Maternal Regulation of Seed Development
Figure 1. Maternal effect mutants produce abnormal seeds when 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. Using maize, we are developing tools for identifying genes that are required before fertilization for post-fertilization development.
We have isolated several maternal effect mutants in maize, which affect seed development when inherited through the embryo sac but not the pollen grain (Figure 1). There are four phenotypic classes of maternal effect mutants: (1) mel1 - characterized by a reduced, etched endosperm and aborted or irregular embryo; (2) bsl1, dkx1, and hrl1 - characterized by a reduced endosperm with a loose pericarp and abnormal embryo; (3) ssc1 and nol1 - 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 2). 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 suggest that this is indeed the case (Figure 3).
Figure 2. Phenotypes of maternal effect mutants. All kernels show the germinal (embryo) face of the seed on the left and a cross section on the right. (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) bsl1. The pericarp is loose and the embryo is indistinct and may have aborted. (D) hrl1. The endosperm is reduced, and the embryo is indistinct and has possibly aborted. (E) dkx1. The pericarp is loose and the embryo is indistinct and may have aborted. (F) ssc1. The endosperm is close to normal while the embryo is reduced or aborted. (G) no legacy1. Embryo is absent or indistinct, similar to ssc1. (H) stunter1. Kernels are morphologically normal but smaller than wild type.
Figure 3. Proposed model for the maternal regulation of basal endosperm transfer layer (BETL) development in wild type and bsl1. In wild type, maternal factor(s) required for correct BETL patterning (blue shading) are putatively localized to the basal region of the central cell before fertilization and lead to basal localization of BETL promoting factors in the endosperm (blue shading). Mutation in bsl1 affects development of the central cell, as indicated by the displacement of the polar nuclei (white circles) away from the central longitudinal axis. We propose that these maternal factor(s) are also displaced in bsl1 central cells, thus altering BETL patterning.