EFB530 Plant Physiology

Phloem translocation and photosynthate partitioning

Water and mineral transport occurs mainly through the xylem, from root to leaves

Transport of photosynthate occurs mainly in the phloem

• girdling a tree has no immediate effect on water transport, but sugar accumulates above the girdle and tissue swells, tissue below girdle dies
• application of ¹⁴CO₂ or ¹⁴C-sucrose, then visualization of path of radioactive tracer indicates that photosynthate moves through phloem sieve elements

sieve elements

• joined together to form sieve tube, sieve plates in between (not in gymnosperms)
• lots of plasmodesmata between sieve element and companion cells
• broken/ruptured sieve tubes are plugged by protein and callose ( ß-1,3 glucose polymer)
• sieve elements lack many organelles (they lack nuclei, vacuoles, Golgi, ribosomes, actin microfilaments, microtubules), DO have mitos, ER, modified plastids, plasma membrane
• associated with companion cells (albuminous cells in gymnosperms)

companion cells/transfer cells

• perform some of the basic cell functions for the sieve-tube members, like protein synthesis, lots of mitos for ATP synthesis, extensive
• some plants have transfer cells with invaginations on sides opposite sieve-tube member, increases surface area for better transport 
• neither companion cells or transfer cells have many or any plasmodesmatal connections with cells opposite the seive elements (they only have plasmodesmatal connections with seive elements)

intermediary cells

• some plants have intermediary cells that have extensive plasmodesmatal connections to bundle sheath/parenchyma cells (as well as to seive elements)

Phloem sap in most plants is very high in sucrose-the main translocated photosynthate

• other sugars may be abundant-raffinose (sucrose+galactose), stachyose (sucrose + 2 galactoses)
• some plants have lots of amino acids (glu, asp) and or amides (gln, asn) in phloem sap
• minerals also cycle through phloem sap

Aphids provide direct access to sieve tubes & phloem sap

• stylets penetrate directly into sieve element, honeydew is sap that comes out
• can use aphid stylets as a tool to study phloem transport and sap

Mechanism of phloem translocation: pressure-flow hypothesis of Ernst Munch (1930)

• movement occurs by bulk flow, driven by a pressure gradient
• concentrated solution adjacent to a more dilute solution-osmotic pressure leads to water influx-turgor pressure builds
• the concentrated solution is in a continuous gradient to a dilute solution with lower turgor pressure
• pressure gradient drives flow (as in a garden hose) from high to low

Accumulation of sucrose in the sieve elements (phloem loading) can occur by two paths

• symplastic= direct movement of sucrose from mesophyll cells to intermediary cells/sieve element through plasmodesmata (abundant plasmodesmatal connections between mesophyll/bundle sheath and intermediary cells)
• apoplastic= release of sucrose out of the mesophyll cells, then active transport to accumulate sucrose in the companion cells (or transfer cells)/sieve element of the phloem (few or no plasmodesmatal connections between mesophyll/bundle sheath and companion or transfer cells)

Apoplastic phloem loading uses the PM H⁺-ATPase to generate a proton gradient

• sucrose is transported against a concentration gradient through a H⁺/sucrose symport carrier protein in the companion cells/sieve element cells

Phloem unloading can also be symplastic or apoplastic

• symplastic in growing tissues like young roots, young leaves
  
• apoplastic unloading into storage tissues, like sugar beet roots
• also apoplastic in seeds (no symplastic connections to embryo)
• often involves conversion - like breaking down sucrose to glucose+fructose (enzyme is invertase)

Apoplastic unloading again requires active transport to accumulate sucrose in the storage cells

Photosynthate partitioning (sinks and sources)

By tracking the movement of ¹⁴C-labeled sugar through the plant, we can characterize the pattern of photosynthate movement

• mature leaves are the principal sources of photosynthate (sucrose), but another source can be a storage organ that is in export mode
• sucrose flows down a pressure gradient to a sink
• sinks can be: young (not fully photosynthetically active) leaves; roots; reproductive tissues-fruits & seeds; storage organs in storage mode

Transport occurs through the most direct phloem connections

• sinks closest to a source receive photosynthate first
• vascular connections usually run vertically, sinks above or below source receive photosynthate first (same orthostichy)
• direction of flow can change through development (sink to source transition) or upon pruning/wounding

Understanding of "sink strength" and regulation of photosynthate partitioning may lead to development of plants that provide greater harvest yield

• by understanding how source cells regulate sucrose vs. starch synthesis
• sink strength=tissue size x activity (determines sink competition)
• sink demand can have a feedback effect on source allocation & activity (reduced sink=lower source allocation) by hormones, turgor pressure

Back to EFB530 Syllabus