Phloem Translocation and Assimilate Partitioning HORT 301 – Plant Physiology October 15 and 17, 2007 Taiz and Zeiger, Chapter 10, Web Topics 10.1-10.10 paul.m.hasegawa.1@purdue.edu 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

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 is outside of the xylem in the shoot Phloem tissue is adjacent to the xylem in the root

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

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

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

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

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

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

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.

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

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

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

“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

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)

Symplastic loading

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) 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

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

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