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.