Thermo- and Photoperiodicity and Involvement of Gibberellins during Day and Night Cycle on Elongation Growth of Begonia x hiemalis Fotsch

The effects of thermo- and photoperiodicity on elongation growth and on endogenous level of gibberellins (GAs) in Begonia x hiemalis during various phases of the day-night cycle have been studied. Plant tissue was harvested during the day and night cycle after temperature and photoperiodic treatments and analyzed for endogenous GAs using combined gas chromatography and mass spectrometry. Elongation growth increased when the difference between day and night temperature (DIF = DT − NT) increased from a negative value (−9.0 and −4.5°C) to zero and with increasing photoperiod from 8 to 16 h. When applied to the youngest apical leaf, gibberellins A₁, A₄, and A₉ increased the elongation of internodes and petioles. GA₄ had a stronger effect on elongation growth than GA₁ and GA₉. In relative values, the effect of these GAs decreased when DIF increased from −9 to 0°C. The time of applying the GAs during a day and night cycle had no effect on the growth responses. In general, endogenous levels of GA₁₉ and GA₂₀ were higher under negative DIF compared with zero DIF. The level of endogenous GA₁ in short day (SD)-grown plants was higher under zero DIF than under negative DIF, but this relationship did not appear in long day (LD)-grown plants. The main effects of photoperiod seem to be a higher level of GA₁₉ and GA₁ at SD compared with LD, whereas GA₂₀ and GA₉ show the opposite response to photoperiod. No significant differences in endogenous level of GA₁, GA₉, GA₁₉, and GA₂₀ were found for various time points during the diurnal day and night cycle. Endogenous GA₂₀ was higher in petiole and leaf compared with stem, whereas there were no differences of GA₁, GA₉, and GA₁₉ between plant parts. No clear relationship was found between elongation of internodes and petioles and levels of endogenous GAs. Received December 26 1996; accepted July 1, 1997     

In vitro morphogenetic responses of Ilex paraguariensis nodal segments treated with different gibberellins and Prohexadione-Ca

One-bud nodal segments of Ilex paraguariensis (yerba mate) were cultured in vitro in a sugar-rich medium with different gibberellins or an inhibitor of their synthesis. Bud sprouting, shoot length (assessed as shoots of less or more than 5 mm) and bud abscission were evaluated after 45 d of culture in a growth chamber at 27±2 °C, with a 14 h photoperiod of white fluorescent light. There was a differential effect of the two types of gibberellins used; the double bond ring-A gibberellins (GA₃ and GA₇) inhibited shoot length, while the non double bond-ring A gibberellins (GA₁ and GA₄) stimulated shoots with a length of more than 5 mm. Prohexadione-Ca (Bx-112; a late step gibberellin biosynthesis inhibitor), at high doses, restrained bud sprouting up to 75%, but lower doses promoted shoot lengthening.     

Integrative regulation of leaf morphogenesis by gibberellic and abscisic acids in the aquatic angiosperm proserpinaca palustris L

A two-hormone system regulating leaf development in the heterophyllous amphibious angiosperm Proserpinaca palustris L. is described. Aerial shoots develop expanded, lanceolate, serrate leaves under long-day photoperiods (LD, 16 h light: 8 h dark), whereas growth under short days (SD, 10 h light: 14 h dark) induces dissected leaf formation. The photoperiodic effect on leaf development of aerial shoots involves changes in endogenous gibberellins (GAs) since plants grown under SD in the presence of GA₃ develop expanded lanceolate serrate leaves. However, when submerged, shoots develop highly dissectedaquatic leaves regardless of photoperiod or GA₃ treatment. In the present study, submerged plants exposed to 1.0 or 5.0 μM abscisic acid (ABA) developed aerial-type leaves typical of the photoperiod under which they were cultured. Both exogenous ABA (5.0 μM) and GA₃ (10 μM) treatments were required for laminar expansion to occur on submerged shoots under SD. It is suggested that (1) leaf development in Proserpinaca is regulated by both endogenous GAs and ABA, and (2) the endogenous status of these phytohormones is modulated by different environmental stimuli of photoperiod and water stress, respectively. The adaptive significance of this mechanism is discussed.     

Stem-swelling and photosynthate partitioning in stem mustard are regulated by photoperiod and plant hormones

The experiment was conducted to study the relationship between stem-swelling and photoperiod and growth hormones by comparing stem swelling with non-stem-swelling stem mustard (Brassica juncea var. tsatsai) plants about their growth characteristics and levels of endogenous gibberellin and cytokinin under different photoperiods. The results here showed that plant biomass was higher in 12-h photoperiod compared to that in long day (LD) and short day (SD), whereas stem growth was much stronger in LD compared to 12-h photoperiod and SD. Exogenous application of 1.0 mM gibberellic acid (GA₃) accelerated stem elongation in SD, but 8.9 μM benzyladenine (BA) failed. The shape of the swollen stem was also found to be associated with day length: a LD promoted stem elongation, while a 12-h photoperiod made the stem oval swollen. Also, stem was shown to have no sign of swelling in plants in SD with a relatively poor growth. The further studies showed that the largest proportion of ¹⁴C photosynthate was allocated to the swelling stems in stem-swelling plants, but to expanded leaves in non-stem-swelling plants, and endogenous gibberellin A₁ (GA₁) and zeatin + zeatin riboside (ZRs) were higher in LD compared to 12-h photoperiod and SD. These results from this experiment indicate that stem growth and swelling is a physiological process of hormonal control, and the photoperiod possibly exerts its influence by altering the balance between the levels of endogenous gibberellins and cytokinins.     

Promotory effect of GA13 on flowering ofAmaranthus — a short day plant

Abstact The three plant types ofAmaranthus namely,A. caudatus f.albiflorus, A. caudatus f.caudatus andA. tricolor var.tristis are qualitative short day plants with critical photoperiods 16.0, 15.5 and 15.0 h, respectively. Gibberellins A₃, A₄₊₇ and A₁₃ affect extension growth, leaf differentiation and floral induction differently. Thus, in all the three plant types ofAmaranthus, whereas, GA₃ and G₄₊₇ enhanced extension growth, GA₁₃ was completely ineffective under both, 24- and 8-h photoperiods. None of the three gibberellins could affect the leaf differentiation. In all the three plant types, flowering was promoted by GA₁₃ and not by other gibberellins tried. GA₁₃ caused promotion was manifested in two manners, firstly by lowering the critical dark period requirement in each inductive cycle, and secondly by shortening the total period taken for the initiation of inflorescence primordia under inductive photoperiods. The floral induction by gibberellins inAmaranthus is contrary to the gibberellin-anthesin concept of Chailakhyan. It is suggested that gibberellins other than GA₃ may be playing an important role in floral morphogenesis of short day plants.     

Effect of Different Gibberellins on the Growth of the Hypocotyl

It has been stated earlier that hypocotyls of different plants show different growth response to added GA₃. It was suggested that this difference may be due to the requirement of some specific gibberellin. Hence hypocotyl growth response of three groups of plants has been studied with different gibberellins: group one showing no or insignificant growth response, group two showing 150–200 per cent growth response and group three showing 300–500 per cent growth response to added GA₃. Eight gibberellins were used, viz., GA₁, GA₂, GA₃, GA₄, GA₅, GA₇, GA₈ and GA₉, to test if this varying response is connected with the requirement of some specific gibberellin. In general, the results obtained do not favour this view. Iberis amara, a plant showing no response to added GA₃, Dianthus sp., a plant showing 150 to 200 per cent response and Lactuca satwa, Antirrhinum majus and Nicotiana tabacum, plants showing 300 to 500 per cent response, were promoted by all the gibberellins tested to a similar extent as by GA₃, with the exception of GA₈ which was inactive in most of the cases.     

Interaction of growth retardants,daylength, and gibberellins A19, A20, and A1 on shoot elongation in birch and alder

Gibberellins A₁₉, A₂₀, and A₁ were applied to seedlings of birch (Betula pubescens Ehrh.) and alder (Alnus glutinosa (L.) Gaertn.) in order to test their ability to counteract growth inhibition induced by growth retardants (ancymidol and BX-112) or short day (SD, 12 h) photoperiod. Ancymidol inhibits early and BX-112 inhibits late steps in gibberellin biosynthesis. BX-112 inhibited stem elongation in both species while ancymidol, applied as a soil drench, was effective in alder only. Growth retardants affected stem elongation mainly by inhibiting elongation of internodes. All three gibberellins were equally active when applied to seedlings treated with ancymidol; however, only GA₁ was able to counteract the growth inhibition induced by BX-112. SD-induced cessation of elongation growth in birch was counteracted by GA₁, and to some degree, by GA₂₀, while GA₁₉ was inactive. SD treatment did not induce cessation of apical growth in alder. These results are consistent with the hypothesis that of gibberellins belonging to the early C-13 hydroxylation pathway, GA₁ is the only active gibberellin for stem elongation.     

LOSS OF GIBBERELLINS BY LEACHING FROM STEMS AND FOLIAGE OF CHRYSANTHEMUM MORIFOLIUM ‘PRINCESS ANNE’

Structurally complex metabolites such as carbohydrates, organic acids, and amino acids are leached from plants by rain and dew. Applied GA₃ (1 mg/ml) absorbed by a 5-cm stem segment of Chrysanthemum morifolium and translocated to a single attached leaf was leached from the leaf with distilled water. To detect endogenous gibberellins in plant leachates, Chrysanthemum morifolium ‘Princess Anne’ plants were leached under 8-hr and 18-hr photoperiods. The leachates were collected and extracted at pH 8.2 with methylene chloride to*** remove inhibitors and at pH 3.0 to remove gibberellins. The extract containing the gibberellins was fractionated on a silica gel column by an eluting phase of 0 to 100% ethyl acetate-n-hexane solutions. The presence of gibberellin-like substances was confirmed by their elution pattern from the silica gel column and by two bioassays, the lettuce hypocotyl and the Rumex bioassays. The results suggest that substances similar to GA_1–3 and GA₅ were leached from plants growing under an 18-hr photoperiod. The leachate from plants growing under an 8-hr photoperiod contained gibberellin-like substances similar to GA_1–3 and GA₆.     

Thermomorphogenic and Photomorphogenic Control of Stem Elongation in Fuchsia Is Not Mediated by Changes in Responsiveness to Gibberellins

Stem elongation in Fuchsia × hybrida was influenced by cultivation at different day and night temperatures or in different light qualities. Internode elongation of plants grown at a day (25°C) to night (15°C) temperature difference (DIF+10) in white light was almost twofold that of plants grown at the opposite temperature regime (DIF−10). Orange light resulted in a threefold stimulation of internode elongation compared with white light DIF−10. Surprisingly, internode elongation in orange light was similar for plants grown at DIF−10 and DIF+10. Flower development was accelerated at DIF−10 compared with DIF+10 in both white and orange light. To examine whether the effects of DIF and light quality on shoot elongation were related to changes in gibberellin metabolism or plant sensitivity to gibberellins (GAs), the stem elongation responses of paclobutrazol-treated plants to applied gibberellins were determined. In the absence of applied gibberellins paclobutrazol (>0.32 μmol plant⁻¹) strongly retarded shoot elongation. This inhibition was nullified by the application of about 10–32 nmol of GA₁, GA₄, GA₉, GA₁₅, GA₁₉, GA₂₀, GA₂₄, or GA₄₄. The results are discussed in relation to possible effects of DIF and light quality on endogenous gibberellin levels and gibberellin sensitivity of fuchsia and their effects on stem elongation. Received October 4, 1997; accepted December 17, 1997     

Responses of ivy (Hedera helix L.) to combinations of gibberellic acid,paclobutrazol and abscisic acid

A reduction in concentration of gibberellins has been implicated in the phase change from juvenile to mature forms of ivy (Hedera helix L.). Attempts were made to increase the effective internal concentration of gibberellins by exogenous application of GA₃, and to decrease them by various applications of abscisic acid (ABA) and paclobutrazol (PP333), alone or in combination with GA₃. ABA and GA₃ were fed directly into the xylem of ivy plants by a wick system (a less drastic procedure than the defoliation or decapitation used by earlier workers) whereas PP333 was applied as a soil drench.Mature ivy responded to the application of GA₃ by reversion to the juvenile form, although this reversion was incomplete with respect to leaf lobing and red (anthocyanin) pigmentation and could occur spontaneously without the application of GA₃. Contrary to expectation, applications of ABA and PP333 caused the stimulation of growth in juvenile ivy. No adult characteristics were induced. As similar concentrations of ABA and PP333 produced severe retardation of growth (which could be alleviated by the application of GA₃) in other species, it is suggested that ivy may be an unsuitable model system for the investigation of phase change in woody plants.     

Phase Change in Hedera helix: Induction of the Mature to Juvenile Phase Change by Gibberellin A3

A sensitive and reproducible method to obtain GA₃ induced morphological reversion of mature Hedera helix to the juvenile form has been developed. Dose response experiments indicate that GA₃ stimulates reversion over a 50–100 fold range with a half maximal response at approximately 0.5 μg GA₃ per plant. The individual characteristics involved in phase change revert to the juvenile form in a sequential manner as GA₃ dose is increased. Variations in light intensity from 1.2–3.6 × 10⁴ lux and temperature from 15 to 26°C do not affect this hormonal response. Other growth regulators including indoleacetic acid, kinetin, abscisic acid and (2-chloroethyl)phosphonic acid (Ethephon) are inactive but other gibberellins (GA₁ and a mixture of A₄–A₇) are active in stimulating reversion. Therefore, the response is specific for gibberellins as a class of hormones but non-specific for a particular form of gibberellin. The significance of this response in relation to juvenility in woody plants is discussed.     

Gibberellin, daylength and temperature effects on floral and vegetative development in white clover

The effects of gibberellin A₃ (GA₃) application on five white clover populations was assessed in both glasshouse and controlled environments. Daylength, temperature and GA₃ interactions were also examined. Gibberellin A₃ did not induce vegetative plants to flower when daylength and temperature requirements were not met. In flowering plants, GA₃ increased the number of reproductive buds per stolon and peduncle length, but did not affect other floral characters. Gibberellin A₃ depressed total stolon numbers, but increased the number of nodes per stolon, internode length, leaf area and petiole length.     

Gibberellin physiology of safflower: endogenous gibberellins and response to gibberellic acid

Endogenous gibberellins (GAs) were extracted from safflower (Carthamus tinctorius L.) stems and detected by capillary gas chromatography-mass spectrometry from which GA₁, GA₃, GA₁₉,, GA₂₀, GA₂₉, and probably, GA₄₄ were detected. The detection of these GAs suggests that the early 13-OH biosynthetic pathway is prevalent in safflower shoots. Deuterated GAs were used as internal standards and GA concentrations were determined in stems harvested at weekly intervals. GA₁ and GA₁₉ levels per stem increased but concentrations per gram dry weight decreased over time. GA₂₀ was only detected in young stem tissue.Gibberellic acid (GA₃) was also applied in field trials and both GA₃ and the GA biosynthetic inhibitor, paclobutrazol, were applied in growth chamber tests. GA₃ increased epidermal cell size, internode length, and increased internode cell number causing stem elongation. Conversely, paclobutrazol reduced stem height, internode and cell size, cell number and overall shoot weight. In field tests, GA₃ increased total stem weight, but decreased leaf weight, flower bud number and seed yield. Thus, GA₃ promoted vegetative growth at the expense of reproductive commitment. These studies collectively indicate a promotory role of GAs in the control of shoot growth in safflower, and are generally consistent with gibberellin studies of related crop plants. Author for correspondence     

The effect of gibberellins on the storage life of plantains

Dipping plantain fruits in gibberellins (GA_4/7 or GA₃) delayed the ripening of individual fruits by approximately 50%, the two treatments being equally effective. Measurement of the preclimacteric period (PCP) by increased respiration showed that plantain fruits vacuum infiltrated with GA_4/7 (10^-5 M) gave an extension of 37% in the PCP under high humidity. No effect was observed at low humidity. The use of gibberellins on plantain storage is discussed.     

Effect of Gibberellic Acid and Monophenols on Flowering of lmpatiens balsamina in Relation to the Number of Inductive and Non-inductive Photoperiodic Cycles

A single treatment of plants with GA₃ (gibberellic acid) is not adequate to cause induction under LD (long day: 24-h photo-period) condition, but its effect is added to the sub-threshold induction caused by one SD (short day: 8-h photoperiod) cycle. Floral bud initiation is hastened, and the number of floral buds and flowers per flowering plant increases in plants receiving a single treatment with the combination GA₃+ SA (salicylic acid) accompanying a single SD cycle. However, the increase on 10 replicate basis is more marked in plants receiving three treatments with the combination GA₃+β-N (β-naphthol) and five treatments with the combination GA₃+ SA accompanying six and 10 SD cycles, respectively. The number of floral buds and flowers decreases with an increase hi the number of SD cycles, but it is higher in plants treated with GA₃, SA or GA₃+β-N than in the water-treated controls. — Under long days, treatment of plants with the combinations GA₃+ SA or GA₃+β-N accelerates the initiation as well as increases the number of floral buds. While a minimum of five treatments with GA₃ or of 25 with SA or β-N alone is needed for floral bud initiation under a 24-h photoperiod, three treatments are adequate to induce floral buds with the combination GA₃+ SA or GA₃+β-N under continuous illumination. Ten or more treatments with these combinations under a 24-h photoperiod produce more flowers than the same treatments under an 8-h photoperiod.     

A Gibberellin-Deficient Brassica Mutant-rosette

A single-gene mutant (rosette [ros/ros]) in which shoot growth and development are inhibited was identified from a rapid cycling line of Brassica rapa (syn campestris). Relative to normal plants, the mutant germinated slowly, had delayed or incomplete floral development, and reduced leaf, petiole, and internode growth. The exogenous application of GA₃ by foliar spray or directly to the shoot tip of rosette resulted in rapid flowering, bolting (shoot elongation), and viable seed production. Shoots of rosette contained endogenous levels of total gibberellin (GA)-like substances (`Tan-ginbozu' dwarf rice assay) of about one-tenth of that of the normal rapid-cycling line of B. rapa which consisted almost entirely of a very nonpolar, GA-like substance which yielded GA₁ and GA₃ upon mild acid hydrolysis. In a normal rapid-cycling B. rapa line, the nonpolar putative GA₁ and GA₃ conjugates were present, but additionally, free GA₁ and GA₃ were abundant and identified by gas chromatography-mass spectrometry-selected ion monitoring. The quantities of free GA₁ and GA₃ in the normal line and in rosette were quantified by GC-MS-SIM using [²H₂]GA₁ as an internal standard. Fourteen-day-old rosette and normal seedlings contained 5.3 and 23.2 ng GA₁ per plant, respectively. At day 21 the rosette plants contained 7.7 and 26.1 nanograms per plant of GA₁ and GA₃, while normal plants contained 31.1 and 251.5 nanograms per plant, respectively. Thus, normal plants contained from four to ten times higher levels of total GA-like substances, GA₁, or GA₃, than rosette. The ros allele results in reduced GA level, yielding the rosette phenotype whose delayed germination and flowering, and reduced shoot growth responses indicate a probable role for endogenous GA₁ and GA₃ in the regulation of these processes in Brassica.     

Gibberellins and auxin regulate soybean hypocotyl elongation under low light and high-temperature interaction

Soybean is an important oilseed crop grown globally. However, two examples of environmental stresses that drastically regulate soybean growth are low light and high-temperature. Emerging evidence suggests a possible interconnection between these two environmental stimuli. Low light and high-temperature as individual factors have been reported to regulate plant hypocotyl elongation. However, their interactive signal effect on soybean growth and development remains largely unclear. Here, we report that gibberellins (GAs) and auxin are required for soybean hypocotyl elongation under low light and high-temperature interaction. Our analysis indicated that low light and high-temperature interaction enhanced the regulation of soybean hypocotyl elongation and that the endogenous GA₃, GA₇, indole-3-acetic acid (IAA), and indole-3-pyruvate (IPA) contents significantly increased. Again, analysis of the effect of exogenous phytohormones and biosynthesis inhibitors treatments showed that exogenous GA, IAA, and paclobutrazol (PAC), 2, 3, 5,-triiodobenzoic acid (TIBA) treatments significantly regulated soybean seedlings growth under low light and high-temperature interaction. Further qRT-PCR analysis showed that the expression level of GA biosynthesis pathway genes (GmGA3ox1, GmGA3ox2 and GmGA3) and auxin biosynthesis pathway genes (GmYUCCA3, GmYUCCA5 and GmYUCCA7) significantly increased under (i) low light and high-temperature interaction and (ii) exogenous GA and IAA treatments. Altogether, these observations support the hypothesis that gibberellins and auxin regulate soybean hypocotyl elongation under low light and high-temperature stress interaction.     

Apical Growth Cessation and Shoot Tip Abscission in Salix

Time course of apical shoot growth and shoot tip abortion in northern ecotypes (lat. 69°39′N, long. 18°37′E) of Salix pentandra and S. caprea have been investigated. In trees more than 15 years old growing under natural climatic conditions apical growth cessation and shoot tip abortion normally occurred in June-July when the day length still was 24 h. Application of GA₃, in spring to the apex effectively delayed growth cessation and shoot tip abortion. Application of kinetin was without effect. First-year seedlings of both species grew continuously at temperatue of 9 to 24°C in 24 h photoperiod. Short days induced apical growth cessation, but two to four (S. pentandra) or three to five (S. caprea) weeks of 12 h photoperiod were required to stop the elongation growth. The results indicated that the critical photoperiod for apical growth cessation in the used ecotype of S. pentandra was 16 to 18 h at 18°C. Short days had a minor effect only on the formation of apical leaf primordia in small seedlings. Development of axillary buds and radial growth were stimulated by short days when compared with long days. Small seedlings of both species (3 to 8 cm high at the start) formed terminal buds in short days, but in large seedlings (more than about 15 cm high) apical growth cessation was accompanied by shoot tip abortion. Abscisic acid applied to the apex or through a leaf did not induce growth cessation in S. pentandra seedlings grown in continuous light. The growth retardants CCC, B-9 and Phosphon D reduced growth rate under continuous light and induced shoot tip abortion in some plants. The effect of CCC was counteracted by GA₃. Apical growth cessation in short days was significantly delayed by a single GA₁ application.     

Selection and Characterization of a Gibberellin-Deficient Mutant of Thlaspi arvense L

Field pennycress (Thlaspi arvense L.) is a winter annual weed with a cold requirement for reproductive development. Previous work in this laboratory has demonstrated that the bolting aspect (rapid stem growth) of reproductive development is mediated by gibberellins (GA). The present paper describes the selection and characterization of a mutant lacking the capacity for thermoinduced stem growth. Seeds of an inbred field pennycress line (CR₁) were treated with the chemical mutagen ethyl methane sulfonate, germinated, and allowed to produce seed. Plants derived from these seeds were screened for reduced stem growth. A mutant line, EMS-141, in which the lack of stem growth can be fully overcome with exogenous GA₃, was selected for further analysis. Other phenotypic abnormalities exhibited by the mutant line include reduced petiole growth, slightly delayed floral initiation, and failure of flowers to develop fully. These are also reversed with exogenous GA₃. Evidence is presented indicating that all of the alterations in growth and development exhibited by EMS-141 are conferred by a recessive mutation of a single nuclear gene. Through quantitative analysis of endogenous GA and GA precursors and a comparison of the abilities of various compounds to restore normal growth when applied to plants of EMS-141, the physiological basis for the mutant phenotype was determined to be the result of highly reduced endogenous GA levels. Moreover, the affected site in GA biosynthesis appears to be the accumulation of ent-kaurene, probably at the level of ent-kaurene synthase. The relative abilities of exogenous GA and GA precursors to restore normal growth of petioles and stems are compared, and the results are used to make inferences on the functions of the two different pathways of GA metabolism that exist in field pennycress.     

Phytochrome inhibits the effectiveness of gibberellins to induce cell elongation in rice

Red light controls cell elongation in seedlings of rice (Oryza sativa L.) in a far-red-reversible manner (Nick and Furuya, 1993, Plant Growth Regul. 12, 195–206). The role of gibberellins and microtubules in the transduction of this response was investigated in the rice cultivars Nihon Masari (japonica type) and Kasarath (indica type). The dose dependence of mesocotyl elongation on applied gibberellic acid (GA₃) was shifted by red light, and this shift was reversed by far-red light. In contrast, coleoptile elongation was found to be independent of exogenous GA₃. Nevertheless, it was inhibited by red light, and this inhibition was reversed by far-red light. The content of the active gibberellin species GA₁ and GA₄ was estimated by radio-immunoassay. In the mesocotyl, the gibberellin content per cell was found to increase after irradiation with red light, and this increase was far-red reversible. Conversely, the cellular gibberellin content in japonica-type coleoptiles did not exhibit any significant light response. Microtubules reoriented from transverse to longitudinal arrays in response to red light and this reorientation could be reversed by subsequent far-red light in both the coleoptile and the mesocotyl. This movement was accompanied by changes in cell-wall birefringence, indicating parallel reorientations of cellulose deposition. The data indicate that phytochrome regulates the sensitivity of the tissue towards gibberellins, that gibberellin synthesis is controlled in a negative-feedback loop dependent on gibberellin effectiveness, and that at least two hormone-triggered signal chains are linked to the cytoskeleton in rice.Abbreviations D darkness - FR far-red light - GA₃ gibberellic acid - GC-SIM gas chromatography-selected ion monitoring - R red light This work was supported by a grant of the Human Frontier Science Organization to P.N. Advice and organizational support by Prof. M. Furuya (Hitachi Advanced Research Laboratory, Hatoyama, Japan) and Prof. N. Murofushi (Department of Agricultural Chemistry, University of Tokyo, Japan) is gratefully acknowledged. Seeds of both rice cultivars were kindly provided by Dr. O. Yatou (Institute for Radiation Breeding, Hitachi-Ohmiya, Japan), and the antiGA₁ Me-antiserum for the radio-immunoassays by Dr. I. Yamaguchi (Department of Agricultural Chemistry, University of Tokyo, Japan).