Microbodies (Glyoxysomes and Peroxisomes) in Cucumber Cotyledons

Abstract

The changes in activities of glyoxysomal and peroxisomal enzymes have been correlated with the fine structure of microbodies in cotyledons of the cucumber (Cucumis sativus L.) during the transition from fat degradation to photosynthesis in light-grown plants, and in plants grown in the dark and then exposed to light. During early periods of development in the light (days 2 through 4), the microbodies (glyoxysomes) are interspersed among lipid bodies and contain relatively high activities of glyoxylate cycle enzymes involved in lipid degradation. Thereafter, these activities decrease rapidly as the cotyledons expand and become photosynthetic, and the activity of glycolate oxidase rises to a peak (day 7); concomitantly the microbodies (peroxisomes) become preferentially associated with chloroplasts.In seedlings grown in the dark for 10 days, the reserve lipid and the glyoxylate cycle enzyme activities persist for a longer time than in the light; correlated with this, there is a continued association of the microbodies with the lipid bodies. When these dark-grown seedlings are then exposed to 51 hours of the light-dark cycle, peroxisomal marker enzymes increase rapidly in activity, and the microbodies become appressed to chloroplasts. We conclude that the characteristic association observed between glyoxysomes and lipid bodies reflects their mutual involvement in net gluconeogenesis through the conversion of fatty acids to carbohydrate, while the close spatial relationship observed between peroxisomes and chloroplasts at later stages of development reflects their mutual involvement in glycolate metabolism.Although glyoxysomal enzyme activities are dropping rapidly while peroxisomal enzyme activities are increasing rapidly during the transition period in the light, the electron microscopic evidence does not indicate that glyoxysomes are being degraded or peroxisomes are being formed. Since in the dark-grown seedlings the activities of peroxisomal enzymes remain low and do not increase as they do in the light, an opportunity is afforded to compare quantitatively any changes in numbers of microbodies per cell with the changes in activities of glyoxysomal enzymes. It is found that the magnitude of the decrease in numbers of microbodies is considerably less than that of the decrease in glyoxysomal enzyme activities between days 4 and 10. When the cotyledons are exposed to light, peroxisomal enzyme activities increase greatly, but again there is no ultrastructural evidence for the synthesis of a new population of microbodies to accommodate this increase. These results allow us to conclude that the developmental transition from glyoxysomal to peroxisomal function almost certainly does not involve the actual replacement of one population of microbodies by another. Rather, the transition probably occurs within existing particles, either by a sequential functioning of two different kinds of microbodies or by a change in enzyme complement within a single population. Our findings with both light- and dark-grown cotyledons favor the latter possibility. The cytoplasmic invaginations into microbodies seen during greening of both light-grown cotyledons and etiolated cotyledons exposed to light may be morphological manifestations of the mechanism by which the microbodies lose or gain enzymes.