Koch K. E., Nolte K. D., Duke E. R., McCarty D. R., Avigne W. T.
Fruit Crops Department, University of Florida, Gainesville, Florida 32611.
Plant Cell. 1992 Jan;4(1):59-69. doi: 10.1105/tpc.4.1.59.
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.
玉米中编码蔗糖合酶的两个基因（Sh1和Sus1）对组织碳水化合物状态的变化表现出明显不同的反应。这种酶被广泛认为是发育中的植物器官进行蔗糖分配、转运和/或代谢的关键。将切除的玉米根尖在不同的糖及一系列浓度下孵育不同时间。Sh1 mRNA在碳水化合物供应有限（约0.2%葡萄糖）的条件下表达量最高。相反，Sus1转录水平在低糖条件下较低或无法检测到，在葡萄糖浓度高10倍（2.0%）时达到峰值。对其他可代谢糖的反应相似，但L-葡萄糖和用甘露醇提高渗透压的影响很小。因此，充足的糖供应增加了Sus1的表达，而糖供应减少则增强了Sh1的表达。在蛋白质水平上，由Sh1和Sus1编码的亚基丰度变化不太明显，但与各自mRNA水平的变化相对应。虽然总酶活性没有显示出净变化，但蔗糖合酶蛋白的细胞定位发生了明显改变。在完整的根中，蔗糖合酶在中柱和根尖中最为普遍。相反，低糖条件有利于在外围细胞中积累，而高糖水平则导致在所有细胞类型中表达升高。这两个蔗糖合酶基因对糖的差异反应提供了一种潜在机制，可根据碳水化合物状态的变化改变酶的分布模式，并根据可利用光合产物的水平调整输入细胞的蔗糖代谢能力。
