Sequential Changes in the Lipids of Developing Proplastids Isolated from Green Maize Leaves

Abstract

Changes in lipid composition were followed as a proplastid develops into a chloroplast. Methods were devised for the isolation of developing proplastids from sections of five different ages from the same 7-day-old maize (Zea mays var. Kelvedon Glory) leaf. Electron micrographs illustrate the homogeneity of the five types of plastid suspension, minimal contamination with other cytoplasmic membranes, and the presence of morphologically intact plastids in the proportions 85% (youngest), 85%, 80%, 70% and 60% (oldest), respectively. Both bundle sheath and mesophyll plastids are well preserved in isolation. Plastid numbers were determined from calibration curves of the chlorophyll content of each type of suspension, and lipid values then expressed as nmoles/10⁶ plastids. Monogalactosyl diglyceride (MGDG), digalactosyl diglyceride (DGDG), sulfoquinovosyl diglyceride, and phosphatidyl glycerol (PG) all increase during plastid development but the rate of increase is different for each lipid. The largest changes are in MGDG (6-fold) and DGDG (4-fold). Phosphatidyl choline shows a continuous decline during plastid development. Phosphatidyl inositol and phosphatidyl ethanolamine were found in all the suspensions in low concentrations (0.4-4.0% of total lipid): calculations showed their presence could not be accounted for by bacterial or mitochondrial contamination. The increase in PG parallels the chlorophyll changes during development and at maturity 1 molecule of PG is present per 3 molecules of chlorophyll. The results are discussed in the context of the molecular structure of the photosynthetic thylakoid membranes.