Enhancement of Gibberellic Acid-stimulated Hypocotyl Elongation of Lettuce (Lactuca sativa L. cv. Grand Rapids) by Phlorizin and Light

The interaction of light, gibberellic acid (GA₃), and phlorizinin the growth of lettuce (Lactuca sativa L. cv. ‘GrandRapids’) hypocotyls was investigated. At all concentrationsof GA₃, phlorizin enhanced GA₃-induced growth at luminous intensitiesabove 50 ft-c (continuous light). Without GA₃, phlorizin hadno effect on hypocotyl growth in the light but it inhibitedgrowth in the dark. Both seedlings and hypocotyl sections respondedto phlorizin in the presence of GA₃. There was no iteractionbetween phlorizin and KCl. Water-growth was severly inhibitedby light. GA₃,-induced growth was slightly inhibited by light,and then only at luminous intensities above 50 ft-c. Thus, relativeto H₂O-growth, GA₃-induced growth increased with increasingluminous intensity up to 450 ft-c, where it reached saturation.It seems that a synergism may exist between light and GA₃ aswell as between phlorizin and GA₃. Lactuca sativa L, lettuce, hypocotyl elongation, gibberellic acid, phlorizin, light     

Hormonal Regulation of Hypocotyl Elongation in Lactuca sativa L.: I. EVIDENCE AGAINST THE INVOLVEMENT OF GIBBERELLINS IN THE ELONGATION OF HYPOCOTYL IN SEEDLINGS GROWN IN THE DARK

The kinetics of extension induced by GA₃¹ in the hypocotyl ofintact seedlings of Lactuca sativa are similar in the dark andin the light, and differs fundamentally from the kinetics ofelongation in the dark without GA₃. Both in continuous lightand in the dark, GA₃-induced promotion starts 24 h after incubation.In the dark, even low concentrations of GA₃, which do not affectthe length measured after 6 d when the extension of hypocotylalmost ceases, remove the lag period of 48 h which precedesextension, and prolong the high rate of elongation. FollowingGA₃ supply the hypocotyl length in the dark and in the lightdoes not differ until 48 h; thereafter the rate of elongationin the light is less, so that the final length of the hypocotylis 40 per cent shorter than that of the dark-grown seedlingswithout GA₃. IAA supplied apically to light-grown seedlings induces a weakpromotion at a concentration of 1 mg l^–1 only. With anincreasing concentration of GA₃ supplied simultaneously, theconcentration of IAA inducing a significant promotion decreases.A combined supply of both these regulators, however, does notrestore the light-mediated inhibition of hypocotyl elongationcompletely. The maximum decrease in hypocotyl length induced by the growthretardants AMO-1618, CCC, and B-9 supplied from the beginningin the dark does not exceed 70 per cent. Saturating doses ofGA₃ supplied in combination with any one of the retardants compensateonly a fraction of the decrease. The results have been interpreted to show that native GAs arenot involved in extension growth in the dark.     

Narrow-band light regulation of ethylene and gibberellin levels in hydroponically-grown <Emphasis Type="Italic">Helianthus annuus</Emphasis> hypocotyls and roots

Both hypocotyl and root growth of sunflower (Helianthus annuus) were examined in response to a range of narrow-band width light treatments. Changes in two growth-regulating hormones, ethylene and gibberellins (GAs) were followed in an attempt to better understand the interaction of light and hormonal signaling in the growth of these two important plant organs. Hydroponically-grown 6-day-old sunflower seedlings had significantly elongated hypocotyls and primary roots when grown under far-red (FR) light produced by light emitting diodes (LEDs), compared to narrow-band red (R) and blue (B) light. However, hypocotyl and primary root lengths of seedlings given FR light were still shorter than was seen for dark-grown seedlings. Light treatment in general (compared to dark) increased lateral root formation and FR light induced massive lateral root formation, relative to treatment with R or B light. Levels of ethylene evolution (roots and hypocotyls) and concentrations of endogenous GAs (hypocotyls) were assessed from both 6-day-old sunflower plants either grown in the dark, or treated with FR, R or B light. Both R and B light had similar effects on hypocotyl and root growth as well as on ethylene and on hypocotyl GA levels. Dark treatment resulted in the highest ethylene levels, whereas FR treatment significantly reduced ethylene evolution for both hypocotyls and roots. R- and B-light treatments elevated ethylene evolution relative to FR light. Endogenous GA₅₃ and GA₁₉ levels in hypocotyls were significantly higher and GA₄₄, GA₂₀ and GA₁ levels significantly lower, for dark and FR light treatments compared to R and B light-treatments. The patterns seen for changes in GA concentrations indicate FR-, R- and B-light-mediated effects [differences] in the metabolism of the early C₂₀ GAs, GA₅₃ → GA₄₄ → GA_19. Surprisingly, GA₂₀, GA₁ and GA₈ levels in hypocotyls were very much reduced by treatment of the plants with FR light, relative to B and R-light treatments, e.g. the increased hypocotyl elongation induced by FR light was correlated with reduced levels of all three of the downstream C₁₉ GAs. The best explanation, albeit speculative, is that a more rapid metabolism, i.e. GA₂₀ → GA₁ → GA₈ → GA₈ conjugates occurs under FR light. Although this study provided no evidence that elevated ethylene evolution by roots or hypocotyls of sunflower is controlling growth via endogenous GA biosynthesis, there are differences between soil-grown and hydroponically-grown sunflower seedlings with regard to trends seen for hypocotyl GA concentrations and both root and hypocotyl ethylene evolution in response to narrow band width R and FR light signaling.     

Light growth inhibition and growth inhibitors in Sakurajima radish seedlings

Growth of Sakurajima radish seedlings (Raphanus sativus var.hortensis f. gigantissimus Makino) was studied in relation tothe effects of light, the effectiveness of plant hormones, andthe variation in endogenous growth inhibitor content Externally applied GAs and IAA had no effect on the elongationof the hypocotyl in the light as well as in the dark. BA wasslightly promotive in the light, but was not so effective asto nullify the light-induced inhibition of elongation. ABA wasstrongly suppressive in the light as well as in the dark. Growth inhibitors in seedlings were extracted, analyzed by thin-layerchromatography and the Sakurajima radish hypocotyl elongationtest, and 3 acidic and 3 neutral inhibitors were detected. Amongthem all of the acidic and one of the neutral inhibitors increasedwith the time period of illumination, whereas the other 2 neutralsubstances remained almost unchanged in the light but decreasedin the dark. Thus the levels of all 6 inhibitors was higherin light-grown seedlings than in dark-grown ones. This suggeststhat light inhibition in Sakurajima radish hypocotyls may becontrolled by the variation in the inhibitor levels in the seedlings. (Received December 1, 1977; )     

Gibberellins control Arabidopsis hypocotyl growth via regulation of cellular elongation

The gibberellins (GAs) are endogenous regulators of plant growth. Experiments are described here that test the hypothesis that GA regulates hypocotyl growth by altering the extent of hypocotyl cell elongation. These experiments use GA-deficient and altered GA-response mutants of Arabidopsis thaliana (L.) Heyhn. It is shown that GA regulates elongation, in both light- and dark-grown hypocotyls, by influencing the rate and final extent of cellular elongation. However, light- and dark-grown hypocotyls exhibit markedly different GA dose-response relationships. The length of dark-grown hypocotyls is relatively unaffected by exogenous GA, whilst light-grown hypocotyl length is significantly increased by exogenous GA. Further analysis suggests that GA control of hypocotyl length is close to saturation in dark-grown hypocotyls, but not in light grown hypocotyls. The results show that a large range of possible hypocotyl lengths is achieved via dose-dependent GA-regulated alterations in the degree of elongation of individual hypocotyl cells.Key words: Arabidopsis, cell elongation, gibberellin (GA), GA mutants, hypocotyl.      

The Role of DNA Synthesis During Hypocotyl Elongation in Light and Dark

Fluorodeoxyuridine, an inhibitor of DNA synthesis, did not affectgermination and post-germinative growth in the aerial part oflettuce and Haplopappus gracilis seedlings when grown in thelight. In the dark, however, elongation of the hypocotyl wasinhibited by fluorodeoxyuridine, strikingly in lettuce and onlyslightly in Haplopappus gracilis. This could imply that thecontrolling mechanism of hypocotyl elongation is in some casesrelated to DNA synthesis, either because mitotic processes (oftenlittle taken into account in considering hypocotyl growth) areinvolved in the elongation of hypocotyls only when they aregrown in the dark, or because DNA synthesis affects cell elongationdirectly, or through the production of a greater number of endopolyploidcells in the dark. Using mainly autoradiographic and cytofluorimetricmethods, these possibilities were tested. Besides lettuce (Lactucasativa L. var. Great Lakes) and H. gracilis (Nutt.) Gray, radish(Raphanus sativus L. var. Tondo rosso quarantino) and soybean(Soya hyspida Sieb. and Zucc. var. Tokyo) seedlings were alsostudied. Fluorodeoxyuridine drastically inhibits cell elongation onlywhen it is preceded or accompanied by mitotic or endomitoticevents. Need for DNA synthesis during hypocotyl elongation,as well as during early post-germinative growth, seems to beof particular importance when endomitotic processes are involved. DNA synthesis, elongation, endoreduplication, fluorodeoxyuridine, Haplopappus gracilis (Nutt.) Gray, Lactuca sativa L., Raphanus sativus L., Soya hyspida Sieb and Zucc     

The Relationship between Levels of Endogenous Gibberellins and the Response of Vigna Hypocotyls to Exogenous Indole-3-Acetic Acid

Segments excised from the upper and the lower parts of cowpea(Vigna unguiculata L.) hypocotyls were compared in terms oftheir responses to exogenous indole-3-acetic acid (IAA) in relationto their endogenous levels of gibberellin. Growth of the segmentswas measured continuously during xylem perfusion with a lineardifferential transformer. IAA induced a burst of elongationin the upper segments but only slight promotion of growth inthe lower segments. Treatment with uniconazole, a potent inhibitorof the biosynthesis of gibberellins, reduced the responsivenessof the upper segments to exogenous IAA to about one half ofthe control value. Pre-perfusion with GA₃ of such segments fortwo hours prior to application of IAA, partially restored theresponsiveness to IAA. Analysis by GC/MS identified GA₁, GA₄,GA₉ GA₂₀ and GA₅₁ as native gibberellins in the hypocotyls ofcowpea seedlings. Analysis by GC/SIM also showed that the physiologicallyactive gibberellins (GA₁ and GA₄) were located mainly in theupper part of the hypocotyl and the treatment with uniconazolemarketly reduced the endogenous level of gibberellins thereto less than 11% of the control level. These results suggestthat levels of endogenous gibberellins possibly control theresponse to IAA in these segments. (Received May 12, 1994; Accepted November 15, 1994)     

Light effects on endogenous levels of gibberellins in photoblastic lettuce seeds

Gibberellin A₁ (GA₁), 3-epi-GA₁ GA₁₇, GA₁₉, GA₂₀, and GA₇₇ were identified by Kovats retention indices and full-scan mass spectra from gas chromatography-mass spectrometry analysis of a purified extract of mature seeds of photoblastic lettuce (Lactuca sativa L. cv. Grand Rapids). Non-13-hydroxylated GAs such as GA₄ and GA₉ were not detected even by highly sensitive radioimmunoassay. These results show that the major biosynthetic pathway of GAs in lettuce seeds is the early-13-hydroxylation pathway leading to GA₁, which is suggested to be physiologically active in lettuce seed germination. Quantification of endogenous GAs in the lettuce seeds by gas chromatography-selected ion monitoring using deuterated GAs as internal standards indicated that the endogenous level of GA₁ increased to a level about three times that of dark control 6 h after a brief red light irradiation, and that far-red light given after red light suppressed the effect of red light. The contents of GA₂₀ and GA₁₉ were not affected by the red light irradiation. Evidence is also presented that 3-epi-GA₁ is a native GA in the lettuce seeds.     

Interaction of Brassinosteroids with Light Quality and Plant Hormones in Regulating Shoot Growth of Young Sunflower and <Emphasis Type="Italic">Arabidopsis</Emphasis> Seedlings

Sunflower hypocotyls elongate as light quality changes from the normal red to far-red (R/FR) ratio of sunlight to a lower R/FR ratio. This low R/FR ratio-induced elongation significantly increases endogenous concentrations of indole-3-acetic acid (IAA) and also of three gibberellins (GAs): GA₂₀, GA₁, and GA₈. Of these, it is likely GA₁ that drives low R/FR-induced growth. Brassinosteroids are also involved in shoot growth. Here we tested three R/FR ratios: high, normal, and low. Significant hypocotyl elongation occurred with this stepwise reduction in R/FR ratio, but endogenous castasterone concentrations in the hypocotyls remained unchanged. Brassinolide was also applied to the seedlings and significantly increased hypocotyl growth, though one that was uniform across all three R/FR ratios. Applied brassinolide increased hypocotyl elongation while significantly reducing (usually) levels of IAA, GA₂₀, and GA₈, but not that of GA₁, which remained constant. Given the above, we conclude that endogenous castasterone does not mediate the hypocotyl growth that is induced by enriching FR light, relative to R light. Similarly, we conclude that the hypocotyl growth that is induced by applied brassinolide does not result from an interaction of brassinolide with changes in light quality. The ability of applied brassinolide to influence IAA, GA₂₀, and GA₈ content, yet have no significant effect on GA₁, is hard to explain. One speculative hypothesis, though, could involve the brassinolide-induced reductions that occurred for endogenous IAA, given IAA’s known ability to differentially influence the expression levels of GA20ox, GA3ox, and GA2ox, key genes in GA biosynthesis.     

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)     

Ethylene-Mediated Regulation of Gibberellin Content and Growth in Helianthus annuus L

Elongation of hypocotyls of sunflower can be promoted by gibberellins (GAs) and inhibited by ethylene. The role of these hormones in regulating elongation was investigated by measuring changes in both endogenous GAs and in the metabolism of exogenous [³H]- and [²H₂]GA₂₀ in the hypocotyis of sunflower (Helianthus annuus L. cv Delgren 131) seedlings exposed to ethylene. The major biologically active GAs identified by gas chromatography-mass spectrometry were GA₁, GA₁₉, GA₂₀, and GA₄₄. In hypocotyls of seedlings exposed to ethylene, the concentration of GA₁, known to be directly active in regulating shoot elongation in a number of species, was reduced. Ethylene treatment reduced the metabolism of [³H]GA₂₀ and less [²H₂]GA₁ was found in the hypocotyls of those seedlings exposed to the higher ethylene concentrations. However, it is not known if the effect of ethylene on GA₂₀ metabolism was direct or indirect. In seedlings treated with exogenous GA₁ or GA₃, the hypocotyls elongated faster than those of controls, but the GA treatment only partially overcame the inhibitory effect of ethylene on elongation. We conclude that GA content is a factor which may limit elongation in hypocotyls of sunflower, and that while exposure to ethylene results in reduced concentration of GA₁ this is not sufficient per se to account for the inhibition of elongation caused by ethylene.     

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     

Dimethipin (2,3-Dihydro-5,6-dimethyl-1,4-dithiin 1,1,4,4-tetraoxide) Effects on Soybean Seedling Growth and Metabolism

Three-day-old dark-grown soybean [Glycine max (L.) Merr.] seedlingswere transferred to 2 mM CaSO₄ or 10^–5 M dimethipin in2 nM CaSO₄ and root-fed via liquid culture. Plants were placedin continuous darkness or in continuous white light (200 µE.m^–2?s^–11,PAR) at 25?C. Dimethipin inhibited root and shoot elongationin dark-grown plants after 24 h and 48 h, respectively. In thelight, root elongation was inhibited also after 24 h, but hypocotylelongation was not significantly affected. Extractable phenylalanineammonia-lyase (PAL) activity per axis in dimethipin-treateddark-grown axes was not generally affected but, in the lightdimethipin caused a significant decrease in PAL activity (24to 96 h). Total soluble hydroxyphenolics in axes were not affectedby dimethipin in light- or dark-grown plants. Anthocyanin andchlorophyll levels were lowered in hypocotyls of dimethipin-treatedplants after 48 to 96 h. Soluble protein in hypocotyls of light-or dark-grown seedlings was not substantially affected by dimethipin.Nitrate reductase (NR) activity (per organ) was generally notaffected by dimethipin in light-grown cotyledons, but in theroots of these seedlings, NR activity was significantly decreased.Proteolytic enzyme activity using three substrates (leucine-p-nitroanilide,LPNA; proline-p-nitroanilide, PPNA; and benzoylarginine-p-nitroanilide,BAPA) indicated little effect on enzyme activities per organin roots and hypocotyls. These data suggest that dimethipinat low concentrations can cause significant growth inhibitionin soybean seedlings grown in either light or darkness and thatfurthermore, extractable activities of some enzymes associatedwith nitrogen metabolism and secondary metabolism are alteredby this chemical. Light also plays a role in the activity ofthis chemical. (Received November 29, 1983; Accepted January 25, 1984)     

Effects of colchicine and lumicolchicine on hypocotyl elongation,respiration rates and microtubules in gibberellic-acid-treated lettuce seedlings

Gibberellic-acid (GA₃)-induced hypocotyl elongation of intact lettuce (Lactuca sativa L.) seedlings was inhibited by colchicine (4×10^-4 M) but not by lumicolchicine (4×10^-4 M). In excised lettuce hypocotyls, GA₃ (10^-5 M) increased respiration over water controls, while both colchicine and lumicolchicine alone, or in combination with GA₃, reduced respiration. Microtubules were present in the hypocotyls of lumicolchicine-treated seedlings but absent in those treated with colchicine. It is suggested that lumicolchicine is a useful drug to discriminate between the metabolic and microtubule-mediated processes in cell morphogenesis.     

Interaction of a Brassinosteroid with IAA and GA3 in the Elongation of Cucumber Hypocotyl Sections

A synthetic brassinosteroid, 22,23(S,S)-homobrassinolide (hBR),was examined for its interaction with IAA and GA₃ in the elongationof hypocotyl sections of light-grown cucumber (Cucumis salivusL. cv. Aonagajibai) seedlings. hBR alone was less active thanIAA. Its optimal concentration was around 10 µM and thelowest effective concentration between 10 and 100 µM,which is more than 100 times higher than that of brassinolide.hBR was more active in sections from younger seedlings. Itsgrowth-promoting effect was negated or greatly reduced by inhibitorsof auxin-induced elongation such as p-chlorophenoxyisobutyricacid and kinetin. hBR acted synergistically with IAA and 2,4-Dbut not with GA₃ showing only an additive effect. Sequentialtreatment of sections with hBR and then with IAA also resultedin synergistic enhancement of auxininduced elongation, but whenthe order of treatment was reversed, hBR was inactive. The synergisticeffect was obtained with 1 h pretreatment with hBR and couldbe reduced by subsequent washing with water. There was no sequentialinteraction between hBR and GA₃. The synergistic pretreatmenteffects of hBR and GA₃ were simply additive to each other. Amembrane-bound ATPase inhibitor, dicyclohexylcarbodiimide, inhibitedthe hBR-induced elongation, but did not affect GA₃-induced elongation.The findings led to the conclusion that brassinosteroids enhanceauxin action and possess growth-promoting activity which isindependent of that of gibberellin. (Received November 9, 1984; Accepted February 18, 1985)     

The role of gibberellins in the growth of floral organs of Pharbitis nil

Evidence that the synthesis of GA₃ is involved in the growthof floral orga'ns of Pharbitis nil is presented. GAs in floralorgans at different developmental stages were surveyed usingTLC followed by the bioassay with two dwarf rice seedlings,‘Tanginbozu’ and ‘Waito-C’. The amountof GAs in the petal and stamen increased rapidly after the petalemerged from calyx, reached a maximum 12 hr before anthesis,then declined markedly thereafter. The GA content in the calyxremained unchanged before and after anthesis, and that in thepistil increased after anthesis. Pharbitis flowers containedat least two active GAs, one of which was probably GA₃, theother appeared to be GA₁₉. GA₃ was detected in relatively largeamounts in both the petal and stamen during their rapid elongation.In the calyx, which showed little increase in fresh weight duringrapid flower growth, GA₉ was the dominant GA. Exogenously suppliedGA₃ promoted elongation of sections in excised young filaments.Sucrose was necessary for definite growth promotion by GA₃.GA₁₉ had little effect on filament elongation, and IAA was ratherinhibitive. (Received July 29, 1972; )     

Effects of Gibberellins on Seed Germination of Phytochrome-Deficient Mutants of Arabidopsis thaliana

Experiments were carried out to explore the involvement of gibberellins(GAs) in the light-induced germination of Arabidopsis thaliana(L.) Heynh, using wild type (WT) and phytochrome-deficient mutants(phyA, phyB and phyAphyB deficient in phytochrome A, B and Aplus B, respectively). Seed germination of WT and phytochrome-deficientmutants was inhibited by uniconazole (an inhibitor of an earlystep in biosynthesis of GA, the oxidation of ent-kaurene) andprohexadione (an inhibitor of late steps, namely, 2rß-and 3rß-hydroxylation). This inhibition was overcomeby simultaneous application of 10^-5 M GA₄. The relative activityof GAs for promoting germination of uniconazole-treated seedswas GA₄>GA₁=GA₉>GA₂₀. The wild type and the phyA and phyBmutants had an increased response to a red light pulse in thepresence of GA₁, GA₄, GA₉, GA₂₀ and GA₂₄ but there were no significantdifferences in activity of each GA between the mutants. Therefore,neither phytochrome A nor hytochrome B appears to regulate GAbiosynthesis from GA₁₂ to GA₄ during seed germination, sincethe conversion of GA₁₂ to GA₉ is regulated by one enzyme (GA20-oxidase). However, GA responsiveness appears to be regulatedby phytochromes other than phytochromes A and B, since the phyAphyBdouble mutant retains the photoreversible increased responseto GAs after a red light pulse. (Received February 13, 1995; Accepted July 11, 1995)     

Light-Regulated Hypocotyl Elongation Involves Proteasome-Dependent Degradation of the Microtubule Regulatory Protein WDL3 in Arabidopsis

Light significantly inhibits hypocotyl cell elongation, and dark-grown seedlings exhibit elongated, etiolated hypocotyls. Microtubule regulatory proteins function as positive or negative regulators that mediate hypocotyl cell elongation by altering microtubule organization. However, it remains unclear how plants coordinate these regulators to promote hypocotyl growth in darkness and inhibit growth in the light. Here, we demonstrate that WAVE-DAMPENED 2–LIKE3 (WDL3), a microtubule regulatory protein of the WVD2/WDL family from Arabidopsis thaliana, functions in hypocotyl cell elongation and is regulated by a ubiquitin-26S proteasome–dependent pathway in response to light. WDL3 RNA interference Arabidopsis seedlings grown in the light had much longer hypocotyls than controls. Moreover, WDL3 overexpression resulted in overall shortening of hypocotyl cells and stabilization of cortical microtubules in the light. Cortical microtubule reorganization occurred slowly in cells from WDL3 RNA interference transgenic lines but was accelerated in cells from WDL3-overexpressing seedlings subjected to light treatment. More importantly, WDL3 protein was abundant in the light but was degraded through the 26S proteasome pathway in the dark. Overexpression of WDL3 inhibited etiolated hypocotyl growth in regulatory particle non-ATPase subunit-1a mutant (rpn1a-4) plants but not in wild-type seedlings. Therefore, a ubiquitin-26S proteasome–dependent mechanism regulates the levels of WDL3 in response to light to modulate hypocotyl cell elongation.     

Seasonal Changes of GA1, GA19 and Abscisic Acid in Three Rice Cultivars

The levels of endogenous gibberellin A₁ and A₁₉ (GA₁ and GA₁₉)and abscisic acid (ABA) in three rice (Oryza sativa L.) cultivars,Nihonbare (normal), Tan-ginbozu (semid-warf) and Tong-il (semi-dwarf),were measured at various stages of internode elongation andear development. GA₁₉ was the main GA in Nihonbare and Tong-ilthroughout the life cycle but was not detected in Tan-ginbozu.The levels of GA₁ in the ears of all three cultivars were lowand reached their maxima after anthesis. Similarly, the earsof all three cultivars contained high ABA levels which peakedafter anthesis. Shoots contained low quantities of ABA throughoutthe life cycle. The roles of GAs and ABA are discussed withrespect to physiological phenomena, such as internode elongation,ear development and dwarfism. (Received May 9, 1981; Accepted July 18, 1981)