Sugar Transport in Plants: Phloem

 Learning Objectives

  • Differentiate between sugar sources and sugar sinks in plant tissues
  • Explain the pressure flow model for sugar translocation in phloem tissue
  • Describe the roles of proton pumps, co-transporters, and facilitated diffusion in the pressure flow model
  • Recognize how different sugar concentrations at sources and different types of sinks affect the transport pathway used for loading or unloading sugars
  • Compare and contrast the mechanisms of fluid transport in xylem and phloem

The information below was adapted from OpenStax Biology 30.5

Sugars move from “source” to “sink”

Plants need an energy source to grow. In growing plants, photosynthates (sugars produced by photosynthesis) are produced in leaves by photosynthesis, and are then transported to sites of active growth where sugars are needed to support new tissue growth. During the growing season, the mature leaves and stems produce excess sugars which are transported to storage locations including ground tissue in the roots or bulbs (a type of modified stem). Many plants lose leaves and stop photosynthesizing over the winter. At the start of the growing season, they rely on stored sugars to grown new leaves to begin photosynthesis again.

Locations that produce or release sugars for the growing plant are referred to as sources. Sugars produced in sources, such as leaves, need to be delivered to growing parts of the plant via the phloem in a process called translocation, or movement of sugar. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. Sinks include areas of active growth (apical and lateral meristems, developing leaves, flowers, seeds, and fruits) or areas of sugar storage (roots, tubers, and bulbs). Storage locations can be either a source or a sink, depending on the plant’s stage of development and the season.

The photosynthates from the source are usually translocated to the nearest sink through the phloem sieve tube elements. For example, the highest leaves will send sugars upward to the growing shoot tip, whereas lower leaves will direct sugars downward to the roots. Intermediate leaves will send products in both directions, unlike the flow in the xylem, which is always unidirectional (soil to leaf to atmosphere). Note that the fluid in a single sieve tube element can only flow in a single direction at a time, but fluid in adjacent sieve tube elements can move in different directions. The direction flow also changes as the plant grows and develops:

  • In the middle of the growing season, actively photosynthesizing mature leaves and stems serve as sources, producing excess sugars which are transported to sinks where sugar use is high. Sinks during the growing season include areas of active growth meristems, new leaves, and reproductive structures. Sinks also include sugar storage locations, such as roots, tubers, or bulbs. At the end of the growing season, the plant will drop leaves and no longer have actively photosynthesizing tissues.
  • Early at the start of the next growing season, a plant must resume growth after dormancy (winter or dry season).  Because the plant has no existing leaves, its only source of sugar for growth is the sugar stored in roots, tubers, or bulbs from the last growing season. These storage sites now serve as sources, while actively developing leaves are sinks. Once the leaves mature, they will become sources of sugar during the growing season.

Overview of Translocation: Transport from Source to Sink

Sugars move (translocate) from source to sink, but how? The most commonly accepted hypothesis to explain the movement of sugars in phloem is the pressure flow model for phloem transport. This hypothesis accounts for several observations:

  1. Phloem is under pressure
  2. Translocation stops if the phloem tissue is killed
  3. Translocation proceeds in both directions simultaneously (but not within the same tube)
  4. Translocation is inhibited by compounds that stop production of ATP in the sugar source

In very general terms, the pressure flow model works like this: a high concentration of sugar at the source creates a low solute potential (Ψs), which draws water into the phloem from the adjacent xylem. This creates a high pressure potential (Ψp), or high turgor pressure, in the phloem. The high turgor pressure drives movement of phloem sap by “bulk flow” from source to sink, where the sugars are rapidly removed from the phloem at the sink. Removal of the sugar increases the Ψs, which causes water to leave the phloem and return to the xylem, decreasing Ψp.

This video provides a concise overview of sugar sources, sinks, and the pressure flow hypothesis:

Transport pathways in sugar translocation

Before we get into the details of how the pressure flow model works, let’s first revisit some of the transport pathways we’ve previously discussed:

  1. Diffusion occurs when molecules move from an area of high concentration to an area of low concentration. Diffusion does not require energy because the molecules move down their concentration gradient (from areas of high to low concentration).
  2. Proton pumps use energy from ATP to create electrochemical gradients, with a high concentration of protons on one side of a plasma membrane. This electrochemical gradient can then be used as a source of energy to move other molecules against their concentration gradients via co-transporters.
  3. Co-transporters are channels that perform a type of secondary active (energy-requiring) transport. Co-transporters move two molecules at the same time: one molecule is transported along (“down”) its concentration gradient, which releases energy that is used to transport the other molecule against its concentration gradient.
    1. Symporters are a type of co-transporter that transports two molecules in the same direction; both into the cell, or both out of the cell.
    2. Antiporters are a type of co-transporter that transports two molecules in opposite directions; one into the cell, and the other out of the cell.

Symporters move two molecules in the same direction; Antiporters move two molecules in opposite directions. Image credit: Khan Academy, https://www.khanacademy.org/science/biology/membranes-and-transport/active-transport/a/active-transportImage modified from OpenStax Biology. Original image by Lupask/Wikimedia Commons.

Each of these transport pathways play a role in the pressure flow model for phloem transport.

Details of the Pressure Flow Model for Phloem Transport

Photosynthates, such as sucrose, are produced in the mesophyll cells (a type of parenchyma cell) of photosynthesizing leaves. Sugars are actively transported from source cells into the sieve-tube companion cells, which are associated with the sieve-tube elements in the vascular bundles. This active transport of sugar into the companion cells occurs via a proton-sucrose symporter; the companion cells use an ATP-powered proton pump to create an electrochemical gradient outside of the cell. The cotransport of a proton with sucrose allows movement of sucrose against its concentration gradient into the companion cells.  occurs.

From the companion cells, the sugar diffuses into the phloem sieve-tube elements through the plasmodesmata that link the companion cell to the sieve tube elements. Phloem sieve-tube elements have reduced cytoplasmic contents, and are connected by a sieve plate with pores that allow for pressure-driven bulk flow, or translocation, of phloem sap.

Phloem is comprised of cells called sieve-tube elements. Phloem sap travels through perforations called sieve tube plates. Neighboring companion cells carry out metabolic functions for the sieve-tube elements and provide them with energy. Lateral sieve areas connect the sieve-tube elements to the companion cells. Image credit: OpenStax Biology.

The presence of high concentrations of sugar in the sieve tube elements drastically reduces Ψs, which causes water to move by osmosis from xylem into the phloem cells. This movement of water into the sieve tube cells cause Ψp to increase, increasing both the turgor pressure in the phloem and the total water potential in the phloem at the source. This increase in water potential drives the bulk flow of phloem from source to sink.

Unloading at the sink end of the phloem tube can occur either by diffusion, if the concentration of sucrose is lower at the sink than in the phloem, or by active transport, if the concentration of sucrose is higher at the sink than in the phloem. If the sink is an area of active growth, such as a new leaf or a reproductive structure, then the sucrose concentration in the sink cells is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly metabolized for growth. If the sink is an area of storage where sugar is converted to starch, such as a root or bulb, then the sugar concentration in the sink is usually lower than in the phloem sieve-tube elements because the sink sucrose is rapidly converted to starch for storage. But if the sink is an area of storage where the sugar is stored as sucrose, such as a sugar beet or sugar cane, then the sink may have a higher concentration of sugar than the phloem sieve-tube cells.  In this situation, active transport by a proton-sucrose antiporter is used to transport sugar from the companion cells into storage vacuoles in the storage cells.

Once sugar is unloaded at the sink cells, the Ψs increases, causing water to diffuse by osmosis from the phloem back into the xylem. This 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.

Sucrose is actively transported from source cells into companion cells and then into the sieve-tube elements. This reduces the water potential, which causes water to enter the phloem from the xylem. The resulting positive pressure forces the sucrose-water mixture down toward the roots, where sucrose is unloaded. Transpiration causes water to return to the leaves through the xylem vessels. Image credit: OpenStax Biology

This video (beginning at 5:03) provides a more detailed discussion of the pressure flow hypothesis:

 

Movement of fluid in xylem vs phloem

It should be clear that movement of sugars in phloem relies on the movement of water in phloem. But there are some important differences in the mechanisms of fluid movement in these two different vascular tissues:

  • Driving force for fluid movement:
    • Xylem: transpiration (evaporation) from leaves, combined with cohesion and tension of water in the vessel elements and tracheids (passive; no energy required)
    • Phloem: Active transport of sucrose from source cells into phloem sieve tube elements (energy required)
  • Cells facilitating fluid movement:
    • Xylem: Non-living vessel elements and tracheids
    • Phloem: Living sieve tube elements (supported by companion cells)
  • Pressure potential
    • Xylem: Negative due to pull from the top (transpiration, tension)
    • Phloem: Positive due to push from source (Ψp increases due to influx of water which increases turgor pressure at source)