Effect of Xyloglucan Nonasaccharide on Cell Elongation Induced by 2,4-Dichlorophenoxyacetic Acid and Indole-3-acetic Acid

Xyloglucan nonasaccharide (XG9) is recognized as an inhibitorof 2,4-D-induced long-term growth of segments of pea stems.In the presence of 10^–5 M 2,4-D, inhibition by 10^–9M XG9 of elongation of third internode segments of pea seedlingswas detected within 2 h after the start of incubation, in someexperiments. Analysis by double-reciprocal (Lineweaver-Burk)plots of elongation in the presence of various concentrationsof 2,4-D, with or without XG9, gave parallel lines, indicatingthat XG9 inhibited 2,4-D-induced elongation in an uncompetitivemanner. XG9 did not influence the 2,4-D-induced cell wall loosening.Thus, XG9 does not fulfill the proposed definition of an "antiauxin". XG9 at 10^–11 to 10^–6 M did not influence IAA-inducedelongation of segments from pea third internodes, azuki beanepicotyls, cucumber hypocotyls, or oat coleoptiles. Inhibitionof IAA-induced elongation by XG9 was not observed even whenthe segments from pea or azuki bean were abraded. Furthermore,fucosyl-lactose at 10^–11 to 10^–4 M did not affectthe IAA-induced elongation of segments of pea internodes orof azuki bean epicotyls. XG9 may be incapable of inhibitingthe IAA-induced cell elongation (especially in oat) or, alternatively,the endogenous levels of XG9 may be so high that exogenouslyapplied XG9 has no inhibitory effect on IAA-induced elongation. (Received February 28, 1991; Accepted May 25, 1991)     

THE INDEPENDENT ACTION OF MORPHACTINS AND GIBBERELLIC ACID ON HIGHER PLANTS

The comparative activity of three morphactins, n-butyl-9-hydroxyfluorene-9-carboxylate (IT 3233), 9-hydroxy-fluorene-9-carboxylicacid (IT 3235) and methyl-2-chloro-9-hydroxy fluorene-9-carboxylate(IT 3456) on the action of gibberellic acid (GA₃) were examinedutilizing dwarf pea (Progress No. 9), dwarf corn (strain d-5),CCC dwarfed Alaska pea, and embryoless barley half-seeds. When applied either prior to or simultaneously with GA₃ themorphactins were without effect in reducing the response ofthe dwarf peas to the gibberellin. In fact IT 3233 and IT 3235in combination with GA₃ produced taller plants than the GA₃controls. Similar results were observed using CCC dwarfed Alaskapeas. The morphactins did, however, produce several morphologicaleffects on the peas. In addition, they were effective in breakingapical dominance in the peas. The morphactins were also withouteffect on the dwarf corn bioassay and no morphological changeswere observed. In the embryoless barley half-seeds the morphactins did notinhibit the response of the tissue to the GA₃. Also, the morphactinsdid not exhibit gibberellin-like properties. The results suggest that morphactins do not compete with gibberellinfor similar sites of action. ¹Published with the approval of the Director of the MichiganAgricultural Experiment Station as Journal Article Number 3917.     

Effect of Hormones on Sucrose Uptake and on ATPase Activity of Citrus sinensis L. Osbeck Leaves

Carbohydrate accumulation in young, fully expanded leaves ofCitrus sinensis L. Osbeck is affected by the presence of thefruitlet on the shoot. Previous work gave evidence that gibberellinsmay be involved in this 'fruit effect'. In the present workwe have studied the effect of gibberellic acid (GA₃) on ¹⁴C-sucroseuptake by leaf discs and whether its action could be due toa modulation of the plasma membrane ATPase, which maintainsthe H⁺ gradient that drives H⁺/sucrose co-transport. The effect of GA₃ on ¹⁴C-sucrose uptake depended on the osmolarityof the assay medium. At 300 mOsm a reduction in the uptake ratewas observed. The inhibitory effect of the hormone disappearedafter preincubating the leaf discs with para-chloromercuri-phenylsulphonicacid (PCMPS), a sulphydril binding inhibitor. ATPase activityof isolated plasma membrane vesicles was inhibited by IAA treatments,while GA₃ or ABA did not affect this enzyme, even after a 3h preincubation period. However, in the absence of a surfactantin the assay medium, GA₃, together with turgor pressure, modulatedplasma membrane ATPase activity, possibly through modificationsof membrane permeability. The hormone effect on ¹⁴ C-sucroseuptake may involve action on the sucrose carrier.Copyright 1994,1999 Academic Press Abscisic acid, Citrus sinensis, gibberellic acid, indoleacetic acid, orange, osmotic pressure, plasma membrane ATPase, ¹⁴C-sucrose uptake     

The regulation of leaf elongation and xyloglucan endotransglycosylase by gibberellin in 'Himalaya' barley (Hordeum vulgare L.)

The potential role of xyloglucan endotransglycosylase (XET)in GA-stimulated cell elongation was investigated during leafexpansion in barley (Hordeum vulgare L.). XET activity in aqueousextracts of leaves was detected in all segments along the elongatingblade of leaf 1 of seedlings, but was at highest levels in basalsegments. Leaf 1 elongation rates of gibberellin (GA)-responsivedwarf mutants were lower than the wild type, and accompaniedby reduced levels of XET activity. Leaf elongation rates ofthe dwarfs increased following treatment with gibberellic acid(GA₃) associated with higher levels of XET activity. The slendermutant, crossed into a dwarfing background, exhibited high ratesof leaf 1 elongation and high levels of XET activity withoutadded GA₃. The elongation of leaf 3 in a GA-responsive dwarfmutant was also studied. Treatment with GA₃ resulted in bladeand sheath lengths being 5-fold and 7-fold (respectively) thelengths of controls, and again there were increases in bladeand sheath XET activities. To investigate the basis for changesin XET activity levels two XET-related cDNA clones were isolated.RNAs detected by the two clones occurred at the highest levelsin basal segments of rapidly elongating leaves, but they haddifferent distribution patterns along the leaf. Overall, thedata indicate that an XET-like activity is detectable in barleyleaves, that the activity level and related. Key words: Gibberellin (GA), leaf elongation, Hordeum vulgare, xyloglucan endotransglycosylase (XET)     

Biological activities of rishitin, an antifungal compound isolated from diseased potato tubers, and its derivatives

Rishitin, a norsesquiterpene alcohol, found in infected, resistantpotato-tuber tissue completely inhibited zoospore germinationand germtube elongation of Phytophthora infestans (MONT.) DEBARY at 10^–3M. There was little difference in sensitivityto rishitin among races of Phytophthora infestans. IAA-inducedelongation of Avcna coleoptile sections and GA₃-induced elongationof wheat leaf sections were also inhibited by rishitin. Theinhibition of IAA-induced elongation of Avena coleoptiles wasrelieved to some extent by increasing IAA concentration. However,little relief of the inhibition of GA₃-induced elongation ofwheat leaf sections was obtained by increasing GA₃ concentration.No plant injury was observed at this concentration of rishitin(10^–3M). Examination of a series of rishitin derivatives indicated thatthe hydroxyl group at C-3 is indispensable for antifungal activity.This activity was intensified by saturating the double bondbetween the rings of rishitin and/or that of the isopropenylgroup at C-7, though activity decreased when oxygenated functionalgroups were introduced into the side chain. Aromatization of the A ring did not lower biological activities.The antifungal activities of most rishitin derivatives almostparalleled their activities as plant growth retardants. However,some compounds without antifungal activity were active as growthretardants. ¹Studies on the phytoalexins (5). (Received August 14, 1968; )     

Activity of Gibberellins A1 to A0 in the Avena First-leaf Bioassay, and Location after Chromatography

Activity curves are determined for gibberellins A₁ to A₀ bythe Avena first-leaf bioassay method. Gibberellins A¹, A₄ andA₅ can be detected at 10^-11 or 10^-10 g/ml and give optimum activityof approximately 230 per cent elongation (water controls = 100per cent). Gibberellins A²A³, and A⁹ can be detected at 10^-3g/mland give optimum activity of approximately 200 per cent. GibberellinsA₆ and A₇ can be detected at 10^-5g/ml; GA₇ gives optimum activityof around 190 per cent. All the gibberellins except GA₈ canbe detected by this bioassay method after chromatography inn-butanol: 1.5 N ammonia (3: 1) and benzene: acetic acid: water(4: 2: 1) when applied to the paper at concentrations from O.Ito µg. The sensitivity of the method is compared withthat of other gibberellin bioassay methods.     

Abscisic Acid and Apical Dominance in Phaseolus coccineus L. The Role of Tissue Age

Abscisic acid (ABA) applied to the decapitated second internodeof runner bean plants enhanced outgrowth of lateral buds onlywhen internode stumps were no longer elongating. Applied toelongating internodes of slightly younger plants, ABA causesinhibition of bud outgrowth. Together with 10^–4 M indol-3-ylacetic acid (IAA), ABA stimulated internode elongation and interactedadditively in the inhibition of bud outgrowth. A mixture of10^–5 M ABA and 10^–6 M gibberellic acid (GA₃ ) causedsimilar effects on internode growth as IAA + ABA, but was mutuallyantagonistic in effect on growth of the lateral buds. Abscisic acid, apical dominance, gibberellic acid, indol-3yl acetic acid, Phaseolus coccineus, bean     

Auxin-gibberellin relationships in their effects on hypocotyl elongation of light-grown cucumber seedlings VI. Promotive and inhibitory effects of kinetin

The interaction of kinetin with IAA and GA₃ on the elongationof hypocotyl sections of Cucumis sativus L. cv. National Picklingwas studied. Kinetin in the concentration range of 10^–7M to 10^–4 M markedly inhibited IAA-induced elongation,while in a lower range from 10^–10 M to 10^–8 M, itsynergistically enhanced IAA-induced elongation. Kinetin alonein this range had no effect. A 5-to 15-min pulse treatment seemsenough to induce the maximum effect for both inhibition andpromotion. Since the magnitude of the maximum inhibition dependedon the concentration and not on the duration of treatment, thereaction in the cell caused by kinetin seemed to be completedwithin a short period. Washing of the sections with distilledwater after kinetin treatment (30 min) did not significantlyeliminate the kinetin effect. This probably indicates that thebinding of kinetin molecules to a supposed acceptor is not reversible.Interaction of kinetin with GA₃ in their pretreatment effectson IAA-induced elongation shows that in the inhibitory concentrationrange, the kinetin effect was partly overcome by GA₃, and thatin the promotive range, the magnitude of the enhancement wasdetermined by kinetin regardless of the presence of GA₃. Theeffect of kinetin seems to dominate over that of GA₃ indicatingthat the modes of their pretreatment effects differ from oneanother. (Received June 24, 1977; )     

Effects of Gibberellic Acid and Naphthaleneacetic Acid on Petiole Senescence and Subtended Peduncle Growth of Cyclamen persicum Mill

Removal of the blade from the leaf subtending the first flowerbud on Cyclamen persicum ‘Swan Lake’ plants causedthe petiole of that leaf to senesce, but had no effect on thegrowth of the flower peduncle in the debladed petiole's axil.A 10 mg NAA l^–1 application generally had no effect onpetiole senescence, peduncle elongation or flowering date whenapplied to the cut end of the petiole after blade removal. A25 mg GA₃ l^–1 application or a combination of 25 mg GA₃l^–1 application or a combination of 25 mg GA₃ l^–1plus 10 mg NAA l^–1 delayed petiole senescence and enhancedpeduncle elongation and subsequent flowering. No treatment significantlyaltered peduncle length at the time of flowering. Cyclamen persicum Mill, ‘Swan Lake’, tissue receptivity, flowering, GA₃, NAA     

The Effect of Gibberellic Acid on Starch Breakdown in Sprouting Tubers of Solanum tuberosum L.

The treatment of potato tubers with 150 µmol dm^–3gibberellic acid (GA₃) stimulated starch breakdown and hexoseaccumulation in tuber tissues and the transfer of dry matterto stems. These effects could not be accounted for by enhancedactivities of starch phosphorylase, amylase and acid invertase.Indeed enzyme activities either declined or remained relativelyconstant as starch degradation and hexose accumulation proceeded.Changes in the rate of starch depletion were related to changesin sink strength and sink type, the onset of tuber initiationin controls causing the rate of starch degradation to exceedthat in GA₃-treated tissues, in which tuberization was inhibited. Solanum tuberosum L., gibberellic acid, starch breakdown     

Effects of Auxin and Gibberellin on Elongation of Young Hyphae in Neurospora crassa

IAA, 2,4-D and GA₃ promoted the elongation of young hyphae inNeurospora crassa at the optimum concentrations of 10^–6,10^–6 and 10^–4 M, respectively. The effects of IAAand GA₃ were additive. (Received June 17, 1983; Accepted December 22, 1983)     

Metabolism and Translocation of Gibberellins in Seedlings of Pharbitis nil. (I) Effect of Photoperiod on Stem Elongation and Endogenous Gibberellins in Cotyledons and Their Phloem Exudates

Gibberellin A₁, (GA₁), GA₁₉, and GA₂₀ in phloem exudates andcotyledons of seedlings of Pharbitis nil cv. Violet, grown underdifferent photoperiodic conditions, were qualitatively and semi-quantitativelyanalyzed by a combination of high performance-liquid chromatography(HPLC) and radioimmunoassays (RIA). The levels of GA₁₉ and GA₂₀were higher in cotyledons from plants grown under dark treatment(DT) conditons of 16 h-light/8 h-dark for 6 days followed by8 h-light/16 h-dark for 3 days than in those grown under continuouslight (CL) for 9 days. This relationship was also observed forthe GAs in phloem exudates, although the levels were much lowerthan in the cotyledons. When GAs were applied to the cotyledons,elongation of the epicotyl was promoted more by GA₂₀ than byGA₁ or GA₁₉, especially under the CL treatment. The relativeeffect of GA₁ and GA₂₀ on the epicotyl elongation was reversedwhen these GAs were applied to epicotyls pre-treated with prohexadione,an inhibitor of 2-oxoglutarate-dependent dioxygenases. ³Present address: Frontier Research Program, The Institute ofPhysical and Chemical Research (RIKEN), 2-1 Hirosawa, Wakoshi,Saitama, 351-01 Japan ⁴Present address: Laboratory of Horticulture, Faculty of Agriculture,Nagoya University, Nagoya, 464-01 Japan     

Effects of Auxin and Gibberellin on Conidial Germination and Elongation of Young Hyphae in Gibberella fujikuroi and Penicillium notatum

In Gibberella fujikuroi and Penicillium notatum, IAA, 2,4-Dand GA₃ promoted conidial germination and the elongation ofyoung hyphae. The promotive effects of IAA and GA₃ were additive.In both fungi, the concentrations of endogenous auxin and gibberellinin the culture media were 10^–10 to 610^–12M. (Received April 27, 1985; Accepted August 12, 1985)     

Further studies of the ability of xyloglucan oligosaccharides to inhibit auxin-stimulated growth

The structural features required for xyloglucan oligosaccharides to inhibit 2,4-dichlorophenoxyacetic acid-stimulated elongation of pea stem segments have been investigated. A nonasaccharide (XG9) containing one fucosyl-galactosyl side chain and an undecasaccharide (XG11) containing two fucosyl-galactosyl side chains were purified from endo-β-1,4-glucanase-treated xyloglucan, which had been isolated from soluble extracellular polysaccharides of suspension-cultured sycamore (Acerpseudoplatanus) cells and tested in the pea stem bioassay. A novel octasaccharide (XG8′) was prepared by treatment of XG9 with a xyloglucan oligosaccharide-specific α-xylosidase from pea seedlings. XG8′ was characterized and tested for its ability to inhibit auxin-induced growth. All three oligosaccharides, at a concentration of 0.1 microgram per milliliter, inhibited 2,4-dichlorophenoxyacetic acid-stimulated growth of pea stem segments. XG11 inhibited the growth to a greater extent than did XG9. Chemically synthesized nona- and pentasaccharides (XG9, XG5) inhibited 2,4-dichlorophenoxyacetic acid-stimulated elongation of pea stems to the same extent as the same oligosaccharides isolated from xyloglucan. A chemically synthesized structurally related heptasaccharide that lacked a fucosyl-galactosyl side chain did not, unlike the identical heptasaccharide isolated from xyloglucan, significantly inhibit 2,4-dichlorophenoxyacetic acid-stimulated growth.     

Identification and Quantification of Cambial Region Hormones of Eucalyptus globulus

Gibberellins GA₁, GA₄, GA₈, GA₉, GA₁₉, GA₂₀, GA₂₉, GA₄₄, GA₈₁,indole-3-acetic acid (IAA) and abscisic acid (ABA) were identifiedin cambial region tissues of Eucalyptus globulus by comparingmass spectra and Kovats retention indices with those of authenticstandards. Using stable isotope labelled internal standardsGA₁₉, GA₂₀ and GA₄₄ were quantified at levels of 2–7 ng(g fr wt)^-1, other GAs were present at levels < 1 ng (g frwt)^-1. Levels of IAA and ABA ranged from 417–1, 140 ng(g fr wt)^-1 and 86–305 ng (g fr wt)^-1 respectively. Thepresence of brassinosteroid-like substances was also indicatedbased on activity in the rice seedling leaf inclination assay. (Received April 28, 1995; Accepted June 20, 1995)     

Relations between DNA, RNA and Protein Synthesis, and the Cellular Basis of the Growth Response in Gibberellic acid-Treated Pea Internodes

1. A study was made of the influence of gibberellic acid (GA₂)on nucleic acid, protein, and cell-wall synthesis in pea internodesin vivo. 2. GA₃-treated fifth internodes finally contained more thantwice as much total RNA and protein as comparable untreatedones, and the contents of RNA and protein were closely relatedto the length of internode cortical cells. 3. Cell elongation, RNA, protein, and cell-wall synthesis werestimulated 24–48 h before there was any demonstrable GA₃effect on DNA synthesis and cell division. 4. Treated fifth internodes finally contained twice as manycortical cells as control internodes, a response that was matchedby a proportionate increase in the amount of DNA. 5. Internodes treated with actinomycin D or cycloheximide failedto elongate in response to GA₃ treatment, indicating that bothRNA and protein synthesis are essential for gibberellin-stimulatedcell elongation to occur in this tissue. 6. 5-fluorodeoxyuridine at concentrations which completely blockcell division did not prevent cells from elongating in the presenceof GA₃. 7. With the possible exception of pectic substances there wasno change in the relative proportions of each of the major cell-wallconstituents in treated, as compared to control internodes.     

Effects of a Plant-Growth Regulator, Prohexadione, on the Biosynthesis of Gibberellins in Cell-Free Systems Derived from Immature Seeds

Inhibition of the biosynthesis of gibberellins by prohexadione,3,5-dioxo-4-propionylcyclo-hexanecarboxylic acid, was studiedwith cell-free systems derived from immature seeds of Cucur-bitamaxima, Phaseolus vulgaris and Pisum sativum. Prohexadione,at a concentration of 10–⁴ M, inhibited C-7 oxidationof GA₁₂-aldehyde, C-20 oxidation of GA₁₅, conversion of C₂₀-gib-berellinsto C₁₉-gibberellins, 3ß-hydroxylation, 2,3-dehydrogenationof GA²⁰, 2,3-epoxidation of GA₅ and 2ß-hydroxylationof GA₉ and GA₂₀. The 3ß-hydroxylase activity appearedto be more sensitive to prohexadione than were the C-20 oxygenaseand the 2ß-hydroxylase activities. The conversionof mevalonic acid to GA₁₂-aldehyde and the 13-hydroxylationof GA₁₂ were not affected by prohexadione at a concentrationof 3 ? 10^–4 M. All of the steps inhibited by prohexadioneare oxidation steps catalyzed by soluble enzymes that require2-oxoglutarate, Fe²⁺ and oxygen, and all of them occur distalto the synthesis of GA₁₂-aldehyde in the biosynthesis of gibberellins. (Received April 4, 1990; Accepted September 14, 1990)     

NaCl-and Gibberellic Acid-induced Changes in the Content of Auxin and the Activities of Cellulase and Pectin Lyase During Leaf Growth in Rice (Oryza sativa)

The interactive effect of NaCl salinity and gibberellic acidin the activities of cellulase and pectin lyase, and on thecontent of auxin and chlorophyll, has been determined duringleaf growth (fifth from base) in rice. The linear growth, chlorophyllcontent, activity of cellulase, and the auxin level of leaveswere markedly decreased when plants were exposed to salt stress(12 dS m^–1). However pectin lyase activity did not registerany significant alteration in the leaves of salt-stressed plantscompared with the control. Treatment of plants with gibberellicacid (GA₃) (10 ppm) increased the leaf growth and chlorophyllcontent with a concomitant rise in the activity of cellulaseunder stressed as well as non-stressed conditions. A markedincrease in the content of auxin was discernible in the leavesof salt-stressed plants treated with GA₃ compared with non-treatedsalinized ones. An appreciable increment in the activity ofpectin lyase in response to GA₃ administration was detectedonly in the leaves of non-stressed plants. These results indicatethat enhancement of cellulase activity and the augmentationof endogenous auxin content may be involved in the stimulationof rice leaf growth by GA₃ under saline conditions. Oryza sativa, rice, leaf growth, NaCl salinity, gibberellic acid, cellulase, pectin lyase, auxin     

Metabolism and Translocation of Gibberellins in the Seedlings of Pharbitis nil (II). Photoperiodic Effects on Metabolism and Translocation of Gibberellins Applied to Cotyledons

We investigated the metabolism and translocation of two gibberellins(GAs), [³H]GA₂₀ and [³H]GA₁, which were applied at low concentrationto the cotyledons of Pharbitis nil (cv. Violet). Seedlings weregrown under three different photoperiodic conditions: continuouslight (CL-CL), continous light followed by short day conditions(CL-DT) and long day conditions followed by short day conditions(DT-DT). Translocation of the applied [³H]GAs from cotyledonsto hypocotyls was promoted by DT for all GAs examined. Whilethe conversion of the translocated [³H]GA₁ to [³H]GA₈ and itsconjugates was rapid in hypocotyl, the conversion of translocated[³H]GA₂₀ to [³H]GA₂₉ was slow. Radioactivity in epicotyls wasdetected much more rapidly on application of [³H]GA₂₀ than of[³H]GA₁, [³H]GA₈ and [³H]GA₂₉ and their conjugates. The conversionof [³H]GA₂₀ to [³H]GA₁ in the epicotyl was more rapid underCL-CL conditions. This result in consistent with the higherlevel of endogenous GA₁ existing in epicotyls under CL-DT thanDT-DT conditions. However, when [³H]GA₁ was applied to the cotyledon,only small amounts of [³H]GA₈ and its conjugates were detectedin the epicotyl regardless of the photoperiodic conditions.This result may suggest that the translocation and metabolismof [³H]GA₂₀ from cotyledons to epicotyl was faster under CL-CLthan DT-DT conditions and may correlate with the increased epicotylelongation of GA₂₀ treated plants under CL-DT than DT-DT conditions. (Received June 28, 1995; Accepted November 2, 1995)     

Effects of Fluorogibberellins on Plant Growth and Gibberellin 3r{beta}-Hydroxylases

3rß-Fluorogibberellin A₉ (3rß-fluoro-GA₉),3rßfluoro-GA₂₀, 3rß-fluorodeoxygibberellinC (3rß-fluoro-DGC) and 13-fluoro-GA₉ were prepared,and their effects on plant growth and gibberellin (GA) 3rß-hydroxyIaseswere examined. 3rß-Fluoro-GA₉ and 3rß-fluoro-GA₂₀promoted the growth of dwarf rice (Oryza sativa L. cv. Tan-ginbozu)seedlings to three times higher than the control seedlings ata dosage of 3 µ plant^–1, and 3rßfluoro-DGCto twice higher at the same dosage. 3rßg-Fluoro-GA₉was active in cucumber (Cucumis sativus L.) hypocotyl assay,its activity being about one-thirtieth as much as that of GA₄.3rß-Fluoro-GAs were active per se in promoting theshoot elongation of rice. 3rß-Fluoro-DGC inhibitedthe 3rß-hydroxylation of [³H₂]GA₉ to [³H]GA₄ by GArß-hydroxylase from bean (Phaseolus vulgaris L.),but 3rß-fluoro-GA₉ and 3rß-fluoro-GA₂₀ didnot show any effects on the enzyme activity. These 3rß-fluoro-GAsalso showed no or only a weak inhibitory effect on the rß-hydroxylasefrom pumpkin (Cucurbita maxima L.). 13-Fluoro-GA₉ promoted growthof rice and cucumber seedlings, and inhibited the 3rß-hydroxylasesfrom both bean and cucumber. 13-Fluoro-GA₉was converted into13-fluoro-GA₄ and 2,3-didehydro-13-fluoro-GA₉, in a cell-freesystem from bean, and conversion of 13-fluoro-GA₉ into 13-fluoro-GA₄was also observed in a cell-free system from pumpkin. Theseresults suggest that 13-fluoro-GA₉ is one of the substratesof GA 3rß-hydroxy-lases, and that 13-fluoro-GA₉ isactive as a result of the conversion to 13-fluoro-GA₄ in riceand cucumber seedlings. (Received October 27, 1997; Accepted March 13, 1998)