Abstract Ethylene is known to interact with auxin in regulating stem growth, and yet evidence for the role of ethylene in tropic responses is contradictory. Our analysis of four mutants of tomato (Lycopersicon esculentum) altered in their response to gravity, auxin, and/or ethylene revealed concentration-dependent modulation of shoot gravitropism by ethylene. Ethylene inhibitors reduce wild-type gravicurvature, and extremely low (0.0005–0.001 μL L−1) ethylene concentrations can restore the reduced gravitropic response of the auxin-resistantdgt(diageotropica) mutant to wild-type levels. Slightly higher concentrations of ethylene inhibit the gravitropic response of all but the ethylene-insensitivenr(never-ripe) mutant. The gravitropic responses of nr and the constitutive-response mutant epi(epinastic) are slightly and significantly delayed, respectively, but otherwise normal. The reversal of shoot gravicurvature by red light in thelz-2(lazy-2) mutant is not affected by ethylene. Taken together, these data indicate that, although ethylene does not play a primary role in the gravitropic response of tomato, low levels of ethylene are necessary for a full gravitropic response, and moderate levels of the hormone specifically inhibit gravicurvature in a manner different from ethylene inhibition of overall growth.
Biophysical parameters related to gibberellin (GA)-dependent stem elongation were examined in dark-grown stem-length genotypes of Pisum sativum L. The rate of internode expansion in these genotypes is altered due to recessive mutations which affect either the endogenous levels of, or response to, GA. The GA deficient dwarf L181 (ls), two GA insensitive semierectoides dwarfs NGB5865 and NGB5862 (Ika and Ikb, respectively) and the `slender' line L197 (la cry[ill]), which is tall regardless of GA content, were compared to the wild-type tall cultivar, Torsdag. Osmotic pressure, estimated by vapor pressure osmometry, and turgor pressure, measured directly with a pressure probe, did not correlate with the differences in growth rate among the genotypes. Mechanical wall properties of frozen-thawed tissue were measured using a constant force assay. GA deficiency resulted in increased wall stiffness judged both on the basis of plastic compliance and plastic extensibility normalized for equal stem circumference. Plastic compliance was not reduced in the GA insensitive dwarfs, though Ika reduced circumference-normalized plasticity. In contrast, in vivo wall relaxation, determined by the pressure-block technique, differed among genotypes in a manner which did correlate with extension rates. The wall yield threshold was 1 bar or less in the tall lines, but ranged from 3 to 6 bars in the dwarf genotypes. The results with the ls mutant indicate that GA enhances stem elongation by both decreasing the wall yield threshold and increasing the wall yield coefficient. In the GA-insensitive mutants, Ika and Ikb, the wall yield threshold is substantially elevated. Plants possessing Ika may also possess a reduced wall yield coefficient.
The Arabidopsis shoot apical meristem starts as a tiny group of fewer than one hundred cells, yet this small group of cells is the source of the aboveground portion of the plant. This simple fact means that a remarkable amount of control (or perception of control signals) in the development of the plant is found in the apical meristem. One reason so little is known about the vegetative apical meristem in Arabidopsis is that it is one of the smallest shoot meristems in the angiosperms (Vaughan 1952). In Arabidopsis , the extremely small size of the apical meristem may mean that some of the processes are regulated differently. For example, a hypothetical morphological gradient that extends over several cells in a large angiosperm meristem could be limited to a gradient in a smaller group of cells or perhaps one cell (i.e., intracellular gradient) in Arabidopsis . However, the size restriction of the Arabidopsis meristem is no longer limiting due to two advances. First, genetic studies using meristem mutants allow the genes and signals involved in various processes to be defined. Second, biochemical studies can be done using an interchangeable system, Brassica oleracea . Cauliflower or B. oleracea var. botrytis is a mutant that contains an immense amplification of shoot meristems (Sadik 1962; Medford et al. 1991). Cauliflower genes are highly homologous to Arabidopsis genes (typically 95% identity), allowing the systems to be interchanged (Medford et al. 1991). Hence, cauliflower (and other related Brassicas ) can be used to collect biochemical quantities of materials, and...
Abstract— Phytochrome influences stem elongation and the mechanism for this is not understood. The levels of indole‐3‐acetic acid (IAA) were analyzed in an leLv genotype of Pisum sativum L. which responded to end‐of‐day far‐red light by doubling growth rate. The IAA levels in epidermal peels increased 40% after far‐red light whereas IAA levels of the entire stem tissue changed insignificantly. This increase was reversible by red light. Under light‐grown conditions, the lv mutation increases stem elongation rates by 2–3‐fold and is thought to block the transduction of a phytochrome signal. Analysis of the short‐term stem elongation kinetics of dark‐ and light‐grown Lv and lv seedlings suggests that lv blocks the action of the light‐stable form of phytochrome. The higher growth rate of lv plants was found to be associated with abnormally high epidermal IAA levels typical of far‐red treated Lv plants. End‐of‐day far‐red treatments did not substantially increase epidermal IAA levels in lv plants. These observations support the view that phytochrome regulation of stem elongation may occur in part through modulation of epidermal IAA levels. The lv mutation may result in increased internode growth in part by blocking the ability of phytochrome to decrease epidermal IAA levels.
Red light causes a reduction in the extension growth of dark-grown seedlings. The involvement of gibberellin in this process was tested by screening a number of gibberellin synthesis and gibberellin response mutants of Pisum sativum L. for the kinetic response of stem growth inhibition by red light. Gibberellin deficient dwarfs, produced by mutant alleles at the Le, Na, and Ls loci, and gibberellin response mutants produced by mutant alleles at the La and Cry(2), Lka, and Lkb loci were tested. Extension growth of expanding third internodes of dark-grown seedlings was recorded with high resolution using angular position transducers. Seedlings were treated with red light at a fluence rate of 4 micromoles per square meter per second either continuously or for 75 seconds, and the response was measured over 9 hours. With certain small exceptions, the response to the red light treatments was similar in all the mutants and wild types examined. The lag time for the response was approximately 1 hour and a minimum in growth rate was reached by 3 to 4 hours after the onset of the light treatment. Growth rate depression at this point was about 80%. Seedlings treated with 75 seconds red light recovered growth to a certain extent. Red/far-red treatments indicated that the response was mediated largely by phytochrome. The similar responses to red light among these wild-type and mutant genotypes suggest that the short-term (i.e. 9 hour) response to red light is not mediated by either a reduction in the level of gibberellin or a reduction in the level or affinity of a gibberellin receptor.
