Making more plant: the biosynthesis of sucrose and starch

Biol 3470: Plant Physiol Biotechnol

Lecture 9Tues. 7 Feb. 2006Chapter 6.1 - 6.2From Buchanan et al., “Biochemistry

and molecular biology of plants”, 2001

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)

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

From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001

Photosynthesis

resp

iratio

n

Glyceraldehyde 3-P

• 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

Photosynthesis is the only input into plant carbon metabolism

glyc

olys

isT

CA

cyc

le

Carbon is allocated from the hexose-P pool according to metabolic need

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.

From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001

Photosynthesis

Glyceraldehyde 3-P

hv + CO2

C skeletons, ATP

Long (root, storage organ) or short term (leaf) C storage

More plant Transport or long term C storage (in root, storage organ)

Energy for biosynthesis (NADPH), nucleic acid precursor (ribose 5-P)

Hexose-P pool

• 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

http://www.sju.edu/biology/ksweb/microsc/mgraphs03.html

Fig. 6.2

Starch and sucrose biosynthesis occur in separate cellular compartments

Starch grain

Grana (stacked thylakoid membranes)

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

(outside chloroplast) (inside chloroplast)

hexose-P pool

triose-P pool

hexose-P pool triose-P pool

photosynthesis_____?

Triose-Ptransporters

Hexose-Ptransporter

From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001

_____?

• 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

Sucrose is synthesized in the cell cytosol from its separate hexose-P pool

UTP

Pi

Glucose 1-P

_________ ic hexose-P pool

From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001

Highly energetically favored∆G = -13 kJ / mol

• 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

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

Sucrose biosynthesis is regulated by the gluconeogenic enzyme fructose 1,6-bisphosphatase (FBPase)

• 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

Translocated photoassimilate from chloroplast

From Buchanan et al., “Biochemistry and molecular biology of plants”, 2001

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 1

1

2

2

33

Export to chloroplast for starch biosynthesis

4

4

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