Arabidopsis thaliana outer ovule integument morphogenesis: Ectopic expression of KNAT1 reveals a compensation mechanism

BMC Plant Biology, Apr 2008

Background The Arabidopsis outer ovule integument is a simple two-cell layered structure that grows around the developing embryo and develops into the outer layer of the seed coat. As one of the functions of the seed coat is the protection of the plant embryo, the outer ovule integument is an example for a plant organ whose morphogenesis has to be precisely regulated. Results To better characterise outer ovule integument morphogenesis, we have isolated some marker lines that show GFP expression in this organ. We have used those lines to identify distinct cell types in the outer integument and to demonstrate similarities between leaves and the outer integument. Using confocal microscopy, we showed that cell sizes and shapes differ between the two cell layers of the outer integument. Expression of KNAT1 in the integuments leads to extra cell divisions specifically in the outer layer of the outer integument. This is being compensated for by a decrease of cell volume in this layer, thus showing that mechanisms exist to control proper ovule integument morphogenesis. Conclusion The Arabidopsis outer ovule integument can be used as a good model system to study the basic principles of plant organ morphogenesis. This work provides new insights into its development and opens new possibilities for the identification of factors involved in the regulation of cell division and elongation during plant organ growth.

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Arabidopsis thaliana outer ovule integument morphogenesis: Ectopic expression of KNAT1 reveals a compensation mechanism

BMC Plant Biology Arabidopsis thaliana outer ovule integument morphogenesis: Ectopic expression of KNAT1 reveals a compensation mechanism Elisabeth Truernit 0 1 Jim Haseloff 1 0 INRA, Centre de Versailles, Institut Jean-Pierre Bourgin, Laboratoire de Biologie Cellulaire , Route de St-Cyr, 78026 Versailles cedex , France 1 University of Cambridge, Department of Plant Sciences , Downing Site, Cambridge CB2 3EA , UK Background: The Arabidopsis outer ovule integument is a simple two-cell layered structure that grows around the developing embryo and develops into the outer layer of the seed coat. As one of the functions of the seed coat is the protection of the plant embryo, the outer ovule integument is an example for a plant organ whose morphogenesis has to be precisely regulated. Results: To better characterise outer ovule integument morphogenesis, we have isolated some marker lines that show GFP expression in this organ. We have used those lines to identify distinct cell types in the outer integument and to demonstrate similarities between leaves and the outer integument. Using confocal microscopy, we showed that cell sizes and shapes differ between the two cell layers of the outer integument. Expression of KNAT1 in the integuments leads to extra cell divisions specifically in the outer layer of the outer integument. This is being compensated for by a decrease of cell volume in this layer, thus showing that mechanisms exist to control proper ovule integument morphogenesis. Conclusion: The Arabidopsis outer ovule integument can be used as a good model system to study the basic principles of plant organ morphogenesis. This work provides new insights into its development and opens new possibilities for the identification of factors involved in the regulation of cell division and elongation during plant organ growth. - Background Fertilised ovules develop into seeds that contain the plant embryo. In Arabidopsis thaliana, three distinct regions can be identified along the proximal-distal axis of the ovule primordium (Figure 1). The most proximal structure of the primordium is the funiculus, which connects the primordium to the placenta. At the distal end of the primordium lies the nucellus in which the megaspore mother cell develops. The chalaza in the central zone of the primordium initiates two integuments, each composed of two cell layers [1,2]. During ovule development, the two integuments grow around the nucellus and, after fertilization, develop into the seed coat that encloses the embryo (Figure 1). Whereas the inner integument initially develops as a radially symmetrical structure that surrounds the nucellus, the outer integument grows only from the side of the ovule primordium that faces the basal end of the carpel (gynobasal side) [1,2]. The outer integument remains two-cell layered throughout seed development [1,2]. At later stages of seed development, cells of the abaxial (outer) layer of the outer integument differentiate termiTFihgeudreev1elopment of ovule integuments in Arabidopsis The development of ovule integuments in Arabidopsis. (A) Two inner and one outer integument grow out from the chalaza (c) during early ovule development. (B) Ovule at stage of fertilization: Integuments have grown around nucellus (n), i.i.1: inner (adaxial) layer of inner integument, i.i.2: outer (abaxial) layer of inner integument, o.i.1: inner (adaxial) layer of outer integument, o.i.2: outer (abaxial) layer of outer integument. nally into highly specialized seed coat cells that contain polysaccharide mucilage [3,4]. The integuments are the only lateral organs produced by the ovule. The evolutionary origin of the integuments is still a matter of debate. The telome theory suggests that integuments originated from the fusion of sterile or fertile branches (telomes) [5,6]. It is generally believed that the inner and outer integument derived independently. While the inner integument most likely originated directly from the fusion of telomes or sporangiophores, the outer integument is believed to have developed later from a cupule, a leaf-like structure surrounding one or more ovules [7,8]. The development of the Arabidopsis outer ovule integument involves the same basic processes required for the formation of other determinate lateral plant organs, such as leaves. The outer ovule integument is an example for an organ of determinate growth and characteristic form in which the rate and direction of cell division and elongation needs to be precisely regulated. Asymmetric growth and differentiation are also essential features of its development. In case of the integuments, proper morphogenesis is especially critical, as an improper curvature or closure would lead to seeds with embryos that are not sufficiently protected. However, it seems that integument extension is relatively sensitive to alterations in cell division or cell expansion. Mutations in SHORT INTEGUMENTS2 (SIN2), for example, lead to shorter integuments du (...truncated)


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Elisabeth Truernit, Jim Haseloff. Arabidopsis thaliana outer ovule integument morphogenesis: Ectopic expression of KNAT1 reveals a compensation mechanism, BMC Plant Biology, 2008, pp. 35, 8, DOI: 10.1186/1471-2229-8-35