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Reevaluating the role of plastidic phosphoglucose isomerase in starch biosynthesis in mesophyll cells. Starch is the main storage carbohydrate in plants and represents the most important carbohydrate in human nutrition. Synthesized in the plastid in autotrophic and heterotrophic organs Starch is a branched homopolysaccharide of α-1,4-linked glucose subunits with α-1,6-linked glucose at the branching points. The use of this polyglucan is becoming increasingly attractive in industrial sectors including production of paper, detergents, bioplastics, bioethanol, etc.. Introduction A classic view of the starch biosynthetic process in photosynthetic tissues pPGI 3PGA/Pi; Redox pPGM AGP SS Sucrose and transitory starch are end products of two segregated pathways taking place in the cytosol and chloroplast, respectively Starch is directly linked to the Calvin cycle by means of plastid phosphoglucose isomerase (pPGI) Kunz et al. (2010) Plant Biol. (Stuttg) 12 Suppl 1: 115-128 The highly regulated ADP-glucose pyrophosphorylase (AGP) is the sole source of ADPG Does plastidial phosphoglucose isomerase link the CalvinBenson cycle to the starch biosynthetic pathway? pPGI is strongly inhibited by light (Heuer et al. (1982) Plant Physiol. 69: 1404-1406) pPGI is strongly inhibited by physiological concentrations of Calvin-Benson intermediates accumulating in the stroma during illumination such as 3-phosphoglycerate (Dietz, K-J. (1985) Biochem. Biophys. Acta 839: 240-248) The stromal G6P/F6P in the illuminated chloroplast is far lower than the equilibrium constant of pPGI (Dietz, K-J. (1985). Biochem. Biophys. Acta 839: 240-248; Sharkey, T.D. and Vassey, T.L. (1988) Plant Physiol. 90: 385-387) Green leaves exposed to 14CO2 for a short period of time synthesize starch with 14C asymmetrically distributed in the glucose (Kandler and Gibbs (1956) Plant Physiol. 31: 411-412; Havir and Gibbs (1963) J. Biol. Chem. 238: 3183-3187; Gibbs and Kandler (1957) Proc. Natl. Acad. Sci. USA 43: 446-451) Leaves pgi1-2 T-DNA insertional mutant impaired in pPGI activity accumulate 10% of the WT starch content, whereas AGP and pPGM null mutants accumulate 1% of the WT starch content (Kunz et al. (2010) Plant Biol. (Stuttg) 12 Suppl 1: 115-128) Identification and molecular characterization of a new pPGI allele (N92274) PGI (3639 bp) A atg B WT exon 6 intron 6 exon 7 (i) genomic DNA ...gct act agg aca act tcg gtaagtagtacttattgc...atttctttcttctttca g atc aag aat aac cct gca ... (ii) mRNA (iii) protein ...gcu acu agg aca acu ucg A T R T T S auc aag aau aac ccu gca ... I K N N P A C pgi1-3 exon 6 intron 6 exon 7 (i) genomic DNA ...gct act agg aca act tcg gtaagtagtacttattgc...atttctttcttctttca aatcaag aat aac cct gca ... (ii) mRNA (iii) protein ...gcu acu agg aca acu ucg A T R T T S aau aac ccu gca ... N N P A Characterization of the N92274 (pgi1-3) mutant Characterization of the N92274 (pgi1-3) mutant Characterization of the N92274 (pgi1-3) mutant N92274 (pgi1-3) is a pPGI null allele Volatiles emitted by different microbial species promote starch accumulation in Arabidopsis Ezquer et al. (2010) Plant Cell Physiol. 51: 1674-1693. Li et al. (2011) Mol. Plant Microb. Interact. 24: 1165-1178. Mutants lacking pPGI accumulate exceptionally high levels of starch when exposed to microbial volatiles Therefore, plastidial phosphoglucose isomerase does not connect the Calvin-Benson cycle with the starch biosynthetic pathway when plants are exposed to microbial volatiles Is GPT2 involved in the accumulation exceptionally high levels of starch when exposed to microbial volatiles? Therefore, GPT2 is not involved in the accumulation of exceptionally high levels of starch when plants are exposed to microbial volatiles CO2 Triose-P CalvinBenson cycle Triose-P 1 FBP 3 2 F6P 4 G6P Starch 10 5´ G1P 9 ADPG Chloroplast GPT2 G6P Cytosol Suggested alternative model of the starch biosynthetic process in mesophyll cells of leaves PGM AGP SuSy Sucrose and starch metabolic pathways are tightly connected by means of the ADPG producing sucrose synthase (SuSy) activity There occurs a yet to be identified ADPG transport machinery in the envelope membranes of plastids Synthesis and breakdown of starch take place simultaneously in the illuminated leaf. AGP and pPGM are involved in the scavenging of glucose units derived from the starch breakdown