Coordination of Chloroplastic Metabolism in N-Limited Chlamydomonas reinhardtii by Redox Modulation (II. Redox Modulation Activates the Oxidative Pentose Phosphate Pathway during Photosynthetic Nitrate Assimilation)

Abstract

l h e onset of photosynthetic NO3-assimilation in N-limited Chlamydomonas reinhardtii increased the initial extractable activity of the glucose-6-phosphate dehydrogenase (C6PDH), the key regulatory step of the oxidative pentose phosphate pathway. l h e total activated enzyme activity did not change upon NO3-resupply. l h e higher activity, therefore, represents activation of existing enzyme. No activation occurred during NH.+ assimilation. Incubation of extracts with D l l reversed the NO3-stimulation of C6PDH activity, indicating that the activation involved redox modulation of G6PDH. Phosphoribulosekinase, an enzyme activated by thioredoxin reduction, was inhibited at the onset of NO3-assimilation. A 2-fold stimulation of O2 evolution and a 70% decrease in the rate of photosynthetic C 0 2 assimilation accompanied the enzyme activity changes. There was an immediate drop in the NADPH and an increase in NADP upon addition of NO3-, whereas NH,+ caused only minor fluctuations in these pools. l h e response of C.reinhardtii to NO3-indicates that the oxidative pentose phosphate pathway was activated to oxidize carbon upon the onset of NO3-assimilation, whereas reduction of carbon via the reductive pentose phosphate pathway was inhibited. lhis demonstrates a possible role for the Fd-thioredoxin system in coordinating enzyme activity in response to the metabolic demands for reducing power and carbon during NO3-assimilation.The assimilation of both N and carbon require the plant cell to supply electrons, ATP, and carbon skeletons. In the light, photosynthetic reactions provide the electrons and ATP, whereas organic carbon is principally supplied from the reductive pentose phosphate pathway (Syrett, 1981;Larsson et al., 1985; Robinson and Baysdorfer, 1985;Le Van Quy et al., 1991). When alga1 cells are grown with sufficient N, the demands of N metabolism are small in comparison to that of carbon metabolism and the interactions between these pathways is difficult to demonstrate. N limitation of Selenastrum minutum, a unicellular green alga, has been shown to increase its capacity for N assimilation relative to photosynthesis and has permitted detailed characterization

Interactions Between Carbon and Nitrogen Metabolism

“…Even in the light, a considerable portion of the reductant for nitrogen assimilation in unicellular algae can come from respiration (Weger and Turpin 1989;Huppe et al 1994). In illuminated leaves, 80% of the reductant necessary to assimilate nitrate into glutamate can be generated directly by the photosynthetic electron transport chain (Fig.…”

“…An elegant effect found in Chlamydomonas is the nitrate-induced activation of the fIrst enzyme of the oxidative pentose phosphate pathway, glucose-6-phosphate dehydrogenase (Huppe et al 1994). As in higher plant leaves, this enzyme undergoes reductive light inactivation via the thioredoxin system.…”

Carbon and Acyl Chain Flux during Stress-induced Triglyceride Accumulation by Stable Isotopic Labeling of the Polar Microalga Coccomyxa subellipsoidea C169

“…The caveat is a tradeoff between cellular TG quantities and reproductive growth. In nature, oil accumulation is linearly correlated with, and may be caused by, a cessation or slowing of the cell cycle (2)(3)(4). Engineering algae to produce oil on demand is needed for improving process yields and necessitates a thorough understanding of the biochemistry underlying oil accumulation for the development of strategies to attain this goal (5).…”

Functional Analyses of Cytosolic Glucose-6-Phosphate Dehydrogenases and Their Contribution to Seed Oil Accumulation in Arabidopsis

“…In plants G6PDH has been most frequently described for its involvement in nitrogen assimilation. The induction of its activity or transcript has been described in various systems including pea (Pisum sativum) roots (Bowsher et al, 1992), Chlamydomonas (Huppe et al, 1994), barley (Hordeum vulgare) roots (Wright et al, 1997), maize (Zea mays) roots (Redinbaugh and Campbell, 1998), tobacco (Nicotiana tabacum) roots and leaves (Debnam et al, 2004), and Arabidopsis (Arabidopsis thaliana; Wang et al, 2003). G6PDH activity is involved in the supply of the reducing equivalents necessary for nitrogen assimilation in root cells (Bowsher et al, 1992;Wright et al, 1997;Jin et al 1998;Esposito et al, 2001Esposito et al, , 2003 and in algae (Huppe and Turpin, 1996).…”