Diurnal modulation of phosphoenolpyruvate carboxylation in pea leaves and roots as related to tissue malate concentrations and to the nitrogen source

Abstract

Phosphoenolpyruvate (PEP) carboxylation was measured as dark CO fixation in leaves and roots (in vivo) or as PEP carboxylase (PEPCase) activity in desalted leaf and roof extracts (in vitro) from Pisum sativum L. cv. Kleine Rheinländerin. Its relation to the malate content and to the nitrogen source (nitrate or ammonium) was investigated. In tissue from nitrate-grown plants, PEP carboxylation varied diurnally, showing an increase upon illumination and a decrease upon darkening. Diurnal variations in roots were much lower than in leaves. Fixation rates in leaves remained constantly low in continuous darkness or high in continuous light. Dark CO fixation of leaf slices also decreased when leaves were preilluminated for 1 h in CO-free air, suggesting that the modulation of dark CO2 fixation was related to assimilate availability in leaves and roots. Phosphoenolpyruvate carboxylase activity was also measured in vitro. However, no difference in maximum enzyme activity was found in extracts from illuminated or darkened leaves, and the response to substrate and effectors (PEP, malate, glucose-6-phosphate, pH) was also identical. The serine/threonine protein kinase inhibitors K252b, H7 and staurosporine, and the protein phosphatase 2A inhibitors okadaic acid and cantharidin, fed through the leaf petiole, did not have the effects on dark CO fixation predicted by a regulatory system in which PEPCase is modulated via reversible protein phosphorylation. Therefore, it is suggested that the diurnal modulation of PEP carboxylation in vivo in leaves and roots of pea is not caused by protein phosphorylation, but rather by direct allosteric effects. Upon transfer of plants to ammonium-N or to an N-free nutrient solution, mean daily malate levels in leaves decreased drastically within 4-5 d. At that time, the diurnal oscillations of PEP carboxylation in vivo disappeared and rates remained at the high light-level. The coincidence of the two events suggests that PEPCase was de-regulated because malate levels became very low. The drastic decrease of leaf malate contents upon transfer of plants from nitrate to ammonium nutrition was apparently not caused by increased amino acid or protein synthesis, but probably by higher decarboxylation rates.

Effects of different ammoniacal nitrogen sources on N-metabolism of the atmospheric bromeliad Tillandsia pohliana Mez

“…The influence of different N compounds on the production of amino acids in plants has been widely studied. In the case of various plant materials it has been shown that a co-relationship exists between the increase of N availability and the production of free amino acids and proteins (Fernandes 1983, Santos & Salema 1991, Perez-Soba et al 1994, and Leport et al 1996. The amino acid, which is often present in greater proportions, as a result of an increase in N concentration, is glutamine, as reported by Barneix et al (1984) in barley (Hordeum vulgare L. cv.…”

Root and Shoot Respiration of Perennial Ryegrass Are Supplied by the Same Substrate Pools: Assessment by Dynamic 13C Labeling and Compartmental Analysis of Tracer Kinetics

“…Malate could have been a major constituent of Q 1 : it is rapidly labeled in leaves (Heber and Willenbrink, 1964), it is translocated to roots (Imsande and Touraine, 1994), and it is thought to be decarboxylated there (Imsande and Touraine, 1994;Stitt et al, 2002). Malate concentration is high in plants growing on nitrate (Leport et al, 1996) and may serve as a control and substrate for the nitrate uptake system (Imsande and Touraine, 1994). If this conclusion is true, then the relevance of Q 1 would depend on nitrogen source.…”

On the Evolutionary Origin of CAM Photosynthesis

“…The evolutionary scenario also predicts that plants that do not possess nighttime organic acid storage (i.e. Leport et al, 1996) are unlikely to evolve toward CAM.…”