NONESSENTIALITY OF CONCURRENT CELL DIVISIONS FOR DEGREE OF POLARIZATION OF LEAF GROWTH. II. EVIDENCE FROM UNTREATED PLANTS AND FROM CHEMICALLY INDUCED CHANGES OF THE DEGREE OF POLARIZATION

Haber, Alan H., and D. E. Foard. (Oak Ridge Natl. Lab., Oak Ridge, Tenn.) Nonessentiality of concurrent cell divisions for degree of polarization of leaf growth. II. Evidence from untreated plants and from chemically induced changes of the degree of polarization. Amer. Jour. Bot. 50(9): 937–944. Illus. 1963.—Tobacco leaves grow with a constant degree of polarization (i.e., ratio of rate of increase in length per mm length to rate of increase in width per mm width). During this growth there is decreasing mitotic activity. Data from the literature, including Sinnott's classic studies with cucurbit fruits, provide additional specific examples of growth of (1) constant degree of polarization during which (2) mitotic activity falls. The generalization that cell division plays no role in maintaining a constant degree of polarization is suggested by the widespread occurrence of these 2 features of growth in determinate organs. These considerations are consistent with our earlier finding that the degree of polarization of growth of the first foliage leaf of wheat is the same in seedlings normally growing with oriented cell divisions and in gamma-plantlets, which are seedlings growing without cell division owing to gamma-irradiation of the grain before sowing. The present work shows that the degree of polarization can be significantly increased by treatment with gibberellic acid and decreased by colchicine, even though it is unaffected by radiation-induced mitotic inhibition. These chemically induced changes in the degree of polarization are, moreover, the same in unirradiated and in gamma-plantlet leaves. We conclude that cell division is essential neither for maintaining the degree of polarization nor for changing the degree of polarization. These considerations lead to 3 biological conclusions, each of which is in harmony with simple geometric considerations: (1) cell divisions do not directly contribute to or cause growth; (2) cell division plays an essential and immediate role in influencing cell forms, but plays a secondary and much less important role in influencing organ form; and (3) there is a fallacy in the usual and accepted manner of interpreting changes in organ size as being due to changes in cell size and changes in cell number.     

Synergism between Gibberellin and Auxin in the Growth of Intact Tomato

Gibberellin A_4&7 was more effective than gibberellic acid in increasing shoot elongation when applied to the apex of intact Lycopersicum esculentum seedlings of Tiny Tim, a dwarf cultivar, and Winsall, a tall cultivar. After 14 days, gibberellic acid and gibberellin A_4&7 stimulated growth of the dwarf more than the tall tomato. In tall tomato the application of indole-3-acetic acid alone (6.1 μg/plant) showed an inhibitory growth effect, but when applied with 17.5 μg per plant of gibberellic acid, it had a synergistic effect at 7 days but not at 14 days. When the auxin concentration was reduced to 0.61 μg per plant a synergistic effect was observed on tall plants at 7 and 14 days between indole-3-acetic acid and gibberellic acid. Application of gibberellin A_4&7 with auxin did not give a synergistic response in tall or dwarf tomato.     

Comparison of endogenous gibberellins and response to applied gibberellin of some dwarf and tall wheat cultivars

Summary A number of dwarf wheat cultivars of the Norin 10 type were compared with several tall forms. Applied gibberellic acid markedly stimulated the growth of seedlings of the tall cultivars but not the growth of dwarf seedlings. Several other gibberellins were also inactive when tested with one dwarf cultivar. De-embryonated grains of all cultivars formed -amylase in response to gibberellic acid. Gibberellic acid caused an increase in soluble carbohydrates in the leaves of the tall cultivars but not in those of the dwarfs.Germinating grains, light-grown seedlings and developing stems of the dwarf cultivars contained more endogenous gibberellin-like activity than those of tall cultivars. It is suggested that the dwarf cultivars have a block to the utilisation of gibberellin in the shoot.     

Gibberellin-auxin interaction in pea stem elongation

Joint application of gibberellic acid and indole-3-acetic acid to excised stem sections, terminal cuttings, and decapitated plants of a green dwarf pea results in a markedly synergistic growth response to these hormones. Synergism in green tall pea stem sections is comparatively small, although growth is kinetically indistinguishable from similarly treated dwarf sections.

Gibberellin-induced growth does not appear to be mediated through its effect on auxin synthesis, since gibberellin pretreatment of dwarf cuttings fails to elicit an enhanced tryptophan-induced growth response of sections, whereas auxin-induced growth is strongly enhanced. Also, tryptophan-gibberellin synergism is not significant in sections and cuttings of green dwarf peas, while auxin-gibberellin synergism is.

Administration of gibberellic acid prior to indole-3-acetic acid results in greatly increased growth. In reversed order, the application fails to produce any synergistic interaction. This indicates that gibberellin action must precede auxin action in growth regulation.

     

Control of tuberisation in potato by gibberellins and phytochrome B

Phytochrome B-deficient plants exhibit increased gibberellin (GA) levels or responsiveness, which may contribute to their elongated growth and reduced chlorophyll levels. We have investigated the effects of applications of gibberellic acid and an inhibitor of gibberellin biosynthesis, ancymidol, on wild-type and phytochrome B-antisense potato (Solanum tuberosum ssp. andigena) plants. The results showed that some phenotypes of the phytochrome B-antisense plants, i.e. increased stem length and reduced chlorophyll, can be mimicked by treating wild-type plants with gibberellic acid. However, another phenotype, i.e. tuberisation response in long days, is mimicked by application of a GA biosynthesis inhibitor ancymidol, thus appearing to be the result of a reduction in the gibberellin levels. A simple increase in gibberellin levels or sensitivity is, therefore, not sufficient to explain the phenotype of the antisense plants.     

The role of gibberellin in regulation of dwarf plants development

The effect of exogenously applied gibberellic (GA₃) acid on developmental processes in dwarf pea and dwarf maize seedlings was studied. Plants responded to the phytohormone by accelerated longitudinal growth rate and apparent shortening of developmental phases. Poly(A)-mRNA population isolated from gibberellin-treated pea or maize seedlings exhibited much higher translational activity per mRNA unit in the cell-free wheat germ system when compared with control, untreated plants. Analysis of in vitro translation products made by means of sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS—PAGE) followed by autoradiography and densitometry revealed markedly increased overall intensity of the labelled polypeptide bands in addition to the new protein bands which started to appear in gibberellin-treated pea and maize seedlings while were still not detectable in the control plants of the same age. The banding pattern of translation products programmed by poly(A)-mRNA extracted from 2 days older untreated pea plants resembled that of the gibberellin-treated 2 days younger seedlings. It is concluded that gibberellic acid applied to dwarfs accelerates not exclusively the longitudinal growth of plants but also promotes their transition to the next developmental phases.     

Some Effects of Amo-1618 on Growth, Peroxidase and α-Amylase Which Cannot Be Easily Explained by Inhibition of Gibberellin Biosynthesis

Growth and peroxidase activity of roots and stems of lentil seedlings were compared after treatment with Amo-1618, alone or in combination with gibberellic acid (GA) at varying concentrations. The peroxidase enhancement in Amo-1618 treated stems could not be attributed to a decrease in the gibberellin content since GA alone had no effect on this enzyme. In other experiments, AMO, at low concentrations, was able to induce α-amylase production in barley aleurone layers; the lag period needed for this induction, was longer than for GA. These facts seem to indicate that some growth retardants might act at least in some cases by mechanisms other than inhibition of gibberellin biosynthesis and reversal of GA action.     

Some treatments affecting growth substances in developing wheat ears

Cytokinin, auxin and gibberellin-like substances were bio-assayed in extracts from developing ears of wheat plants grown in various conditions. Changes in cytokinin activity along the ears may be related to the earlier flowering in the middle of the ear. Ears on the main stems of plants from which all the tillers had been removed contained less cytokinin than the main-stem ears of normal tillered plants. When grain development was stopped by preventing fertilization of the ovules the ear contained more cytokinin than normal ears. With de-tillered plants, removing flag leaves before anthesis increased cytokinin, gibberellin and auxin in the ears; later removal of flag leaves did not affect cytokinin but decreased gibberellin in the ears. Conversely, removing ears before anthesis did not affect cytokinin or auxin in the flag leaves, but their gibberellin was less than that of flag leaves on intact plants. Treatment of wheat ears with zeatin did not affect grain weight or number per ear which supports the conclusion that the growth substances in the ear may be adequate for normal grain growth.     

Plant growth regulator activities of stereochemical isomers of triadimenol

Etiolated seedlings of wheat ( Triticum aestivum L. cv. Jubilar) were treated with individual isomers of triadimenol in order to determine the biochemical basis for plant growth retardation. The Is, 2R isomer showed the highest activity as a plant growth retardant, followed by the 1R, 2s form. The inhibition of growth was not relieved by exogenous gibberellic acid suggesting a mode of action different from inhibition of gibberelln synthesis. Labelling of sterols with radioactive acetate and methionine demonstrated a strong inhibition of sterol synthesis, most likely at the step of C-14 demethylation of obtusifoliol. The extent of growth inhibition was accompanied with the potency of individual isomers to inhibit sterol synthesis. The inhibition of gibberellin synthesis appears of less importance for growth retardation.     

Correlative Studies on Plant Growth and Metabolism II. Effect of Light and of Gibberellic Acid on the Changes in Protein and Soluble Nitrogen in Lettuce Seedlings

Protein and soluble nitrogen distribution in different parts of lettuce seedling was studied in light and darkness and in presence and absence of gibberellic acid. In dark, applied gibberellic acid failed to show any marked effect on the nitrogen changes in lettuce. Light inhibits translocation of nitrogen reserves from the cotyledons. Gibberellic acid reverses the light inhibition of longitudinal growth but has no effect on the inhibition of translocation from the cotyledons. Light grown, gibberellic acid treated seedlings exhibit a pattern of protein and soluble-N which is characteristic of the dark grown seedlings. Thus gibberellic acid not only causes morphological reversal of light inhibition but also shifts the nitrogen metabolism of light grown plants, close to that of plants grown in darkness.     

Interaction of gibberellic acid and photoperiod on leaf sheath elongation in normal and gibberellin-insensitive genotype of wheat

Effect of gibberellic acid (GA₃) on leaf sheath elongation in a normal (cv. Møystad) and a gibberellin(GA)-insensitive (cv. Siete Cerros) genotype of wheat ( Triticum aestivum L.) were studied at 18 and 12°C under short (SD, 12 h) or long (LD, 24 h) photoperiod. Leaf sheath length in cv. Møystad was signficantly increased by exogenous GA₃ both under SD and LD. LD alone stimulated leaf sheath elongation and the combined effect of LD and GA₃ was additive, and there was no statistically signficant interaction between photoperiod and GA₃ concentrations. Leaf sheath length in cv. Siete Cerros was not significantly affected by GA₃ under any conditions. However, there was a highly significant stimulation of leaf sheath elongation by LD in cv. Siete Cerros as well. These results indicate that stimulation of elongation growth in wheat leaves by LD is not mediated by gibberellin.     

Phytochrome and hormonal control of expansion and greening of etiolated wheat leaves

Summary Unrolling of etiolated wheat leaf segments is stimulated by short periods of exposure to red light. Both gibberellic acid and kinetin will stimulate unrolling in the dark, whereas abscisic acid (ABA) inhibits the unrolling response to these two hormones and to red light. Exposure to 5 minutes of red light leads to a rapid increase in endogenous gibberellin levels in etiolated wheat leaves, and this increase is followed by a rapid decline. Pre-treatment with ABA inhibits the increase in gibberellin levels in response to red light, but the ihibitory effect of ABA on unrolling cannot be ascribed only to its effect on gibberellin levels. Pre-treatment with red light reduces the lag-phase in chlorophyll development when wheat leaf segments are subsequently exposed to white light; the effect of red light may be replaced by pre-treatment with kinetin, but gibberellic acid is relatively ineffective in this respect.     

The Promotion of Indole-3-acetic Acid Oxidation in Pea Buds by Gibberellic Acid and Treatment

Terminal buds of dark-grown pea (Pisum sativum) seedlings have an indole-3-acetic acid oxidase which does not require Mn(2+) and 2,4-dichlorophenol as cofactors. Oxidase activity is at least 50 times higher in buds of tall peas than in dwarf seedlings. Administration of gibberellic acid to dwarf peas stimulates both growth and indoleacetic acid oxidase activity to the same levels as in tall seedlings. By contrast, indoleacetic acid oxidation assayed in the presence of Mn(2+) and 2,4-dichlorophenol proceeds at similar rates regardless of gibberellin application. Treatment of tall peas with the growth retardant AMO-1618 reduces growth and oxidase activity. Such treated seedlings are indistinguishably dwarf. The enzyme does not appear to be polyphenol oxidase, nor do the results suggest that reduced activity in dwarf buds is due to higher levels of a dialyzable inhibitor. The peroxidative nature of the oxidase is probable.     

Morphactin-gibberellin interaction in lettuce seed germination and seedling growth

Morphactin-butylester (a flourene-9-carboxylic acid derivative) inhibited seed germination of two strains of lettuce. Morphactin induced inhibition of germination could be partially or wholly reversed by simultaneous addition of gibberellic acid. However, gibberellic acid played very little part in reversing the inhibitory effect of morphactin on seedling growth. It is concluded that gibberellin can not reverse all the growth effects induced by morphactin.     

Gibberellin Substitution for the Requirement of the Cotyledons in Stem Elongation in Pisum sativum Seedlings

The removal of the cotyledons from 8-day-old light-grown Pisum sativum cv. Alaska seedlings caused a reduction in the rate of stem elongation to 50% of the intact control value. Gibberellic acid restored the stem elongation rate of decotylized plants to the level of the intact controls. The effect of decotylization was to lower both the rate of node formation and the rate of internode elongation. The steady state rate of internode elongation was reduced to 50% of the control rate by decotylization. Applied gibberellic acid did not restore the normal rate of node formation nor the lag in internode elongation caused by decotylization, but gibberellic acid did restore the normal steady state rate of internode elongation. Analysis of variance demonstrated an interaction between the cotyledons and applied gibberellic acid. 2-Isopropyl-4-dimethylamino-5-methyl phenyl-1-piperidine carboxylate methyl chloride inhibited internode elongation to the same extent in both intact and decotylized plants. The results indicate that the cotyledons are an effective source of gibberellin for the young pea seedling.     

Regulation of root growth by gibberellin in Lemna minor

Hormonal control of root growth was studied in Lemna minor. Although addition of gibberellic acid (GA3) to the culture medium did not promote the root growth, a gibberellin biosynthesis inhibitor, uniconazole P (Un-P), significantly inhibited root growth. Both length and diameter of roots in Un-P-treated plants were significantly smaller than those in control plants, mainly caused by inhibition of cell division. In epidermal cells, the length was slightly decreased and the width increased by Un-P treatment, indicating inhibition of elongation growth. GA3 completely nullified the inhibition caused by Un-P. Transverse cortical microtubules (CMTs) of epidermal cells in the elongation zone were significantly fragmented by treatment with Un-P, but not by that in the presence of GA3. The cellulose microfibril array in the Un-P-treated cells was more random and more oblique than that in the control cells. These results suggested that root growth in L. minor is regulated by endogenous gibberellin.     

The effects of temperature and the Rht3 dwarfing gene on growth, cell extension, and gibberellin content and responsiveness in the wheat leaf

The effects of low temperature and the Rht3 dwarfing gene onthe dynamics of cell extension in leaf 2 of wheat were examinedin relation to gibberellin (GA) content and GA-responsivenessof the extension zone. Leaf 2 of wild-type (rht3) wheat closelyresembled that of the Rht3 dwarf mutant when seedlings weregrown at 10C. The maximum relative elemental growth rate (REGR)within the extension zone in both genotypes was lower at 10Cthan at 20C, but the position with respect to the leaf basewas unaffected by temperature. The size of the extension zoneand epidermal cell lengths were similar in both genotypes at10C. Growth at 20C, instead of 10C, increased the lengthof the extension zone beyond the point of maximum REGR in thewild type, but not in the Rht3 mutant. Increasing temperatureresulted in longer epidermal cells in the wild type. Treatingwild-type plants at 10C with gibberellic acid (GA₃) also increasedthe length of the extension zone, but the Rht3 mutant was GA-non-responsive.However, the concentrations of endogenous GA₁ and GA₃ remainedsimilar across the extension zone of wild-type plants grownat both temperatures, despite large differences in leaf growthrates. The period of accelerating REGR as cells enter the extensionzone, and the maximum REGR attained, are apparently not affectedby GA. It is proposed that GA functions as a stimulus for continuedcell extension by preventing cell maturation in the region beyondmaximum REGR and that low temperature increases the sensitivitythreshold for GA action. Key words: Cell extension, gibberellin, Rht3 dwarfing gene, temperature, wheat leaf     

The induction of sensitivity to gibberellin in aleurone tissue of developing wheat grains

Aleurone tissue from undried immature developing wheat grains (Triticum aestivum L. cv. Sappo), normally insensitive to gibberellic acid, can be made to respond to the hormone by a series of temperature treatments. Incubation of the de-embryoed grains at temperatures above 27° C for at least 8 h causes the tissue to become sensitive. Prolonged incubation at temperatures below 27° C does not effect a change in sensitivity. In addition to the requirement for exposure to an elevated temperature for a period of several hours the tissue must also subsequently be subjected to a period at a lower temperature for just a few seconds for the response to be observed. Once sensitized, the tissue remains responsive to gibberellic acid for substantial periods of time. Exposure of the tissue to temperatures which induce sensitivity to gibberellic acid also results in an increased leakage of amino acids. It is suggested that the increase in sensitivity to gibberellin requires two separate processes to take place. One could be a homeoviscous adaptation of the cell membranes in response to elevated temperature, the other a subsequent, permanent change in conformation of membrane components.     

Gibberellic Acid-Promoted Lignification and Phenylalanine Ammonia-lyase Activity in a Dwarf Pea (Pisum sativum)

The effects of gibberellic acid on lignification in seedlings of a dwarf and a tall cultivar of pea (Pisum sativum) grown under red or white light or in the darkness, were studied. Gibberellic acid (10⁻⁶-10⁻⁴ m) promoted stem elongation in both light and dark and increased the percentage of lignin in the stems of the light-grown dwarf pea. The gibberellin had no effect on the lignin content of the tall pea although high concentrations (10⁻⁴ m) promoted growth of the tall plants. Time course studies indicated that the enhanced lignification in the gibberellin-treated dwarf plants occurred only after a lag period of several days. It was concluded that gibberellic acid-enhanced ligmification had no direct relation to gibberellic acid-promoted growth. The activity of phenylalanine ammonia-lyase (E.C. 4.3.1.5) was higher in gibberellin-treated dwarf plants grown under white or red light than in untreated dwarf plants. Gibberellic acid had no detectable effect on the activity of this enzyme when the plants were grown in darkness, just as it had no effect on lignification under dark conditions. The data suggest that in gibberellin-deficient peas the activity of phenylalanine ammonia-lyase is one of the limiting factors in lignification.     

Suppression and promotion of growth by ethylene in rice seedlings depends on ambient humidity

We examined the effect of ethylene on the growth of rice seedlings (Oryza sativa L.) at various degrees of humidity. Ethylene significantly suppressed the growth of shoots when applied to seedlings grown under 30% relative humidity (RH), but promoted the growth of shoots when applied to seedlings grown under 100% RH. The application of gibberellic acid (GA₃) promoted the elongation of shoots in seedlings grown under 30% and 100% RH. Ethylene inhibited the shoot elongation induced by GA₃ at 30% RH, but enhanced the elongation induced by GA₃ at 100% RH. These results indicate that ethylene can either promote or suppress the growth of rice shoots depending on ambient humidity, and that these actions of ethylene may be mediated through modulating the responsiveness of shoots to gibberellin.