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Chapter 15

Long-Distance Transport

 

The long-distance transport of solutes in vascular plants follows two functionally distinct pathways, the xylem and the phloem, which closely parallel each other throughout most of the plant body. Volume flow in the xylem is much greater than in the phloem and is driven by the tension resulting from transpirational water loss. Solute concentration in the phloem is high; movement is driven by a turgor differential between regions of phloem loading (“sources”) and unloading (“sinks”). Thus, solutes in the xylem move upward to sites of photosynthesis, whereas movement in the phloem is metabolically directed and variable in direction, depending on the relative positions of sources and sinks.
      The principal translocated sugar in most crop species is sucrose. In some species (mostly herbaceous), sucrose is released into the minor vein apoplast and is accumulated in the phloem by H+ -sucrose cotransport across the plasma membranes of the sieve element/companion cell complex (“apoplastic loading”), resulting in a substantially higher concentration of sucrose in the conducting cells than in the leaf mesophyll. In other species (mostly woody), sugars follow a fully symplasmic pathway to the SE/CC complex (“symplasmic loading,” which has no active accumulation step). In symplasmic loaders translocating the raffinose series of oligosaccharides, however, the RSOs are synthesized in companion cells, where they reach much higher concentrations than in the mesophyll.
      In most sinks, assimilates exit the sieve tube as a bulk flow of solution via plasmodesmata. This step has a high hydraulic resistance and is accompanied by a large pressure drop, making it effectively irreversible. Controls on the location, number, and resistance of these plasmodesmal “leaks” presumably play an important role in assimilate partitioning.
      Transport of solutes to and from the xylem and phloem occurs largely by cell-to-cell movement via plasmodesmata. Until recently, these structures seemed essentially static, acting as molecular sieves to prevent the movement of proteins and RNA while allowing passage of solutes smaller than 800 Da. This view continues to have some validity, but requires certain fundamental elaboration's. Some proteins and RNAs can move into and out of the phloem conducting cells, which are enucleate and lack protein synthetic machinery. Plasmodesmata in the postphloem symplasmic pathway of sink tissues have MELs of at least 10 kDa, presumably an important factor for accommodating high-solute fluxes there. Several endogenous proteins, including some in phloem exudate, can up-regulate the symplasmic MEL in leaf mesophyll to about 20 kDa. Developmental coordination in meristems has been shown to involve proteins that mediate not only their own transport between cell layers but also that of their own mRNA. Observations on plasmodesmal gating implicate ATP in the regulation of the MEL, and both actin and myosin have been localized to plasmodesmata. Several approaches are yielding insight into the identity of other plasmodesmal proteins. Although their role as molecular sieves remains intact, plasmodesmata must also be viewed as dynamic structures capable of altering their MEL and of interacting with and transporting specific proteins and RNAs.


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