WILD-TYPE OVULE DEVELOPMENT IN ARABIDOPSIS-THALIANA - A LIGHT-MICROSCOPE STUDY OF CLEARED WHOLE-MOUNT TISSUE

A detailed morphological description of wild-type ovule development in Arabidopsis thaliana is presented. The entire process from the format ion of the ovule protrusion until the eight-nuclear endosperm stage is described. The study is based on a light-microscopical analysis of st ained and subsequently optically cleared whole-mount ovules. It is sup plemented by confocal laser scanning microscopy of propidium iodide-st ained whole-mount ovules. It has been shown that the combination of bo th techniques eliminates the need for sections to a large extent, and hence allows the rapid morphological inspection of a large number of o vules in Arabidopsis. The ovule constitutes a relatively simple organ. During development, three discrete major pattern elements are laid do wn along the proximal-distal axis: the nucellus at the distal end (har bors the megaspore/gametophyte lineage), the chalaza (flanked by the i nteguments) and the funiculus (includes the vascular strand) at the pr oximal end. These three pattern elements already appear at a very earl y stage, when the initially formed protrusion, consisting of files of uniform cells, is transformed into a patterned primordium. Subsequent morphogenesis results in the manifestation of the morphological charac ters of each pattern element. It was possible to dissect this developm ental process into distinct, morphologically discernible steps at a hi gh resolution. A classification scheme of ovule developmental stages i s proposed, which is based on ovule-specific, discrete, and easy-to-sc ore markers.