NaCl–promoted Respiration and Cell Division in Halophilism of a Halophyte, the Common Ice Plant Mesembryanthemum crystallinum L.

The common ice plant (Mesembryanthemum crystallinum L.), a halophytic vegetable plant, shows the maximum growth in the salinity environment where almost all crops die. The NaCl-promoted growth enhancement called halophilism is an essential trait of increased salt tolerance. To elucidate the factors associated with halophilism, we investigated the effects of NaCl on respiration and genes of cell-division-related factors in suspension-cultured cells. Respiration rate increased with the increase of concentration of NaCl, being maximal at 100 mM NaCl. Metabolite analysis showed that contents of metabolites in glycolysis such as glucose, fructose 6-phosphate, and lactic acid were reduced in the NaCl treated cells. The contents of TCA cycle metabolites such as pyruvate (or oxaloacetic acid), citric acid (or isocitric acid), succinic acid, fumaric acid, and malic acid were reduced in NaCl-treated cells at 21 d after treatment, indicating that the metabolic flux of the TCA cycle is activated to increase respiration and production of NADH and FADH(2), which are electron donors for ATP synthesis. Expression analysis showed that the expression of eight genes for complex of ETC (electron transport chain complex), ATP synthase, and TCA cycle enzymes was induced in the salt-treated cells within 6-48 h after the onset of treatment. Moreover, we found that among five cell cycle-related genes that we newly annotated, the expression of a gene associated with the progression of the G1 (McCycD3;1), a gene for B2-type cyclin-dependent kinase (McCDKB2;1), and its cyclin partner forming an active kinase complex in G2/M phase (McCycD4;1) were stimulated by NaCl. We suggested that these factors contributed to the enhanced cell growth in the common ice plant halophilism.