explain how companion cells transfer assimilates to phloem sieve tubes, with reference to proton pumps and cotransporter proteins

Transport in Plants – Topic 7 (Cambridge AS/A‑Level)

Learning Objectives

• Describe the structure of xylem and phloem, including all major cell types and their distribution in stems, roots and leaves.
• Explain the apoplastic and symplastic pathways for water, minerals and assimilates, and the role of the Casparian strip.
• Describe transpiration, the cohesion‑tension mechanism and the factors that regulate stomatal aperture.
• Define water potential (Ψ) and calculate its components, noting the sign of pressure potential in xylem.
• State the mass‑flow (pressure‑flow) hypothesis for phloem transport and cite experimental evidence.
• Explain how companion cells load assimilates into sieve‑tube elements, with reference to H⁺‑ATPases and H⁺‑coupled cotransporters.
• Outline the mechanisms of phloem unloading at sink tissues.

1. Structure of Vascular Tissue

2. Pathways for Water, Minerals and Assimilates

• Apoplastic pathway – movement through cell walls and intercellular spaces; no crossing of plasma membranes.
• Symplastic pathway – movement from cytoplasm to cytoplasm via plasmodesmata; each cell crossed requires at least one membrane crossing.

In roots, the Casparian strip (a band of suberin and lignin in the radial wall of endodermal cells) blocks the apoplastic route, forcing water and dissolved ions to cross the plasma membrane and enter the symplast before reaching the stele. This selective barrier is essential for mineral uptake and for maintaining ion homeostasis.

Root pressure (generated by active uptake of ions into the xylem and subsequent osmotic influx of water) can contribute to upward water movement, especially at night or in low‑transpiration conditions, and explains the exudation observed when a cut stem is placed in water.

3. Transpiration and the Cohesion‑Tension Mechanism

1. Water evaporates from mesophyll cells into intercellular air spaces and exits through stomata (transpiration).
2. Evaporation creates a negative water potential in the leaf (Ψ ≈ –0.5 MPa), producing a tension that is transmitted down the continuous water column of the xylem.
3. Cohesion (hydrogen bonding between water molecules) and adhesion (attraction of water to the hydrophilic walls of xylem vessels) maintain the column without breaking.
4. Stomatal aperture is regulated by guard‑cell turgor, which changes in response to:

   • Light‑induced K⁺ uptake (opening).
   • Abscisic acid (ABA)‑induced K⁺ loss and Ca²⁺ influx (closing).
   • Leaf water status (hydraulic signals).

4. Water Potential (Ψ)

Definition: Water moves from regions of higher water potential to regions of lower water potential.

Equation: Ψ = Ψ_s + Ψ_p

• Ψ_s = solute (osmotic) potential (always negative; becomes more negative as solute concentration increases).
• Ψ_p = pressure potential (positive in turgid cells, negative in xylem under tension).

Example calculation (simplified):

• Soil water: Ψ_s = –0.2 MPa, Ψ_p ≈ 0 → Ψ_soil ≈ –0.2 MPa.
• Leaf mesophyll: Ψ_s = –0.5 MPa, Ψ_p = +0.3 MPa → Ψ_leaf ≈ –0.2 MPa.
• Water moves from soil to leaf because Ψ_soil > Ψ_leaf.

5. Mass‑Flow (Pressure‑Flow) Hypothesis for Phloem Transport

1. Loading (source) – sucrose is actively loaded into sieve‑tube elements, raising the solute concentration to ~0.5 M.
2. Osmotic influx of water from the adjacent xylem raises the turgor pressure in the SE‑CC complex to ~+0.5 MPa.
3. Unloading (sink) – sucrose is removed (by metabolism or export), lowering solute concentration and turgor pressure to ~+0.1 MPa.
4. The resulting pressure gradient drives bulk flow of phloem sap from source to sink (mass‑flow).

Experimental support includes:

• Aphid stylet measurements showing a pressure difference of ~0.4 MPa between source and sink.
• Radio‑labelled carbon (^14C) movement in cut stems demonstrating unidirectional bulk flow.
• Observation that severing a phloem strand stops flow downstream while water continues to move in the xylem.

6. Companion‑Cell‑Mediated Phloem Loading (Apoplastic Route)

6.1. Why Companion Cells?

• Contain nuclei, mitochondria and abundant cytoplasm → can generate ATP.
• Form a dense symplasmic connection with a single sieve‑tube element via numerous plasmodesmata.
• Host the transport proteins (H⁺‑ATPase, sucrose‑H⁺ symporters, amino‑acid antiporters) required for loading.

6.2. The Proton Gradient – Driving Force

• Plasma‑membrane H⁺‑ATPase hydrolyses ATP and pumps H⁺ from the CC cytosol to the apoplast.
• This creates a low‑pH, positively charged apoplast (pH ≈ 5.5) and a more negative interior (inside‑negative membrane potential).
• The resulting proton‑motive force (PMF) powers secondary active transporters that move sucrose and amino acids against their concentration gradients.

6.3. Cotransporter Proteins

• Sucrose‑H⁺ symporters (SUTs / SUCs) – typically transport 1 sucrose + 1 H⁺ (occasionally 2 H⁺) into the CC cytosol.
• Amino‑acid‑H⁺ antiporters (AAPs, LHTs) – exchange external H⁺ for internal amino acids, allowing simultaneous loading of nitrogenous compounds.
• Both families are regulated by phosphorylation, sugar status and hormonal signals (e.g., auxin, cytokinin).

6.4. Step‑by‑Step Mechanism (Apoplastic Loading)

1. Photosynthesis in mesophyll cells produces sucrose, which diffuses into the apoplast surrounding the CC‑SE complex.
2. The H⁺‑ATPase pumps protons out, lowering apoplastic pH and establishing the PMF.
3. SUT proteins bind sucrose + H⁺ from the apoplast.
4. Conformational change releases sucrose and H⁺ into the CC cytosol, increasing cytosolic sucrose concentration.
5. Sucrose diffuses through the numerous plasmodesmata into the adjacent sieve‑tube element (SE).
6. Osmotic influx of water from the xylem raises turgor pressure in the SE‑CC complex, creating the pressure gradient for bulk flow.
7. Protons that entered the CC are continuously exported by the H⁺‑ATPase, maintaining the gradient.

6.5. Energetics

Overall reaction for a typical 1 H⁺ sucrose symporter:

\$\text{Sucrose}{\text{apoplast}} + \text{H}^{+}{\text{apoplast}} + \text{ATP} \;\longrightarrow\; \text{Sucrose}{\text{cytosol}} + \text{H}^{+}{\text{cytosol}} + \text{ADP} + P_i\$

ATP is used indirectly to maintain the proton gradient; the actual transport of sucrose is secondary active (driven by the PMF).

7. Phloem Unloading at Sink Tissues

• Apoplastic unloading – sucrose exits the SE via SWEET (facilitated diffusion) or sucrose‑H⁺ antiporters, enters the apoplast, then is taken up by sink cells (often using H⁺‑symporters).
• Symplastic unloading – abundant plasmodesmata allow direct diffusion of sucrose from the SE into sink parenchyma; common in rapidly growing tissues such as root tips and developing fruits.
• Regulation involves:

  • Hormones (auxin, cytokinin, gibberellin) that modify plasmodesmal conductivity or transporter expression.
  • Developmental cues (e.g., transition from sink to source during leaf maturation).

• Once inside sink cells, sucrose may be:

  • Converted to starch (storage organ).
  • Metabolised for respiration.
  • Used as a carbon source for biosynthesis (cellulose, lipids, proteins).

8. Comparison of Loading Strategies

9. Key Points to Remember

• Companion cells provide the ATP needed for the plasma‑membrane H⁺‑ATPase, establishing the proton gradient that drives secondary active loading.
• The proton gradient is the central energy source for the uptake of sucrose, amino acids and other assimilates.
• SUT proteins are the classic H⁺‑coupled sucrose symporters; their activity determines the rate of phloem loading.
• Loading raises the osmotic pressure in the SE‑CC complex, drawing water in from the xylem and generating the pressure gradient that drives long‑distance phloem transport.
• Unloading can be apoplastic or symplastic, depending on the species, the nature of the sink and hormonal regulation.