Immunofluorescence Microscopical Observation of Cortical Microtubule Arrangement as Affected by Gibberellin in d5 Mutant of Zea mays L.

The relationship between the dwarf habit of growth and corticalmicrotubule (MT) orientation as affected by gibberellin wasexamined using a gibberellin responding dwarf mutant of Zeamays L. (d₅). The 4 mm portion of the mesocotyl below the coleoptilarnode of dark-grown seedlings was divided into four 1 mm segments.MTs were observed by means of immunofluorescence microscopyon the tangential surface of the epidermal cells (EP-t), theradial surfaces of epidermal (EP-r) and cortex cells (C-r) inboth normal and d₅ seedlings. MT arrangement in EP-t was transverse,oblique and/or longitudinal. In lower regions, cells with transverselyoriented MTs decreased, while those with obliquely/longitudinallyoriented MTs increased. The frequency of the occurrence of transverselyoriented MTs was much higher in normal than in d₅. In EP-r almostall the cells of the four regions had transversely orientedMTs. In d₅ seedlings only a few percent of cells had obliquelyoriented MTs. In C-r all the cells of the four regions of normalseedlings had transversely oriented MTs, while in d₅, 20% ofthe uppermost region cells showed oblique/longitudinal orientationand more in lower regions. By GA₃ (100µM) treatment for24 h, the growth of d₅ mesocotyls was stimulated, and the orientationof MTs in EP-t, EP-r and C-r of any regions became transverse. ¹Present address: Department of Forestry, Faculty of Agriculture,the University of Tokyo, Yayoi-1-Chome, Bunkyo-ku, Tokyo, 113Japan (Received August 16, 1990; Accepted January 22, 1991)     

Effects of Gibberellin on the Arrangement and the Cold Stability of Cortical Microtubules in Epidermal Cells of Pea Internodes

Longitudinal microtubules are predominant in epidermal cellsof the 3rd internodes of dwarf pea (Pisum sativum L. cv. LittleMarvel) seedlings. In more than 50% of the cells, cortical microtubulesare running parallel to the cell axis. GA₃ promotes elongation of the internodes and gives rise toa predominance of transverse microtubules. In more than 60%of the GA₃-treatd cells, cortical microtubules are running transverseto the cell axis. Longitudinal microtubules in the GA₃-untreated cells are resistantto low-temperature treatment, but transverse microtubules inthe GA₃-treated cells are sensitive to it. Longitudinal microtubulesare present in GA₃-treated epidermal cells with low frequency.They are resistant to low-temperature treatment. Longitudinal, oblique and transverse microtubules are presentwith almost the same frequency in epidermal cells of the 3rdinternodes of tall pea (cv. Early Alaska) seedlings. GA₃ promoteselongation of the internodes also in tall pea seedlings, butit does not alter the direction of cortical microtubules sodistinctly as it does in dwarf pea seedlings. As in dwarf pea seedlings, longitudinal microtubules are resistantto low-temperature treatment, and transverse microtubules aresensitive to it in tall pea seedlings. (Received September 19, 1986; Accepted December 26, 1986)     

Internode Length in Pisum. A New Allele at the le Locus

In Pisum sativum L. a third, more severe, allele at the internodelength locus le is identified and named le^d. Plants homozygousfor le^d possess shorter internodes and appear relatively lessresponsive to GA₂₀ than comparable le (dwarf) plants. Gene le^dmay act by reducing the 3ß-hydroxylation of GA₂₀ tothe highly active GA₁ more effectively than does gene le. Theresults indicate that le is a leaky mutant and therefore thatendogenous GA₁ influences internode elongation in dwarf (le)plants. Pisum sativum, peas, internode length, genetics, gibberellin, dwarf elongation     

Evidence for the Translocation of Gibberellin A3 and Gibberellin-Like Substances in Grafts between Normal, Dwarf1 and Dwarf5 Seedlings of Zea mays L.

Approach grafts were made between the cut surfaces of mesocotylsfrom normal and dwarf seedlings of Zea mays L. (maize). Thedwarfs were the non-allelic single gene gibberellin mutants,dwarf₁ and dwarf₅. The graft combinations were normal-normal,normal-dwarf₁, normal-dwarf₅, dwarfi-dwarf₁, dwarf₅-dwarf₅,and dwarf₁-dwarf₅. The grafts were used to demonstrate the movementof gibberellin-like substances across the union. GA₃, addedto one member of the graft, resulted in leaf-sheath elongationto both members. Gibberellin-like elongation was observed inthe dwarf₁ and dwarf₅ members of normal-dwarf₁ and normal-dwarf₅grafts. Also, in some cases, elongation was observed in thedwarf₁ and dwarf₅ members of dwarf₁-dwarf₅ grafts, especiallyin the dwarf₅ members. The results are discussed in terms of:the gibberellin biosynthetic pathway in maize, the control ofthis pathway by the d₅ and d₁ mutant genes, and the movementof gibberellin-like substances in the plant. Anatomical observationof grafted seedlings after the resonse was observed shows thatcallus cells developed and bridged across the interface of theautografts. There was also evidence for vascular differentiationacross the graft union. ¹ Dedicated to late Prof. J. Ashida. (Received December 17, 1982; Accepted February 9, 1983)     

Stabilization of Cortical Microtubules in Maize Mesocotyl Cells by Gibberellin A3

Effects of GA₃ on the stability of cortical microtubules (MTs)were studied in mesocotyl cells of etiolated maize seedlings.Propyzamide, an MT-disrupting agent specific for plant tubulin,disrupted cortical MTs in cells in the upper regions of mesocotylsof GA₃-untreated seedlings and caused swelling of the cells.GA₃ prevented propyzamide from disrupting MTs and from causingsuch swelling. Chilling of mesocotyls at 4°C for 60 minbrought about the disruption of cortical MTs in cells in theupper regions of mesocotyls of GA₃-untreated seedlings, butnot in cells in corresponding regions of GA₃-treated seedlings,suggesting that treatment with GA₃ increased the stability ofcortical MTs in maize mesocotyl cells. Cortical MTs in protoplastsisolated from mesocotyls of GA₃-untreated seedlings failed towithstand chilling at 0°C for 90 min, while those in protoplastsisolated from mesocotyls of GA₃-treated seedlings withstoodchilling successfully. It appears that the cell wall is notinvolved in the stabilization of cortical MTs by GA₃ in maizemesocotyl cells. (Received July 6, 1993; Accepted November 29, 1993)     

Osmotic potential of the epidermal cells of cucumber hypocotyls as affected by gibberellin and cotyledons

By using light-grown cucumber (cv. Aonagajibae) seedlings withfully expanded cotyledons, the osmotic potential of the epidermalcells of the hypocotyl, as determined by the plasmolysis methodwas examined in relation to the effects of gibberellin (GA₇)and cotyledons. GA₇ at a dosage level of 0.05 µg/plantapplied to the apical bud lowered the osmotic potential; thedecrease was significant 5 hr after the GA₇ treatment. Removalof the cotyledons, covering them with aluminum foil, or applyinglinuron, a DCMU analogue, to them caused an increase in thepotential. In light-grown cucumber seedlings, the maintenanceof a lower osmotic potential in the epidermal cells of the hypocotylwas concluded to be regulated by the photosynthetic activityof the cotyledon and also by gibberellin. ¹Noriko Katsu, Kitano-Haitsu 203, Saginuma 1-9-10, Takatsu-ku,Kawasaki 218, Japan. (Received May 30, 1980; )     

Distribution of Endogenous Gibberellins in Vegetative Shoots of Rice

Levels of endogenous gibberellins in rice seedlings (Oryza sativaL., cv. Nipponbare) were compared between young and old leavesat the 4- and 5-leaf stages. The levels of GA₁, GA₁₉ and GA₅₃were higher in the youngest leaf than in older leaves at the5-leaf stage, but they did not differ significantly betweenthe leaf sheath and the leaf blade. At the 4-leaf stage, thelevel of GA₁, was highest in the third leaf sheaths which containedyoung elongating tissues. These results indicate that gibberellinsare synthesized in young vegetative tissues to promote theirelongation growth. The levels of GA₁ in the youngest leaf sheathsof two cultivars of dwarf rice, Tan-ginbozu and Waito-C, wereapproximately 10% of that in the normal rice at the 5-leaf stage.This result could explain the retardation of shoot elongationin these dwarf cultivars. (Received February 15, 1995; Accepted June 1, 1995)     

Uptake of Gibberellin Methyl Esters by Spores and Induction of Dark Germination in Lygodium japonicum

In the fern Lygodium japonicum, the effect of the exogenousapplication of two gibberellin methyl esters, gibberellin A₄methyl ester (GA₄Me) and gibberellin A₂₀ methyl ester (GA₂₀Me)on spore germination in the dark and uptake of GA₄Me and GA₂₀Meby spores was investigated. Tritiated GA₄Me and GA₂₀Me wereprepared and used as radioactive tracers. The activity of GA₄Mewas more than 100-fold that of GA₂₀Me for the induction of sporegermination. When treated for 24 h, the activity for inducingspore germination remained after removal of the gibberellinmethyl esters from the medium. The amount of GA₄Me taken upby spores was more than three times that of GA₂₀Me throughoutthe 24 h time course of treatment. The uptake of both gibberellinmethyl esters was proportional to the external concentrationfor the range of concentrations between 10^–9 M and 10^–6M. When treated with the tritiated gibberellin methyl estersat 10^–6 M and 10^–7 M for 24 h, most of the gibberellinmethyl esters taken up by the spores were not metabolized. Althoughthe uptake of the two gibberellin methyl esters differed by3- to 5- fold, their abilities to induce spore germination differedby more than 100-fold. Therefore, the difference in the activityof the two gibberellin methyl esters regarding the inductionof spore germination could not be explained solely by the differencein their uptake. (Received January 11, 1988; Accepted May 26, 1988)     

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.     

Biological Activities of the Methyl Ester of Gibberellin A73, a Novel and Principal Antheridiogen in Lygodium japonicum

The synthetic methyl ester of GA₇₃ (GA₇₃-Me) and the naturalantheridiogen of Lygodium japonicum showed almost the same activityto induce the formation of antheridia in dark-grown protonemataof L. japonicum at concentrations of 10^-14 M and higher. Thus,it appears that the principal antheridiogen in L. japonicumis GA₇₃-Me. GA₇₃-Me inhibited formation of ar-chegonia in light-grownprothallia of L. japonicum at concentrations of 10^-11 M andhigher and induced germination of spores in the dark in thisspecies at the same range of concentrations. GA₇₃(free acidform) promoted growth of seedlings of dwarf rice and hypocotylsof cucumber seedlings at dosages of and above 1 and 100 ng/plant,respectively. Eight compounds related to GA₇₃-Me, includingantheridiogens of Anemia phyllitidis and Anemia mexicana, wereactive in inducing an-theridial formation in L. japonicum, althoughtheir activities were considerably lower than that of GA₇₃-Me. (Received August 24, 1988; Accepted November 28, 1988)     

Responses of normal and dwarf Pharbitis nil to an extended dark period and gibberellic acid

The effects of a 24 hr short day, a 24 hr long day, and a 48hr short day were analyzed with regard to flowering and stemgrowth of normal and dwarf Pharbitis nil, and were comparedto effects of these photoperiodic treatments plus applied GA₃.Both short day treatments produced the same number of flowersper plant after seven cycles. The applied GA₃ was effectivein overcoming the growth deficiency of the dwarf; however, theextended dark period of the 48 hr short day and applied GA₃were both required to enhance a flowering response in the dwarfequal to that of the normal. These results indicate that somefactor is present during the extended dark period which enhancesflowering. ¹ This work was supported by NSF Grant GB-7510 and State supportedresearch TTU-191-4771 to M. W. C. ² Present address: Department of Biology, Union University,Jackson, TN 38301, U.S.A. (Received September 4, 1979; )     

Gibberellins, Amylase, and the Onset of Heterosis in Maize Seedlings

Rood, S. B. and Larsen, K. M. 1988. Gibberellins, amylase, andthe onset of heterosis in maize seedlings.—J. exp. Bot.39: 223–233. The possible involvement of gibberellins and amylase in heterosisof maize seedlings was investigated in two parental inbreds,CM7 and CM49, and their single cross F₁ hybrid, CM7xCM49. Germinationof all three genotypes was complete within 36 h after the onsetof imbibition. By 48 h, heterosis (hybrid vigour) for increasedshoot and root length was consistently observed. The endogenousconcentration of gibberellin A₁ (GA₁) was measured in 48 h seedlingsby gas chromatography-mass spectrometry with selected ion monitoring(GC-SIM) using [²H₂]-GA₁ as an internal standard. The GA₁ concentrationwas highest in the hybrid (59 ng g^–1 dry wt.), intermediatein CM49 (9.0 ng g^–1), and lowest in CM7 (<5.0 ng g^–1).Amylase activities in all three genotypes were very low at 24h, but increased during the next 24 h, after which time amylaseactivity in the hybrid was significantly higher than that ofeither parental inbred. Inhibitors of gibberellin (GA) biosynthesis,AMO-1618 or CCC, inhibited germination, shoot and root growth,and amylase activity in all three genotypes. Conversely, exogenousgibberellic acid (GA₃) increased amylase activity, particularlyin the inbred CM7. Amylase isozymes were separated through polyacrylamidegel electrophoresis and generally similar profiles of starchdegrading enzymes were observed in the three genotypes. SinceGA is known to control a-amylase biosynthesis in some cereals,these results are consistent with the hypothesis that GAs areinvolved in the regulation of heterosis in maize. A higher endogenousGA₁ concentration in the hybrid could result in increased amylaseactivity in the hybrid seedlings and consequently, more rapidstarch hydrolysis which fuels heterosis for early growth. Key words: Amylase, germination, gibberellic acid, Gibberellin A₁, heterosis, hybrid vigour, Zea mays     

Change of gibberellin and abscisic acid content during fruit development of Prunus persica

The change of endogenous gibberellin and ABA content was measuredduring the fruit development of Prunus persica. GA₅, GA₃₂ andtwo unidentified gibberellins, were found throughout the developmentalstages. GA₅, GA₃₂ and ABA showed maximum peaks in their contentsixty days after full bloom, which suggests that they play importantroles in fruit development. ¹ To whom correspondence should be addressed. (Received December 12, 1975; )     

Partial Purification and Characterization of Gibberellin 3{beta}-hydroxylase from Immature Seeds of Phaseolus vulgaris L.

Gibberellin 3/ß-hydroxylase,a 2-oxoglutarate-dependentdioxygenase that catalyzes the hydroxylation of GA₂₀ to GA₁,was purified 313-fold from immature seeds of Phaseolus vulgarisL. The mol wt of the enzyme was estimated to be 42,000 by gelfiltration HPLC and SDS-polyacrylamide gel electrophoresis.The enzyme exhibited maximum activity at pH 7.7. The Km valuesfor [2,3-³H]GA20 and [2,3-³H]GA, were 0.29µu and 0.33µm, respectively. The enzyme requires 2-oxoglutarate asa cosubstrate; the K_m value for 2-oxoglutarate was 250µMusing [³H]- GA₂₀ as a substrate. Fe²⁺ and ascorbate significantlyactivated the enzyme at all purification steps, while catalaseand BSA activated the purified enzyme only. The enzyme was inhibitedby divalent cations Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Cd²⁺ and Hg²⁺.3ß-Hydroxylation of [³H]- GA₂₀ was also inhibitedby non-radioactive GA₅, GA₉,GA₁₅, GA₂₀ and GA₄₄. The possiblesite of 3ß-hydroxylation in gibberellin biosynthesisis discussed in terms of the substrate specificity of partiallypurified gibberellin 3ß-hydroxylase. (Received February 29, 1988; Accepted June 3, 1988)     

Effect of GA3 on the Molecular Mass of Polyuronides in the Cell Walls of Alaska Pea Roots

The role of cell wall matrix polysaccharides in gibberellin-regulatedroot growth is unknown. We examined pectic polysaccharides frompea roots treated with or without gibberellin A₃ (GA₃) in thepresence of ancymidol, an inhibitor of gibberellin biosynthesis.Pectic polymers solubilized by CDTA (trans-l,2-cyclohexanediamine-N,N,N',N'-tetraaceticacid) at 23°C and subjected to gel permeation analysis exhibitedhigh polydispersity with a molecular mass in excess of 500 kDa.Subsequent extraction of cell walls with CDTA at 100°C solubilizedpolymers with an average mol mass of 10 to 40 kDa. Subjectingthe high molecular mass pectic polymers extracted at 23°Cto 70–100°C for 2h generated 10 to 40 kDa fragments,similar in size distribution to those solubilized directly fromcell walls by CDTA solutions at 100°C. Pectic polymers from(GA₃+Anc)-treated roots were of higher average mol mass thanthose from Anc-treated roots in both the elongation zone andin the basal maturation zone. Since (GA₃+Anc)-treated rootselongate more quickly than Anc-treated roots [Tanimoto (1994)Plant Cell Physiol. 35:1019], the slender, GA₃-treated rootsmay produce and deposit highly integrated pectins more rapidlythan the thicker, Anc-treated roots in the elongating or elongatedcell walls. ²Present address: Horticultural Sciences Department, POB 110690IFAS, University of Florida, Gainesville, FL 32611-0690 U.S.A.     

Changes in the Sugar Content of the Leaf Sheath of Dwarf Maize Seedlings Associated with Gibberellin-Induced Growth

Changes in the time in sugar and starch content of the firstleaf sheath were compared among normal, d₅, and GA₁-treated(1µg/plant) d₅ seedlings of Zea mays L., from which endospermshad been removed. The patterns of changes in the levels of thesecarbohydrates were more or less similar to one another amongthe three groups of seedlings. Generally, the levels decreasedtoward the time of maturation of the leaf sheath except forthe level of sucrose which showed a sharp increase followedby a decrease. There was an inverse relationship over time betweengrowth and levels of glucose and fructose. Although treatmentwith GA₁ resulted in a decrease in sucrose content, it had practicallyno significant effect on the levels of the other sugars. Inthe leaf sheath of d₅, growth is probably not limited by thelevels of these sugars, and GA-induced elongation does not seemto have any significant correlation with the changes in thelevels of sugars. ¹Present address: Department of Botany, Suri Vidyasagar College,Suri, Dt. Birbhum, West Bengal, India. (Received March 18, 1988; Accepted November 21, 1988)     

Unusual Branching in the Seedlings of Lotus japonicus--Gibberellins Reveal the Nitrogen-sensitive Cell Divisions within the Pericycle on Roots

We studied the effects of several plant-growth regulators onthe induction of nodule-like structures on roots of Lotus japonicus,which has been proposed as a candidate for a leguminous plantfor molecular genetic analysis. Contrary to our expectations,the addition of gibberellin A₃ (GA₃) at concentrations of 10^-4M to 10^-4 M resulted in the formation of nodule-like structureson roots when seedlings were plated on nitrogen-free Fahraeusagar medium. GA₄ also induced such outgrowths but was less activethan GA₃. Application of an inhibitor of auxin transport, N-(1-naphthyl)-phthalamicacid (NPA) and of kinetin, which have been reported to inducepseudonodules in other legumes, had no effect on L. japonicus.Microscopic observations showed that GA₃-induced nodule-likestructures were caused by cell divisions within the pericycleon the roots. In addition, the outgrowths elicited by GA₃ couldbe completely suppressed by the addition of 15 mM potassiumnitrate or ammonium nitrate. These results show that the pericyclecells of the roots of L. japonicus are specifically sensitiveto gibberellins and that potential for cell division might bemodulated by nitrogen compounds. We also examined the effectsof ancymidol and uniconazole [S-3307; (E)-1-(4-chIorophenyl)-4,4-dimethyl-2-(1,2,4-triazol-1-yl)-1-penten-3-ol],two synthetic plant-growth retardants. Both compounds at 3 x10^-5 M significantly increased the number of stunted lateralroots. The unusual branching could not be counteracted by theexogenous addition of GA₃ but by 10^-6 M brassinolide. We discussthe physiological role of brassinolide in the initiation oflateral roots. (Received August 4, 1995; Accepted March 11, 1996)     

A lack of correlation between the biological activity and rate of metabolism of ent-(3H)-17-kaurenoic acid by seedlings of dwarf rice cv. Tan-ginbozu

Significant leaf sheath elongation occurred within 24 hr afterapplication of 10 µg (0.67, µCi) of ent-(³H)-17-kaurenoicacid (KA) to individual seedlings of dwarf rice cv. Tan-ginbozu,but this growth was unaccompanied by production of significantlevels of radioactivity in more polar, acidic, ethyl acetate-solublemetabolites of (³H)-KA. However modest levels of radioactivityappeared in the highly water-soluble fraction by hour 24, subsequentto the most rapid phase of KA-induced growth. Growth continuedand by hour 48 was accompanied by the appearance of small amountsof radioactivity in polar, acidic products. It would appearthat KA per se, and not its metabolic products, may be responsiblefor the leaf sheath elongation noted at hour 24. On the speculation that it might be a metabolite of KA, gibberellinA₁₄ (GA₁₄) was applied simultaneously with (³H)-KA to individualrice seedlings. Several changes in the metabolism of ³H-KA inthe presence of GA₁₄ were noted, and GA₁₄ antagonized the KA-inducedsheath elongation. ¹Present address: Botany Department, Rhodes University, Grahamstown,6140, South Africa. ²Present address: Crop Science Department, University of Saskatchewan,Saskatoon, Sask. S7N OWO, Canada. (Received May 12, 1975; )     

The presence of gibberellin A20 in Stevia rebaudiana and its significance for the biological activity of steviol

Gibberellin-like substances of stems and leaves from Steviarebaudiana were analyzed and gibberellin A₂₀ was identifiedby gas chromatography-mass spectrometry. The presence of GA₂₀in S. rebaudiana is significant for the interpretation of thegibberellin activity of steviol. It indicates that steviol,the C-13 hydroxykaurenoic acid, may function as a precursorfor C-13 hydroxy-gibberellins and not as a gibberellin analog. ¹ This work was supported by National Science Foundation GrantGB 17304 to M. R. and by a Research Grant-in-Aid from SigmaXi to L. M. A. The research described is from a dissertationsubmitted by L. M. A. in partial fulfillment of the requirementsfor the Ph.D. degree. ² Present address: Laboratory of Plant Morphogenesis, Departmentof Biology, Manhattan College, Bronx, N. Y. 10471, U. S. A. (Received June 12, 1978; )     

Promotion of mesocotyl growth in etiolated rice seedlings by 4-ethoxy-1-(p-tolyl)-s-triazine-2,6(1H,3H)-dione

4-Ethoxy-1-(p-tolyl)-s-triazine-2,6(1H,3H)-dione (TA) promoted mesocotyl growth in dark-grown rice (Oryza sativa L.) seedlings. In cultivars of the japonica type TA alone showed a small promotive effect and TA+gibberellic acid(GA₃) had a marked synergistic effect, while in other cultivars, mostly of the indica type, TA alone showed a great promotive effect and TA+GA₃ had only an additive effect. In cv. Nato, a typical representative of cultivars showing the second type of response, the concentration of TA giving the greatest growth promotion was around 0.1–0.2 mM. In Nato seedlings treated with TA at 0.1 mM, the mesocotyls continued to elongate for 6 days and reached about 75 mm in length, while the mesocotyls of control seedlings grew to a maximum of about 10 mm and growth was limited to the first 3 days after planting. The TA-induced mesocotyl elongation was mainly the consequence of increased cell multiplication in the meristematic area immediately below the coleoptilar node. GA₃, abscisic acid (ABA) and ethylene also stimulated mesocotyl growth in dark-grown Nato seedlings but their effects were much smaller than those of TA. ABA, like GA₃, had an additive effect with TA, but ethylene suppressed the effect of TA and resulted in increased lateral expansion in the upper region of the mesocotyls of TA-treated seedlings.Abbreviations ABA abscisic acid - GA(s) gibberellin(s) - GA₃ gibberellic acid - TA 4-ethoxy-1-(p-tolyl)-s-triazine-2,6(1H,3H)-dione Part 5 in the series Plant Growth-regulating Activities of Isourea Derivatives and Related Compounds; Part 4=Ogawa et al. (1978)