made to characterize proteins that may be involved in phloem loading of noncarbohydrate materials
[258].
C. Symplastic Pathway
The apoplastic pathway of phloem loading was favored by most phloem physiologists in the early 1980s.
A few, however, held a different view. Lucas and coworkers repeated much of Giaquinta’s work and ex-
tended it to include additional tests and additional species. Their studies [259–263] led to the conclusion
that absorption of exogenously supplied sugars was accomplished by a sugar retrieval system that is nor-
mally used to absorb sugars that leak from cells. Some of this sugar would be loaded into phloem. These
investigators further concluded that the symplast is the prime pathway of phloem loading for many plants.
They also suggested that partitioning of sucrose between cytosol and vacuole of source cells may control
availability of assimilate to the phloem loading system [263].
Contrary to Giaquinta’s [244] findings that PCMBS inhibited vein loading in sugar beet leaves
whether they were supplied with^14 CO 2 or [^14 C]sucrose, Madore and Lucas [264] found that PCMBS in-
hibited vein loading of Ipomoea tricolorleaf disks at pH 5.0 only when [^14 C]sucrose was supplied but not
when the system was labeled via photosynthesis by^14 CO 2. From all of these data, Madore and Lucas con-
cluded that apoplastic phloem loading occurs in some species whereas symplastic phloem loading occurs
in others. Turgeon and coworkers [265–267] have since presented data that support this conclusion.
Van Bel, Gamalel, and coworkers pointed out that the frequency of plasmodesmata connecting ST-
CC complex with other leaf cells varies by close to two orders of magnitude [268–270]. Therefore, they
categorized vein types into those that have frequent plasmodesmata connections between ST-CC complex
and other cells as type 1 veins and those with few such connections as type 2 veins. They also indicated
that there are intermediate types, so they added subcategories (types 1/2, 2a, and 2b). Type 1 vein com-
panion cells have an intermediary cell structure, companion cells of 2b vein have a transfer cell structure,
and 2a companion cells retain the type 2 characteristic of containing few plasmodesmata between com-
panion cells and phloem parenchyma, yet have smooth walls. Various vein types are represented in
Figure 1.
It was then demonstrated that plants in which PCMBS inhibited phloem loading of carbon from pho-
tosynthesis contain type 2 minor veins and plants in which PCMBS did not inhibit phloem loading have
type 1 minor veins [268,271]. These workers concluded that plants with type 1 vein anatomy use the sym-
plastic pathway as the prime pathway of phloem loading, those with type 2 anatomy use the apoplastic
436 HENDRIXFigure 1 Typical minor vein structures: (a) open type (type 1) with intermediary cells (IC) as companion cell
(Hydrangea petiolaris), (b) closed type (type 2b) with transfer cells (TC) as companion cells (Pisum satiuum),
and (c) composite type (1/2b) with intermediary and transfer cell in a single minor vein (Acanthus mollis). XV,
xylem vessel; PP, phloem parenchyma; the central unmarked elements are sieve elements. (From Ref. 269.)