THE STRUCTURE OF GRANA IN FLOWERING PLANTS

A model of granum structure, consisting of multiple helical frets wound around each cylindrical granum, is proposed as a representation of granim structure for flowering plants in general. This model was originally formulated for grana in the mesophyll plastids of Zea mays and was subsequently extended to grana of Phaseolus vulgaris by additional studies. The model is now shown to be applicable also to grana in the plastids of Cannabis sativa, Elodea canadensis, Nicotiana rustica, Pisum sativum, and Spinacia oleracea. This sample of seven angiosperms includes genera commonly studied by other workers irn their ultrastructural investigations of plastid structure. IT HAS LONG been recognized that the chloroplasts of flowering plants contain dense regions, the grana, embedded in an amorphous matrix, the stroma. Electron micrographs of chloroplasts have revealed that the grana seen in the light microscope are stacks of closely packed circular compartments. The grana are interconnected by a system of more loosely arranged membranes, the frets (Weier, 1961). The 3-dimensional relationship of a granum to the adjacent fretwork has been a matter of considerable debate. Some workers have visualized the membrane systems of chloroplasts simply as interleafed large and small discs, with the smaller discs stacked to give the localized, dense grana and the large discs extending among the grana to bind them together (Wehrmeyer, 1961; Wettstein, 1961). Weier and Thomson (1962) first demonstrated a helical arrangement of connections between the compartments of a granum and its adjacent frets. Heslop-Harrison (1963) suggested that this helical pattern of fret connections is the manifestation of a tilted arrangement of the frets with respect to the grana. He proposed a model in which planar fret membranes intercepted a cylindrical granlum, with the locus of connections between a fret and the compartments of the granum describing an ellipse. The two halves of the ellipse on either side of its major axis each appear as a segment of a helix. Wehrmeyer (1964) determined that a fret (stroma membrane) has a spiral-cyclical arrangement around a granum. He theorized that a fret increases the height of a granum stack by overtopping the end of the granum to form 1 Received for publication 22 August 1968. This research was supported by National Science Foundation Grant GB-6520. 2 Formerly Judith Ann Reighard. additional compartments. The spiral-cyclical (i.e., helical) form of the fret was supposed to relate to its role in the enlargement of the granum. Heslop-Harrison (1966) subsequently adopted more or less this same point of view. Paolillo and Falk (1966) discovered multiple hlelical frets on the grana of corn and argued that if the frets have a morphogenetic function it remains impossible to designate which fret of the many around a granum should be held responsible for the growth of the granum. Paolillo and Reighard (1967) showed that the multiple frets around a granum in a chloroplast of the bean plant are equivalent to each other in the quantity and quality of connections to granum compartments. They concluded that in its simplest form, the theory of spiral cyclical growth as proposed by Wehrmeyer (1964) does not account for the structure of grana that have multiple helical frets wound around them. If the discovery of the multiple helical frets is to lead to some improvement in our theories of granum structure and development, it should be demonstrated that their occurrence is a general phenomenon in the grana of chloroplasts of flowering plants. The present communication is devoted to the demonstration of a common plan of construction in the grana of various