1. Phytochrome: regulation of
Skotomorphogenesis
Seed germination
Genes, enzymes and signal transduction
Embryo and
Seed development
Photomorphogenesis
Photoreceptors
Phytochrome
Cells and cell growth
Phytochrome: regulation of
light responses
Photosynthesis: light reaction
Photosynthesis: carbon fixation
Photorespiration
Nitrogen fixation
Plant nutrition
Respiration
Phloem translocation
Sucrose, starch, cellulose

2. Carbohydrates: where are they coming from, where are they going???

3. Phloem Translocation, and Photosynthate Allocation and Partitioning
HORT 301 – Plant Physiology
October 29, 2010
Taiz and Zeiger, Chapter 10
Phloem translocation - bidirectional transport of photosynthate, assimilated nutrients, signaling molecules and redistribution of water and molecules
Sources to sinks

4. Phloem loading and unloading Photosynthate allocation and partitioning
Lecture topics:
Phloem translocation
Phloem loading and unloading
Photosynthate allocation and partitioning
Photosynthate moves in the phloem
Taiz & Zeiger (2010) Web Topic 10.1.A

5. Phloem tissue is adjacent to the xylem
Proximity of the xylem and phloem is critical for translocation
Stem (perennial)
Leaf

6. Phloem - sieve (tube) elements and companion cells
Sieve elements – protoplasmic w/modified plastids and ER
Companion cells – sieve element metabolic functioning, phloem loading and unloading

7. Sieve plates – pores predominantly at the ends of sieve elements
Sieve elements are interconnected cells
Callose plugs seal sieve pores

8. Companion cells facilitate phloem loading and unloading
Interconnected with sieve elements
Ordinary companion cell
Transfer cell
OCC and TC – minimal interconnection w/adjacent cells – apoplastic to symplastic uptake
IC – interconnection to adjacent cells, symplastic uptake-
Intermediary cell

9. Source to sink translocation
Source – photosynthate production (availability) is greater than metabolic need, export
Sink – requires photosynthate, import

10. Change in photosynthate distribution in sink leaves by removal of sources
Taiz & Zeiger (2010) Web Topic 10.1.C

11. Phloem sap sampling for analysis
Taiz & Zeiger (2010) Web Topic A

12. Sugars are a primary component of sap

13. Sugars are translocated in non-reducing form – sucrose is the predominant photosynthate

14. Pressure flow model for phloem translocation from source to sink
Source – phloem loading increases ψp
Sink – phloem unloading decreases ψp
Ψp gradient between source and sink drives solution movement
Ψw gradient between sieve elements and xylem vessel elements recirculates the solution

15. Phloem translocation requires minimal energy – passive transport

16. Phloem loading via symplastic or apoplastic pathways
Symplastic loading is passive transport
Apoplastic loading is active transport

17. Sucrose-H+ symporter involved in phloem loading via an apoplastic pathway – active transport
Symporter is driven by the H+ gradient generated by the plasma membrane H+-ATPase

18. Polymer trapping model for symplastic phloem loading – passive uptake
Sucrose diffuses from the bundle sheath/mesophyll cells into intermediary cells
Sucrose is synthesized into raffinose, concentration decreases in intermediary cells
More sucrose enters the intermediary cells
Raffinose cannot move to bundle sheath/mesophyll cells because plasmodesmata are too small

19. Phloem unloading by symplastic and apoplastic pathways into sinks
Developing leaves, roots, tubers and reproductive organs
Unloading, short-distance transport and sink loading

20. Leaf transition from sink to source
14C import (dark) from a source leaf

21. Sucrose-H+ symporter gene expression in source and sink leaves
AtSUC2 promoter::GUS reporter fusion – phloem loading

22. Photosynthate allocation and partitioning
Allocation – regulation of carbon assimilate distribution into various metabolic pathways or sink cells
Partitioning – distribution of photosynthates, assimilation products, ions, hormones, signaling molecules (proteins and perhaps mRNAs)

23. Allocation is regulated by sinks and sources
Partitioning of triose-phosphates
Sink strength