Organic acid accumulation may inhibit N2 fixation in phosphorus-stressed lupin nodules.
Nodulated lupins (Lupinus angustifolius cv. Wonga) were hydroponically grown under conditions of low phosphate (LP) or adequate phosphate (HP) to assess the effect of phosphoenolpyruvate carboxylase (PEPC)-derived organic acids on nitrogen assimilation in LP nodules. LP conditions are linked to altered organic acid metabolism, by the engagement of PEP metabolism via PEPC. In LP nodules, the enhanced organic acid synthesis may reduce the available organic carbon for nitrogen assimilation. The diversion of carbon between the organic acid- and amino acid pools was assessed through key nodular enzymes and (14)CO(2) metabolism. Under LP conditions, increased rates of organic acid synthesis via PEPC and malate dehydrogenase (MDH), coincided with reduced nitrogen assimilation via aspartate aminotransferase (AAT), aspartate synthetase (AS) and glutamine synthetase (GS)/glutamate synthase (GOGAT) activities. There was a preferential metabolism of nodular (14)CO(2) into organic acids and particularly into malate. High malate levels were associated with reduced N(2) fixation and synthesis of amino acids. These results indicate that phosphorus deficiency can enhance malate synthesis in nodules, but that excessive malate accumulation may inhibit N(2) fixation and nitrogen assimilation.
Le Roux
M R
MR
Plant Physiology Group, South African Herbal Science and Medicine Institute, University of the Western Cape, Private Bag X17, Belleville 7535, South Africa.
Khan
S
S
Department of Health Sciences, Faculty of Health and Wellness Science, Cape Peninsula University of Technology, PO Box 652, Cape Town 8000, South Africa.
Valentine
A J
AJ
Plant Physiology Group, South African Herbal Science and Medicine Institute, University of the Western Cape, Private Bag X17, Belleville 7535, South Africa.
D000143:Acids / Q000378:metabolism*; D002244:Carbon / Q000378:metabolism; D002250:Carbon Radioisotopes; D031297:Lupinus / Q000254:growth & development / Q000378:metabolism*; D009586:Nitrogen Fixation / Q000187:drug effects* / Q000502:physiology; D010101:Oxygen Consumption; D010758:Phosphorus / Q000494:pharmacology*; D018526:Plant Transpiration; D053204:Root Nodules, Plant / Q000187:drug effects* / Q000378:metabolism*
D000143:Acids; D002250:Carbon Radioisotopes; D010758:Phosphorus; D002244:Carbon