Alteration of Photosynthate Partitioning by High-Level Expression of Phosphoglucomutase in Tobacco Chloroplasts

Abstract

Plastidial phosphoglucomutase (PGM) plays an important role in starch synthesis and degradation. Nonetheless, the impact of enhanced plastidial PGM activity on metabolism in photosynthetic tissue is yet to be elucidated. In this study, we generated transplastomic tobacco plants overproducing Arabidopsis thaliana plastidial PGM (AtptPGM) in chloroplasts and analyzed the consequent metabolic and physiological parameters in the transplastomic plants. AtptPGM accumulated in the chloroplasts to up to 16% of total soluble protein in the leaves. PGM activity in leaves increased 100-fold relative to that of wild-type plants. The transplastomic plants were phenotypically indistinguishable in their growth rates, photosynthetic activities, and starch synthesis from wild-type plants, but hexose partitioning in the light period was dramatically different. Furthermore, alteration of extracellular invertase activity was observed in the lower leaves of the transplastomic plants. These observations suggest that high-level expression of plastidial PGM alters hexose partitioning in light periods via modification of extracellular invertase activity.Key words: chloroplast; hexose partitioning; Nicotiana tabacum; phosphoglucomutase; transplastomic plant Phosphoglucomutase (PGM, EC 2.7.5.1) is a widely distributed enzyme in all life forms, catalyzing the nearequilibrium reversible interconversion between glucose 1-phosphate (Glc1P) and glucose 6-phosphate (Glc6P). There are two types of PGMs in plant cells: one cytosolic and the other plastidial.1) Cytosolic PGM participates in sucrose and hexose sugar metabolism and provides intermediates for glycolysis and building blocks for cellular components.2) In contrast, plastidial PGM functions in starch synthesis and degradation. 3)Starch is synthesized in leaf chloroplasts during the day via photosynthetic CO 2 fixation and is degraded at night as a major source of the sucrose required for dark metabolism.4) Lack of plastidial PGM results in a starchless phenotype characterized by a significant decrease in starch content and a high level of soluble sugar accumulation in the leaves during the day. 5,6) These mutants exhibit repressed carbon assimilation and altered growth characteristics. 5,7,8) A transgenic potato expressing the plastidial PGM gene in the antisense orientation has the same lowered rate of photosynthesis of starchless mutants as other plant species, as well as altered compartmentation of photosynthetic metabolites in the cytosol and plastids relative to the wildtype. 9,10) Plastidial PGM, therefore, not only catalyzes an essential reaction for starch synthesis, but also regulates photosynthesis through leaf starch synthesis.Enzymes that catalyze the near-equilibrium reversible reaction in carbohydrate synthesis were thought not to exert much influence on pathway flux, but recent studies have demonstrated that such enzymes as plastidial aldolase, 11,12) transketolase 13) and isocitrate lyase 14) do exert significant metabolic control. Expressing bacterial PGM in the potato c...

ARSENATE INDUCED CHLOROSIS 1/ TRANSLOCON AT THE OUTER ENVOLOPE MEMBRANE OF CHLOROPLASTS 132 Protects Chloroplasts from Arsenic Toxicity

“…When As(V) was absent, five proteins were more abundant in aic1 acd1-3 than in cad1-3. Among these proteins, three were involved in photosynthesis, one functioning in starch synthesis and degradation (Uematsu et al, 2012) and one as an antioxidant for catalyzing the breakdown of hydrogen peroxide into water and oxygen (Mhamdi et al, 2010; Table 3). By contrast, four proteins were less abundant in aic1 acd1-3 than in cad1-3, including one chloroplast organization-related protein, two photosynthesis proteins, and one involved in compound metabolism (Table 3).…”

Integrated mRNA and Small RNA Sequencing Reveals a microRNA Regulatory Network Associated with Starch Biosynthesis in Lotus (Nelumbo nucifera Gaertn.) Rhizomes

“…Thus far, many genes associated with rhizome development have been researched in lotus. Among them, SUS [ 10 ], UDP [ 11 ], PGM [ 12 , 13 ], AGP [ 14 ] are closely related to the synthesis of total starch, GBSS [ 15 , 16 ] is a key gene affecting amylose synthesis; SS [ 17 , 18 ], SBE [ 17 ] and DBE [ 19 , 20 ] are involved in the synthesis of amylopectin.…”

Identification and Analysis of <I>PGM</i> and <I>SUS</i> Gene Families in Leymus Chinensis Seed Transcriptome

“…Overexpression of PGM can effectively Molecular Plant Breeding 2021, Vol.12, No.7, 1-8 http://genbreedpublisher.com/index.php/mpb increase starch synthesis. Uematsu et al (2012a) overexpressed pPGM through tobacco (Nicotiana tabacum L.) leaves. Compared with wild-type plant leaves, they found that the activity of pPGM was improved in the leaves of transgenic plants, and the content of starch is also increased by 2 to 3 times (Vriet et al, 2010), which indicates that pPGM plays a role in promoting starch synthesis.…”