Sucrose Synthase Localization during Initiation of Seed Development and Trichome Differentiation in Cotton Ovules

Sucrose synthase in cotton (Gossypium hirsutum L.) ovules was immunolocalized to clarify the relationship between this enzyme and (a) sucrose import/utilization during initiation of seed development, (b) trichome differentiation, and (c) cell-wall biosynthesis in these rapidly elongating "fibers." Analyses focused on the period immediately before and after trichome initiation (at pollination). Internal tissues most heavily immunolabeled were the developing nucellus, adjacent integument (inner surface), and the vascular region. Little sucrose synthase was associated with the outermost epidermis on the day preceding pollination. However, 1 d later, immunolabel appeared specifically in those epidermal cells at the earliest visible phase of trichome differentiation. The day following pollination, these cells had elongated 3- to 5-fold and showed a further enhancement of sucrose synthase immunolabel. Levels of sucrose synthase mRNA also increased during this period, regardless of whether pollination per se had occurred. Timing of onset for the cell-specific localization of sucrose synthase in young seeds and trichome initials indicates a close association between this enzyme and sucrose import at a cellular level, as well as a potentially integral role in cell-wall biosynthesis.