Events surrounding the early development of Euglena chloroplasts : 18. Structure of the developing proplastid in the first hours of illumination from serial sections of wild-type cells

Abstract

The structure of the proplastid in dark-grown nondividing cells of wild-type Euglena gracilis var. bacillaris and the changes which occur during the early hours of illumination have been studied using serial sections and transmission electron microscopy. The proplastid in dark-grown cells is a polyp-like structure with a number of tubular arms, each containing a noncrystalline prolamellar body composed of irregular tubules, at its extremity. The proplastid with membrane whorls attached is in intimate contact with the surrounding cytoplasm and is in close juxtaposition to microbody-like organelles and mitochondria. The plastid assumes a more globular shape after 30 min of illumination. During this period membrane whorls, similar to those described above, are found as transient structures at the interior of the prolamellar bodies and are found to be closely associated with mitochondria. Membrane whorls are also observed to penetrate to the cores of the microbodies. The thylakoids are attached to the remaining irregular tubular regions of the prolamellar body which surround the membrane whorls. There is a very close association of plastids, mitochondria, microbodies, and Golgi. After 1 to 2 hr the membrane whorls associated with the interior of the prolamellar bodies begin to disappear to be replaced by elongated straight tubules which, in turn, also disappear to leave a cavity surrounded by the remains of the original irregular tubules. There is a progressive pairing of thylakoids with the deposition of a dense matrix between the pairs; a different region of high density can be seen on either side of the paired thylakoids where single thylakoids adjoin them near the point of connection to the irregular tubules of the prolamellar bodies. No relations among the membrane whorls, straight tubules, and thylakoids have been detected. By 24 hr of development paired thylakoids are the rule. Reconstructions from serial sections are presented showing the three-dimensional structure of the proplastid before and after illumination.