Ribosomes and Polysomes in Cucumber Leaves during Growth and Senescence

Abstract

The quantity of RNA in the ribosomal fraction of the first leaf of cucumber (Cucumis sativus) increases during growth, reaches a maximum before the final fresh weight is attained, and then decreases. The main changes are in the free ribosome fraction, the quantity of membrane-bound ribosomes remaining about constant. Few 65.5S chloroplast ribosomes are present in small leaves; however, they increase in quantity rapidly during growth and form about half of the ribosomes present in the mature fully green leaf. The cytoplasmic ribosomes have a sedimentation coefficient of 77.6S. Ribonuclease-sensitive polysomes were present in leaves of all ages except possibly the very oldest. The proportion of ribosomes in polysome form decreases during growth and then remains roughly constant during senescence. Following maturation of the leaf, the rate of incorporation of 'P into ribosomal-fraction RNA begins to decline. This decline could account for the loss of ribosomes during the early stages of senescence. The possibility that leaf ribonuclease might be responsible for the final, more rapid loss of RNA, is discussed.The quantity of protein in a leaf increases during growth but then, at the time of senescence, it declines, although the evidence suggests that the leaf has not completely lost the power to synthesize protein (14). In view of their role in protein synthesis it was thought that a study of leaf ribosomes might throw some light on the mechanism controlling protein metabolism in leaves.Two classes of ribosome are found in leaf tissues, 70S in the chloroplasts and 80S in the cytoplasm (8). Most studies have been made with young tissues, and little is known about the changes that occur with aging. A decrease in the proportion of polysomes during growth has been indicated in Chinese cabbage leaves (6) and maize shoots (10), and an electron microscope study revealed the presence of polysome-like clusters in aging cucumber cotyledons (5). An over-all loss of ribosomes during aging has been observed in several plant tissues both by electron microscopy (1, 4,5,13) and from biochemical studies (6,15) Preparation of Ribosomes and Polysomes. The leaves (10 to 40 g) were rapidly washed, blotted dry, and weighed; then they were cooled and ground by pestle and mortar with enough acid-washed sand to facilitate grinding and with one to two times their volume of tris-magnesium buffer containing 0.05 M tris, pH 7.8, and 0.01 M magnesium acetate. The low molarity of the grinding medium allowed chloroplast lysis. The homogenate was squeezed through four layers of muslin, and the material in the muslin was extracted three times with fresh buffer and the extracts combined. This preparation and all subsequent handling of extract or ribosomes were made at about 0 C unless otherwise stated. Two types of preparations were made from the extract. Differential Centrifugation. To determine the quantity of free and membrane-bound ribosomes, the extract was divided into halves, and Triton X-100 in tris-magnesium buffer was added to one o...

Ribosomal Rna, Fraction I Protein Synthesis, and Ribulose Diphosphate Carboxylase Activity in Developing and Senescing Leaves of Cucumber

“…On the other hand, changes in 80S ribosome content and the proportions of these ribosomes as polyribosomes were relatively less pronounced until late in the senescence process. Similar observations on polyribosomes have been made by Eilam, Butler and Simon (1971) and J. A.…”

Exclusion of plastid nucleoids and ribosomes from stromules in tobacco and Arabidopsis

“…However, plastid ribosomes appear to be distributed throughout the stroma in many electron microscope images (Jacobson et al. , 1963; Eilam et al. , 1971; Gunning and Steer, 1975), and a significant proportion of the ribosomes (variously estimated at 20–50%) is associated with the thylakoid membranes (Falk, 1969; Chua et al.…”

RNA function and phosphorus use by photosynthetic organisms

“…This accords with the longer generation of the embryophytes than of the microalgae. For embryophytes there are data for DNA, RNA, with some studies of rRNA distinguishing between monosomes and polysomes and/or whether ribosomes are free or protein-bound, and studies of protein synthesis during leaf development (Loening and Ingle, 1967 ; Eilam et al, 1971 ; Lin and Stocking, 1978 ; Makrides and Goldthwaite, 1981 ; Dean and Leech, 1982 ). There are also measurements of the P concentration in apical tissues, leaves and stems (Moody and Edwards, 1978 ).…”

“…There are also measurements of the P concentration in apical tissues, leaves and stems (Moody and Edwards, 1978 ). Eilam et al ( 1971 ) examined developing Cucumis sativus leaves, showing that the ribosome concentration peaked at about 14 days when the leaf had attained more than half of its final fresh weight. There was a predominance of larger cytosolic ribosomes early in leaf development, with an increasing fraction of the 70S (mainly plastidic) ribosomes, so that about 40% of the rRNA is in the 70S ribosomes.…”