1. Phloem Translocation and Assimilate Partitioning HORT 301 – Plant Physiology October 27, 2008 Taiz and Zeiger, Chapter 10 Web Topics 10.1, 10.5-10.7, 10.9 &10.10 paul.m.hasegawa.1@purdue.edu Phloem translocation – facilitates movement of photosynthetic products to cells in storage organs/tissues or in growing tissues/organs Phloem anatomy – cellular structure facilitates translocation 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 Source - net carbon fixation, fixation is greater than respiratory and/or storage requirements Sink – net import of photosynthetic product, high respiration (growth) and/or storage metabolism Phloem transport is bidirectional – primarily organic molecules (C and N sources) in solution (water), but also mineral nutrients, hormones and other signaling molecules, etc.

3. Phloem anatomy – translocation is facilitated by cellular structure Phloem tissue is adjacent to the xylem, outside of the xylem in leaves and stems

4. Secondary xylem (wood) and phloem (bark) in a perennial stem, primary and you secondary phloem conduct translocation, adjacent to the cambium (specialized meristem)

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

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

7. Translocated solution is highly viscous, referred to as sap P-proteins (some lectins) and callose (β-1, 3-glucan) seal sieve element pores upon injury, during dormancy, and when cells become non- conducting to minimize sap loss sealed sieve element pore Callose seals pore more permanently than lectins

8. Companion cells – adjacent to sieve elements Facilitate loading (and unloading) of sap into sieve elements in minor veins of leaves (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, passive transport Companion cells also carry out metabolic and regulatory functions for 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 – phloem translocation moves photosynthate (products of photosynthesis) from a source to a sink Translocation may be upwards or downwards Source – an exporting organ because carbon availability is greater than utilization, e.g. photosynthetically active leaves or 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. Factors that affect source to sink translocation: Proximity – as a generalization, 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 at the same developmental age

13. Translocation pathways have plasticity – removal of sinks results in greater accumulation into other sinks, e.g. removal of leaf #6 and other leaves results in greater partitioning into leaf #2 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. Pressure flow model for phloem translocation - source to sink translocation in phloem sieve elements is driven by a pressure gradient, pressure-driven bulk (mass) flow Positive pressure gradients between sieve elements at the source (higher) and sink (lower) drives solute movement from source to sink

15. Source sieve elements - phloem loading of solutes (solute transport into sieve elements) causes a more negative solute (osmotic) potential (ψ s ) and a concomitant more negative water potential (ψ w ) Water moves into the sieve elements from the xylem along the ψ w gradient and turgor pressure (ψ p ) increases Source Sink

16. Sink sieve elements - phloem unloading into sink cells (solute transport out of sieve elements) causes a less negative ψ s and a concomitant less negative ψ w Ψ w in sieve elements becomes less negative than ψ w in the adjacent xylem cells, ψ p decreases and water moves from sieve elements into the xylem vessels Then, source sieve elements have a higher ψ p than sink sieve elements and there is pressure-driven bulk flow of solutes (with water) from source to sink Resistances (sieve plates and sieve areas) in the translocation pathway maintain the pressure gradient, i.e. pores facilitate sufficient resistance to allow the maintenance of pressure gradients Source Sink

17. Bulk (mass) flow (not osmosis) because membranes are not crossed, symplastic interconnections from the source to the sink sieve elements 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 from source cells and unloading into sink cells – facilitated by companion cells Phloem loading from photosynthetically active source cells – chloroplasts in photosynthetically active mesophyll cells to sieve elements in leaves Triosphosphate molecules are transported from chloroplasts to the cytosol in mesophyll cells and are converted to sucrose Sucrose moves symplastically (plasmodesmata) from mesophyll cells into cells near the companion cell-sieve element complex

19. Sugars move initially through a symplastic pathway but concentrate in companion cells via (A) symplastic or (B) apoplastic transport (Passive transport) (Active transport) Symplastic – passive transport Apoplastic – active transport at the phloem parencyhyma cell-companion cell interface

20. 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 on companion cell on the side adjacent to the parenchyma cells Transport from the companion cell to sieve elements is through plasmodesmata (passive)

21. Symplastic loading – via intermediary cells adjacent to the sieve element, sugars, sugar alcohols or/and oligosaccharides Apoplastic loading – primarily sucrose

22. 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 – metabolically active regions (e.g. shoot or root tip, young leaves) or storage organs (e.g. seeds, tubers, fruits, etc.) Apoplastic – apoplastic unloading from sieve element-companion cell complex or apoplastic movement along the transport pathway, active transport from the apolast back into cells Phloem unloading and transport into sink cells may be: Symplastic (through plasmodesmata only), passive diffussion

23. 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 (pools) in source or sink cells e.g. to sucrose or starch Partitioning – differential distribution of photosynthetic products into sinks

24. Allocation in source leaves is regulated – triose phosphates are allocated to renew photosynthetic intermediates, or to starch or sucrose production Pi/triose phosphate composition regulates starch or sucrose biosynthetic enzymes in mesophyll cells 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

25. 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, storage product biosynthesis) Hormones, sugars, and turgor pressure are among the different signals that coordinate activities of sources and sinks, precise mechanisms are not established “Signals” travel from different organs through the vascular system, long distance transport, evidence indicates most often in the phloem Partitioning efficiency from vegetative sinks into storage organs translates into productivity for many crops