Wild‐type ovule development in Arabidopsis thaliana: a light microscope study of cleared whole‐mount tissue

Abstract

A detailed morphological description of wild‐type ovule development in Arabidopsis thaliana is presented. The entire process from the formation of the ovule protrusion until the eight‐nuclear endosperm stage is described. The study is based on a light‐microscopical analysis of stained and subsequently optically cleared whole‐mount ovules. It is supplemented by confocal laser scanning microscopy of propidium iodide‐stained whole‐mount ovules. It has been shown that the combination of both techniques eliminates the need for sections to a large extent, and hence allows the rapid morphological inspection of a large number of ovules in Arabidopsis. The ovule constitutes a relatively simple organ. During development, three discrete major pattern elements are laid down along the proximal‐distal axis: the nucellus at the distal end (harbors the megaspore/gametophyte lineage), the chalaza (flanked by the integuments) and the funiculus (includes the vascular strand) at the proximal end. These three pattern elements already appear at a very early 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 characters of each pattern element. It was possible to dissect this developmental process into distinct, morphologically discernible steps at a high resolution. A classification scheme of ovule developmental stages is proposed, which is based on ovule‐specific, discrete, and easy‐to‐score markers.