Browsing by Author "Le Roux, Marcellous Remarque"

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    Phosphoenolpyruvate (PEP) metabolism in roots and nodules of Lupinus angustifolius under P stress
    (Stellenbosch : Stellenbosch University, 2004-03) Le Roux, Marcellous Remarque; Valentine, A. J.; Botha, F. C.; Stellenbosch University. Faculty of AgriSciences. Dept. of Genetics. Institute for Plant Biotechnology (IPB).
    ENGLISH ABSTRACT: This study investigated the activities of several of the enzymes involved in the alternative route of PEP metabolism via PEPc (EC 4.1.1.31). This reaction circumvents the adenyl ate-controlled PK (EC 2.7.1.40) reaction of the conventional glycolytic network under conditions of P stress. It was hypothesized that the synthesis of pyruvate under Pi stress would induce the PEPc alternative route and that C for pyruvate synthesis would primarily be imported via this route. This was assessed by looking at how total enzyme activities are perturbed under P stress and also by following the route of radioactive labelled 14C02 under sufficient (2 mM) and deficient P (2 JlM) conditions in either roots or nodules. The significance of the pathway under P stress, was further assessed by determining pool sizes of pyruvate that was synthesized from PEPc-derived C. The experiments were conducted under glasshouse conditions, as two separate studies: one to investigate the phenomenon of Pi stress and its consequences for PEPc-derived C metabolism, and the other one to study the enzymes involved. Seeds of Lupinus angustifolius (cv. Wonga) were inoculated with Rhizobium sp. (Lupinus) bacteria and grown in hydroponic culture. Tanks were supplied with either 2 JlM P04 (LP) or 2 mM P04 (control) and air containing 360 ppm CO2. Roots experienced pronounced P stress with a greater decline in Pi, compared to nodules. LP roots synthesized more pyruvate from malate than LP nodules, indicating the engagement of the PEPc route under Pi stress. In this regard, pyruvate is considered as a key metabolite under Pi stress. The role of pyruvate accumulation under Pi stress, was further highlighted by the metabolism of PEP via both the PK and PEPc routes. The enhanced PK activities supported these high pyruvate levels. Under P stress, PEPc activities increased in roots but not in nodules and these changes were not related to the expression of the enzyme. Root and nodular PEPc were not regulated by expression, but possibly by posttranslational control. The novelty of our results for symbiotic roots demonstrates that using metabolically available Pi is indeed a more sensitive indicator ofP stress. These results show that under Pi stress, nodules are able to maintain their Pi and adenylate levels, possibly at the expense of the root. It is suggested that nodules do not experience P stress to the same extent as roots or alternatively function optimally under conditions of low P availability. The increase in concentration of pyruvate synthesized from malate, indeed suggest that under LP conditions there is an increase requirement for pyruvate. It is clear from this data that the operation of bypass route in nodules should be investigated further. Nevertheless, this study provided incentives for understanding the role of C pathways in Ni-fixation, in particular under conditions ofP limitation.