Phloem Translocation in Plants

Phloem Translocation: Pathways and Patterns

• Phloem and xylem are the two long-distance transport pathways in plants.
• Sieve elements (sieve tube elements in angiosperms, sieve cells in gymnosperms) conduct sugars and other photoassimilates.
• Phloem also contains companion cells, parenchyma cells (storage and release of food molecules), fibers, sclereids (protection and strengthening), and laticifers (latex-containing cells).
• Mature sieve elements lack nuclei, tonoplasts, and many other cellular structures, but retain modified mitochondria, plastids, and smooth endoplasmic reticulum. Their walls are non-lignified.

Sieve Elements and Transport

• Sieve elements have sieve areas in their cell walls with pores connecting conducting cells.
• Sieve plates, located on end walls of sieve tube elements, have larger pores enabling transport between cells.
• P-protein, a phloem protein, is found in various forms and helps seal damaged sieve elements.
• Callose, a β-1,3-glucan, seals sieve pores in response to damage or stress.

Source-Sink Relationships

• Sources produce more photosynthates than they need; sinks import carbohydrates for growth or storage.
• Examples of sources: mature leaves, storage organs during export.
• Examples of sinks: roots, tubers, developing fruits, immature leaves.
• Source-to-sink pathways are complex, influenced by proximity, developmental stage, and vascular connections.
• Proximity: upper leaves supply shoot tips, lower leaves supply roots.
• Development: root and shoot apices are major sinks during vegetative growth; fruits become dominant during reproductive development.
• Vascular Connections: Sources preferentially supply sinks with direct connections. Anastomoses provide alternative pathways.

Translocated Materials

• Water is the most abundant substance transported in the phloem.
• Sucrose is the most commonly transported sugar (non-reducing).
• Nitrogen is transported primarily as amino acids and amides (glutamate, aspartate, glutamine, asparagine).

Photosynthate Allocation and Partitioning

• Allocation: regulation of fixed carbon into metabolic pathways.
• Partitioning: differential distribution of photosynthates within the plant.
• Allocation includes:
  • Synthesis of storage compounds (starch, primarily stored in chloroplasts).
  • Metabolic utilization (energy needs, carbon skeletons for synthesis).
  • Synthesis of transport compounds (sugars for export, some temporary storage in the vacuole).
• A sink's ability to store or metabolize sugars affects its competition for photosynthates. This competition determines photosynthate partitioning and influences growth patterns.
• Turgor pressure in sieve elements may coordinate loading and unloading rates. Plant hormones and nutrients also play a signaling role. Increased transport needs must not compromise essential functions.
• Increased photosynthesis in a source leaf generally leads to increased translocation. Control points include allocation of triose phosphates to:
  • Calvin cycle regeneration.
  • Starch synthesis.
  • Sucrose synthesis and distribution (between transport and storage pools).
• Competition between sinks (e.g. reproductive vs. vegetative tissues) for photosynthates significantly impacts resource allocation. Removing a sink increases translocation to other sinks.