The two genes encoding sucrose synthase in maize (Sh1 and Sus1) show markedly different responses to changes in tissue carbohydrate status. This enzyme is widely regarded as pivotal to sucrose partitioning, import, and/or metabolism by developing plant organs. Excised maize root tips were incubated for varying periods in different sugars and a range of concentrations. The Sh1 mRNA was maximally expressed under conditions of limited carbohydrate supply (~0.2% glucose). In contrast, Sus1 transcript levels were low or nondetectable under sugar-depleted conditions and peaked at 10-fold greater glucose concentrations (2.0%). Responses to other metabolizable sugars were similar, but L-glucose and elevation of osmolarity with mannitol had little effect. Plentiful sugar supplies thus increased expression of Sus1, whereas reduced sugar availability enhanced Sh1. At the protein level, shifts in abundance of subunits encoded by Sh1 and Sus1 were much less pronounced but corresponded to changes in respective mRNA levels. Although total enzyme activity did not show net change, cellular localization of sucrose synthase protein was markedly altered. In intact roots, sucrose synthase was most prevalent in the stele and apex. In contrast, sugar depletion favored accumulation in peripheral cells, whereas high sugar levels resulted in elevated expression in all cell types. The differential response of the two sucrose synthase genes to sugars provides a potential mechanism for altering the pattern of enzyme distribution in response to changing carbohydrate status and also for adjusting the sucrose-metabolizing capacity of importing cells relative to levels of available photosynthetic products.

The two genes encoding sucrose synthase in maize (Sh1 and Sus1) show markedly different responses to changes in tissue carbohydrate status. This enzyme is widely regarded as pivotal to sucrose partitioning, import, and/or metabolism by developing plant organs. Excised maize root tips were incubated for varying periods in different sugars and a range of concentrations. The Sh1 mRNA was maximally expressed under conditions of limited carbohydrate supply (~0.2% glucose). In contrast, Sus1 transcript levels were low or nondetectable under sugar-depleted conditions and peaked at 10-fold greater glucose concentrations (2.0%). Responses to other metabolizable sugars were similar, but L-glucose and elevation of osmolarity with mannitol had little effect. Plentiful sugar supplies thus increased expression of Sus1, whereas reduced sugar availability enhanced Sh1. At the protein level, shifts in abundance of subunits encoded by Sh1 and Sus1 were much less pronounced but corresponded to changes in respective mRNA levels. Although total enzyme activity did not show net change, cellular localization of sucrose synthase protein was markedly altered. In intact roots, sucrose synthase was most prevalent in the stele and apex. In contrast, sugar depletion favored accumulation in peripheral cells, whereas high sugar levels resulted in elevated expression in all cell types. The differential response of the two sucrose synthase genes to sugars provides a potential mechanism for altering the pattern of enzyme distribution in response to changing carbohydrate status and also for adjusting the sucrose-metabolizing capacity of importing cells relative to levels of available photosynthetic products.

No abstract

The role of sucrose synthase in translocation and sucrose partitioning remains unresolved despite extensive study of its association with elevated carbon import. Although growing interest has centered on its involvement in sucrose metabolism by importing cells (2, 17), additional evidence also supports a possible vascular function (1,3,6). Recently, Yang and Russell (19) suggested that the promoter for one of the sucrose synthase genes in maize (shrunken 1 gene) was "phloem specific" in transgenic tobacco plants. However, other studies have indicated expression of sucrose synthase genes may be more complex and appears to be sensitive to changes in metabolism and/or environment (14, 15). Nonetheless, immunohistological evidence has indicated a greater abundance of sucrose synthase protein in vascular areas of young roots (1). The levels of total protein also tend to be greater in vascular tissues (13), however, leaving the extent of sucrose synthase activity in vascular tissues unresolved.The possibility that sucrose synthase activity may be greater in vascular bundles has been difficult to address because few opportunities exist for isolation of vascular strands (4). Most plant tissues have fine networks of vascular bundles that cannot be effectively separated from adjacent parenchymal cells. Hawker and Hatch (6) MATERIALS AND METHODS'Marsh' grapefruit (Citrus paradisi Macf.) were sampled from the outer, southern canopy of 55-year-old trees in a commercial orchard in Lake Wales, FL, during late August to early September (before completion of the expansive phase of growth) during four consecutive growing seasons. Samples for immediate dissection were also obtained during the fourth growing season from 10-year-old containerized trees grown in Gainesville, FL. Results from these fruit were similar to those from the orchard-grown trees and were included in overall means.Fruit were washed, sectioned longitudinally, peeled, and separated into individual segments. These were dissected into tissues pictured in Figure 1