plant 3+4 Flashcards
1
Q
Sink organs
A
- non-photosynthesising organs and tissues
- rely on the import of photoassimilates for growth and development
- young leaves, roots, flowers, fruits, seeds, vegetative storage organs, meristems
2
Q
Source organs
A
- photosynthetic leaves
- Export photoassimilates to sink organs
3
Q
What is translocated through the phloem?
A
- Large variation between species.
- Carbohydrates, hormones, amino acids, some inorganic ions, RNAs and proteins, secondary metabolites.
4
Q
Sieve elements (SE)
A
- Highly specialised cells
- Long distance sugar conducting cells
- Living cells
- Devoid of almost all organelles
- Non-metabolically active
5
Q
Companion cells (CC)
A
- Load SE with sugars from producing cells (parenchyma)
- Perform metabolic functions lost from SE
- Energy (ATP)
6
Q
SE have sieve pores and form sieve tubes
A
- Pores in cell walls of the SE – sieve plate.
- Allow the flow/transport of sap.
- Diameter 1-15μm.
- Individual SE cells assemble into files forming sieve tubes.
7
Q
Sieve element and companion cell connection
A
- Connection through plasmodesmata.
- Often branched and complex.
- Symplastic transport of solutes to sieve elements.
- Close functional relationship to sieve elements
8
Q
Phloem loading in source tissues – apoplastic route
A
- Through the apoplast.
- The region outside the plasma membrane, made of cell walls of neighbouring cells.
- No plasmodesmata.
- Sucrose moves out of mesophyll cells into the apoplast
- Sucrose taken up into the phloem through an energy-requiring sucrose transporter: a sucrose-H+ symporter.
9
Q
Phloem loading in source tissues – symplastic route
A
- Through plasmodesmata joining neighbouring cells.
- Sucrose then moves through the plasmodesmata joining the companion cells and sieve elements.
- Sucrose may be converted to larger oligosaccharides e.g. raffinose, which then move to the sieve elements.
- Oligosaccharides are too large to diffuse back to mesophyll cells; “trapped” in the phloem.
10
Q
Pressure flow model of solutes in the phloem
A
- Loading of sieve element with sucrose, increases solute concentration, reduces Ψ.
- Ψ in xylem higher than in sieve element; water moves into sieve element.
- ΨP in sieve element increases.
- High pressure
- Unloading of sieve element at the sink, reduces solute concentration.
- Water moves into the xylem.
- ΨP in sieve element decreases.
- Low pressure
11
Q
Phloem unloading in sinks
A
- Through the symplast, through plasmodesmata.
Or, - Sucrose exported to the apoplast through efflux proteins, then taken up into the sink cell through sucrose transporters.
Or, - Sucrose exported to the apoplast through efflux proteins, and cleaved to glucose and fructose by acid invertase. Monosaccharides taken up into the sink cell through monosaccharide transporters.
* Any route may be used.
12
Q
Defects in loading, transport or unloading affect growth of sink organs
A
- GSL7 is a callose synthase required for the lining of sieve elements with callose.
- In mutant gsl7 plants sieve pore size is smaller than the wild-type.
- Mutant gsl7 plants have restricted flow of sucrose through the phloem.
- Reproductive organs are much smaller than those on wild-type plants.
- SWEET11, 12 and 15 are required for Suc transport across apoplastic barriers in developing seeds.
- Mutant sweet11;12;15 embryos develop slower and are smaller than wild-type
13
Q
Partitioning
A
differential distribution of photoassimilates within the plant.
14
Q
Sink strength
A
the ability of an organ to draw photoassimilates toward itself.
Sink strength = (sink size) X (sink activity)
Affected by, e.g.:
o proximity to source
o developmental stage
15
Q
Sucrose synthesis from triose phosphate (TP)
A
- TP exits the chloroplast through the transporter TPT, in exchange for inorganic phosphate.
- Sucrose synthesis in the cytoplasm.
- Tightly regulated
