Regional Localization of Suspensor mRNAs during Early Embryo Development

Abstract

We investigated gene activity within the giant embryos of the scarlet runner bean (Phaseolus coccineus) to gain understanding of the processes by which the apical and basal cells become specified to follow different developmental pathways after division of the zygote. We identified two mRNAs, designated G564 and C541, that accumulate specifically within the suspensor of globular-stage embryos. G564 mRNA accumulates uniformly throughout the suspensor, whereas C541 mRNA accumulates to a higher level within the large basal cells of the suspensor that anchor the embryo to the surrounding seed tissue. Both G564 and C541 mRNAs begin to accumulate shortly after fertilization and are present within the two basal cells of embryos at the four-cell stage. In contrast, at the same stage, these mRNAs are not detectable within the two descendants of the apical cell. Nor are they detectable within cells of the embryo sac before fertilization, including the egg cell. We used a G564/beta-glucuronidase reporter gene to show that the G564 promoter is activated specifically within the basal region and suspensor of preglobular tobacco embryos. Analysis of the G564 promoter identified a sequence domain required for transcription within the suspensor that contains several copies of a conserved motif. These results show that derivatives of the apical and basal cells transcribe different genes as early as the four-cell stage of embryo development and suggest that the apical and basal cells are specified at the molecular level after division of the zygote.

Using Genomics to Study Legume Seed Development

“…The most common legume models are peanut (Arachis hypogaea), Lotus (Lotus japonicus), Medicago (Medicago truncatula), soybean (Glycine max), scarlet runner bean (SRB; Phaseolus coccineus), common bean (Phaseolus vulgaris), pea (Pisum sativum), and broad bean (Vicia faba). The latter five species have been used historically to study seed and embryo development (Goldberg et al, 1989;Johnson et al, 1994;Coste et al, 2001;Weterings et al, 2001;Weber et al, 2005).…”

“…For example, some legume seeds are giants and are excellent models for developmental studies, particularly during early stages of seed development. The large size of SRB globular-stage seed and embryo allows manipulation and isolation of embryonic regions, such as the embryo proper and suspensor, using hand-dissection techniques (Walbot et al, 1972;Sussex et al, 1973;Weterings et al, 2001). Due to their size, large quantities of cells and tissues from these SRB embryonic regions can be obtained, facilitating molecular and biochemical studies.…”

“…Our laboratory has utilized SRB as a model system (Weterings et al, 2001) to identify genes and regulatory networks programming early embryo developmental events using a genomics approach (Figs. 3 and 4).…”

“…We have focused on the question of how the embryo-proper and suspensor regions are specified from the apical and basal cells of a two-cell embryo, respectively ( Fig. 4A; Weterings et al, 2001). SRB is unique in this regard because of its giant seed and large embryo ( Fig.…”

“…2A), permitting hand dissection of embryo-proper and suspensor regions at early developmental stages (Fig. 4, B and C; Walbot et al, 1972;Sussex et al, 1973;Weterings et al, 2001). We prepared cDNA libraries from hand-dissected embryo-proper and suspensor regions of SRB globular-stage embryos ( Fig.…”