Cytochemical and Developmental Changes in Microbodies (Glyoxysomes) and Related Organelles of Castor Bean Endosperm

Abstract

Structural changes in endosperm cells of germinating castor beans were examined and complemented with a cytochemical analysis of staining with diaminobenzidine (DAB) . Deposition of oxidized DAB occurred only in microbodies due to the presence of catalase, and in cell walls associated with peroxidase activity . Seedling development paralleled the disappearance of spherosomes (lipid bodies) and matrix of aleurone grains in endosperm cells . 6 to 7 days after germination, a cross-section through the endosperm contained cells in all stages of development and senescence beginning at the seed coat and progressing inward to the cotyledons . Part of this aging process involved vacuole formation by fusion of aleurone grain membranes . This coincided with an increase in microbodies (glyoxsomes), mitochondria, plastids with an elaborate tubular network, and the formation of a new protein body referred to as a dilated cisterna, which is structurally and biochemically distinct from microbodies although both apparently develop from rough endoplasmic reticulum (ER) . In vacuolate cells microbodies are the most numerous organelle and are intimately associated with spherosomes and dilated cisternae . This phenomenon is discussed in relation to the biochemical activities of these organelles. Turnover of microbodies involves sequestration into autophagic vacuoles as intact organelles which still retain catalase activity . Crystalloids present in microbodies develop by condensation of matrix protein and are the principal site of catalase formerly in the matrix .

Endoplasmic Reticulum as the Site of Lecithin Formation in Castor Bean Endosperm

“…The elongated vesicular nature of the membranes and dense association of ribosomes is quite similar to that seen in the electron micrographs prepared from intact endosperm tissue (29,42) .…”

“…The fact that inclusion of Mg++ in the gradients yielded enzymatically active fractions of higher density comprised of rough endoplasmic reticulum, and the prevalence of similar structures in sections of living cells (29,42) strongly suggest that this is the location of the enzyme in vivo. Membranes of mitochondria and glyoxysomes from castor bean endosperm contain lecithin as a major structural phospholipid (23,25) .…”

“…12 g/cm 3 provided evidence that the membranes in this fraction were derived from endoplasmic reticulum and thus that this cellular component is the site of lecithin synthesis in vivo . Electron micrographs of cells of castor bean endosperm have shown that the endoplasmic reticulum is richly endowed with ribosomes and is predominantly "rough" in character (29,42) . Studies on cell-free preparations from rat liver have shown that the controlled use of EDTA results in dissociation of the ribosomes from the membranes and further that the attachment can be maintained by appropriate concentrations of Mg++ (38) .…”

Autophagy-Related Proteins Are Required for Degradation of Peroxisomes inArabidopsisHypocotyls during Seedling Growth

“…Whether pexophagy occurs in plant cells is not clear (Hu et al, 2012), despite evidence supporting it. First, electron microscopic observation suggested pexophagy in castor bean (Ricinus communis) endosperm (Vigil, 1970), although similar observations have not been reported. Second, Atg genes encoding core autophagic machineries are highly conserved in various eukaryotes, including dicots (Hanaoka et al, 2002;Doelling et al, 2002) and monocots (Chung et al, 2009), and reverse genetic studies using Arabidopsis demonstrated their functions in plant autophagy (reviewed by Kim et al, 2012).…”

Programmed cell death during plant growth and development

“…Less is known, however, about the aleurone pcd pathway. Cell death in aleurone and endosperm cells during germination of seeds appears to involve autophagic vacuoles (Vigil, 1970;Reid and Meier, 1972), and, in wheat aleurone, is preceded by an increase in cytosolic calcium levels (Kuo et al, 1996). It was also recently discovered that death of wheat aleurone cells is prevented by treatment with okadaic acid and hence appears to depend on protein phosphatase activity (Kuo et al, 1996).…”