THE DIFFERENTIAL EXPRESSION OF SUCROSE SYNTHASE IN RELATION TO DIVERSE PATTERNS OF CARBON PARTITIONING IN DEVELOPING COTTON SEED

Developing cotton (Gossypium hirsutum L.) seed exhibits complex patter ns of carbon allocation in which incoming sucrose (Suc) is partitioned to three major sinks: the fibers, seed coat, and cotyledons, which sy nthesize cellulose, starch, and storage proteins or oils, respectively . In this study we investigated the role of Suc synthase (SuSy) in the mobilization of Suc into such sinks. Assessments of SuSy gene express ion at various levels led to the surprising conclusion that, in contra st to that found for other plants, SuSy does not appear to play a role in starch synthesis in the cotton seed. However, our demonstration of functional symplastic connections between the phloem-unloading area a nd the fiber cells, as well as the SuSy expression pattern in fibers, indicates a major role of SuSy in partitioning carbon to fiber cellulo se synthesis. SuSy expression is also high in transfer cells of the se ed coat facing the cotyledons. Such high levels of SuSy could contribu te to the synthesis of the thickened cell walls and to the energy gene ration for Suc efflux to the seed apoplast. The expression of SuSy in cotyledons also suggests a role in protein and lipid synthesis. In sum mary, the developing cotton seed provides an excellent example of the diverse roles played by SuSy in carbon metabolism.