plant 3+4 Flashcards
Sink organs
- 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
Source organs
- photosynthetic leaves
- Export photoassimilates to sink organs
What is translocated through the phloem?
- Large variation between species.
- Carbohydrates, hormones, amino acids, some inorganic ions, RNAs and proteins, secondary metabolites.
Sieve elements (SE)
- Highly specialised cells
- Long distance sugar conducting cells
- Living cells
- Devoid of almost all organelles
- Non-metabolically active
Companion cells (CC)
- Load SE with sugars from producing cells (parenchyma)
- Perform metabolic functions lost from SE
- Energy (ATP)
SE have sieve pores and form sieve tubes
- 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.
Sieve element and companion cell connection
- Connection through plasmodesmata.
- Often branched and complex.
- Symplastic transport of solutes to sieve elements.
- Close functional relationship to sieve elements
Phloem loading in source tissues – apoplastic route
- 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.
Phloem loading in source tissues – symplastic route
- 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.
Pressure flow model of solutes in the phloem
- 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
Phloem unloading in sinks
- 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.
Defects in loading, transport or unloading affect growth of sink organs
- 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
Partitioning
differential distribution of photoassimilates within the plant.
Sink strength
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
Sucrose synthesis from triose phosphate (TP)
- TP exits the chloroplast through the transporter TPT, in exchange for inorganic phosphate.
- Sucrose synthesis in the cytoplasm.
- Tightly regulated
Sucrose synthesis controls the rate of C fixation
- TPT regulates the rate of TP export to the cytoplasm for sucrose synthesis.
- High rates of TP export: less TP remains in the Calvin cycle; restricts C fixation.
- Low rates of TP export: less Pi enters the chloroplast, less ATP available for the Calvin-Benson-Bassham cycle; restricts C fixation.
Starch
an alternative fate for fixed C.
Made of amylose and amylopectin (glucose polymers).
* Large, insoluble, semicrystalline granules.
* Large variation among species in the amount of fixed C sequestered as starch.
Starch synthesis in leaves during the day
- Synthesised during the day, in chloroplasts.
- Substrates from the Calvin cycle.
- Some plants sequester ~50% of the fixed C as starch. Others rely more on sucrose synthesis.
- Starch and sucrose synthesis tightly linked.
- Low rates of sucrose synthesis - high rates of starch synthesis.
A shortage of photoassimilates inhibits growth
- Little or no sucrose available for export from source leaves and transport through the phloem.
- C starvation.
- Growth of sink organs (e.g. roots) inhibited or even seizes.
- An acute response to starvation.
Starch degradation sustains growth at night
starch excess1 (sex1); also gwd1:
o No starch degradation at night.
o No sucrose supply at night.
o Carbon starvation .
o Silique growth is reduced.
When do plants initiate flowering?
- The decision to initiate flowering is critical.
- Determines plant fitness in terms of reproductive success.
- Depends on integration of internal and external cues: o Age of the plant o Environmental signals: daylength (photoperiod), light quality, temperature (e.g. vernalization)
- Involves changes in the developmental programme in the SAM.
- Transition to an inflorescence meristem
Plant development has three phases
Juvenile phase
Adult vegetative phase
Adult reproductive phase
Juvenile phase
Not able to form reproductive organs
o Length varies between species
Adult vegetative phase
o Able to form reproductive organs under inductive conditions
o Leaf morphology, thorniness, root system, e
Adult reproductive phase
o Flowering, seed production
Phase transition to flowering in the apex
- Integration of internal and environmental cues.
- Phase transition to reproductive growth.
- Changes in the morphology and the developmental program of the meristem.
- Meristem converted to an inflorescence meristem.
- No more vegetative organs are produced.
Two types of meristems
inflorescence meristem (IM, )
floral meristem (FM; )
floral meristem (FM; )
produces floral organs of a single flower. Determinate.
inflorescence meristem (IM, )
can be determinate or indeterminate.
Genetic control of flower initiation and development
- Flowering time genes. Determine when flowering starts. Link to environmental conditions. e.g. FLOWERING LOCUS T (FT)
- Floral meristem identity genes. Commit meristems to produce floral rather than vegetative structures. e.g. LEAFY (LFY), APETALA1 (AP1), etc.
- Floral organ identity genes. Control floral organ development (sepals, petals, stamens, carpels). e.g. ABC genes (AP1, AP2, AP3, PISTILATTA, AGAMOUS)
Floral meristem identity – LEAFY
- The TF LEAFY (LFY) is involved in determining floral meristem identity. Expressed at FMs.
- Mutant lfy plant: instead of FMs and flowers, it forms a shoot with leaves.
- Overexpression of LFY: IM converted to a FM, produces a terminal flower
Floral meristem identity – AP1 and CAL
- The TFs APETALA1 (AP1) and CAULIFLOWER (CAL) determine floral meristem identity. Expressed at FMs.
- Mutant ap1;cal plants: apical meristem produces new IMs but no FMs.
- The new IMs produce IMs again. The apex is converted to a mass of IMs.
- Similar to cauliflower, Romanesco broccoli.
Regulatory interactions conferring floral meristem identity
- Expression pattern of FM identity genes and their interactions determine FM identity.
- TERMINAL FLOWER1 (TFL1) is expressed at the IM.
- TFL1 represses LFY, AP1/CAL expression at the IM. Maintains IM identity.
- LFY, AP1/CAL expressed at the FM, and suppress TFL1. Maintain FM identity.
- Loss of TFL1, converts IM to FM. Terminal flower.
Key transcription factors promote floral organ identity
- The ABC model of floral organ identity specification.
- Within the floral meristem, key transcription factors specify floral organs.
- Encoded by the ABC genes.
- Different whorls of floral organs are produced from different combinations of the ABC genes.
