Histochemistry and fine structure of developing wheat aleurone cells.

Developing aleurone cells can first be distinguished 10 days after anthesis beneath the degenerating nucellus as somewhat cuboidal cells with extremely thin walls and large nuclei. Ribosomes are very abundant but little endoplasmic reticulum (ER) is apparent. By 14 days the cell walls are intensely autofluorescent, possibly due to the presence of a ferulic acid-carbohydrate complex. At this stage the cytoplasm is characterized by the presence of large vacuoles, many of which contain small, electron-dense inclusions, presumably the beginnings of the phytin globoids (Type I inclusions) of mature aleurone grains. The paired appearance of many of the cells suggests that they are dividing periclinally, the innermost cells destined to become part of the starchy endosperm. By 4 weeks the cell walls have greatly thickened, ER and mitochondria have proliferated, and the vacuoles, which subsequently give rise to mature aleurone grains, contain a second type of inclusion (Type II inclusion) embedded in a protein matrix. Although the walls remain uniformly autofluorescent, an intensely stained inner wall can be distinguished readily from the outer wall. By 5 weeks the aleurone grains are almost completely surrounded by lipid droplets and contain numerous Type I inclusions. The cells change little in appearance from 6 weeks to maturity. At the latter stage the inner and outer walls are quite distinct and the cytoplasm is densely packed with aleurone grains which are completely surrounded by lipid droplets and interspersed with occasional plastids and numerous mitochondria with rather indistinct cristae.