In discussing phloem loading, several characteristics of the system must be considered: (1) loading
is selective, (2) concentrations of oligosaccharides and certain amino acids are higher in the ST-CC than
in other cells, (3) as a result, the ST-CC has an osmotic potential more negative than that of adjacent cells,
and (4) these conditions (2 and 3) result in the ST-CC having a higher pressure than other source cells.
Pathways available for phloem loading are apoplastic (cell wall) and symplastic (plasmodesmata).
The conditions listed above indicate that metabolic energy must be involved in phloem loading. This en-
ergy requirement is most easily explained by invoking the apoplastic pathway. However, the mechanism
of transport through plasmodesmata is not sufficiently understood to specify a mechanism of energy in-
put for the symplastic pathway. In 1987, Delrot [236] and Van Bel [237] separately published papers in
which they discussed the merits of each proposed pathway.
B. Apoplastic Pathway
Serious early steps in developing understanding of phloem loading were taken by Gunning and Pate [238]
and Gunning [239] in their study of transfer cells associated with the phloem of minor veins of several
families. Transfer cells have a large number of cell wall intrusions toward the interior of cells resulting in
a large surface between the symplast and apoplast. Other plant structures that transfer materials between
apoplast and symplast contain transfer cells [240]; therefore, it seemed reasonable to assume that phloem
transfer cells are involved in phloem loading.
Sucrose supplied to the apoplast of sugar beet leaves was readily translocated through the phloem
[241]. Glaquinta [242] using sugar beet and Robinson and Hendrix [243] using wheat demonstrated that
asymmetrically labeled sucrose supplied to mature leaves did not have its label randomized as would be
expected if supplied sucrose was hydrolyzed and then resynthesized before phloem loading. Taken to-
gether, these data clearly indicate that sucrose can be absorbed from the apoplast directly into the translo-
cation system.
Giaquinta [244] demonstrated that [^14 C]sucrose supplied to sugar beet leaf disks was accumulated in
minor veins, and he also demonstrated that the nonpenetrating sulfhydryl reagent p-chloromercuriben-
zenesulfonic acid (PCMBS) inhibited absorption of sucrose without altering absorption of glucose, fruc-
tose, or 3-O-methylglucose. In addition PCMBS did not alter the rate of photosynthesis [244,245]. At
about the same time, Gunning [239] cited several papers which indicated that companion cells that dif-
ferentiated as transfer cells were involved in phloem loading.
Giaquinta also demonstrated that the optimum apoplastic pH for phloem loading was between 5.0
and 6.0 [245,246]. Furthermore, he demonstrated [245] that changing the apoplastic pH from 5.0 to 8.0
more than doubled Kmfor sucrose absorption but did not change Vmax. Bush [247], using plasma mem-
brane vesicles, demonstrated a 1:1 relationship between Hand sucrose transport. More recently,
Lemoine et al. [248] demonstrated that plasma membrane vesicles from mature sugar beet leaves accu-
mulated four times as much sucrose as vesicles from immature leave when a proton motive force was ap-
plied. Even though these vesicles were from entire leaves, there was a clear indication of differences be-
tween sink and source leaf plasma membranes. On that basis, a model for phloem loading was proposed
in which a sieve tube plasmalemma adenosinetriphosphatase (ATPase) pumps protons out of the ST-CC
into the apoplast [249]. Sucrose secreted into the apoplast by mesophyll cells would then be loaded into
the ST-CC complex by a sucrose-proton cotransporter against the sucrose concentration gradient using
the free energy gradient of protons that had been established by the ATPase. The charge gradient estab-
lished by that proton-ATPase would also account for the high concentration of potassium in sieve tubes
(discussed earlier).
Expression of an H-sucrose symport has been demonstrated in Arabidopsisand tobacco plants
[250,251]; also, suppression of this protein inhibited sucrose transport [251]. Shakya and Sturm [252]
demonstrated the existence of two sucrose-Hsymports, one expressed in source leaves and the other ex-
pressed in storage cells. Furthermore, Botha and Cross [253] used dye infusion and plasmodesmata fre-
quency along with electrical potential differences to demonstrate the physiological isolation of the ST-CC
from other leaf cells.
All of these data support the mechanistic model proposed in the early 1980s for apoplastic phloem
loading as reviewed by Giaquinta [2,3]. Furthermore, work on the isolation of sucrose binding and H-
ATPase proteins from membranes of source leaves together with characterization of the mode of con-
trol of these proteins [254–257] will aid us in understanding phloem loading. Efforts are also being
PRODUCTION-RELATED ASSIMILATE TRANSPORT 435