Sugar Levels Modulate Differential Expression of Maize Sucrose Synthase Genes

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.