s galactosidases, showing that carboxyterminal fusions of protein coding regions with s-galactosidase may be of wider application for plant studies. PPDK and NADP-ME proteins accumulate during the first seven days of salt stress. This indicates that differential regulation of gene expression rather than enzyme activation accounts for the increase of apparent activity. Two subunit isoforms of PEPCase are present in C3 leaves, roots and calli. They can be distinguished by their apparent molecular weights on SDS gels (Mr approximately 110,000 and 100,000). Only the 100-kDa subunit accumulates during CAM induction.