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. The use of this polyglucan is becoming increasingly attractive in industrial sectors including production of paper, detergents, bioplastics, bioethanol, etc.. Starch is a branched homopolysaccharide of α-1,4-linked glucose subunits with α-1,6-linked glucose at the branching points. Synthesized in the plastid in autotrophic and heterotrophic organs

Introduction A classic view of the starch biosynthetic process in photosynthetic tissues  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: pPGI  The highly regulated ADP-glucose pyrophosphorylase (AGP) is the sole source of ADPG AGP SS 3PGA/Pi; Redox pPGM

Does plastidial phosphoglucose isomerase link the Calvin- Benson cycle to the starch biosynthetic pathway?  pPGI is strongly inhibited by light (Heuer et al. (1982) Plant Physiol. 69: )  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: )  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: ; Sharkey, T.D. and Vassey, T.L. (1988) Plant Physiol. 90: )  Green leaves exposed to 14 CO 2 for a short period of time synthesize starch with 14 C asymmetrically distributed in the glucose ( Kandler and Gibbs (1956) Plant Physiol. 31: ; Havir and Gibbs (1963) J. Biol. Chem. 238: ; Gibbs and Kandler (1957) Proc. Natl. Acad. Sci. USA 43: )  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: )

Identification and molecular characterization of a new pPGI allele (N92274)

Characterization of the N92274 (pgi1-3) mutant

N92274 (pgi1-3) is a pPGI null allele Characterization of the N92274 (pgi1-3) mutant

Volatiles emitted by different microbial species promote starch accumulation in Arabidopsis Ezquer et al. (2010) Plant Cell Physiol. 51: Li et al. (2011) Mol. Plant Microb. Interact. 24:

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?

Triose-P Chloroplast 5´ G6P G1P ADPG Starch Cytosol Triose-P FBP F6P G6P GPT2 CO 2 Benson Calvin- cycle Therefore, GPT2 is not involved in the accumulation of exceptionally high levels of starch when plants are exposed to microbial volatiles

 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 AGP SuSy PGM Suggested alternative model of the starch biosynthetic process in mesophyll cells of leaves  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