SUCROSE SYNTHASE LOCALIZATION DURING INITIATION OF SEED DEVELOPMENT AND TRICHOME DIFFERENTIATION IN COTTON OVULES

Sucrose synthase in cotton (Gossypium hirsutum L.) ovules was immunolo calized to clarify the relationship between this enzyme and (a) sucros e import/utilization during initiation of seed development, (b) tricho me differentiation, and (c) cell-wall biosynthesis in these rapidly el ongating ''fibers.'' Analyses focused on the period immediately before and after trichome initiation (at pollination). Internal tissues most heavily immunolabeled were the developing nucellus, adjacent integume nt (inner surface), and the vascular region. Little sucrose synthase w as associated with the outermost epidermis on the day preceding pollin ation. However, 1 d later, immunolabel appeared specifically in those epidermal cells at the earliest visible phase of trichome differentiat ion. 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, re gardless of whether pollination per se had occurred. Timing of onset f or the cell-specific localization of sucrose synthase in young seeds a nd trichome initials indicates a close association between this enzyme and sucrose import at a cellular level, as well as a potentially inte gral role in cell-wall biosynthesis.