Thesis (MSc)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: This study aimed to assess the contribution of anaplerotic C prOVISIOn VIa
phosphoenolpyruvate carboxylase (pEPc, EC 4.1.1.31), during Pi stress in the root and
nodule components of Lupinus angustifolius. The role of PEPc in DIC metabolism in
roots and nodules of phosphate-starved plants was studied. The symbioses involving
leguminous plants and species of Rhizobium and Bradyrhizobium bacteria form an
integral part of effective management ofN in the environment. In agricultural settings,
roughly 80% of this biologically fixed N2 comes from this type of symbiotic
relationship. Nitrogen-fixing bacteria in concert with legumes fix atmospheric nitrogen,
which is then available to the infected plant. Worldwide, legumes are grown on
approximately 250 Mha and they fix about 90 Tg (90 billion tons) of N, per year. The
overall stoichiometry for nitrogen assimilation in the nodule requires one molecule of
oxaloacetate to be converted to one molecule of asparagine per dinitrogen molecule
fixed. One possible source for the required oxaloacetate is the reaction catalysed by
PEPc. The reaction catalysed by PEPc is a major source of anaplerotic carbon for the
plant and it is expected that this reaction will be even more important to plants under Pi
stress, as the reaction is not ATP-dependent.
Seeds of Lupinus angustifolius (cv. Wong a) were inoculated with Rhizobium sp.
(Lupinus) bacteria and grown in hydroponic culture. Tanks were supplied with either 2
11MP04 (LP) or 2 mM P04 (HP) and air containing 360 ppm CO2. Roots experienced
pronounced P stress with a greater decline in Pi, compared to nodules. 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. LP roots also synthesised more
pyruvate from malate than LP nodules. The role of pyruvate accumulation under Pi
stress, was further highlighted by the metabolism of PEP via both the pyruvate kinase
(PK, Ee 2.7.1.40) and PEPc routes. The enhanced PK activities supported these high
pyruvate levels.
The results show unequivocally that nodules do not experience P stress to the same
extent as roots. Implications of the findings are that nodules require low P to function
normally. Maintenance of phosphate levels in nodules may be at the expense of host. It
can be suggested that when nodules are P-starved they can become aggressive
scavengers for available P and even out-compete roots. / AFRIKAANSE OPSOMMING: Die doel van hierdie studie was om die bydrae van anaplerotiese koolstof-voorsiening
via fosfo-enolpirovaatkarboksilase (pEPc, EC 4.1.1.31), tydens fosfaatstremming in die
wortels en wortelknoppies van Lupinus angustifolius te bepaal. Die rol van PEPc in die
metabolisme van opgeloste anorganiese koolstofdioksied in fosfaat-beperkte wortels en
wortelknoppies is ondersoek. Die simbiose tussen peulplante en spesies van Rhizobium
en Bradyrhizobium bakterieë vorm 'n integrale deel van die doeltreffende bestuur van
stikstof in die omgewing. In die landbou word ongever 80 %van biologies-gefikseerde
stikstof deur hierde simbiotiese verhouding geproduseer. Stikstotbindende bakterieë, in
simbiose met peulplante, fikseer atmosferiese stikstof, wat dan beskikbaar is vir die
geïnfekteerde plant. Wêreldwyd fikseer peulplante ongeveer 90 biljoen ton stikstof per
jaar. Die algehele stoïgiometrie vir stikstof-fiksering in wortelknoppies vereis dat een
molekule oksaalsuur na een molekule asparagien omgesit word per stikstofmolekule wat
gefikseer word. Een moontlike bron vir die benodigde oksaalsuur is die reaksie wat
deur PEPc gekataliseer word. Die reaksie wat deur PEPc gekataliseer word is 'n
belangrike bron van anaplerotiese koolstof vir die plant en dit word vermoed dat hierdie
reaksie van nog groter belang sal wees vir plante onder fosfaatstremming, omdat die
reaksie nie ATP-afhanklik is nie.
Sade van Lupinus angustifolius (cv. Wonga) is geïnokuleer met Rhizobium sp.
(Lupinus) bakterieë en gekweek in waterkultuur. Tenke is voorsien met óf 2 !lM P04
(LP), óf 2 mM P04 (HP) en lug wat 360 ppm CO2 bevat het. Wortels het skerp
fosfaatstremming ervaar, met 'n groter afname in Pi, vergelykbaar met wortelknoppies. Tydens fosfaatstremming het die aktiwiteit van PEPc toegeneem in wortels, maar nie in
wortelknoppies nie en hierdie veranderinge was nie verwant aan die uitdrukking van die
ensiem nie. PEPc van wortels en wortelknoppies is nie gereguleer deur uitdrukking nie,
maar moontlik deur post-translasie kontrole. Wortels onder 'n lae-fosfaat voorsiening
het ook meer pirodruiwesuur vanaf malaat gesintetiseer as wortelknoppies. Die rol van
pirodruiwesuur-akkumulering tydens fosfaatstremming is verder beklemtoon deur die
metabolisme van PEP via beide die pirovaatkinase- (PK, EC 2.7.1.40) en PEPc- roetes.
Die verhoogde PK-aktiwiteite verklaar hierdie hoër vlakke van pirodruiwesuur.
Die resultate toon ondubbelsinnig dat wortelknoppies me tot dieselfde mate
fosfaatstremming ervaar as wortels nie. Dit impliseer dat wortelknoppies min fosfaat
benodig om normal te funksioneer. Handhawing van fosfaatvlakke in wortelknoppies
mag ten koste van die wortel wees. Dit is moontlik dat, wanneer wortelknoppies
fosfaatbeperk is, hulle aggressiewe opruimers word vir beskikbare fosfaat en selfs beter
funksioneer as die wortels.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:sun/oai:scholar.sun.ac.za:10019.1/50184 |
| Date | 03 1900 |
| Creators | Ward, Caroline (Caroline Linda) |
| Contributors | Valentine, A. J., Botha, F. C., Stellenbosch University. Faculty of Science. Dept. of Botany and Zoology. |
| Publisher | Stellenbosch : Stellenbosch University |
| Source Sets | South African National ETD Portal |
| Language | en_ZA |
| Detected Language | English |
| Type | Thesis |
| Format | 110 p. : ill. |
| Rights | Stellenbosch University |
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