Changes In Elongation Rate Research Articles

Comparative Growth and Gravitropsim Studies in Auxin Response Mutants of Arabidopsis thaliana

We used a video digitizer system to measure the kinetics of promotion and inhibition of root elongation by applied auxin in roots of intact seedlings of Arabidopsis thaliana. We determined the dose response relationship for auxin action on elongation in roots of wild type seedlings and seedlings of four mutants with altered auxin responsiveness. Roots of all four mutants maintained more rapid rates of elongation in the presence of inhibitory concentrations of auxin than did roots of the wild type. The latent period preceding the change in elongation rate after auxin application was the same in the mutants as in the wild type indicating that the differences in the auxin response between the wild type and the mutants are not due to differences in hormone uptake. Low concentrations of auxin induced substantial promotion of root elongation in the wild type and in the mutants aux1-7 and axr2 but not in axr1-3 or axr1-12. The mutants aux1-7 and axr2 showed no gravitropic curvature when stimulated at 90°. The mutants axr1-3 and axr1-12 exhibited rates of gravitropism intermediate between the non-responsive mutants and the wild type. For these intermediate mutants there was an inverse relationship between growth rate and rate of gravitropic curvature.

Action spectra of the light-growth response of Phycomyces

The light-growth response of Phycomyces blakesleeanus (Burgeff) is a transient change in elongation rate of the sporangiophore caused by a change in light intensity. Previous investigators have found that the light-growth response has many features in common with phototropism; the major difference is that only the light-growth response is adaptive. In order to better understand the light-growth response and its relationship to phototropism, we have developed a novel experimental protocol for determining light-growth-response action spectra and have examined the effect of the reference wavelength and intensity on the shape of the action spectrum. The null-point action spectrum obtained with broadband-blue reference light has a small peak near 400 nm, a flat region from 430 nm to 470 nm, and an approximately linear decline in the logarithm of relative effectiveness above 490 nm. The shape of the action spectrum is different when 450-nm reference light is used, as has been shown previously for the phototropic-balance action spectrum. However, the action spectrum of the light-growth response differs from that for phototropic balance, even when the same reference light (450 nm) is used. Moreover, for the light-growth response, the relative effectiveness of 383-nm light decreases as the intensity of the 450-nm reference light increases; this trend is the opposite of that previously found for phototropic balance. The dependence of the lightgrowth-response action spectrum on the reference wavelength, its difference from the phototropic-balance action spectrum, and the reference-intensity dependence of the relative effectiveness at 383 nm may be attributable to dichroic effects of the oriented photoreceptor(s), and to transduction processes that are unique to the light-growth response.

Early growth of the root system in Allium cepa

The development of the root system in Allium cepa was followed for periods of up to 25 days after emergence of the radicle from the seed. Apical meristem activity, in terms of mitotic index, the percentage of cells undergoing DNA synthesis, and cell doubling time, was maximal within several days of emergence in both the primary and adventitious roots, but it gradually decreased in the subsequent period of growth. These changes in proliferative activity in individual roots were paralleled by concurrent changes in elongation rate. Thus, the period of maximum cell production, as well as that of elongation, was of limited duration in each root. Throughout the period of study, the most recently emerged root displayed the highest level of proliferative activity, relative to that found in the other roots on the plant. Changes in the rate of growth of individual roots appeared to be correlated with the early growth of the succeeding root. Key words: Allium, onion, root development, meristem, primordium.

EFFECT OF AGING ON GROWTH AND FLOWERING OF THE TERMINAL BUD OF PHARBITIS NIL

The tendency of the terminal bud of Pharbitis nil to flower after one long night was studied in the context of growth changes occurring with aging. Seedlings were grown under constant light and temperature, selected for uniformity and subjected to a 16‐hr dark period at ages ranging from 3 to 14 days. A decline in percent terminals flowering was observed after day 6, to zero at day 10. Flowering of the terminal was obtained in some plants, thereafter. Floral index of development scored lower for terminals flowering on days 7–9 and 11–14 than on days 3–6. Loss of the tendency to flower was associated with a decline in rate of leaf production in the terminal bud and a change in rate of elongation of the epicotyl axis. The data lend support to a nutrient‐diversion hypothesis in the control of flowering in Pharbitis.

Effect of Aging on Growth and Flowering of the Terminal Bud of Pharbitis nil

The tendency of the terminal bud of Pharbitis nil to flower after one long night was studied in the context of growth changes occurring with aging. Seedlings were grown under constant light and temperature, selected for uniformity and subjected to a 16-hr dark period at ages ranging from 3 to 14 days. A decline in percent terminals flowering was observed after day 6, to zero at day 10. Flowering of the terminal was obtained in some plants, thereafter. Floral index of development scored lower for terminals flowering on days 7–9 and 11–14 than on days 3–6. Loss of the tendency to flower was associated with a decline in rate of leaf production in the terminal bud and a change in rate of elongation of the epicotyl axis. The data lend support to a nutrient-diversion hypothesis in the control of flowering in Pharbitis.

The Phototropic Responses ofAvenaColeoptiles

Macleod, K., Firn, R. D. and Digby, J. 1986. The phototropic responses of Avena coleoptiles.—J. exp. Bot. 37: 542–548. A number of studies of the elongation rate changes causing phototropism have been made but the findings of different groups have not been entirely Consistent. Studies of oat coleoptile phototropism in response to first-positive and second-positive doses indicate that no single pattern of elongation rate changes causes phototropism even in a single species. The relative effect of phototropic stimulation on the elongation rate at the shaded or the illuminated side of coleoptiles subject to unilateral illumination depends on physiological state of the cell-for instance its position in the elongation zone or whether it has been given red light recently. Models of phototropism will have to account for such a diversity of phototropic responses. The importance of making full elongation rate measurements has once again been demonstrated.

Cell repair after liver injury: Stimulation of RNA synthesis, engaged polymerases, number of RNA transcribing molecules, and elongation rate in postischemic liver nuclei

Nuclei isolated from liver cells recovering from reversible (nonnecrogenic) ischemia show an increased RNA synthesis which is essentially sustained by the enhanced activity of the “engaged” (endogenous template-dependent) polymerases; “free” (exogenous templatedependent) polymerases do not change in activity. Chromatin isolated from postischemic nuclei, assayed with Escherichia coli polymerase, shows a normal template activity. Under experimental conditions known to maximize elongation, such as the presence of heparin and high ionic strength, the difference between normal and postischemic nuclei persists, suggesting that stimulation of RNA synthesis during the postischemic period may be caused in part by an increase in the number of polymerase molecules engaged in transcription. But under the same conditions stimulation of RNA synthesis decreases, implying that changes in elongation rate may also play a role in the postischemic effect. Measurements of [ 3H]UMP, derived from internal nucleotides, and [ 3H]uridine, derived from 3′-termini, and subsequent calculation of polynucleotide elongation rate and number of transcribing polymerase molecules confirm that postischemic recovery enhances RNA synthesis mainly by increasing the number of transcribing polymerases and in addition by stimulating the elongation rate of the α-amanitin-resistant polymerase I+III forms.

Translational elongation rate changes in encephalomyocarditis virus-infected and interferon-treated cells.

Infection of mouse L cells with encephalomyocarditis virus results in a rapid inhibition of host protein synthesis before the synthesis of viral proteins. Although no alterations in initiation factor activities have been demonstrated in encephalomyocarditis virus-infected mouse cells, a defect in polypeptide chain elongation has been shown to occur in infected cell extracts. We investigated the significance of this elongation defect in the host shutoff phenomenon in vivo. Average polypeptide chain elongation rates were measured at various times after infection. Interferon was used as a reagent to separate temporarily the virus-induced alterations. Encephalomyocarditis virus infection of L cells was shown to lead to a progressive reduction in the elongation rate. Whereas interferon pretreatment delayed the decrease in elongation rate in a dose-dependent manner, it failed to alter the kinetics of host shutoff, suggesting that slowing of elongation steps played no significant role in this phenomenon. In addition, interferon pretreatment of either mock-infected or virus-infected cells led to no elongation defect that could be attributed to interferon action.