Development of Microbodies in Sunflower Cotyledons and Castor Bean Endosperm during Germination

Abstract

In cotyledons of sunflower seedlings glyoxysomal and peroxisomal enzymes exhibit different rates of development during germination. The total activity of isocitrate lyase, a glyoxysomal marker enzyme, rapidly increased during the first 3 days, and then decreased 89% by day 9. Exposure to light accelerated this decrease only slightly. The specific activity of glyoxysomal enzymes (malate synthetase, isocitrate lyase, citrate synthetase, and aconitase) in the microbody fraction from sucrose density gradients increased between days 2 and 4 about 2- to 3-fold, and thereafter it remained about constant in light or darkness.Total activity of the peroxisomal enzymes increased slowly in the dark during the first 4 days of germination and thereafter remained at a constant level of activity in the dark or increased 2-fold in 24 hours of light. The specific activties of glycolate oxidase, hydroxypyruvate reductase, and serine-glyoxylate aminotransferase in the isolated microbody fraction increased about 10-fold between days 2 and 4 in the dark and then remained constant or increased again 10-fold after an additional 48 hours in the light.The total activity of the common microbody marker, catalase, developed similarly to isocitrate lyase, but decreased only 72% by day 9. The specific activities of enzymes (catalase, malate dehydrogenase, and aspartate aminotransferase) common to both microbody systems were 10- to 1000-fold greater than those of other enzymes. It is proposed that malate and aspartate may be involved in hydrogen transport between microbodies and other cellular sites.Glutamate-glyoxylate aminotransferase was very active in microbodies from castor bean endosperm and sunflower cotyledons. The specific activity of this aminotransferase developed similarly to glyoxysomal enzymes in the dark but further increased in the light, as did peroxisomal enzymes.The microbody fraction of castor bean endosperm germinated in the dark for 5 days contained both glyoxysomal and peroxisomal enzymes of similar specific activity.Adjacent to the microbody fraction on sucrose gradients from sunflower cotyledons were etioplasts at slightly lower densities and protein bodies at similar and higher densities. Their presence in the microbody fractions resulted in artificially low specific activities.