Evidence that a Proliferation of the Endoplasmic Reticulum Precedes the Formation of Glyoxysomes and Mitochondria in Germinating Castor Bean Endosperm

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Abstract
Excised castor bean endosperm halves incubated with CDP-[Me-14C]choline actively incorporated this compound into membrane phosphatidylcholine. The capacity of the tissue to synthesize phosphatidyl-[14C]choline increased during the first 3 d of germination and subsequently declined. At the onset of germination phosphatidyl-[l4C]choline was exclusively recovered in the ER membrane fraction. The rate of incorporation into the ER membranes increased strikingly during the first 24 h of germination while that into mitochondria and glyoxysomes remained low. At later developmental stages an increasing proportion of the newly synthesized phosphatidyl-[14C]choline was present in mitochondria and glyoxysomes; the rate of incorporation into the membranes of these organelles increased while that into the ER membrane began to level off. The kinetics of CDP-[14C]choline incorporation into membrane phosphatidylcholine of the major organelle fractions of 3-d-old endosperm tissue showed that the ER was immediately labelled, whereas a lag period preceded the labelling of mitochondria and glyoxysomes.

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