Histological study of seed coat development in Arabidopsis thaliana

A detailed analysis of Arabidopsis seed coat development using light and tr ansmission electron microscopy revealed major morphological changes associa ted with the transition of the integuments into the mature seed coat. By th e use of a metachromatic staining procedure, cytological events such as the production of phenolic compounds and acidic polysaccharides were followed. Immediately after fertilization, the cells of the inner epidermis of the i nner integument became vacuolated and subsequently accumulated pigment with in them. This pigment started to disappear from the cytoplasm at the torped o stage of the embryo, as it became green. During the torpedo stage, mucila ge began to accumulate in the cells of the external epidermis of the outer integument. Furthermore, starch grains accumulated against the central part of the inner periclinal wall of these cells, resulting in the formation of small pyramidal domes that persisted until seed maturity. At the maturatio n stage, when the embryo became dormant and colourless, a new pigment accum ulation was observed in an amorphous layer derived from remnants of crushed integument layers. This second pigment layer was responsible for the brown seed colour. These results show that seed coat formation may proceed in a coordinated way with the developmental phases of embryogenesis.