LTP2 transcripts can be detected in the proembryo-stage in all cells of the outer cell layer of the embryo proper, but are absent in the subtending suspensor (Fig. A). Both hemispheres of the proembryo therefore specify different peripheral cells, and the LTP2 molecular marker allows us to distinguish between the suspensor and the embryo proper early in maize embryogenesis. The difference between the suspensor and the embryo proper observed in outer cell layers presumably reflects functional differences, which may reside in nutrient uptake by the suspensor versus separation of the embryo proper from surrounding endosperm development.
A series of longitudinal sections through late proembryo/early transition-stage embryos showed that distribution of the LTP2 transcript in the protoderm is not completely radial symmetrical. At the adaxial side of the embryo proper, the LTP2 transcript is absent in a few protodermal cells above the suspensor (Fig. B). This position coincides with the prospective SAM anlage, which at this developmental stage is neither detectable histologically nor by analyzing Knotted1 (Kn1) expression pattern. The absence of LTP2 transcripts in the embryonic protodermal region, which is specified to form the L1 layer of the prospective SAM, indicates that the embryo proper is regionalized before the meristem marker Kn1 is activated.
During further stages of development
expression remains confined to the outer cell layer of embryonic organs
like the scutellum and the coleoptile (Fig.
C). In the epidermis of true leaves, as in the L1 layer of the SAM,
expression is generally absent (Fig.
D) This observation provides molecular evidence that the coleoptile
is in lineage with the scutellum but is not a derivative of the SAM. Absence
of the LTP2 transcript in the L1 layer of the SAM and leaves also
shows that epidermal cell fate is different from the identity of cells
comprising the outer cell layer in the maize proembryo. In the late proembryo/early
transition stage lack of LTP2 expression above the prospective SAM
anlage indicates that epidermal cell fate may be realized prior to activation
of the Kn1 meristem marker. The LTP2 radial asymmetry in
the proembryo can be taken as first evidence that despite the irregular
cell division pattern, positional information is imposed very early in
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