KÖLN, GERMANY
Universität Köln
The expression pattern of Lipid Transfer Protein 2 (LTP2) gene indicates regionalisation in the proembryo and confirms the coleoptile to be in lineage with the scutellum --Bommert P, Werr W In a pattern formation process during early plant embryogenesis at least two patterns are superimposed. Firstly, an apical-basal axis is established, and, secondly a radial pattern defining the inside and the outside of the embryo is superimposed. In maize as in most angiosperms, the apical and basal poles of the embryo are fixed by the first asymmetric division of the zygote oriented perpendicular to the chalazal/micropylar axis of the embryo sac. The upper, chalazal daughter cell gives rise to the embryo proper while the lower, micropylar cell develops to the suspensor. In contrast to Arabidopsis or other dicots, successive divisions of the zygote do not follow a predictable pattern in maize, but result in an undifferentiated club-shaped cell mass, the proembryo. The first evidence of morphological differentiation becomes visible in the early-transition stage with the appearance of a distinct outer cell layer, the embryonic protoderm (Randolph, J., Agric. Res. 53:881-916, 1936). Slightly later in the midtransition stage the anlage of the shoot apical meristem (SAM) is histologically detectable as a group of small, densely packed cells at the adaxial surface of the embryo. The SAM itself has a tunica/corpus organization composed of at least two clonally distinct cell layers the outer L1 Layer and subtending L2 and L3 Layers, but it is still unclear whether maize organizes a distinct L2 layer. Here we report on the expression pattern of the LTP2 gene (Sossountzov et al., Plant Cell 3:923-933, 1991) during the early stages of embryogenesis.

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 LTP2 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, LTP2 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 maize embryogenesis.
 


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