Phosphoenolpyruvate Provision to Plastids Is Essential for Gametophyte and Sporophyte Development inArabidopsis thaliana   

Abstract

Restriction of phosphoenolpyruvate (PEP) supply to plastids causes lethality of female and male gametophytes in Arabidopsis thaliana defective in both a phosphoenolpyruvate/phosphate translocator (PPT) of the inner envelope membrane and the plastid-localized enolase (ENO1) involved in glycolytic PEP provision. Homozygous double mutants of cue1 (defective in PPT1) and eno1 could not be obtained, and homozygous cue1 heterozygous eno1 mutants [cue1/eno1 (+/2)] exhibited retarded vegetative growth, disturbed flower development, and up to 80% seed abortion. The phenotypes of diminished oil in seeds, reduced flavonoids and aromatic amino acids in flowers, compromised lignin biosynthesis in stems, and aberrant exine formation in pollen indicate that cue1/eno1(+/2) disrupts multiple pathways. While diminished fatty acid biosynthesis from PEP via plastidial pyruvate kinase appears to affect seed abortion, a restriction in the shikimate pathway affects formation of sporopollonin in the tapetum and lignin in the stem. Vegetative parts of cue1/eno1(+/2) contained increased free amino acids and jasmonic acid but had normal wax biosynthesis. ENO1 overexpression in cue1 rescued the leaf and root phenotypes, restored photosynthetic capacity, and improved seed yield and oil contents. In chloroplasts, ENO1 might be the only enzyme missing for a complete plastidic glycolysis. INTRODUCTIONPhosphoenolpyruvate (PEP) plays a central role in plant metabolism. As an intermediate of glycolysis, PEP is indispensable for energy metabolism in the cytosol and delivers ATP and pyruvate by the action of cytosolic pyruvate kinase (PK) (Plaxton, 1996;Givan, 1999). Pyruvate can be fed into the citric acid cycle, yielding NADH for respiratory ATP generation (Fernie et al., 2004). Inside the plastids, PEP may act as a precursor for at least four metabolic pathways ( Figure 1A).Together with erythrose 4-phosphate, PEP is fed into the shikimate pathway, which delivers essential aromatic amino acids and a large number of secondary plant products. The initial steps of the shikimate pathway are exclusively localized within the plastid stroma (Herrmann, 1995;Schmid and Amrhein, 1995;Herrmann and Weaver, 1999). Inside the stroma, PEP can also be sequentially metabolized to pyruvate and acetyl-CoA by plastid PK and the pyruvate dehydrogenase complex (Reid et al., 1977;Elias and Givan, 1979; Lernmark and Gardeströ m, 1994) and thus enter the biosynthesis of fatty acids (Dennis, 1989;Ohlrogge and Jaworski, 1997), which are quantitatively important for triacylglycerol production in oil seeds (e.g., Voelker and Kinney, 2001;Rawsthorne, 2002;Ruuska et al., 2002). Like the shikimate pathway, the de novo biosynthesis of fatty acids for membranes and storage lipids is localized to the plastids (Ohlrogge et al., 1979;Ohlrogge and Jaworski, 1997). Moreover, stromal pyruvate can act as a precursor for the synthesis of branched-chain amino acids (Schulze-Siebert et al., 1984) and together with glyceraldehyde 3-phosphate for the mevalonate-independent way (2-C-methyl-...

A Noninvasive Platform for Imaging and Quantifying Oil Storage in Submillimeter Tobacco Seed

“…The supply of this metabolite from the cytosol to the plastid is catalyzed by PPT (Knappe et al, 2003), which, at least in soybean seed, is present at the same site as lipid accumulates (Borisjuk et al, 2005). In Arabidopsis, the down-regulation of PPT depresses lipid accumulation (Prabhakar et al, 2010). Our work demonstrated that its up-regulation raised lipid accumulation by approximately 13%.…”

Pollen tube growth: where does the energy come from?

“…In approximately 40% of the ovules, the embryo sac was diminished in size or even absent. 63 Besides the already mentioned plastidial enzymes, also mitochondrial proteins could be identified which are involved in pollen vitality and ovule or embryo development, respectively. For instance, GmAOX2b antisense plants exhibit increased rates of pollen abortion in vivo and reduced rates of pollen germination in vitro.…”

“…However, a heterozygous knock-out for eno1 in the homozygous cue1 (defective in PPT1) background (ccEe) possesses a reduced number of pollen containing a large portion of underdeveloped, non-vital, non-germinating pollen grains. 63 In addition, some of the mutant pollen accumulate a massive amount of starch in plastids indicating that starch degradation and subsequent glycolysis is inhibited. Therefore, no ATP can be generated via plastidial glycolysis to satisfy the high energy demand during pollen development and elongation.…”

“…During the last years, various research articles were published where it was shown that a knock-out of genes coding for plastid-localized glycolytic enzymes and related proteins specifically affect male gametophyte development. [63][64][65][66] During the final stage of pollen maturation, starch biosynthesis is critical because starch represents a source of energy for pollen germination and elongation and, in addition, serves as a checkpoint of pollen maturity (e.g., it was shown that pollen non-viability is associated with starch deficiency). [67][68][69] Starch degradation and following triose-phosphate oxidation with coupled ATP generation in plastids require a continuous regeneration of NAD C (Fig.…”

The specific role of plastidial glycolysis in photosynthetic and heterotrophic cells under scrutiny through the study of glyceraldehyde-3-phosphate dehydrogenase

“…In this sense, the presence of a complete glycolytic plastidial pathway in chloroplasts, specifically its lower part catalyzed by phosphoglycerate mutase and enolase, has even been questioned. [2][3][4] The reactions of plastidial/chloroplastic phosphoglycerate kinase (PGK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are particularly interesting because different isoforms of the same enzyme catalyze the same reactions, but in the opposite direction, of the plastidial glycolysis and Calvin cycle. Plastidial glycolytic GAPDH (GAPCp) converts glyceraldehyde-3-phosphate (GAP) into 1,3-bisphosphoglycerate (1,3-bisPGA), which is then converted into 3-phosphoglycerate (3-PGA) by plastidial PGK (PGKp).…”