Loss of Cytosolic Phosphoglucomutase Compromises Gametophyte Development in Arabidopsis

Abstract

Cytosolic phosphoglucomutase (cPGM) interconverts glucose-6-phosphate and glucose-1-phosphate and is a key enzyme of central metabolism. In this study, we show that Arabidopsis (Arabidopsis thaliana) has two cPGM genes (PGM2 and PGM3) encoding proteins with high sequence similarity and redundant functions. Whereas pgm2 and pgm3 single mutants were undistinguishable from the wild type, loss of both PGM2 and PGM3 severely impaired male and female gametophyte function. Double mutant pollen completed development but failed to germinate. Double mutant ovules also developed normally, but approximately half remained unfertilized 2 d after pollination. We attribute these phenotypes to an inability to effectively distribute carbohydrate from imported or stored substrates (e.g. sucrose) into the major biosynthetic (e.g. cell wall biosynthesis) and respiratory pathways (e.g. glycolysis and the oxidative pentose phosphate pathway). Disturbing these pathways is expected to have dramatic consequences for germinating pollen grains, which have high metabolic and biosynthetic activities. We propose that residual cPGM mRNA or protein derived from the diploid mother plant is sufficient to enable double mutant female gametophytes to attain maturity and for some to be fertilized. Mature plants possessing a single cPGM allele had a major reduction in cPGM activity. However, photosynthetic metabolism and growth were normal, suggesting that under standard laboratory conditions cPGM activity provided from one wild-type allele is sufficient to mediate the photosynthetic and respiratory fluxes in leaves.Flowering plants have specialized tissues to support the development of the haploid male and female gametophytes (the pollen and ovules, respectively) during sexual reproduction and subsequently the developing seeds. Understanding the route of import, metabolism, and utilization of the delivered carbohydrates is fundamental for understanding plant reproduction and may inform strategies for improving yield and nutritional value in our seed crops. The gametophytes are essentially heterotrophic, dependent on the carbohydrate supply from the parent plant. However, current knowledge about the pathways by which carbohydrates are imported and used to support gametophyte and subsequent seed development is rather fragmented.Suc is the predominant carbohydrate transported in the phloem of most higher plants (including Arabidopsis [Arabidopsis thaliana]) and is the primary source of energy for developing gametophytes and seeds. In general, Suc can enter heterotrophic cells (Fig.

Regulation of Leaf Starch Degradation by Abscisic Acid Is Important for Osmotic Stress Tolerance in Plants

“…Samples of the neutralized soluble fraction (200 mL) were applied to sequential 1.5-mL columns of cation exchanger Dowex 50 W and anion exchanger Dowex 1 (Sigma-Aldrich). Neutral compounds were eluted with 5 mL of water, lyophilized, redissolved in 200 mL of water, and separated on a CarboPac PA20 column on an ICS-3000 system (Dionex) as previously described (Egli et al, 2010). Peaks were identified by coelution with known malto-oligosaccharide standards and areas were determined using the instrument's Chromeleon software.…”

Loss of Cytosolic Phosphoglucose Isomerase Affects Carbohydrate Metabolism in Leaves and Is Essential for Fertility of Arabidopsis

“…Impairment or loss of function of enzymes contributing to the cytosolic hexose phosphate pool has recently been investigated for the Glc1P-forming enzyme PGM (Egli et al, 2010). The Arabidopsis genome encodes three PGM isoforms, with PGM1 localized to plastids and PGM2 and PGM3 localized to the cytosol (Caspar et al, 1985;Egli et al, 2010).…”

“…The Arabidopsis genome encodes three PGM isoforms, with PGM1 localized to plastids and PGM2 and PGM3 localized to the cytosol (Caspar et al, 1985;Egli et al, 2010). Analyses of transfer DNA (T-DNA) mutants showed that homozygous pgm2/pgm3 double mutants were nonviable because of impaired gametophyte development.…”

“…Analyses of transfer DNA (T-DNA) mutants showed that homozygous pgm2/pgm3 double mutants were nonviable because of impaired gametophyte development. However, pgm2 and pgm3 single mutants grew like ecotype Columbia (Col-0) wild-type plants, indicating overlapping functions of PGM2 and PGM3 (Egli et al, 2010).…”

Molecular genetic analysis of glucan branching enzymes from plants and bacteria in Arabidopsis reveals marked differences in their functions and capacity to mediate starch granule formation

“…Neutral sugars were eluted from sequential ion exchange columns of Dowex 50W and Dowex 1 (Sigma-Aldrich) and analyzed by high-performance anion-exchange chromatography with pulsed amperometric detection (HPAEC-PAD) as described by Egli et al (2010). Standard solutions containing known amounts of maltose (5 mM to 0.8 mM) and MOS DPs of three to seven (25 nM to 0.8 mM) were used to identify and quantify MOSs.…”