Most of ADP·glucose linked to starch biosynthesis occurs outside the chloroplast in source leaves

Abstract

Sucrose and starch are end products of two segregated gluconeogenic pathways, and their production takes place in the cytosol and chloroplast of green leaves, respectively. According to this view, the plastidial ADP⅐glucose (ADPG) pyrophosphorylase (AGP) is the sole enzyme catalyzing the synthesis of the starch precursor molecule ADPG. However, a growing body of evidences indicates that starch formation involves the import of cytosolic ADPG to the chloroplast. This evidence is consistent with the idea that synthesis of the ADPG linked to starch biosynthesis takes place in the cytosol by means of sucrose synthase, whereas AGP channels the glucose units derived from the starch breakdown. To test this hypothesis, we first investigated the subcellular localization of ADPG. Toward this end, we constructed transgenic potato plants that expressed the ADPG-cleaving adenosine diphosphate sugar pyrophosphatase (ASPP) from Escherichia coli either in the chloroplast or in the cytosol. Source leaves from plants expressing ASPP in the chloroplast exhibited reduced starch and normal ADPG content as compared with control plants. Most importantly however, leaves from plants expressing ASPP in the cytosol showed a large reduction of the levels of both ADPG and starch, whereas hexose phosphates increased as compared with control plants. No pleiotropic changes in photosynthetic parameters and maximum catalytic activities of enzymes closely linked to starch and sucrose metabolism could be detected in the leaves expressing ASPP in the cytosol. The overall results show that, essentially similar to cereal endosperms, most of the ADPG linked to starch biosynthesis in source leaves occurs in the cytosol.

Arabidopsis Responds to Alternaria alternata Volatiles by Triggering Plastid Phosphoglucose Isomerase-Independent Mechanisms

“…Chloroplasts from mature leaves also possess a yet to be identified ADPG transport machinery (Pozueta-Romero et al, 1991). Taking into account that a sizable pool of ADPG linked to starch biosynthesis has a cytosolic localization in leaves (Baroja-Fernández et al, 2004;Bahaji et al, 2011Bahaji et al, , 2014a, it is likely that the starch biosynthesis observed in the mesophyll cells of VC-treated mature leaves partly involves the cytosolic production of ADPG, its subsequent transport into the chloroplast, and, then, conversion into starch (Fig. 6).…”

Regulation of Starch Biosynthesis in Response to a Fluctuating Environment

“…It was suggested that in leaves, ADP-Glc is synthesized in the cytosol by Suc synthase, using ADP and Suc as substrates, and then imported into the chloroplast for starch synthesis (Baroja-Fernández et al, 2004;Muñ oz et al, 2005 Schematic representation of the pathway of starch biosynthesis, its subcellular compartmentation, and distribution of flux control in photosynthetic leaves (A) and heterotrophic tissues (B). The reactions of the pathway of starch biosynthesis are catalyzed by the following enzymes: 1, phosphoglucoisomerase; 2, PGM; 3, AGPase; 4, SS; 5, SBE; 6, starch-debranching enzyme; 7, inorganic pyrophosphatase; 8, Suc synthase; 9 UDP-Glc pyrophosphorylase; 10, fructokinase; 11, ATP/ADP translocator; 12, Glc-6-P/Pi translocator; 13, cytosolic AGPase; and 14, ADP-Glc/ADP translocator (steps 13 and 14 are highlighted to be specific for cereal endosperm).…”

Plastidial Glyceraldehyde-3-Phosphate Dehydrogenase Deficiency Leads to Altered Root Development and Affects the Sugar and Amino Acid Balance in Arabidopsis

“…Data presented are the arsenate-dependent activities. Measurements of ADP-Glc pyrophosphorylase, starch phosphorylase, Suc synthase, total starch synthase, and Suc phosphate synthase activities were assayed as described by Baroja-Fernández et al (2004). We define 1 unit of enzyme activity as the amount of enzyme that catalyzes the production of 1 mmol of product per minute.…”