Plant Physiol. (1967) 42, 47-54

Gibberellin- Interaction in Pea Stem Elongation' Ralph Ockerse2 and Arthur W. Galston Department of Biology, Yale University, New Haven, Connecticut 06520

Summlary. Joint application of and indole-3-acetic acid to excised stem sections, terminal cuttings, and decapitated plants of a green dwarf pea results in a markedly synergistic growth response to these . Synergism in green tall 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 on auxin synthesis, since gibberellin pretreatment of dwarf cuttings fails to elicit an enhanced tryptophan-induced growth response of sections, whereas auxin-induced growth 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 greatly increased growth. In reversed order, the application fails to produce any synergistic interaction. This indicates that gibberellin action must precede aulxin action in growth regulation.

Both gibberellins and contribute to growth pronounced in green sections. Three types of in length of excised etiolated Alaska pea epicotyl hypotheses have been advanced to explain the syn- sections (4, 7, 16) and green pea stem sections (1,3) ergistic interaction between GA3 and IAA: A) although gibberellin is less effective in green sec- gibberellin may act by promoting the synthesis of tions. \Vhen gibberellin and auxin are applied auxin (10), B) gibberellin may protect auxin from together to etiolated Alaska sections, growth is inactivation in the plant (1,15), and C) gibberellin either equal to or less than the sum of the separate may cause the formation of some substance, such as responses (17). This has led to the conclusion a nucleic acid, which interacts with auxin in promo- that gibberellin and auxin make independent con- tion of growth (12,23). tributions to stem growth throtugh different bio- The work reported in this paper is part of aii chemical pathways. In contrast to these results, a investigation which dealt with a reexamination of small degree of atuxin-gibberellin synergism has auxin-gibberellin interactions in a tall and dwarf been demonstrated in sections from Progress No. 9 pea cultivar, and a reevaluation of the results in pea seedlings grown in weak red light (21). the light of recent knowledge. The results pre- Joint application of the 2 hormones to green sented here -serve- -to- define the system- and we pea tissuies, such as Alaska stem sections (3) and hope to present further data in a subsequent paper Meteor stem sections (1) results in a marked syner- (Ockerse and Galston, in preparation). gistic interaction between auxin and gibberellin, being more pronounced in the latter (dwarf) cul- Materials and Methods tivar. These results, together with those derived from the petiole of single-node cuttings of sweet Light-Grown Peas. A tall cultivar (Alaska) potato (11) indicate that gibberellin-induced growth and a dwarf cultivar (Progress No. 9) of pea, is dependent on auxin. It thus appears that auxin- Pistin, sativumt L., purchased from Asgrow Inc., gibberellin synergism is a phenomenon character- Orange, Connecticut, were used in this investiga- istic of sections from dwarf peas, and is more tion. Seeds were soaked in tap water for 2 to 3 hours and sown in coarse Zonolite vermiculite in perforated 10 X 10 X 8 cm plastic boxes. The plants were grown at 230 under illumination of 1 This investigation w-as supported in part by a United States Public Health Service Fellowship (GM-24,349) about 1500 ft-c supplied by a combination of 90 % to R.O. from the National Institute of General Medical "Daylight" and "White" fluorescent and 10 % in- Sciences and in part by a Yale University Fellowship. candescent light energy. The seedlings were auto- This material is abstracted from a thesis presented by matically subirrigated twice daily with a nutrient R.O. to the Graduate School of Yale University in par- tial fulfillment of the requiremen-ts for the Ph.D. degree. solution consisting of 120 g Hyponex (Hydroponics - Present address: Department of Biology, Hope Col- Chemicals Company, Copley, Ohio) per 100 liters lege, Holland, 'Michigan 49423. tap water. 47 48 In experiments with intact planits, a 1.0 M,J drop recturvature. The sections were grown and meas- of IAA purchased from Regis Chemical Company, tired as described for green sections, except that Chicago, Illinois, or GA, obtained from Plant Pro- they- were placed in the dark anid the growth tection Ltd., Yalding, Kent, in 0.05 % Tween 20 me(lium had 2 % WNT/V suicrose instead. was placed on the first leaf of 7 day old plants and Etiolated cuittiings were obtained from 7-day-old the internodes were meastured 10 days following dxwarf peas xx-ith a third internode of 20 to 30 mm, application. Suibapical 5 mm sections were excised severed below the second node. The cuittinigs -xere with a douible-bladed cutting tool from 12 to 15 placed in 50 ml beakers (20 culttings to a beaker) day old Alaska peas whose apex was barely show- containing 25 ml of solution of the test compouinds. ing above the folded stipulies at node 7, and of 14 The initial aind final lengths of the third iinternode to 16 day old dwarf peas whose apex had just not were measuired with a metric scale ruiler. protruded from between the folded stipules at the All statistics involved the t-test of paired repli- seventh node. The sections were placed directly cates (20) and each replicate N-as the average of in 5 cm petri dishes, 10 sections to a dish, con- 10 sectioins, cuittings, or decapitated seedlings. taining 5 ml of medium, consisting of 25 mm \Vhere appropriate, the standard error is in(licated potassiulm phosphate (pH 6.1) bulffer andl 1 % in the figuires. All experiments wvere performed \V/V sucrose with or withouit the test compouinds. at least 3 times. L-Tryptophan was obtaiined from The sections were inculbated for about 20 or 44 Sigma Chemical Company. houirs on a reciprocating shaker (40 cycles/min) in continuous light and their length measured to Results the nearest 0.1 mm uinder a dissecting m;croscope equipped with an ocuilar micrometer. Rcspontses of intact Peai Steiii to Gibbcrellin aniid Aux.int. Green Progress No. 9 seedlings treated Terminal cuittings of dwarf peas were obtained with high levels of GA., are virtually idlentical in from 13 to 15 day old plants with sixth internodes appearance with uintreated green Alaska peas. This 7 to 10 mm long and severed juist above the fifth can be illuistrated for the sixth internode when node. The fifth internodes of 20 cuittings were growth in length is plotted as a functioin of GA3 inserted through a perforated alulminulm foil cover (losage (fig 1). In contrast to greein Progress No. placed over a 14.5 cm petri (lish half. These were 9 seedlings, GA3 had a relatively small effect on filled with 250 ml soluitions of the test compouinds. growth promotion of the internode of green Alaska Everv 12 hours the contents was restored to the peas. Treatment of both cuiltivars with IAA had original voltume with distilled water. The initial no effect oIn the growth of the interinodes. These and final lengths of the sixth internodes were resuilts suiggest that the amouint of endogenouoll gib- measured with a metric scale ruiler. berellin is limiting the growth of Progress No. 9 In work with decapitated plants, the apex was peas. severed from the 2 to 3 mm long seventh internodes Effects of Au.xin, Gibberellini, and Tryptopha a of dwarf plants whose sixth internodes were 9 to on Growth of Stem1i Section1s. The dose-response 11 mm long. A plastic capsuile wsith a capacity of cuirv-es of green Progress No. 9 anid Alaska pea 0.7 ml (obtained from LKB Instruments Inc., sections to IAA, GA3, or L-tryptophan are sutm- Washington, D.C.) was perforatedl at its base, filled marized in figuire 2. In confirmation of previouis with a solution of the test compouind and inserte(d work (3), Alaska section length increases with snugly over the seventh internodle stuimp. The increasing IAA concentrations in excess of 0.1 yi sixth internode length was measuredl after 20 or 44 with an optimum near 0.1 mat. By contrast, houirs w ith a metric scale ruiler. Progress No. 9 sections react less to IAA, and a Etiolated Peas. Seeds were soaked in tap water satuirating level is attained betweeni 0.01 andI 0.1 in darkness for 2 to 3 hoturs andl sown in plastic m.i. \Vith G.A3, sections of both culti-ars show trays containing prewashed, water-saturated ver- a small anlcl essentially similar growth response. micuilite (Mica-Gro Type B suipplied by California With L-tryptophan, dwarf section length increases Produlcts Company, Cambridge, Massachulsetts). with concentrations in excess of 1 j,M and the (lose- The trays were placed in a darkened cabinet in a response cturxve is comparable to that of ITAA-treated (lark room maintained at 27° +- 1°. Three days Alaska sectioins. This may suiggest that dwarf prior to harvest of the epicotyls the seedlings were sections possess an enzyme system which converts watered with 1 liter of tap water. A dim green L-tryptophan to an aulxin (5, 10). light furnished by a 1 5-watt Sylvaniia green fluo- Gibberellin-Auxin Synergismii in Grcen .Sectionzs. rescent tube, wrapped in 3 layers each of amber It has been shown that gibberellin and aulxinl can and green DuPont cellophane, was used sparingly promote growth synergistically in green pea sections in the handling of the seedlings. Subapical 5 mm (1,3). WVhen supplied to Progress No. 9 sections S, sections (16) were excised from the third inter- at satulrating concentrations, GA., and IA.A show node of 7-day-old Alaska peas with an internode a high degree of synergistic interaction (table I length of 25 to 40 mm and reclirv7ed terminal buds andl fig 3). At 1 umM IAA, IAA-induicedl growth or Progress No. 9 see(lliilgs wrhich (lo nlot show suich is inhibite(d aindI canl be partially reversed 1l, GA3. OCKERSE AND GALSTON-GIBBERELLIN-AUXIN INTERACTION IN PEA STEM ELONGATION 49

80 GA3

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

60 - CM0 0 z Zso z .x 0 (- I oz 40

I- DWARF CULTIVAR O 30

20 F

IAA

6 5 4 o --A I I - v V 0.175 0.35 1.75 3.5 17.5 35 175 350 3500 -LOG MOLAR CONCENTRATION MPG PER PLANT 1 2

10

° 6 z

- 7 1. 5

TIME INHOURS TIME INHOURS 3 4 FIG. 1. Growth of the sixth internode of green tophan (TTP). Sections allowed to grow for 20 hours Progress No. 9 (A, A) and Alaska (0, *) peas in FIG. 3. Kinetics of elongation of green Progress response to various levels of GA3 or IAA. No. 9 pea stem sections. Treatments are as indicated. FIG. 2. Dose-response of green Progress No. 9 (A) FIG. 4. Kinetics of elongation of green Alaska pea and Alaska (Q) stem seftj,A%AA, GA1, and tryp- stenm sections. Treatmeints are as indicated. 50 PLANT P HYSIOLOGY Table I. IAA-GA3 Synergism in Green Progress No. 9 Pea Stem Sections, Cuttings, and Decapitated Plants Growth in excess of control (mm) IAA + mM Time IAA Conitrol GA3 0.1 GA3 Material (hr) (MM) A L (mm) IAA (0.1mM) Calc Obs Obs-Galc Sections 21¼2 2.93 -0.70 0.78 0.0B 0.77 0.69* 20¼2 2.42 0 1.38 0.86 2.24 3.77 1.53* * 45¼ 4.23 -1.38 1.01 -0.37 0.50 0.87 47V/2 3.81 50 0.83 0.73 1.56 5.22 3.66*** Cuttings 21 2.81 5 0.81 2.03 2.84 6.42 3.58*** 21 2.47 50 2.26 2.37 4.63 5.10 0.47 20 2.53 100 3.30 2.41 5.71 5.84 0.13 45 4.00 5 0.50 4.31 4.81 9.16 4.35*|* 44 3.82 50 2.67 5.51 8.18 8.96 0.78 45 3.93 100 4.70 5.01 9.71 9.35 -0.36 Decapitated 22 2.41 plants -0.03 2.63 2.60 3.70 1.10** 10 0.29 2.92 3.65 0.73* 43 4.59 -0.10 7.70 7.60 9.31 1.71*** 10 -0.97 6.73 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- Kinetics of Growth of Green Alaska and Prog- tions from 0.1 to 1 ,UM IAA, and is consistent with ress No. 9 Sections. The time course of section a similar observation reported in wheat coleoptile growth for Progress No. 9 and Alaska peas in sections (22). This inhibition can be reversed to various media is shown in figures 3 and 4 respec- a growth promotion by a 4 houir pretreatment of tively. Growth of control sections is essentially the sections in basal medium alone (14). At stipra- similar for both cultivars. After a lag-phase of optimal IAA levels, the degree of synergism is about 9 hours, growth proceeds at a constant rate only slightly smaller. Fuirthermore, GA, concen- and diminishes at about 25 hours from the start. trations between 10 m1umI andl 1 mm are eqIually With GA3, a small growth stimulation is produiced. effective in the presence of IAA (14). The major difference in growth between Alaska Synergism, expresse(d as a percent of total ancl Progress No. 9 sections is seen in their re- growth, can alrea(ly be seen after a 2 houir incuiba- sponse to IAA which is considerably smaller for tion an(l increases progressively with time (14). the dwarfs. This is (Ilie to both a smaller growthl After 48 houirs almost one qtuarter of the sectioln rate anid a prematuire cessation of growth. The length can be attrilbUted to synergism. By conitrast, initial growth rate in (lwarf and Alaska sections is synergism in Alaska pea sections is very smnall and maintained for 11 an(l 17 hoturs respectively. Wheni can only be demonstrate(d wheni TAA levels are GA.. and IAA are stupplied together, Alaska sections suboptimal (fig 4). show a small promotion of IAA-induced growth. It has been reported that GA. and tryptophan With dwarf sections, GA, markedly enhances IAA- interact synergistically in coleoptile section induced growth by increasing the initial growth growth (19), and that this synergism is dtue to rate and delaying the onset of decline in growth. the effect of GA, on the enzymatic conversion of Such treated dwarf sections now follow exactly tryptophan to atlxin (10). In Progress No. 9 pea the same kinetics as Alaska sections treated with sections, however, synergism (htie to GA, and both hormones. Tryptophan-indtuced growth of tryptophan (table II) is not statistically significant. Progress No. 9 sections is only slightly enhanced OCKERSE AND GALSTON-GIBBEREL.LIN-AUXIN INTERACTION IN PEA STEM ELONGATION 51 Table II. Effect of Concentration of Tryptophan (TTP) on Tryptophan-GA3 Synergism in Green Progress No. 9 Sections, Cuttings, and Decapitated Plants Growth in. excess of control (mm) TTP + 0.1 mM GA3 Time TTP Control GA3 Material (hr) (NI) A L (mm) TTP (O.lmM) Calc Obs Obs-Calc Sections 21 3.30 10-6 0.10 0.85 0.95 1.15 0.20 10-5 1.40 2.25 2.00 -0.25 21 2.50 10-4 1.50 0.80 2.30 2.70 0.40 10-3 3.20 4.00 3.75 -0.25 45 4.30 0.25 0.45 0.70 0.95 0.25 10-5 1.65 2.10 2.10 0.00 43 3.35 10-4 1.65 0.55 2.20 2.65 0.45 103 3.75 4.30 4.25 -0.05 Cuttings 22 2.50 105 -0.17 1.87 1.70 2.04 0.34 10-4 -0.01 1.86 1.90 0.04 10-3 -0.20 1.67 2.00 0.33 44 3.11 10-5 0.31 4.86 5.17 4.89 -0.28 10-4 0.43 5.29 4.63 -0.66 lor3 0.49 5.35 4.65 -0.70 Decapitated 20 1.85 plants 10-4 0.53 1.34* 1.87 2.72 0.85** 44 4.59 10-4 0.25 4.78* 5.03 5.71 0.68

* GA3 = 0.1 sM. ** Difference significant at the 5 % level. by GA3 (fig 3). With tryptophan alone, growth ment indicating that the tissue still has the potential does not begin until after about 9 hours and GA, for growth. Also, with increasing pretreatment appears to shorten this lag-period by 2 to 3 hours. times to 20 hours in IAA, the degree of GA.-IAA Effects of Pre- and Post-treatment of Dwarf interaction is markedly reduced if GA3 is directly Sections with Gibberellic Acid. During the initial added to the medium (14). These results strongly hours of growth of green dwarf sections, the rates suggest that GA3 action must precede IAA action of IAA-induced growth with and without GA3 are in growth regulation. The hypothesis advanced about 0.33 mm/hour and 0.27 mm/hour respectively by Kefford and Goldacre (8) that auxin prepares when the hormones are supplied from the start. the tissue for subsequent gibberellin action needs to Preincubation in basal medium with or without be modified. GA3 markedly enhances subsequent IAA-induced Growth Response of Dwarf Cuttings to Tryp- growth (fig 5). This growth stimulation without tophan, Indole-3-Acetic Acid, or Gibberellic Acid. GA3 is mainly due to a higher growth rate (0.40 Cuttings were also used as test objects since they mm/hr) following the addition of auxin. With resemble the intact plant more closely (14). Con- GA., the growth rate is not only greater (0.54 centrations of IAA in excess of 1 pM are growth mm/hr) but the duration of elongation is consider- promotive, showing that in this system, IAA can ably prolonged, and the sections still grow after move from base to apex. At the optimum of 0.1 50 hours. With lengthier pretreatment times, the mM IAA the internodes were swollen in appearance. growth rates are even higher. Thuls, after a 21 \Vith GA3, a 0.1 gM concentration was saturating houir pretreatment with or withouit GA3, growth and higher levels were equally effective. Tryp- rates of 0.76 mm/hour and 0.45 mm/hour respec- tophan was without any effect even at a I m-, tively can be attained. Preincubation, especially concentration. in the presence of GA3, brings about some change The kineti,s of growth of the sixth internode which enhances the response of dwarf sections to are the same 'or untreated cuttings and intact auxin. plants (fig 7). With GA3, elongation is imme- By contrast, pretreatment of sections with IAA diately enhanced and is linear with time during and transfer to basal medium with or without GA.. the 45 houir period. \Vith auxin, growth starts results in a reduction in growth rate (fig 6). Re- at a high rate but falls gradually, until after 45 introduction of IAA again leads to growth enhance- hours no further growth is detected. 52 PLANT PHYSIOLOGY Gibberellin-A*x-in Synergism in Green Cutttings. Joint application of GA3 and IAA to dwarf cuittings results in a synergistic growth response to these hormones only when the concentration of I A (5 j,Ma) is suboptimal (table I). At an optimal concentration of IAA (0.1 mM) the IAA-GA3 in- teraction becomes merely additive (no synergism). As with excised stem sections, GA3 and tryptophan do not interact synergistically (table II).

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

10

ro 7 ' ,+ Ge-~~~~~~~~~~~5X0~JIAA 8 _-o 0 5 10 15 20 25 30 35 40 45 50 TIME INHOYJS

FIG. 6. Effect of a 4 hour pretreatment of green Progress No. 9 pea stem sections in IAA following tran-sfer to basal medium with or without GA3. IAA b 0~~~~~~~~~~~~~~1GA4G3 reintroduced 4 hours following transfer. ilar (14). As in cuttings, low concentrations of IAA and GA3 interact synergistically, albeit to a CONTROL - KA slightly smaller extent (table I). In contrast with cuittings, a small degree of synergism between GA, and tryptophan can be obtained in decapitated peas. Synergism in Etiolated Peas. In confirmation of previouis work (7, 17), no GA3-IAA synergism could be demonstrated in etiolated Alaska sections (14). The results for etiolated dwarf sections are presented in table III. Synergism is significant

0 5 10 15 20 25 30 35 40 45 50 55 60 when the IAA and GA3 concentrations are 0.1 ,UM TIMlE IN JRS 5 and 0.1 mm respectively. Increasing the GA3 con-

FIG. 5. Effect of a 5 hlour p)reltreatment of greenq 14 Progress No. 9 pea sltem sections inl basal medlium with o)r withoult GA3 on IAA-induced growth. 13 12

Effect of Gib4erl lin-Pretretment On Growth 1

Indlucedl by Aufxin (mnd Tryptopill(mn. Pretreatment 10 of culttings with GA:i markedly enhanlces aulxin-in- a 9 (lced growth of sub2sequently excised sections but z rFedces tryptophan-inEft ace(l growth as compared w_ 8 with similarly treate(m sections from culttings pre- - 7 treate(l withGlistilled water (fig 8). IfAAcl acts 2 y promoting the synthesis of auxicn (o1) from tryptophanl, anl increase(l sulpply of tryptophan shouldduead to growthenhancemdelit. Pretreatment of ctuttings with tryptophan or JAA dloes not alter 3 the auxin-inddced growth of subsequently excised sections,ceut markedly inhibits tryptophan-induced growth. Here also, the expected GA3-IAA inter- actions in GA3-treated sections from attxin-pre- 0 5 10 15 20 25 30 35 40 45 50 treated cwttings is- not observed. TIME IN HOURS 7 Synergism in Green Decapitated Dwvarf Peas. FIG. 7. Kinetics of growth of The dose-response curves of the sixth the sixth internode of internode green Progress No. 9 cuttings, treated with 0.1 mM of decapitated dwarf peas anor dwarf cuttings to GA3, 0.1 mm IAA, or distilled wvater, and untreated sAct iAA, and to tryptophan are essentipaly sim- intact plants. Initial internode length was about 9.0 mm. 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 and Stem Sections Growtlh in excess of control (nmni) IAA + Ti.e GA., IAA Conitrol GA. (0.1 11mM) Material (lir) (Al) A L (mlm) IAA (0.1lM) Calc Obs Obs-Calc Sections 19.5 2.66 10-8 -0.09 1.54 1.45 1.60 0.15 10-7 0.44 1.98 2.40 0.42: 0.80 2.34 2.31 -0( (03 10-' 0.70 2.24 1.94 -0.30 2.90 10-7 0.86 0.98* 1.84 2.99 1.154':* Cuttinigs 20 9.8 5x10- 0.2 10.4 10.6 13.9 3 3-:<:!: 5X10-7 0.9 11.3 8.9 2.4 5 X 10-6 0.2 10.6 7.6 -3.0 .44 13.2 5X10-8 0.1 17.1 i7.1 21.7 4.5** 5X10-7 0.5 17.6 19.7 2.1 5 X 10-6 -0.4 16.7 17.2 -0.5 * GAi3 = 1 mm.h ** Difference significant at the 5 % level. Difference significant at the 0.1 % level. Discussion

In the past, convincing stildies by KIuse ( I 1) ain(I Brian aii( Hemming (1) have (lenionistrated ui z0a 3 that responise to gibberellin depend(s on the avail- cc ability of auxin. The converse, that auxini actioni i is also dependent on gibberellin is probable but a 'A more difficuilt to demonstrate. If the assuimptioni a;. 2 9 is correct that Progress No. 9 and Alaska peas U. 0 (liffer in their endogenouis gibherellin contenlt, a fn'A dwarf-tall relationship wouild be expected to be U LU z mirrored in the response of sections to aulxin. The -1 -d I resuilts indicate that IAA-induice(d growth is indee(d less in Progress No. 9 sections, becauise of a lowver rate of growth maintained over a shorter time period. With GA3 alone, growth of sections in both cultivars is promoted buit little, whereas in GA3 IAA TTP GA3 IAA TTP GA3 IAA TTP GA3 IAA TTP H20 - PRETREATED IAA - PRETREATED GA3 - PRETREATED TTP - PRETREATED combination with IAA, elongationi of Progress No. 8 9 sections is synergistically enhailce(d anld kinetically FIG. 8. Effect of a 15 lhour pretreatment of green equial to that of Alaska sections. This weakens thc Progress No. 9 cuttings in distilled '\-ater, 50 IAA, IA-m thesis that GA3 and I -kA make ilndepenidenit coln- 0.1 m-m GA3, or 0.1 mM tryptophan (TTP) on eloniga- tion of subsequently excised sectionis, inicuibated with trilbutions to growth (17). 0.1 mm GA3, 0.1 m-r TTP, or 50 IA:r IAA. Sections Sinlce aulxinl is kniownl to be forme(d in the apex allow ed to grow for 20 hours. ani(l youlng foliage an(d tranisportedl to the elonigatilng region, anid exogenolusly suipplie(l atlxini is requiired cenitrationi to 1 mAi considerably enhlaniced synergism, for sectioni growth, it is possible that in the initact while increasing IAA concentrationis decreased the planit gibberellini acts by enihanlcinig the synlthesis synergistic effect. of aulxiin in the apical regioni. This was first suig- Gibberellin-auxin synergism is also significant geste(l by Kuraishi and MuIiir (9) ho reported When etiolated dwarf cuttings are employed (table that GA3inot only inicreased the amouniit of diffllSil)le III). Low IAA concentrations have little effect atlxin from the apex buit that this increase preceded oni growth of the third internode whereas, at 50 growth. Suibse(quienit work showe(d that GAX enll- ,A.i growth is inhibited. With GA3, a synergistic hanced the efficienlcy of conversion of trvptophan effect is evident when IAA is present at 0.05 A\. into auixin by enzyme preparations from GA3- No synergism cotuld be demonstrated in etiolate1-d treated tissues (10). While wve do not wish to Alaska pea cuttings (14). question the results of Kuraishi and Muir, our 54 PLANT PHYSIOLOGY evidence does not support the view that gibberellin 4. GALSTON, A. W. AND H. WARBURG. 1959. An controls growth through its action on auxin syn- analysis of auxin-gibberellin interaction in pea thesis for the following reasons. Firstly, sections stem tissue. Plant Physiol. 34: 16-22. wvhich showed a growth response to tryptophan, 5. GORDON, S. A. 1961. The biogenesis of auxin. In Handbuch der Pflanzenphysiologie. W. Ruhland, presumably by conversion of tryptophan to auxin, ed. Springer-Verlag, Berlin. Vol. XIV. shoved neither significant GA3-tryptophan syn- 6. KAMERBEEK, G. A. 1956. Peroxidase content of ergism, nor enhancement of tryptophan-induiced dwarf types and giant types of plants. Acta Botan. growth in sections from GA3-pretreated cuttings. Neeri. 5: 257-63. Seconidly, no tryptophan-GA3 synergism could be 7. KATO, J. 1958. Studies on the physiological effect (lemonostrated in cuttings btut a small degree of of gibberellin. II. On the interaction of gib- syniergism was obtained in decapitated plants, lack- berellin with auxins and growth inhibitors. Physiol. ing their main centers of auxin synthesis. Thirdly, Plantarum 11: 10-15. aI synergism could be demon- 8. KEFFORD, N. P. AND P. L. GOLDACRE. 1961. The marked IAA-GA3 changing concept of auxin. Am. J. 48: strated in all these cases. If GA3 acts solely via 643-50. auxini synthesis, then at best an additive response 9. KURAISHI, S. AND R. M. MUIR. 1964. The rela- would be expected from combinations of IAA and tionship of gibberellin and auxin in plant growth. GA3. Finally, GA3-induced growth of dwarf cut- Pilant Cell Physiol. 5: 61-69. tings is almost immediate and does not appear to 10. KURAISHI, S. AND R. M. MUIR. 1964. The mech- go through a lag period. Thuis, in our view it anism of gibberellic action in the dwarf pea. Plant' wouild be difficult to determine whether the increase Cell Physiol. 5: 259-71. in auxin production precedes, and therefore causes 11. KUSE, G. 1958. Necessity of auxin for the growth effect of gibberellin. Botan. Mag. Tokyo 71: growth. 151-59. In dwarf cuttings, the continued decline in the 12. MASUDA, Y. AND N. YANAGISHIMA. 1965. Further aulxini-induced growth rate from the time of ap- studies on the RNA functional in auxin action. plication could be due to the premature onset of Plant Cell Physiol. 6: 17-23. the matuiration phase. This is not attributed to 13. MCCUNE, D. C. AND A. W. GALSTON. 1959. In- a depletion of auxin since repeated transfer to verse effects of gibberellin on peroxidase activity freslhly prepared solutions does not alter the growth and growth in dwarf strains of peas and corn. response (14). Aln explanation may be presented Plant Physiol. 34: 416-18. onl the basis of auixin-peroxidase interaction. The 14. OCKERSE, R. 1966. A study of auxin-gibberellin interactions in peas. Ph.D. Thesis, Yale Univer- additioni of high levels of auxin may induce the sity, New Haven, Connecticut. synithesis of more peroxidase which then leads 15. PILET, P. E. 1957. Action des gibberellines sur to its own destrttction (2,18). By contrast, GA3- l'activite auxin-oxydasique de tissus cultive in vitro. inidtuced growth in cuttings is linear with time. Compt. Rend. Acad. Sci. (Panis) 245: 1327-28. In dwarf sections, which presumably have a higher 16. PURVES, W. K. AND W. S. HILLMAN. 1958. Re- peroxidase content than Alaska sections (6, 13), sponse of pea stem sections to indoleacetic acid, GA3 treatment delays the onset of the maturation gibberel,lic acid, and sucrose as affected by lengtl phase of aulxin-induced growth, which now approxi- and distance from apex. Physiol. Plantarum 11: mates auxin-indtuced growth in Alaska sections. 29-35. 17. PURVES, W. K. AND W. S. HILLMAN. 1959. Ex- This may explain why in cuttings and decapitated perimental separation of gibberellin and auxin seedlings GA3 and IAA interact synergistically only actions in etiolated pea epicotyl sections. Physiol. if the IAA level is suboptimal. This is supported Plantarum 12: 786-98. further by the observation that in auxin-treated 18. RICARD, J. AND J. NARI. 1963. Adaptation et mode dwarf sections the degree of synergism decreases d'action des auxineoxydases. Compt. Rend. 155: as the time of GA3-post-treatment increases. We 846-49. hope to present further data on this question. 19. SASTRY, K. K. S. 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Physiol. tive studies on the growth and light sensitivity of Plantarum 19: 167-76. green and etiolated pea stem sections. In: Light 23. VARNER, J. E. AND G. RAM CHANDRA. 1964. Hor- and Life. W. D. McElroy and B. Glass, eds. Johns monal control of enzyme synthesis in barley endo- Hopkins Press, Baltimore. sperm. Proc. Natl. Acad. Sci. 52: 100-06.