THE MINIATURE1 SEED LOCUS OF MAIZE ENCODES A CELL-WALL INVERTASE REQUIRED FOR NORMAL DEVELOPMENT OF ENDOSPERM AND MATERNAL CELLS IN THE PEDICEL

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 et hyl methanesulfonate-induced mn1 mutants with altered enzyme activity and (2) a near-linear relationship between geneldose and invertase act ivity and the CWI-2 protein. In addition, molecular analyses showed th at the cBNA clone incw2 maps to the Mn1 locus and differentiates the s ix ethyl methanesulfonate-induced mn1 mutants of independent origin in to two classes when RNA gel blot analyses were used. We also report tw o unexpected observations that provide significant new insight into th e physiological role of invertase and its regulation in a developing s eed. First, a large proportion of total enzyme activity (similar to 90 %) was dispensable (i.e., nonlimiting). However, below the threshold l evel of similar to 6% of wild-type activity, the endosperm enzyme cont rolled both the sink strength of the developing endosperm as well as t he developmental stability of maternal cells in the pedicel in a rate- limiting manner. Our data also suggest an unusually tight coordinate c ontrol between the cell wall-bound and the soluble forms of invertase, which are most likely encoded by two separate genes, presumably throu gh metabolic controls mediated by the sugars.