Translocation

• Translocation


  Movement in the Phloem
• Translocation could refer to the transport of substances in the xylem and phloem, but it is more commonly connected with the transport of assimilates in the phloem tissue
• Translocation within phloem tissue

  The transport of assimilates from source to sink and requires the input of metabolic energy (ATP)
• Phloem sap


  The liquid being transported within phloem sieve tubes, consisting of sugars (mainly sucrose), water, and other dissolved substances such as amino acids, hormones and minerals
• Sources of assimilates


  • Green leaves and green stem (photosynthesis produces glucose which is transported as sucrose)
  • Storage organs eg. tubers and tap roots (unloading their stored substances at the beginning of a growth period)
  • Food stores in seeds (which are germinating)
• Sinks (where assimilates are required)

  • Meristems (apical or lateral) that are actively dividing
  • Roots that are growing and/or actively absorbing mineral ions
  • Any part of the plant where the assimilates are being stored (eg. developing seeds, fruits or storage organs)
• The loading and unloading of the sucrose from the source to the phloem, and from the phloem to the sink is an active process
• Translocation of assimilates can be slowed down or even stopped at high temperatures or by respiratory inhibitors
• Translocation of assimilates is not fully understood yet by scientists
• Studies used to understand translocation

  • Collecting and studying the sap from plants with 'clotting' sap (eg. castor oil plants)
  • Using aphids to collect the sap-after the aphid inserts its stylet(tubular mouthpart) scientists remove the aphids head and collect the sap that continues to flow
  • Using radioactively labeled metabolites (eg. Carbon-14 labeled sugars) which can be traced during translocation
  • Advances in microscopes enabling the adaptations of companion cells to be seen
  • Observations about the importance of mitochondria to the process of translocation
• Assimilates


  Substances moved through a plant by the process of translocation, from source to sink
• Sucrose


  The form in which carbohydrates are generally transported in plants, as it allows for efficient energy transfer and increased energy storage, and is less reactive than glucose
• Sucrose loading mechanism


  1. Sucrose molecules may move by the symplastic pathway (through the cytoplasm and plasmodesmata) or the apoplastic pathway (through the cell walls)

  2. If taking the apoplastic pathway, modified companion cells pump hydrogen ions out of the cytoplasm via a proton pump and into their cell walls, creating a concentration gradient that allows a cotransporter protein to carry sucrose into the companion cell against the concentration gradient

  3. The sucrose then moves into the sieve tubes via the plasmodesmata from the companion cells
• Companion cells


  • Have infoldings in their cell surface membrane to increase the available surface area for the active transport of solutes and many mitochondria to provide the energy for the proton pump
• Sucrose unloading mechanism


  1. Sucrose is actively transported out of the companion cells and then moves out of the phloem tissue via apoplastic or symplastic pathways

  2. To maintain a concentration gradient in the sink tissue, sucrose is converted into other molecules through metabolic reactions requiring enzymes (eg. invertase which hydrolyses sucrose into glucose and fructose)