A re-evaluation of the relative roles of two invertases, INCW2 and IVR1, in developing maize kernels and other tissues

We have examined the relative abundance and distribution of the transcripts and protein products of a cell wall gene (Incw2) and a soluble invertase gene (Ivr1) to better understand their relative roles during maize (Zea mays L.) kernel development. In developing kernels the steady-state levels of Incw2 transcript increased dramatically from 0 to 12 d after pollination, while Ivr1 transcript, in contrast to a previous report, was undetectable. Consistent with the RNA expression data, the IVR1 protein could not be detected in kernel extracts using antisera raised to a synthetic peptide. Fractionation of the soluble form of invertase from developing kernels by isoelectric focusing and protein blots suggested that the enzyme activity was due to contamination of the cell wall invertase protein. A similar observation was made in a maize cell suspension culture in which Ivu1 RNA, but not IVR1 protein, was significantly modulated by sugars in the medium. Protein-blot analyses of the soluble enzyme activity suggested that changes in the enzyme activity are attributable to a cell wall invertase protein in the soluble fraction. Based on the collective evidence, we propose that the cell wall, but not the soluble invertase, is critical to heterotrophic sinks such as cell suspension cultures and developing kernels.