Glucose-1-Phosphate Transport into Protoplasts and Chloroplasts from Leaves of Arabidopsis

Abstract

Almost all glucosyl transfer reactions rely on glucose-1-phosphate (Glc-1-P) that either immediately acts as glucosyl donor or as substrate for the synthesis of the more widely used Glc dinucleotides, ADPglucose or UDPglucose. In this communication, we have analyzed two Glc-1-P-related processes: the carbon flux from externally supplied Glc-1-P to starch by either mesophyll protoplasts or intact chloroplasts from Arabidopsis (Arabidopsis thaliana). When intact protoplasts or chloroplasts are incubated with [U-14C]Glc-1-P, starch is rapidly labeled. Incorporation into starch is unaffected by the addition of unlabeled Glc-6-P or Glc, indicating a selective flux from Glc-1-P to starch. However, illuminated protoplasts incorporate less 14C into starch when unlabeled bicarbonate is supplied in addition to the 14C-labeled Glc-1-P. Mesophyll protoplasts incubated with [U-14C]Glc-1-P incorporate 14C into the plastidial pool of adenosine diphosphoglucose. Protoplasts prepared from leaves of mutants of Arabidopsis that lack either the plastidial phosphorylase or the phosphoglucomutase isozyme incorporate 14C derived from external Glc-1-P into starch, but incorporation into starch is insignificant when protoplasts from a mutant possessing a highly reduced ADPglucose pyrophosphorylase activity are studied. Thus, the path of assimilatory starch biosynthesis initiated by extraplastidial Glc-1-P leads to the plastidial pool of adenosine diphosphoglucose, and at this intermediate it is fused with the Calvin cycle-driven route. Mutants lacking the plastidial phosphoglucomutase contain a small yet significant amount of transitory starch.

Metabolic Fluxes in an Illuminated Arabidopsis Rosette

“…First, bioinformatic analysis of Arabidopsis metabolite profiles revealed patterns suggestive of microcompartmentation of metabolism within organelles (Krueger et al, 2011), as previously postulated for a pool of UDPG that is involved in cellulose synthesis (Amor et al, 1995). Secondly, recent studies in Arabidopsis and potato implicate the presence of as yet unidentified G1P transporters at the plastid and plasma membranes (Fettke et al, 2011). Further analyses will be required to fully understand the exact mechanisms by which the active and inactive pools of these metabolites are separated.…”

Double Knockout Mutants of Arabidopsis Grown under Normal Conditions Reveal that the Plastidial Phosphorylase Isozyme Participates in Transitory Starch Metabolism

“…1). Both zones are undetectable in PHS2-deficient Arabidopsis mutants but present in the mutants lacking PHS1 (Lu et al, 2006;Fettke et al, 2011a). It is, however, not clear what actually causes the partitioning between the immobile and the more mobile state.…”

Starch Turnover and Metabolism during Flower and Early Embryo Development

“…The phosphoglucose isomerase (PGI) converts Fru-6-P to Glc-6-P; then, Glc-6-P is converted to Glc-1-P by phosphoglucomutase (PGM); Glc-1-P is then metabolized to ADP-Glc by ADP-Glc pyrophosphorylase (AGPase). However, there is ongoing discussion over the origin of the residual starch (3%-15%) in some null mutants of the plastidic PGI-PGM-AGPase pathway (Niewiadomski et al, 2005;Streb et al, 2009;Kunz et al, 2010;Bahaji et al, 2015), the significance of the observed uptake of exogenously applied Glc-1-P to isolated chloroplasts and its conversion to starch (Fettke et al, 2011), the disputed possibility of an alternative cytosolic route of ADP-Glc synthesis by Suc synthase (SUS) and its potential translocation to the chloroplast (Muñoz et al, 2005;Barratt et al, 2009;Baroja-Fernández et al, 2012;Smith et al, 2012), and the recently suggested PGI1-independent starch synthesis (Bahaji et al, 2015). Starch synthesis in nonphotosynthetic plastids relies on the translocation of carbon from the cytosol, mainly in the form of hexose phosphate (Flügge, 1999).…”