RECONSTRUCTIONS OF THE GRANA‐FRETWORK SYSTEM OF A CHLOROPLAST

The grana-fretwork system was reconstructed from serial sections representing 0.5 ,um slice, in profile view, from the midregion of a chloroplast of Nicotiana tabacum. Reconstructions show grana in multiple strata. The fretwork integrates the grana in all three dimensions. Large grana are visualized as complex structures consisting of two or more eccentrically stacked solids that vary in height and diameter. The close spacing and overlapping positions of grana in the plastid explain why phase microscopy cannot be used to demonstrate adequately the numerous strata of grana within a chloroplast. The grana are not far enough apart to allow for successful optical sectioning of a plastid in face view. In profile view no distinct grana can be resolved because the grana-fretwork system has the aspect of a "honeycomb." Thus, observations with a light microscope are not adequate to determine the arrangement of grana in a chloroplast, and the recently-proposed model showing all grana of a plastid to be arranged in a single spiral ribbon must be rejected as incompatible with properties of real plastids. SOME YEARS AGO we published a model of the arrangement of membranes in chloroplasts, based on the study of serial sections of known orientation in three-dimensional space (Paolillo, 1970). The grana of plastids were idealized as cylinders, and each granum was shown to be surrounded by multiple helical frets, all of which were determined to be right-handed helices. It was further argued that such a model could be extrapolated throughout a plastid. Whereas, the helical pattern of the fretwork is evident and regular in the vicinity of each granum, the degree and smoothness of integration of the fretwork between grana was said to depend on local variations (Paolillo, 1970). However, the grana of a plastid were said to be integrated laterally and vertically by the fretwork throughout the extent of the threedimensional grana-fretwork system. The model of grana with multiple helical frets has been accepted into the literature (Park and Sane, 1971; Gunning and Steer, 1975; Coombs and Greenwood, 1976) without any particular controversy, most likely because our model accounts for and conserves previous observations. For example, observations on the variability of grana margins (Weier and Thomson, 1962), on the tilted arrangement of fret membranes (Heslop-Harrison, 1963), and on I Received for publication 16 July 1979; revision accepted 16 November 1979. The authors express their gratitude to Dr. J. Aist, Department of Plant Pathology, Cornell University, for the use of the Zeiss Nomarski interference contrast microscope. the spiral-helical relationships of membranes in plastid fragments (Wehrmeyer, 1964) all prepared the way for the formulation of our model (Paolillo and Falk, 1966; Paolillo and Reighard, 1967; Paolillo, 1970) as a correct interpretation of the three-dimensional arrangement of the grana-fretwork system in a fully developed chloroplast. Our model was originally illustrated as idealized drawings (Paolillo and Reighard, 1967; Paolillo, 1970). A related drawing appears in the text by Gunning and Steer (1975). Although these drawings depend entirely on what can be seen in electron micrographs, they represent abstractions or general cases rather than the results of direct reconstructions from serial sections. In the present paper, we present illustrations from a three-dimensional (3-D) reconstruction of all the grana and parts of grana in a 0.5 ,um slice from the midregion of a chloroplast, and from the reconstructions by overlays of tracings, from the same series of sections. In this way we illustrate the spatial arrangement and 3-D shapes of the grana, and the complexity of the interconnections among grana in the grana-fretwork system. Incentive to perform the task of reconstructing a portion of a real plastid was provided by the publication of a model of plastid structure that represents the grana as if they are all arranged in one layer, in a single and more-orless flat, spiral-helical ribbon (Wildman, Atchison and Pringle, 1974; Jope, 1976; Jope et al., 1977; Jope, Atchison and Pringle, 1980; Wildman, Jope and Atchison, 1980). Rose (1979) has referred to this model as possibly