Cheng W. H., Taliercio E. W., Chourey P. S.
Program in Plant Molecular and Cellular Biology, University of Florida, Gainesville, Florida 32611-0680.
Plant Cell. 1996 Jun;8(6):971-983. doi: 10.1105/tpc.8.6.971.
Collective evidence demonstrates that the Miniature1 (Mn1) seed locus in maize encodes an endosperm-specific isozyme of cell wall Invertase, CWI-2. The evidence includes (1) isolation and characterization of ethyl methanesulfonate-induced mn1 mutants with altered enzyme activity and (2) a near-linear relationship between gene/dose and invertase activity and the CWI-2 protein. In addition, molecular analyses showed that the cDNA clone incw2 maps to the Mn1 locus and differentiates the six ethyl methanesulfonate-induced mn1 mutants of independent origin into two classes when RNA gel blot analyses were used. We also report two unexpected observations that provide significant new insight into the physiological role of invertase and its regulation in a developing seed. First, a large proportion of total enzyme activity (~90%) was dispensable (i.e., nonlimiting). However, below the threshold level of ~6% of wild-type activity, the endosperm enzyme controlled both the sink strength of the developing endosperm as well as the developmental stability of maternal cells in the pedicel in a rate-limiting manner. Our data also suggest an unusually tight coordinate control between the cell wall-bound and the soluble forms of invertase, which are most likely encoded by two separate genes, presumably through metabolic controls mediated by the sugars.
多项证据表明，玉米中的微型1（Mn1）种子位点编码一种细胞壁转化酶CWI-2的胚乳特异性同工酶。证据包括：（1）对甲磺酸乙酯诱导的酶活性改变的mn1突变体进行分离和表征；（2）基因剂量与转化酶活性以及CWI-2蛋白之间存在近似线性关系。此外，分子分析表明，cDNA克隆incw2定位于Mn1位点，当使用RNA凝胶印迹分析时，可将甲磺酸乙酯诱导的六个独立起源的mn1突变体分为两类。我们还报告了两个意外发现，这为深入了解转化酶在发育种子中的生理作用及其调控提供了重要的新见解。首先，大部分总酶活性（约90%）是可有可无的（即非限制性的）。然而，低于野生型活性约6%的阈值水平时，胚乳酶以限速方式控制着发育中胚乳的库强以及花梗中母细胞的发育稳定性。我们的数据还表明，细胞壁结合型和可溶性形式的转化酶之间存在异常紧密的协同控制，它们很可能由两个不同的基因编码，大概是通过糖介导的代谢控制实现的。
