Gibberellin-Auxin Interaction in Pea Stem Elongation' · Gibberellin-Auxin Interaction in Pea Stem Elongation' ... This has led to the conclusion ... OCKERSE AND GALSTON-GIBBERELLIN-AUXIN

Plant Physiol. (1967) 42, 47-54

Gibberellin-Auxin Interaction in Pea Stem Elongation'Ralph Ockerse2 and Arthur W. Galston

Department of Biology, Yale University, New Haven, Connecticut 06520

Summlary. Joint application of gibberellic acid and indole-3-acetic acid to excisedstem sections, terminal cuttings, and decapitated plants of a green dwarf pea resultsin a markedly synergistic growth response to these hormones. Synergism in greentall pea stem sections is comparatively small, although growth is kinetically indis-tinguishable from similarly treated dwarf sections.

Gibberellin-induced growth does not appear to be mediated throtugh its effect onauxin synthesis, since gibberellin pretreatment of dwarf cuttings fails to elicit anenhanced tryptophan-induced growth response of sections, whereas auxin-inducedgrowth is strongly enhanced. Also, tryptophan-gibberellin synergism is not signifi-cant in sections and cuttings of green dwarf peas, while auxin-gibberellin synergism is.

Administration of gibberellic acid prior to indole-3-acetic acid results in greatlyincreased growth. In reversed order, the application fails to produce any synergisticinteraction. This indicates that gibberellin action must precede aulxin action ingrowth regulation.

Both gibberellins and auxins contribute to growthin length of excised etiolated Alaska pea epicotylsections (4, 7, 16) and green pea stem sections (1,3)although gibberellin is less effective in green sec-tions. \Vhen gibberellin and auxin are appliedtogether to etiolated Alaska sections, growth iseither equal to or less than the sum of the separateresponses (17). This has led to the conclusionthat gibberellin and auxin make independent con-tributions to stem growth throtugh different bio-chemical pathways. In contrast to these results, asmall degree of atuxin-gibberellin synergism hasbeen demonstrated in sections from Progress No. 9pea seedlings grown in weak red light (21).

Joint application of the 2 hormones to greenpea tissuies, such as Alaska stem sections (3) andMeteor stem sections (1) results in a marked syner-gistic interaction between auxin and gibberellin,being more pronounced in the latter (dwarf) cul-tivar. These results, together with those derivedfrom the petiole of single-node cuttings of sweetpotato (11) indicate that gibberellin-induced growthis dependent on auxin. It thus appears that auxin-gibberellin synergism is a phenomenon character-istic of sections from dwarf peas, and is more

1 This investigation w-as supported in part by a UnitedStates Public Health Service Fellowship (GM-24,349)to R.O. from the National Institute of General MedicalSciences and in part by a Yale University Fellowship.This material is abstracted from a thesis presented byR.O. to the Graduate School of Yale University in par-tial fulfillment of the requiremen-ts for the Ph.D. degree.

- Present address: Department of Biology, Hope Col-lege, Holland, 'Michigan 49423.

pronounced in green sections. Three types ofhypotheses have been advanced to explain the syn-ergistic interaction between GA3 and IAA: A)gibberellin may act by promoting the synthesis ofauxin (10), B) gibberellin may protect auxin frominactivation in the plant (1,15), and C) gibberellinmay cause the formation of some substance, such asa nucleic acid, which interacts with auxin in promo-tion of growth (12,23).

The work reported in this paper is part of aiiinvestigation which dealt with a reexamination ofauxin-gibberellin interactions in a tall and dwarfpea cultivar, and a reevaluation of the results inthe light of recent knowledge. The results pre-sented here -serve- -to- define the system- and wehope to present further data in a subsequent paper(Ockerse and Galston, in preparation).

Materials and Methods

Light-Grown Peas. A tall cultivar (Alaska)and a dwarf cultivar (Progress No. 9) of pea,Pistin, sativumt L., purchased from Asgrow Inc.,Orange, Connecticut, were used in this investiga-tion. Seeds were soaked in tap water for 2 to 3hours and sown in coarse Zonolite vermiculite inperforated 10 X 10 X 8 cm plastic boxes. Theplants were grown at 230 under illumination ofabout 1500 ft-c supplied by a combination of 90 %"Daylight" and "White" fluorescent and 10 % in-candescent light energy. The seedlings were auto-matically subirrigated twice daily with a nutrientsolution consisting of 120 g Hyponex (HydroponicsChemicals Company, Copley, Ohio) per 100 literstap water.

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PLANT PHYSIOLOGY

In experiments with intact planits, a 1.0 M,J dropof IAA purchased from Regis Chemical Company,Chicago, Illinois, or GA, obtained from Plant Pro-tection Ltd., Yalding, Kent, in 0.05 % Tween 20was placed on the first leaf of 7 day old plants andthe internodes were meastured 10 days followingapplication. Suibapical 5 mm sections were excisedwith a douible-bladed cutting tool from 12 to 15day old Alaska peas whose apex was barely show-ing above the folded stipulies at node 7, and of 14to 16 day old dwarf peas whose apex had just notprotruded from between the folded stipules at theseventh node. The sections were placed directlyin 5 cm petri dishes, 10 sections to a dish, con-taining 5 ml of medium, consisting of 25 mmpotassiulm phosphate (pH 6.1) bulffer andl 1 %\V/V sucrose with or withouit the test compouinds.The sections were inculbated for about 20 or 44houirs on a reciprocating shaker (40 cycles/min)in continuous light and their length measured tothe nearest 0.1 mm uinder a dissecting m;croscopeequipped with an ocuilar micrometer.

Terminal cuittings of dwarf peas were obtainedfrom 13 to 15 day old plants with sixth internodes7 to 10 mm long and severed juist above the fifthnode. The fifth internodes of 20 cuittings wereinserted through a perforated alulminulm foil coverplaced over a 14.5 cm petri (lish half. These werefilled with 250 ml soluitions of the test compouinds.Everv 12 hours the contents was restored to theoriginal voltume with distilled water. The initialand final lengths of the sixth internodes weremeasured with a metric scale ruiler.

In work with decapitated plants, the apex wassevered from the 2 to 3 mm long seventh internodesof dwarf plants whose sixth internodes were 9 to11 mm long. A plastic capsuile wsith a capacity of0.7 ml (obtained from LKB Instruments Inc.,Washington, D.C.) was perforatedl at its base, filledwith a solution of the test compouind and inserte(dsnugly over the seventh internodle stuimp. Thesixth internode length was measuredl after 20 or 44houirs w ith a metric scale ruiler.

Etiolated Peas. Seeds were soaked in tap waterin darkness for 2 to 3 hoturs andl sown in plastictrays containing prewashed, water-saturated ver-micuilite (Mica-Gro Type B suipplied by CaliforniaProdulcts Company, Cambridge, Massachulsetts).The trays were placed in a darkened cabinet in a(lark room maintained at 27° +- 1°. Three daysprior to harvest of the epicotyls the seedlings werewatered with 1 liter of tap water. A dim greenlight furnished by a 1 5-watt Sylvaniia green fluo-rescent tube, wrapped in 3 layers each of amberand green DuPont cellophane, was used sparinglyin the handling of the seedlings. Subapical 5 mmS, sections (16) were excised from the third inter-node of 7-day-old Alaska peas with an internodelength of 25 to 40 mm and reclirv7ed terminal budsor Progress No. 9 see(lliilgs wrhich (lo nlot show suich

recturvature. The sections were grown and meas-tired as described for green sections, except thatthey- were placed in the dark anid the growthme(lium had 2 % WNT/V suicrose instead.

Etiolated cuittiings were obtained from 7-day-olddxwarf peas xx-ith a third internode of 20 to 30 mm,severed below the second node. The cuittinigs -xereplaced in 50 ml beakers (20 culttings to a beaker)containing 25 ml of solution of the test compouinds.The initial aind final lengths of the third iinternodewere measuired with a metric scale ruiler.

All statistics involved the t-test of paired repli-cates (20) and each replicate N-as the average of10 sectioins, cuittings, or decapitated seedlings.\Vhere appropriate, the standard error is in(licatedin the figuires. All experiments wvere performedat least 3 times. L-Tryptophan was obtaiined fromSigma Chemical Company.

Results

Rcspontses of intact Peai Steiii to Gibbcrellinaniid Aux.int. Green Progress No. 9 seedlings treatedwith high levels of GA., are virtually idlentical inappearance with uintreated green Alaska peas. Thiscan be illuistrated for the sixth internode whengrowth in length is plotted as a functioin of GA3(losage (fig 1). In contrast to greein Progress No.9 seedlings, GA3 had a relatively small effect ongrowth promotion of the internode of green Alaskapeas. Treatment of both cuiltivars with IAA hadno effect oIn the growth of the interinodes. Theseresuilts suiggest that the amouint of endogenouoll gib-berellin is limiting the growth of Progress No. 9peas.

Effects of Au.xin, Gibberellini, and Tryptopha aon Growth of Stem1i Section1s. The dose-responsecuirv-es of green Progress No. 9 anid Alaska peasections to IAA, GA3, or L-tryptophan are sutm-marized in figuire 2. In confirmation of previouiswork (3), Alaska section length increases withincreasing IAA concentrations in excess of 0.1 yiwith an optimum near 0.1 mat. By contrast,Progress No. 9 sections react less to IAA, and asatuirating lev el is attained betweeni 0.01 andI 0.1m.i. \Vith G.A3, sections of both culti-ars showa small anlcl essentially similar growth response.With L-tryptophan, dwarf section length increaseswith concentrations in excess of 1 j,M and the (lose-response cturxve is comparable to that of ITAA-treatedAlaska sectioins. This may suiggest that dwarfsections possess an enzyme system which convertsL-tryptophan to an aulxin (5, 10).

Gibberellin-Auxin Synergismii in Grcen .Sectionzs.It has been shown that gibberellin and aulxinl canpromote growth synergistically in green pea sections(1,3). WVhen supplied to Progress No. 9 sectionsat satulrating concentrations, GA., and IA.A showa high degree of synergistic interaction (table Iandl fig 3). At 1 umM IAA, IAA-induicedl growthis inhibite(d aindI canl be partially reversed 1l, GA3.

48

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OCKERSE AND GALSTON-GIBBERELLIN-AUXIN INTERACTION IN PEA STEM ELONGATION

- Vv

0.175 0.35 1.75 3.5 17.5 35 175 350

MPG PER PLANT

3500

z0

(-I

6 5 4

-LOG MOLAR CONCENTRATION

1 2

z

7

TIME INHOURS 3

FIG. 1. Growth of the sixth internode of green

Progress No. 9 (A, A) and Alaska (0, *) peas inresponse to various levels of GA3 or IAA.

FIG. 2. Dose-response of green Progress No. 9 (A)and Alaska (Q) stem seftj,A%AA, GA1, and tryp-

TIME INHOURS

4tophan (TTP). Sections allowed to grow for 20 hours

FIG. 3. Kinetics of elongation of green ProgressNo. 9 pea stem sections. Treatments are as indicated.

FIG. 4. Kinetics of elongation of green Alaska pea

stenm sections. Treatmeints are as indicated.

80

60 -

49

CM00z

Zso

z

.x

o 40

I-

O 30

GA3

.... IAA ~~~~~~~TALLCULTIVAR

DWARF CULTIVAR

20 F

IAA

--A I Io

10

° 6

-

1. 5



Table I. IAA-GA3 Synergism in Green Progress No. 9 Pea Stem Sections, Cuttings, and Decapitated Plants

Growth in excess of control(mm)

ConitrolA L (mm)

2.93

2.42

4.23

3.81

2.81

2.47

GA3IAA (0.1mM)

-0.70

1.38

-1.38

0.83

0.81

2.262.53

3.304.00

0.503.82

2.673.93

4.702.41

-0.030.29

4.59-0.10-0.97

0.78

0.86

1.01

0.73

2.03

IAA + 0.1 mM GA3Calc

0.0B

2.24

-0.37

1.56

2.84

2.37 4.63

2.41 5.71

4.31 4.81

5.51 8.18

5.01 9.71

2.63 2.602.92

7.70 7.606.73

Obs Obs-Galc

0.77 0.69*

3.77 1.53* *

0.50 0.87

5.22 3.66***

6.42 3.58***

5.10 0.47

5.84 0.13

9.16 4.35*|*

8.96 0.78

9.35 -0.36

3.70 1.10**3.65 0.73*

9.31 1.71***8.53 1.80***

* Difference significant at the 5 % level.** Difference signiflicant at the 0.5 % level.*** Difference significant at lower than the 0.1 % level.

This was obtained in all experiments at concentra-tions from 0.1 to 1 ,UM IAA, and is consistent witha similar observation reported in wheat coleoptilesections (22). This inhibition can be reversed toa growth promotion by a 4 houir pretreatment ofthe sections in basal medium alone (14). At stipra-optimal IAA levels, the degree of synergism isonly slightly smaller. Fuirthermore, GA, concen-

trations between 10 m1umI andl 1 mm are eqIuallyeffective in the presence of IAA (14).

Synergism, expresse(d as a percent of totalgrowth, can alrea(ly be seen after a 2 houir incuiba-tion an(l increases progressively with time (14).After 48 houirs almost one qtuarter of the sectiolnlength can be attrilbUted to synergism. By conitrast,synergism in Alaska pea sections is very smnall andcan only be demonstrate(d wheni TAA levels are

suboptimal (fig 4).It has been reported that GA. and tryptophan

interact synergistically in oat coleoptile sectiongrowth (19), and that this synergism is dtue tothe effect of GA, on the enzymatic conversion oftryptophan to atlxin (10). In Progress No. 9 pea

sections, however, synergism (htie to GA, andtryptophan (table II) is not statistically significant.

Kinetics of Growth of Green Alaska and Prog-ress No. 9 Sections. The time course of sectiongrowth for Progress No. 9 and Alaska peas invarious media is shown in figures 3 and 4 respec-tively. Growth of control sections is essentiallysimilar for both cultivars. After a lag-phase ofabout 9 hours, growth proceeds at a constant rateand diminishes at about 25 hours from the start.With GA3, a small growth stimulation is produiced.The major difference in growth between Alaskaancl Progress No. 9 sections is seen in their re-

sponse to IAA which is considerably smaller forthe dwarfs. This is (Ilie to both a smaller growthlrate anid a prematuire cessation of growth. Theinitial growth rate in (lwarf and Alaska sections ismaintained for 11 an(l 17 hoturs respectively. WheniGA.. and IAA are stupplied together, Alaska sectionsshow a small promotion of IAA-induced growth.With dwarf sections, GA, markedly enhances IAA-induced growth by increasing the initial growthrate and delaying the onset of decline in growth.Such treated dwarf sections now follow exactlythe same kinetics as Alaska sections treated withboth hormones. Tryptophan-indtuced growth ofProgress No. 9 sections is only slightly enhanced

Material

Sections

Cuttings

Decapitatedplants

Time(hr)

21¼2

20¼2

45¼

47V/2

21

21

20

45

44

45

22

43

IAA(MM)

0

50

5

50

100

5

50

100

10

10

50 PLANT PHYSIOLOGY



OCKERSE AND GALSTON-GIBBEREL.LIN-AUXIN INTERACTION IN PEA STEM ELONGATION

Table II. Effect of Concentration of Tryptophan (TTP)Sections, Cuttings, a

51on Tryptophan-GA3 Synergism in Green Progress No. 9nd Decapitated Plants

Growth in. excess of control(mm)

Time(hr)

21

21

TTP(NI)

10-610-5

10-410-3

45

10-543

10-4103

2210510-410-3

4410-510-4lor3

2010-4

4410-4

ControlA L (mm)

3.300.101.40

2.501.503.20

4.300.251.65

3.351.653.75

2.50-0.17-0.01-0.20

3.110.310.430.49

1.850.53

4.59

GA3TTP (O.lmM)

TTP + 0.1 mM GA3

Calc

0.85 0.952.25

Obs

1.152.00

0.80 2.30 2.704.00 3.75

0.45 0.70 0.952.10 2.10

0.55 2.20 2.654.30 4.25

1.87 1.70 2.041.86 1.901.67 2.00

4.86 5.17 4.895.29 4.635.35 4.65

1.34* 1.87 2.72

0.25 4.78* 5.03 5.71

* GA3 = 0.1 sM.** Difference significant at the 5 % level.

by GA3 (fig 3). With tryptophan alone, growthdoes not begin until after about 9 hours and GA,appears to shorten this lag-period by 2 to 3 hours.

Effects of Pre- and Post-treatment of DwarfSections with Gibberellic Acid. During the initialhours of growth of green dwarf sections, the ratesof IAA-induced growth with and without GA3 are

about 0.33 mm/hour and 0.27 mm/hour respectivelywhen the hormones are supplied from the start.Preincubation in basal medium with or withoutGA3 markedly enhances subsequent IAA-inducedgrowth (fig 5). This growth stimulation withoutGA3 is mainly due to a higher growth rate (0.40mm/hr) following the addition of auxin. WithGA., the growth rate is not only greater (0.54mm/hr) but the duration of elongation is consider-ably prolonged, and the sections still grow after50 hours. With lengthier pretreatment times, thegrowth rates are even higher. Thuls, after a 21houir pretreatment with or withouit GA3, growthrates of 0.76 mm/hour and 0.45 mm/hour respec-

tively can be attained. Preincubation, especiallyin the presence of GA3, brings about some changewhich enhances the response of dwarf sections toauxin.

By contrast, pretreatment of sections with IAAand transfer to basal medium with or without GA..results in a reduction in growth rate (fig 6). Re-introduction of IAA again leads to growth enhance-

ment indicating that the tissue still has the potentialfor growth. Also, with increasing pretreatmenttimes to 20 hours in IAA, the degree of GA.-IAA

interaction is markedly reduced if GA3 is directlyadded to the medium (14). These results stronglysuggest that GA3 action must precede IAA action

in growth regulation. The hypothesis advanced

by Kefford and Goldacre (8) that auxin preparesthe tissue for subsequent gibberellin action needs to

be modified.Growth Response of Dwarf Cuttings to Tryp-

tophan, Indole-3-Acetic Acid, or Gibberellic Acid.Cuttings were also used as test objects since theyresemble the intact plant more closely (14). Con-

centrations of IAA in excess of 1 pM are growthpromotive, showing that in this system, IAA can

move from base to apex. At the optimum of 0.1

mM IAA the internodes were swollen in appearance.\Vith GA3, a 0.1 gM concentration was saturatingand higher levels were equally effective. Tryp-tophan was without any effect even at a I m-,

concentration.The kineti,s of growth of the sixth internode

are the same 'or untreated cuttings and intactplants (fig 7). With GA3, elongation is imme-diately enhanced and is linear with time duringthe 45 houir period. \Vith auxin, growth startsat a high rate but falls gradually, until after 45hours no further growth is detected.

Material

Sections

Cuttings

Decapitatedplants

Obs-Calc

0.20-0.25

0.40-0.25

0.250.00

0.45-0.05

0.340.040.33

-0.28-0.66-0.70

0.85**

0.68



PLANT PHYSIOLOGY

Gibberellin-A*x-in Synergism in Green Cutttings.Joint application of GA3 and IAA to dwarf cuittingsresults in a synergistic growth response to thesehormones only when the concentration of I A(5 j,Ma) is suboptimal (table I). At an optimalconcentration of IAA (0.1 mM) the IAA-GA3 in-teraction becomes merely additive (no synergism).As with excised stem sections, GA3 and tryptophando not interact synergistically (table II).

11 .{0~~~~~~~~~~~~~~A3-[AA

10

ro 7 ' ,+ Ge-~~~~~~~~~~~5X0~JIAA8 _-o

b 0~~~~~~~~~~~~~~1GA4G3

CONTROL - KA

0 5 10 15 20 25 30 35 40 45 50 55 60

TIMlE IN JRS 5

FIG. 5. Effect of a 5 hlour p)reltreatment of greenqProgress No. 9 pea sltem sections inl basal medlium witho)r withoult GA3 on IAA -induced growth.

Effect of Gib4erl lin-Pretretment On GrowthIndlucedl by Aufxin (mnd Tryptopill(mn. Pretreatmentof culttings with GA:i markedly enhanlces aulxin-in-(lced growth of sub2sequently excised sections butrFedces tryptophan-inEft ace(l growth as comparedwith similarly treate(m sections from culttings pre-treate(l withGlistilled water (fig 8). IfAAcl actsy promoting the synthesis of auxicn (o1) from

tryptophanl, anl increase(l sulpply of tryptophanshouldduead to growthenhancemdelit. Pretreatmentof ctuttings with tryptophan or JAA dloes not alterthe auxin-inddced growth of subsequently excisedsections,ceut markedly inhibits tryptophan-inducedgrowth. Here also, the expected GA3-IAA inter-actions in GA3-treated sections from attxin-pre-treated cwttings is- not observed.

Synergism in Green Decapitated Dwvarf Peas.The dose-response curves of the sixth internodeof decapitated dwarf peas anor dwarf cuttings tosAct iAA, and to tryptophan are essentipaly sim-

0 5 10 15 20 25 30 35 40 45 50TIME INHOYJS

FIG. 6. Effect of a 4 hour pretreatment of greenProgress No. 9 pea stem sections in IAA followingtran-sfer to basal medium with or without GA3. IAAreintroduced 4 hours following transfer.

ilar (14). As in cuttings, low concentrations ofIAA and GA3 interact synergistically, albeit to aslightly smaller extent (table I). In contrast withcuittings, a small degree of synergism between GA,and tryptophan can be obtained in decapitated peas.

Synergism in Etiolated Peas. In confirmationof previouis work (7, 17), no GA3-IAA synergismcould be demonstrated in etiolated Alaska sections(14). The results for etiolated dwarf sections arepresented in table III. Synergism is significantwhen the IAA and GA3 concentrations are 0.1 ,UMand 0.1 mm respectively. Increasing the GA3 con-

14

13

12

1

10

a 9z

w_ 8

- 72

3

0 5 10 15 20 25 30 35

TIME IN HOURS

40 45 50

FIG. 7. Kinetics of growth of the sixth internode ofgreen Progress No. 9 cuttings, treated with 0.1 mM

GA3, 0.1 mm IAA, or distilled wvater, and untreatedintact plants. Initial internode length was about 9.0 mm.

7

52



OCKERSE AND GALSTON--GIBBERELLIN-AUXIIN INTERACTION IN PEA STEM ELONGATION 53

Table III. Effect of IAA Concentration on IAA-GA3 Synergism in Etiolated Progress No. 9 Pea Cutttings andStem Sections

Growtlh in excess of control(nmni)

IAA Conitrol(Al) AL (mlm) IAA

2.6610-810-7

10-'

10-7

5x10-5X10-75 X 10-6

5X10-85X10-75X 10-6

2.90

9.8

13.2

-0.090.440.800.70

GA.

(0.1lM) Calc

1.54 1.451.982.342.24

IAA + GA.,(0.1 11mM)Obs Obs-Calc

1.602.402.311.94

0.86 0.98* 1.84 2.99

0.2 10.4 10.6 13.90.9 11.3 8.90.2 10.6 7.6

0.10.5

-0.4

17.1 i7.117.616.7

21.719.717.2

0.150.42:

-0( (03-0.30

1.154':*

3 3 -:< :!:2.4

-3.0

4.5**2.1

-0.5

GAi3 = 1 mm.hDifference significant at the 5 % level.Difference significant at the 0.1 % level.

GA3 IAA TTP GA3 IAA TTP GA3 IAA TTP GA3 IAA TTP

H20 - PRETREATED IAA - PRETREATED GA3 - PRETREATED TTP - PRETREATED

FIG. 8. Effect of a 15 lhour pretreatment of green

Progress No. 9 cuttings in distilled '\-ater, 50 IA-m IAA,0.1 m-m GA3, or 0.1 mM tryptophan (TTP) on eloniga-tion of subsequently excised sectionis, inicuibated with0.1 mm GA3, 0.1 m-r TTP, or 50 IA:r IAA. Sectionsallow ed to grow for 20 hours.

cenitrationi to 1 mAi considerably enhlaniced synergism,while increasing IAA concentrationis decreased thesynergistic effect.

Gibberellin-auxin synergism is also significantWhen etiolated dwarf cuttings are employed (tableIII). Low IAA concentrations have little effectoni growth of the third internode whereas, at 50,A.i growth is inhibited. With GA3, a synergisticeffect is evident when IAA is present at 0.05 A\.No synergism cotuld be demonstrated in etiolate1-dAlaska pea cuttings (14).

Discussion

In the past, convincing stildies by KIuse ( I 1)ain(I Brian aii( Hemming (1) have (lenionistratedthat responise to gibberellin depend(s on the avail-ability of auxin. The converse, that auxini actioniis also dependent on gibberellin is probable butmore difficuilt to demonstrate. If the assuimptioniis correct that Progress No. 9 and Alaska peas(liffer in their endogenouis gibherellin contenlt, a

dwarf-tall relationship wouild be expected to bemirrored in the response of sections to aulxin. Theresuilts indicate that IAA-induice(d growth is indee(dless in Progress No. 9 sections, becauise of a lowverrate of growth maintained over a shorter timeperiod. With GA3 alone, growth of sections inboth cultivars is promoted buit little, whereas in

combination with IAA, elongationi of Progress No.8 9 sections is synergistically enhailce(d anld kinetically

equial to that of Alaska sections. This weakens thcthesis that GA3 and I -kA make ilndepenidenit coln-trilbutions to growth (17).

Sinlce aulxinl is kniownl to be forme(d in the apex

ani(l youlng foliage an(d tranisportedl to the elonigatilngregion, anid exogenolusly suipplie(l atlxini is requiiredfor sectioni growth, it is possible that in the initactplanit gibberellini acts by enihanlcinig the synlthesisof aulxiin in the apical regioni. This was first suig-geste(l by Kuraishi and MuIiir (9) ho reportedthat GA3inot only inicreased the amouniit of diffllSil)leatlxin from the apex buit that this increase precededgrowth. Suibse(quienit work showe(d that GAX enll-hanced the efficienlcy of conversion of trvptophaninto auixin by enzyme preparations from GA3-treated tissues (10). While wve do not wish toquestion the results of Kuraishi and Muir, our

Material

Sections

Cuttinigs

Ti.e(lir)

19.5

20

.44

*

**

53

uia 3z0cc

ia'A

a;. 2

9U.0'AfnULUz-1-d I



PLANT PHYSIOLOGY

evidence does not support the view that gibberellincontrols growth through its action on auxin syn-thesis for the following reasons. Firstly, sectionswvhich showed a growth response to tryptophan,presumably by conversion of tryptophan to auxin,shoved neither significant GA3-tryptophan syn-ergism, nor enhancement of tryptophan-induicedgrowth in sections from GA3-pretreated cuttings.Seconidly, no tryptophan-GA3 synergism could be(lemonostrated in cuttings btut a small degree ofsyniergism was obtained in decapitated plants, lack-ing their main centers of auxin synthesis. Thirdly,aI marked IAA-GA3 synergism could be demon-strated in all these cases. If GA3 acts solely viaauxini synthesis, then at best an additive responsewould be expected from combinations of IAA andGA3. Finally, GA3-induced growth of dwarf cut-tings is almost immediate and does not appear togo through a lag period. Thuis, in our view itwouild be difficult to determine whether the increasein auxin production precedes, and therefore causesgrowth.

In dwarf cuttings, the continued decline in theaulxini-induced growth rate from the time of ap-plication could be due to the premature onset ofthe matuiration phase. This is not attributed toa depletion of auxin since repeated transfer tofreslhly prepared solutions does not alter the growthresponse (14). Aln explanation may be presentedonl the basis of auixin-peroxidase interaction. Theadditioni of high levels of auxin may induce thesynithesis of more peroxidase which then leadsto its own destrttction (2,18). By contrast, GA3-inidtuced growth in cuttings is linear with time.In dwarf sections, which presumably have a higherperoxidase content than Alaska sections (6, 13),GA3 treatment delays the onset of the maturationphase of aulxin-induced growth, which now approxi-mates auxin-indtuced growth in Alaska sections.This may explain why in cuttings and decapitatedseedlings GA3 and IAA interact synergistically onlyif the IAA level is suboptimal. This is supportedfurther by the observation that in auxin-treateddwarf sections the degree of synergism decreasesas the time of GA3-post-treatment increases. Wehope to present further data on this question.

Literature Cited

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2. GALSTON, A. W. AND L. Y. DALBERG. 1954. Theadaptive formation and physiological significanceof indoleacetic acid oxidase. Am. J. Botany 41:373-0.

3. GALSTON, A. W. AND R. KAue 1961. GCnpara-tive studies on the growth and light sensitivity ofgreen and etiolated pea stem sections. In: Lightand Life. W. D. McElroy and B. Glass, eds. JohnsHopkins Press, Baltimore.

4. GALSTON, A. W. AND H. WARBURG. 1959. Ananalysis of auxin-gibberellin interaction in peastem tissue. Plant Physiol. 34: 16-22.

5. GORDON, S. A. 1961. The biogenesis of auxin. InHandbuch der Pflanzenphysiologie. W. Ruhland,ed. Springer-Verlag, Berlin. Vol. XIV.

6. KAMERBEEK, G. A. 1956. Peroxidase content ofdwarf types and giant types of plants. Acta Botan.Neeri. 5: 257-63.

7. KATO, J. 1958. Studies on the physiological effectof gibberellin. II. On the interaction of gib-berellin with auxins and growth inhibitors. Physiol.Plantarum 11: 10-15.

8. KEFFORD, N. P. AND P. L. GOLDACRE. 1961. Thechanging concept of auxin. Am. J. Botany 48:643-50.

9. KURAISHI, S. AND R. M. MUIR. 1964. The rela-tionship of gibberellin and auxin in plant growth.Pilant Cell Physiol. 5: 61-69.

10. KURAISHI, S. AND R. M. MUIR. 1964. The mech-anism of gibberellic action in the dwarf pea. Plant'Cell Physiol. 5: 259-71.

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12. MASUDA, Y. AND N. YANAGISHIMA. 1965. Furtherstudies on the RNA functional in auxin action.Plant Cell Physiol. 6: 17-23.

13. MCCUNE, D. C. AND A. W. GALSTON. 1959. In-verse effects of gibberellin on peroxidase activityand growth in dwarf strains of peas and corn.Plant Physiol. 34: 416-18.

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15. PILET, P. E. 1957. Action des gibberellines surl'activite auxin-oxydasique de tissus cultive in vitro.Compt. Rend. Acad. Sci. (Panis) 245: 1327-28.

16. PURVES, W. K. AND W. S. HILLMAN. 1958. Re-sponse of pea stem sections to indoleacetic acid,gibberel,lic acid, and sucrose as affected by lengtland distance from apex. Physiol. Plantarum 11:29-35.

17. PURVES, W. K. AND W. S. HILLMAN. 1959. Ex-perimental separation of gibberellin and auxinactions in etiolated pea epicotyl sections. Physiol.Plantarum 12: 786-98.

18. RICARD, J. AND J. NARI. 1963. Adaptation et moded'action des auxineoxydases. Compt. Rend. 155:846-49.

19. SASTRY, K. K. S. AND R. M. MUIR. 1965. Effectsof gibberellic acid on the utilization of auxin pre-cursors by apical segments of the Avena coleop-tile. Plant Physiol. 40: 294-98.

20. SNEDEcOR, G. W. 1956. Statistical methods. 5tledition. Iowa State University Press, Ames, Iowa.

21. STOWE, B. B. 1959. Similar activating effeots oflipids on cytochromes and on plant hormones.Biochem. Biophys. Res. Commun. 1: 86-90.

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Gibberellin-Auxin Interaction in Pea Stem Elongation' · Gibberellin-Auxin Interaction in Pea Stem Elongation' ... This has led to the conclusion ... OCKERSE AND GALSTON-GIBBERELLIN-AUXIN