Sugar Transport in Plants: Phloem

translocation

movement of sugar through plant phloem

• sugars move from sources to sinks

source

any structure in a plant that either produces (like a leaf) or releases (like a storage bulb) sugars for the growing plant

sink

any location where sugar is delivered for use in a growing tissue or storage for later use

• growing tissues: apical and lateral meristems; developing leaves, flowers, seeds, and fruits

• storage locations: roots, tubers, and bulbs

a storage location can either be a source of a sink depending on the plant’s stage of development and the season:

• middle of growing season

  • sources: mature leaves and stems which are actively photosynthesizing and producing excess sugars

  • sinks: areas of active growth meristems, new leaves, and reproductive structures like flowers and seeds; sugar storage locations like roots, tubers, or bulbs

• end of growing season

  • plants drop leaves and no longer have photosynthesizing tissue- growing season may end either due to onset of winter or onset or dry season depending on climate

• start of next growing season

  • dt no existing leaves → only source of sugar for growth is sugar stored in roots, tubers, or bulbs from previous growing season- storage sites now serve as sources

  • actively developing leaves are the sinks

  • once leaves mature → they become sources of sugar during the growing season

the pressure flow model accounts for several observations:

1. the fluid in the phloem is under high positive pressure

2. translocation stops if the phloem tissue is killed

3. translocation can proceed in both directions simultaneously (but not within the same tube)

4. translocation is inhibited by compounds that stop ATP production in the sugar source

pressure flow model

• high conc of sugar at source creates a low Ψ_s

→ low solute potential draws water into phloem from adjacent xylem

→ movement of water into phloem creates a high Ψ_p in the phloem

→ high turgor pressure forces movement of phloem sap from source to sink through process called '“bulk flow”

→ the sugars moved via bulk flow are rapidly removed from the phloem at the sink

→ removal of the sugars at the sink increases the Ψ_s → causes water to leave phloem and return to xylem → decreases Ψ_p at the sink

diffusion

go from area of high conc to low conc

proton pumps

use energy from ATP to create electrochemical gradients → used to move molecules against conc gradients via co-transporters

co-transporters

move two molecules at same time- one down its conc gradient, which releases energy to transport other molecules against conc gradient

A. symporters: transport two molecules in same direction (both into cell; both out of cell)

B. antiporters: transport two molecules in opposite directions (one into cell and other out of cell)

transport pathways in pressure flow model

• photosynthates (sucrose) are produced in parenchyma cells of photosynthesizing leaves

→ sugars are actively transported (ATP) from source cells into the sieve-tube companion cells

• companion cells use ATP-powered proton pump to create EG outside cell than proton-sucrose cotransporter to move proton down its CG with sucrose against its CG and into the companion cells

→ active transport of sugar into the companion cells allows the companion cells to accumulate higher conc of sugar than the photosynthesizing leaves

→ in companion cells, the sugar diffuses down its CG into the phloem sieve-tube elements (connected by pores) through the plasmodesmata

different mechanisms for unloading sucrose into sink cells

1. if sink is an area of active growth: sucrose conc in the sink cells is usually lower than in phloem sieve-tube elements bc the sink sucrose is rapidly metabolized for growth (passive transport)

2. if sink is an area of storage where sugar is converted to starch (root, bulb): sugar conc in sink lower than in the phloem sieve-tube elements bc the sink sucrose is rapidly converted to starch for storage (passive transport)

3. if sink is an area of storage where sugar is stored as sucrose (sugar beet, sugar cane): sink has high conc of sugar than phloem sieve-tube cells → active transport by proton-sucrose cotransporter (relies on ATP-powered proton pump) transports sugar from companion cels into storage vacuoles in the storage cells

once sugar unloaded at sink cells:

the Ψs increases, causing water to diffuse by osmosis from the phloem back into the xylem

• movement of water out of the phloem causes Ψp to decrease, reducing the turgor pressure in the phloem at the sink and maintaining the direction of bulk flow from source to sink

driving force for bulk flow (fluid movement) in phloem vs xylem

• phloem: active transport of sucrose at source

• xylem: transpiration from leaves

site of bulk flow (cells facilitating fluid movement) in phloem vs xylem

phloem: living sieve tube elements

xylem: non-living vessel elements and tracheids (cohesion)

type of pressure potential in sap in phloem vs xylem

phloem: positive (push from source; pressure; Ψ_p increases)

xylem: negative (pull from top, tension; Ψ_p decreases)