Ribonucleic Acid and Protein Synthesis as Essential Processes for Cell Elongation

Reports of changes in RNA and protein metabolism associated with cell expansion are numerous. Results often depend on whether studies on cell expansion were done with intact plants or with excised parts. A net increase in protein content is usually associated with intact cell extension (6, 15, 17, 30), while excised tissues usually show a net decrease (9, 18, 30, 33). A possible exception to the latter phenomenon was reported by Thimann and Loos (32) for excised potato and artichoke tissue. In these tissues, auxin stimulated the synthesis of protein over a 4-day period. Burroughs and Bonner (8) demonstrated protein synthesis associated with only slight variation in protein content during growth of excised coleoptile tissue. However, auxin did not affect the amount of C14-amino acid incorporated into protein of the coleoptile tissue during 6 hours. Changes in specific activities and amounts of certain enzymes occur in association with cell elongation (3, 7, 14, 27, 28). Studies on changes in RNA metabolism associated with cell expansion are equally complex. There is usually a progressive increase in RNA content per cell from the apical meristem through the zone of radial enlargement (15,17), but in corn mesocotyl tissue (19), there was a net decrease in RNA per cell associated with cell expansion and maturation. The association of RNA with various cell organelles, as measured by differential centrifugation, changes in certain tissues during cell expansion and maturation (10, 19, 23, 29). In excised tissues, a net decrease in RNA is usually associated with cell expansion (10, 20, 33, 35). With corn mesocotyl, growth-promoting concentrations of auxin enhanced the rate of RNA breakdown (20, 33), but this was not observed in soybean hypocotyl tissue (18, 21). In the expanding zone of excised soybean hypocotyl, auxin enhanced C14-nucleotide incorporation into RNA (21), but this did not occur with corn mesocotyl (20). The differences in RNA metabolism in elongating zones of soybean hypocotyl and corn mesocotyl may be attributable to differences in the degradative metabolism of RNA in these tissues. Corn mesocotyl has been shown to have more ribonuclease activity than soybean hypocotyl (34), and low concentrations of auxin enhance ribonuclease activity in corn mesocotyl during excised growth (30). There is little or no change in ribonuclease activity of soybean hypocotyl during excised growth with or without auxin. The growth rate of corn root tips correlates positively with the RNA content of the tissue (35). Woodstock and Skoog (36) hypothesized that the amount of RNA synthesis by the cells of the root apex determines their subsequent rate of elongation. They also showed that IAA inhibited growth and RNA synthesis in the corn root. Many lines of evidence support the view that RNA and protein synthesis may function during cell expansion, although no definitive experiments lhave been done. Developments in the field of protein and nucleic acid biochemistry provide a background of information on specific inhibitors which facilitate the study of RNA and protein syntheses. First, the antibiotic actinomycin D inhibits DNA-dependent synthesis of RNA by RNA polymerase at low concentrations (16) and leads to an inhibition of protein synthesis (13, 22). DNA synthesis is inhibited only at higher concentrations (16). Secondly, puromycin is an effective inhibitor of protein synthesis (1, 26). Presumably by formation of abnormal messenger RNA, 8-azaguanine inhibits protein synthesis in bacteria while only slightly affecting RNA synthesis (9). Also, the demonstration that part or all messenger or informational RNA is unstable (rapid turnover) in many systems (22, 25) would implicate continued RNA synthesis for the normal complete development of a cell. Using the elongating zone of the soybean hypocotyl and actinomycin D, puromycin, and 8-azaguanine as specific inhibitors, the essentiality of RNA and protein synthesis for cell elongation was investigated. The data show that RNA and protein synthesis are essential for cell elongation in soybean hypocotyl tissue. There also is suggestive evidence that the role of auxin in regulating cell elongation may be associated with the control of RNA and/or protein synthesis.