Download Plastidial phosphoglucose isomerase does not link the Calvin

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