strates reduce the hydrostatic pressure in the phloem and allow continued phloem transport by bulk flow.
In addition, in expanding cells, some of the water for expansion may come from the phloem, allowing fur-
ther dissipation of the phloem turgor pressure.
VI. MOLECULAR APPROACHES TO PHLOEM TRANSPORT:
SUCROSE TRANSPORTERSDevelopments in molecular techniques have allowed characterization of sucrose transporters and con-
struction of transgenic plants to evaluate the impact of sucrose transporters on phloem loading and un-
loading [89–91]. Surprisingly, there is as yet no information on species with a type 1 minor vein config-
uration. The availability of these genes should greatly facilitate localization of the gene products within
source leaves with either type 1 or type 2 companion cells and should allow confirmation of the role of
sucrose-proton contransport in phloem loading in different plant species.
So far, studies of the sucrose carrier genes have shown that antisense mutants of the SUT1 gene in
tobacco and potato accumulate soluble sugars and starch in source leaves [92,93], which clearly demon-
strates the importance of this class of transporter for facilitating transport of sucrose into the phloem cells.
Other studies have demonstrated impaired flowering and tuber yield [94], and it is possible that other de-
velopmental processes will be found to be affected by sucrose transport capabilities of the plant.
There is no question of the strength of molecular techniques for potentially increasing crop biomass
or yield, but it is important to note that manipulation of genes involved in biochemical pathways may be
difficult to achieve. Self-regulation, biofeedback, and alternative pathways for precursors, intermediates,
and metabolites of sucrose production and degradation products may be altered in response to changes in
sucrose export, delivery, and utilization patterns in source tissues such that the “theoretical” trait desired
is buried within emergent properties of engineered crop species.
VII. FUTURE PERSPECTIVES
Our understanding of the physiology of carbon allocation, partitioning, and phloem transport in crop
plants is still evolving. With the advent of and continued progress in molecular biology techniques, we
may be able to answer once-difficult phloem transport questions. The ability to modify plants genetically
in highly specific ways through molecular approaches should continue to revolutionize the study of crop
physiology. In combination with conventional physiology studies, these techniques should allow signifi-
cant progress to be made in our understanding of assimilate transport processes in crop plants, and indeed
there is still much to be learned.
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