Phloem Translocation and Assimilate PartitioningHORT 301 – Plant Physiology

October 15 and 17, 2007Taiz and Zeiger, Chapter 10, Web Topics 10.1-10.10

paul.m.hasegawa.1@purdue.eduClass 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

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

Phloem anatomy (cellular structure) – translocation capacity is due to the anatomy

Phloem tissue is adjacent to the xylem in the root

1.1 Schematic representation of the body of a typical dicot (Part 4) 1.1 Schematic representation of the body of a typical dicot (Part 3)

Phloem is outside of the xylem in the shoot

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

10.2 Transverse section of a 3-year-old stem of an ash (Fraxinus excelsior) tree 10.1 Transverse section of a vascular bundle of trefoil, a clover (Trifolium)

Sieve elements - primary conducting cells of phloem

Sieve tube elements - highly differentiated cells in angiosperms, focus of the lecture

Retain plasma membrane, mitochondria, plastids and endoplasmic reticulum

10.3 Schematic drawings of mature sieve elements (sieve tube elements)

Sieve cells - less specialized cells in gymnosperms

Sieve elements are specialized cells - protoplasmic (living) w/o a nucleus, tonoplast, Golgi apparatus and ribosomes

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

10.5 Sieve elements and open sieve plate pores

Photosynthetic products - translocated throughout the plant

Solution is referred to as sap, can be highly viscous

10.6 Electron micrograph of a sieve area (sa) linking two sieve cells of a conifer (Pinus resinosa)

P-proteins (some lectins) and callose (β-1, 3-glucan) seal sieve element pores upon injury or during dormancy, minimize sap loss and translocation

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

Companion cell types:Ordinary companion cell – plasmodesmatal connection to the sieve element but minimal plasmodesmatal connection to surrounding cells

10.7 Electron micrographs of companion cells in minor veins of mature leaves (Part 1)

Transfer cell – ordinary companion cell-like, w/finger-like membrane protrusions that increase plasma membrane surface area, minimal plasmodesmatal connection to surrounding cells

10.7 Electron micrographs of companion cells in minor veins of mature leaves (Part 2)

Intermediary cell – numerous plasmodesmatal connections to surrounding cells

10.7 Electron micrographs of companion cells in minor veins of mature leaves (Part 1)

Ordinary companion and transfer cells facilitate apoplastic transport of sap into sieve elements

Intermediary cells facilitate symplastic transport into sieve elements

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)

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

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.

10.8 Source-to-sink patterns of phloem translocation

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

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 10.10 Pressure-flow model of translocation in the phloem

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)

10.10 Pressure-flow model of translocation in the phloem

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

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

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

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

1.1 Schematic representation of the body of a typical dicot (Part 2)

“Sugars” concentrate at the site of loading

Sugars move initially through a symplastic pathway but concentrate in companion cells via apoplastic or symplastic transport

10.14 Schematic diagram of pathways of phloem loading in source leaves (Part 2)

Apoplastic loading

Apoplastic loading – sucrose is transported into the companion cell by a secondary active sucrose-H+ co-transporter in the plasma membrane

10.16 ATP-dependent sucrose transport in sieve element loading

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)

Symplastic loading

10.14 Schematic diagram of pathways of phloem loading in source leaves (Part 1)

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

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)

ORApoplastic - type 1 – unloading from sieve element-companion cell complex is apoplastic

or type 2 – apoplastic transport occurs in other cells in the transport pathway 10.18 Pathways for phloem unloading and short-distance transport

Regardless the “process” is energy dependent based on inhibitor studies

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

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 10.21 A simplified scheme for starch and sucrose synthesis during the day

Sink tissues compete for available translocated photosynthate10.8 Source-to-sink patterns of phloem translocation

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