Maternal Regulation of Seed Development
 Figure 7. Maternal effect mutants produce abnormal maternally inherited but normal seed when paternally inherited. mel1 is shown for example. Proper seed development depends on gene activity not only from the embryo and endosperm but also from the maternal embryo sac and the maternal sporophyte. (Evidence suggests that most genes from the paternal genome are silenced very early in seed development.) Using maize, we are developing tools for identifying genes that are required for normal embryo sac development and/or function. Nuclear-type endosperm development of maize and many other angiosperms begins as a syncytium with a single layer of cortical nuclei. Pattern formation of the endosperm may therefore occur by the asymmetric localization of determinants within a common cytoplasm. Patterning could also be achieved by specific localization of maternal factors in the central cell before fertilization. Our recent results with the baseless1 mutant (described below) suggest that this is indeed the case. We have isolated 7 maternal effect mutants in maize, which affect seed development when inherited through the embryo sac but not the pollen grain (Figure 7). There are four phenotypic classes of maternal effect mutants: (1) mel1 - characterized by a reduced, etched endosperm and aborted or irregular embryo; (2) bsl1 and hrl1 - characterized by a reduced endosperm with a loose pericarp and abnormal embryo; (3) ssc1 , nol1 , and nol2 - characterized by a reduced but mostly normal endosperm and an absent or aborted embryo; and (4) stt1 - characterized by a miniature but morphologically normal kernel (Figure 8). Several of these mutants affect endosperm patterning as well as seed growth (Figure 9).  Figure 8. Phenotypes of maternal effect mutants. All kernels show the germinal (embryo) face of the seed. (A) Wild type. The embryo is white in this genetic background, oval shaped, and covers most of the germinal surface of the seed. (B) mel1. The endosperm is etched on the crown, and the embryo is smaller than wild type and has an irregular outline. (C) baseless1. The pericarp is loose and the embryo is indistinct and may have aborted. (D) sans scion1. Mature seeds lack any discernible embryo. (E) heirless1. The pericarp is l oose and the embryo is indistinct and has possibly aborted, similar to baseless1. (F) no legacy1. Embryo is absent or indistinct, similar to sans scion1. (G) no legacy2. Embryo defects are similar to sans scion1 and no legacy1. (H) stunter1. Kernels are morphologically normal but smaller than wild type.
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