Phosphoenolpyruvate (PEP) metabolism in roots and nodules of Lupinus angustifolius under P stress

[摘要] ENGLISH ABSTRACT: This study investigated the activities of several of the enzymes involved in the alternativeroute of PEP metabolism via PEPc (EC 4.1.1.31). This reaction circumvents theadenyl ate-controlled PK (EC 2.7.1.40) reaction of the conventional glycolytic networkunder conditions of P stress. It was hypothesized that the synthesis of pyruvate under Pistress would induce the PEPc alternative route and that C for pyruvate synthesis wouldprimarily be imported via this route. This was assessed by looking at how total enzymeactivities are perturbed under P stress and also by following the route of radioactivelabelled 14C02 under sufficient (2 mM) and deficient P (2 JlM) conditions in either rootsor nodules. The significance of the pathway under P stress, was further assessed bydetermining pool sizes of pyruvate that was synthesized from PEPc-derived C. Theexperiments were conducted under glasshouse conditions, as two separate studies: one toinvestigate the phenomenon of Pi stress and its consequences for PEPc-derived Cmetabolism, and the other one to study the enzymes involved. Seeds of Lupinusangustifolius (cv. Wonga) were inoculated with Rhizobium sp. (Lupinus) bacteria andgrown in hydroponic culture. Tanks were supplied with either 2 JlM P04 (LP) or 2 mMP04 (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 theengagement of the PEPc route under Pi stress. In this regard, pyruvate is considered as akey 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. Theenhanced PK activities supported these high pyruvate levels. Under P stress, PEPcactivities increased in roots but not in nodules and these changes were not related to theexpression of the enzyme. Root and nodular PEPc were not regulated by expression, butpossibly by posttranslational control.The novelty of our results for symbiotic roots demonstrates that using metabolicallyavailable Pi is indeed a more sensitive indicator ofP stress. These results show that underPi stress, nodules are able to maintain their Pi and adenylate levels, possibly at theexpense of the root. It is suggested that nodules do not experience P stress to the sameextent as roots or alternatively function optimally under conditions of low P availability.The increase in concentration of pyruvate synthesized from malate, indeed suggest thatunder LP conditions there is an increase requirement for pyruvate. It is clear from thisdata that the operation of bypass route in nodules should be investigated further.Nevertheless, this study provided incentives for understanding the role of C pathways inNi-fixation, in particular under conditions ofP limitation.

[发布机构] Stellenbosch University