Developmental Changes in Ribosomal Ribonucleic Acid and Fraction I Protein in Wheat Leaves

Abstract

In light-grown wheat (Triticumn aestivum L.) seedlings, the amount of chloroplast and cytoplasmic ribosomal RNA increased to a maximum in the first leaf near the end of its growth and declined by about 60% in the following 3 days. While total ribosomal RNA was declining, labeled uracil was still incorporated into cytoplasmic ribosomal RNA, but the rate of incorporation into chloroplast ribosomal RNA fell by more than 80%, as did the incorporation of labeled leucine into fraction I protein. Either there is greater replacement of cytoplasmic ribosomal RNA than chloroplast ribosomal RNA in mature leaves, or chloroplasts are able to repress the incorporation of exogenous precursor when there is no net synthesis of RNA.In leaves there are two major systems of protein synthesis (2, 18), one using the ribosomes of the cytoplasm, and the other using the smaller ribosomes of the chloroplast (4,19,20). Although it is difficult to estimate relative amounts of the two classes of ribosomes directly, they each contain characteristic species of RNA which can be separated and estimated using polyacrylamide gel electrophoresis (6,8). The larger subunit of chloroplast rRNA in particular is somewhat unstable during isolation (6). Therefore, in order to study the net changes in rRNA at different stages of the development of wheat leaves, we have measured the amounts of the smaller subunits of chloroplast and cytoplasmic rRNA. We have compared the changes in rRNA levels with changes in the level of fraction I protein, which is probably made on the chloroplast ribosomes (20). In addition, the ability of wheat leaves to incorporate exogenous precursor into chloroplast rRNA, cytoplasmic rRNA, and fraction I protein has been measured at different stages of development. MATERIALS AND METHODSGrowth of Wheat. Wheat (Triticum aestivum L., var. Timgalen) was grown in vermiculite with a 24 hr cycle of 12-hr light and dark, at 22 C and 16 C, respectively. Light (about 9,000 lux) was supplied by equal numbers of Philips MCFER warm white and Sylvania Grolux fluorescent tubes. Sowing and harvesting was midway through the light period, and the nutrient solution used was that of Hoagland and Arnon (5).Incorporation of Radioisotopes. Leaves were dissected from the coleoptile and cut at the ligule, surface sterilized with 0.1% cetyltrimethylammonium bromide, thoroughly rinsed with sterile distilled water, and cut into 3-mm sections. Tissue (0.5 g) was incubated with 1 ml of precursor solution for 6 hr with intermittent shaking in light at 22 C. 5-3H-Uracil (240 ,uc/ml; 1 c/mmole) was used as a precursor of RNA, and 'H-leucine, generally labeled (100 ,uc/ml; 50 mc/mmole), was used as a precursor of protein. Incubation was begun 3 hr before the middle of the light period.Extraction of RNA. RNA was prepared using Method B of Loening and Ingle (8) and purified by the method of Ralph and Bellamy (17). Total RNA and DNA were estimated by the method of Smillie and Krotkov (21).Extraction of Fraction I Protein. Leaves were frozen in liquid nitrogen a...

Characterization of the 32,000 Dalton Chloroplast Membrane Protein

“…Active synthesis of RuBPCase in the distal section of mature leaves has also been reported in wheat (7). These two cases stand in contrast to the marked reduction in incorporation of labeled precursors into carboxylase in a number of other plant species at about the time that leaf expansion ceases (8,9,24).…”

Incorporation of 14C-Photosynthate into Protein during Leaf Development in Young Populus Plants

“…Marked variations in physiological activity with leaf maturation and aging have been reported, including changes in photosynthesis and photosynthate translocation (2,3,12,14,24), photorespiration (10,21), protein and enzyme synthesis (4,6,8,15,24,26), and nucleic acid metabolism (17,19,23,26). Our understanding of leaf developmental physiology, however, remains rudimentary.…”

Incorporation of 14C-Leucine into Apple Leaf Protein and Its Inhibition by Protein Synthesis Inhibitors during Growth and Senescence

“…Although protein synthesis capacity remained high in apple leaf tissue, the total and specific activities of some enzymes, such as TDH(NAD)' and TDH(NADP), began to decline when total protein declined (19) and photosynthetic capacity in other leaf types also occurs at the time of initial protein decline, the onset of senescence (7,10,11,17). It has been suggested that the initial decline of protein and the concurrent loss of activity and lack of incorporation of "4C-amino acids into fraction I protein and other enzymes is due to the cessation or reduction of chloroplastic protein synthesis (2,12,23).…”