Endoplasmic Reticulum of Mung Bean Cotyledons

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Abstract
Homogenates of mung bean cotyledons were subjected to equilibrium density centrifugation on linear sucrose gradients and the positions of the various organelles determined by assay of marker enzymes. Measurement of phospholipid distribution on such gradients showed that the major peak of phospholipid at a density of 1.11-1.13 g/cm3 coincided with the position of the endoplasmic reticulum (ER), confirming ultrastructural evidence that storage parenchyma cells are rich in ER. Germination and seedling growth were accompanied by a rapid decline in ER-associated phospholipid but a marked increase in the ER marker enzyme NADH cytochrome c reductase. Similar experiments with developing seeds indicated that the amount of ER-associated phospholipid increases during cotyledon expansion reaching a maximum during seed maturation. There was no subsequent decline during seed desiccation instead of ER-associated phospholipid levels were maintained in the dry seed until germination when catabolism was initiated 12-24 h after the start of imbibition. The observed ER breakdown may not be an expression of the overall senescence of the cotyledons, but may represent the dismantling of the extensive rough ER used for reserve protein synthesis during cotyledon development.