[The role of inorganic phosphate in the regulation of the phosphoenolpyruvate carboxylase of Mesembryanthemum crystallinum L]

The stimulation of CAM activity in Mesembryanthemum crystallinum in nitrate and phosphate-deficient conditions

Plants of Mesembryanthemum crystallinum L. were grown from seed in phosphate-sufficient and phosphate-deficient conditions. Separate plants were subjected to nitrate deficiency by removing nitrate from the rooting medium of previously nitrate-sufficient plants. Subsequently, they were also irrigated with 400 mM NaCl to induce CAM. Plants of M. crystallinum with induced CAM that had been grown in complete nutrient medium were deinduced by rehydration. Nitrate and phosphate deficiencies caused enhanced CAM activity prior to salt treatments. After salinisation, phosphate deficiency also caused higher background levels of malate. Prolonged nitrate deficiency reduced CAM activity. Endogenous P₁ levels in phosphate-sufficient plants correlated with CAM activity. However in phosphate-deficient plants CAM occurred without a significant rise in P₁ content. Levels of endogenous P₁ appeared more related to nitrate deficiency than to CAM or salt treatment. Nitrate and phosphate deficiencies and salt treatment could all cause a nitrate deficiency within the leaf, causing high endogenous P₁ when available in the rooting medium. It is speculated that a change in nitrogen status could play a role in the initiation of CAM induction in M. crystallinum. A further possibility is that CAM could be induced by reduced water potential due to dehydration and increased ion content in saline conditions, and by an inhibition of growth and concomitant accumulation of solutes in phosphate- and nitrate-deficient conditions.

Development of C4 photosynthesis in sugar cane: Changes in properties of phosphoenolpyruvate carboxylase during greening

Phosphoenolpyruvate (PEP) carboxylase extracted from etiolated and greening sugar cane (a Saccharum hybrid) displayed different properties in terms of DEAE-cellulose elution profile and activation by glucose-6-phosphate. During the first 20 hours of greening, no increase in extractable PEP carboxylase activity was observed, but ionic and allosteric properties of the enzyme changed, becoming similar to those found for the enzyme from light-grown cane. Density labelling studies with deuterium oxide provided no evidence for either de novo synthesis or turnover of the enzyme during this period, or during the following 24 hours of greening. These results are discussed in relation to enzyme control mechanisms and the development of the C4 pathway in photosynthesis.