1. Phloem Translocation and Assimilate Partitioning
HORT 301 – Plant Physiology
October 15 and 17, 2007
Taiz and Zeiger, Chapter 10, Web Topics
Class Handout
Phloem translocation – facilitates movement of photosynthetic products to storage organs or growing tissues
Phloem anatomy – cellular structure and function
Source to sink translocation – pressure-flow movement of photosynthetic products
Phloem loading and unloading – substances and processes
Photosynthetic product allocation and partitioning – regulation and distribution into sinks

2. Phloem translocation – movement of photosynthetic products from a net carbon source to a net carbon sink via the phloem
Xylem – transports water and mineral nutrients from roots to shoots
Source - net carbon fixation, sink – net import of photosynthetic product
Phloem transport is bidirectional - organic molecules (primarily) in solution (water), other nutrients, signaling molecules

3. Phloem anatomy (cellular structure) – translocation capacity is due to the anatomy
Phloem is outside of the xylem in the shoot
Phloem tissue is adjacent to the xylem in the root

4. Perennial stem illustrating secondary xylem and phloem (bark), primary phloem is the functional tissue, adjacent to the cambium
Primary phloem in leaves– organized into vascular bundles

5. Sieve elements - primary conducting cells of phloem
Sieve tube elements - highly differentiated cells in angiosperms, focus of the lecture
Sieve cells - less specialized cells in gymnosperms
Sieve elements are specialized cells - protoplasmic (living) w/o a nucleus, tonoplast, Golgi apparatus and ribosomes
Retain plasma membrane, mitochondria, plastids and endoplasmic reticulum

6. Sieve element pores (sieve plate or lateral sieve area) - interconnect the elements longitudinally (plate) and radially (area) forming a symplastic connection from shoot to root
Sieve area pores - <1 µm to 10 µm in diameter, sieve plate pores are much larger

7. Photosynthetic products - translocated throughout the plant
Solution is referred to as sap, can be highly viscous
P-proteins (some lectins) and callose (β-1, 3-glucan) seal sieve element pores upon injury or during dormancy, minimize sap loss and translocation

8. Companion cells – adjacent to sieve elements in mature source leaves
Facilitate loading of sap into sieve elements in minor veins (phloem loading)
Companion cells and sieve elements derive from a common progenitor cell but are products of specialized differentiation
Plasmodesmata interconnect the symplasm of the sieve elements and companion cells
Companion cells also carry out metabolic and regulatory functions for the sieve elements

9. Companion cell types:
Ordinary companion cell – plasmodesmatal connection to the sieve element but minimal plasmodesmatal connection to surrounding cells
Transfer cell – ordinary companion cell-like, w/finger-like membrane protrusions that increase plasma membrane surface area, minimal plasmodesmatal connection to surrounding cells

10. Intermediary cell – numerous plasmodesmatal connections to surrounding cells
Ordinary companion and transfer cells facilitate apoplastic transport of sap into sieve elements
Intermediary cells facilitate symplastic transport into sieve elements

11. Source to sink translocation – photosynthetic products transported in the phloem (translocation) moves from a source (net photo-assimilation) to a sink (metabolically active or storage)
Translocation may be upwards or downwards
Source – an exporting organ because carbon availability is greater than utilization, e.g. photosynthetically active leaf, storage organs (tubers) at the exporting phase of development.
Sink – storage organ (seed) or an organ that does not produce enough photosynthate for its metabolic demand (young leaves)

12. Source to sink pathways and affecting factors – specific physical pathways interconnect particular sources and sinks
Proximity – upper leaves provide photosynthate to the shoot apex and lower leaves provide for the root
Development – fruits become a dominant sink during reproduction, storage roots export as vegetative tissue starts to develop after over-wintering
Vascular connections – source leaves preferentially supply sinks that have a direct vascular connection, often directly above or below but not necessarily by developmental age

13. Translocation pathways have plasticity – removal of sinks results in greater accumulation into those that remain
Translocation is expressed as velocity – linear distance traveled per unit time OR mass transfer rate – quantity of material passing through a given cross section (area) per unit time.

14. The pressure flow model proposes that translocation in phloem sieve elements from a source to a sink is driven by a pressure gradient, pressure-driven bulk (mass) flow
Source to sink pressure gradient - positive pressure gradient is between sieve elements at the source (higher) and sink (lower), source to sink

15. Source sieve elements - phloem loading of solutes (solute transport into sieve elements) causes a lower (more negative) solute (osmotic) potential (ψs) and a concomitant more negative water potential (ψw)
Water moves into the sieve elements from the xylem along the ψw gradient and turgor pressure (ψp) increases

16. Sink sieve elements - phloem unloading (solute transport exported from phloem) causes a higher ψs (less negative) and concomitant higher ψw (less negative)
When sieve element ψw becomes less negative than ψw in the adjacent xylem cells - water moves from the sieve element to the xylem causing a decrease in turgor in the sieve element
Source sieve elements have a higher hydrostatic pressure than sink sieve elements, pressure-driven bulk flow of the solutes (with water)
Resistances (sieve plates and sieve areas) in the translocation pathway maintain the pressure gradient – pores are sufficient resistance to allow the maintenance of pressure gradients

17. Bulk (mass) flow instead of osmosis because membranes are not crossed, symplastic interconnections from the source to the sink
Solution movement in the phloem is based on the pressure gradient and not the water potential gradient because it is a bulk flow process and not osmosis, i.e. conforms to the laws of thermodynamics
Transport in the phloem is bidirectional but requires different sieve element translocation units, i.e., there is no bidirectional transport in an individual cell

18. Phloem loading (source) and unloading (sink) – movement into and out of the sieve elements
Sources - photosynthetically active cells (photosynthate production > respiratory carbon use) or storage cells at a different stage of development (e.g. seeds during germination)
Sinks – storage organs, e.g. seeds, roots and tubers

19. Phloem loading - chloroplasts to sieve elements
Triose phosphates – are transported from the chloroplasts to cytosol (mesophyll cells), converted to sucrose
Sucrose moves in small veins from the mesophyll cell to cells near the companion cell-sieve element complex

20. “Sugars” concentrate at the site of loading
Sugars move initially through a symplastic pathway but concentrate in companion cells via apoplastic or symplastic transport
Apoplastic loading

21. Apoplastic loading – sucrose is transported into the companion cell by a secondary active sucrose-H+ co-transporter in the plasma membrane
Sucrose-H+ co-transporter and H+-ATPase (generate H+ gradient) are localized companion cell on the side adjacent to the parenchyma cells
Transport from the companion cell to sieve elements is through plasmodesmata (passive)

22. Symplastic loading

23. Symplastic loading – via intermediary cells adjacent to the sieved element, sugars, sugar alcohol or/and oligosaccharides
Intermediary cells adjacent to the sieve element
Apoplastic loading – primarily sucrose

24. Phloem unloading and storage – unloading of sugars from the sieve element-companion cell complex near the sink, short distance transport and storage in sinks
Sinks – growth and other metabolically active regions (e.g. shoot or root tip, young leaves) or storage organs (e.g. seeds, tubers, fruits, etc.)
Phloem unloading and transport into sink cells may be symplastic (through plasmodesmata only) OR
Apoplastic - type 1 – unloading from sieve element-companion cell complex is apoplastic
or type 2 – apoplastic transport occurs in other cells in the transport pathway
Regardless the “process” is energy dependent based on inhibitor studies

25. Photosynthetic product allocation and partitioning - regulation of photosynthesis and photosynthate utilization is highly coordinated in plants
Allocation - regulation of fixed carbon diversion into various metabolic pathways in sources (organs that export photosynthetic products) or sinks (organs that import photosynthetic products), e.g. to sucrose or starch
Partitioning – differential distribution of photosynthetic products into sinks

26. Allocation in source leaves is regulated – triose phosphates are allocated to renew photosynthetic intermediates, or starch or sucrose production.
Pi/triose phosphate composition regulates starch or sucrose biosynthetic enzymes
Low Pi concentration in the cytosol limits export of triose phosphate from the chloroplasts
Triose phosphate export from the chloroplasts leads to greater production of sucrose

27. Sink tissues compete for available translocated photosynthate
Sink strength regulates translocation and is a function of sink size (e.g., fruit number) and activity (e.g. metabolic activity, invertase, sucrose splitting enzyme)
Hormones, sugars, and turgor are among the different signals that coordinate activities of sources and sinks, precise mechanisms are not established
Partitioning efficiency from vegetative sinks into storage organs translates into productivity for many crops
“Signals” travel from different organs through the vascular system, long distance transport, evidence indicates most often in the phloem