1. Making more plant: the biosynthesis of sucrose and starch
Biol 3470:
Plant Physiol Biotechnol
Lecture 9
Tues. 7 Feb. 2006
Chapter
From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001

2. Making sucrose and starch is linked with photosynthesis
Sucrose and starch are the ultimate products of the light-independent reactions (non-recycled C)
This “fixed” carbon can be used for:
Building new plant parts by using their C skeletons directly for the biosynthesis of
Amino acids (protein)
Cell wall (polysaccharides)
Fatty acids/triglycerides
Secondary metabolites for defense (terpenes, alkaloids)
Nucleic acids (purines, pyrimidines)
ATP synthesis via respiration (glycolysis, the TCA cycle and oxidative phosphorylation)

3. Plant cells regulate the relative amount of carbon exported or stored in the leaf
This is known as a________
Much of the photoassimilate is translocated immediately as sucrose in phloem
Many plants store excess fixed C in mesophyll cells to fuel mesophyll cell metabolism
starch in chloroplasts (soybean, spinach, tobacco)
sucrose in vacuole (wheat, barley, oats)
The plant must “decide” how to allocate its C from the 3-carbon carbohydrate (G3P) made in the PCR cycle
This depends on the metabolic need of sinks in the plant

4. Photosynthesis is the only input into plant carbon metabolism
The metabolism of carbon as carbohydrates has a central role in plant biosynthetic reactions
Photosynthesis provides the fixed carbon
Sucrose and starch are synthesized in different compartments
starch in chloroplast stroma
sucrose in mesophyll cytosol
Interchanging of this C through the hexose-phosphate pool feeds many metabolic pathways
glycolysis
Glyceraldehyde 3-P
Photosynthesis
respiration
TCA cycle
From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001

5. Carbon is allocated from the hexose-P pool according to metabolic need
Transport or long term C storage (in root, storage organ)
More plant
hv + CO2
From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001
Hexose-P pool
Photosynthesis
Glyceraldehyde 3-P
Long (root, storage organ) or short term (leaf) C storage
Energy for biosynthesis (NADPH), nucleic acid precursor (ribose 5-P)
C skeletons, ATP
Let’s focus on the inputs and outputs of the hexose-P pool
Note how C flows from “source” (the light-independent reactions or PCR cycle) to multiple C “sinks” (starch, sucrose, C skeleton generation for making new molecules, respiration)
Cells in different stages of their life cycle require different sources of C
Rapidly dividing cells need PPP nucleic acid precursors and energy
Biosynthesizing cells need NADPH
Etc.

6. Starch and sucrose biosynthesis occur in separate cellular compartments
There are separate hexose-P pools in the stroma and mesophyll cell cytosol
Interchange between these pools occurs via a triose-phosphate translocator in the chloroplast membrane
Starch is synthesized in the chloroplast stroma
Large starch grains are evident in electron micrographs of chloroplasts
Starch synthesis requires withdrawal of hexose-P from the mesophyll cellular pool
Using hexose-P for starch synthesis requires its a_________ by ATP
Only occurs in actively photosynthesizing cells
Fig. 6.2
Starch grain
Grana (stacked thylakoid membranes)

7. There are numerous control points that regulate the allocation of carbon
Specialized plastids called a________ store massive amounts of starch long-term
These plastids can also import and export hexose-P via a special translocator
This reflects their changing/dynamic role as a sink during seed development and source during germination
High [hexose-P]: favors starch synthesis during seed development
High inorganic P levels (substrate for starch degrading enzymes): favors starch breakdown during seed germination
The hexose-P transporter controls source/sink status of amyloplast
Note the large number of potential control points for regulating the flow of C between starch and sucrose!
Each is controlled by one or more enzymes
(inside chloroplast)
_____?
(outside chloroplast)
hexose-P pool
Triose-P
transporters
triose-P pool
Hexose-P
transporter
hexose-P pool
triose-P pool
_____?
photosynthesis
From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001

8. Sucrose is synthesized in the cell cytosol from its separate hexose-P pool
_________ ic hexose-P pool
Unlike starch, sucrose is synthesized in the cytosol of mesophyll cells
a/k/a photoassimilate: a disaccharide of glucose + fructose
Recall that sucrose may be
Stored
Translocated to sinks
Converted to starch via the chloroplast hexose-P pool
Metabolized via respiration to generate C skeletons and ATP
Main synthesis pathway uses hexose-P pool substrates:
Vast majority of sucrose is synthesized by sucrose phosphate synthase (SPS) and sucrose phosphate phosphatase (SPP)
Sucrose synthesis is energetically favored and allows its accumulation to high levels
Glucose 1-P
UTP Pi
Highly energetically favored
∆G = -13 kJ / mol
From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001

9. Starch or sucrose?
How does the plant “choose” whether to make sucrose or starch?
Traditionally, it was thought that carbohydrate metabolism was governed by source-sink relationships – sink removal causes backup of starch in leaves
This is false!
We now know that starch is needed in leaves for metabolic needs of the plant during dark period
Plants allocate photoassimilate between sucrose and starch in ways unrelated to:
Sink capacity of storage organs
Metabolic capability of chloroplasts in leaves to synthesize starch
Control of carbon distribution needed
Need to balance photoassimilate supply and utilization to avoid:
Depleting G3P pool in chloroplasts: keep the ____ cycle spinning!
Decreasing RuBP regeneration
Impairing carbon fixation and growth

10. Sucrose biosynthesis is regulated by the gluconeogenic enzyme fructose 1,6-bisphosphatase (FBPase)
From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001
Translocated photoassimilate from chloroplast
The activity of the cytosolic enzyme FBPase appears to control sucrose biosynthesis
It is the first irreversible reaction in conversion of triose-P to hexose-P
Observed that
tissues with higher FBPase activity accumulate more sucrose
tissues with lower FBPase activity accumulate less sucrose

11. The activity of FBPase is inhibited by the regulatory metabolite fructose 2,6-bisphosphate
Here’s how this regulation works:
A high sucrose synthesis rate results in high cytosolic Pi levels
This in turn slows the sucrose export rate
The hexose-P pool size grows as its utilization slows
This favors Fru 2,6-P2 synthesis
High Fru 2,6-P2 inhibits FBPase from adding more to the hexose-P pool
This slows sucrose biosynthesis
The hexose-P is converted to triose-P
High triose-P levels favor the breakdown of Fru 2,6-P2, returning C to the hexose-P pool
This releases FBPase from inhibition and once again favors sucrose biosynthesis
Export to chloroplast for starch biosynthesis 1 2 3 4 3 4 2 1

12. The size of the hexose-P pool also directly controls the rate of photosynthesis
Note that inhibition of FBPase also favors the export of triose-P to the chloroplast!
This favors starch over sucrose biosynthesis
Increasing the sizes of the hexose-P pools also feedback inhibits both the light-dependent and -independent reactions of photosynthesis
Less Pi available to make ATP in the photosynthetic E.T.C.
This in turn reduces O2 evolution and CO2 fixation
The balance between allocation to starch or sucrose synthesis is delicate and very dynamically controlled millisecond to millisecond
Controlled by compartmental levels of triose-P and Pi
Part of a complicated signal transduction network between chloroplast and cytosol
Dual role
Provide energy for growth and C-skeletons
Exchange information about metabolic status cytosol  chloroplast