Planta (Berl.) 85, 238--249 (1969)

The Specificity of the Auxin Transport System R. HElaTEL MSU/AEC Plant Research Laboratory, Michigan State University, East Lansing M. L. EVANS Kalamazoo College, Kalamazoo, Michigan A. C. LEOPOLD Department of Horticulture, Purdue University, Lafayette, Indiana

H. M. SELL Department of Biochemistry, Michigan State University, East Lansing Received December 13, 1968

Summary. In an effort to examine the specificity of the auxin transport system, the movement of a variety of growth substances and of auxin analogues through corn eo]eoptile sections was measured in both the basipetal and acropetal directions. In contrast to the basipetal, polar transport of the auxins indoleaeetie acid (IAA) and 2,4-dichlorophenoxyacetie acid, no such movement was found for benzoic acid or for gibberellin A i. A comparison of the ~- and /~-isomers of naphthaleneacetie acid showed that the growth-active ~-form is transported, but not the inactive f?-analogue. Both the dextro ( + ) and leave (--) isomer of 3-indole-2-methylacetic acid showed the basipetal movement characteristic of IAA, the dextro isomer being more readily transported than the (--)-form. In this instance, too, the transport was roughly proportional to the growth promoting activity. The antiauxin p-chlorophenoxyisobutyric acid inhibited auxin transport as it inhibited auxin-induced growth. These results agree with the hypothesis that processes involved in auxin transport are closely linked to or even identical with the primary auxin action. A. Introduction I t has been s u g g e s t e d t h a t t h e " r e c o g n i t i o n s i t e " for p r i m a r y a u x i n a c t i o n could be i d e n t i c a l w i t h t h e specific site f o r a u x i n t r a n s p o r t (HE,TEL a n d LEOPOLD, 1963). This w o u l d i m p l y t h a t g r o w t h - i n a c t i v e analogues of a u x i n s s h o u l d n o t be t r a n s p o r t e d . T h e e x p e r i m e n t s r e p o r t e d in t h e following p a p e r b e a r o u t this prediction.

B. Materials and Methods a) Chemicals. 3-Indoleacetie acid (IAA) was purchased from Nutritional Biochemical Corporation, Cleveland, Ohio. ~-Naphthaleneacetic acid (cr

and

The Specificity of Auxin Transport

239

fi-naphthaleneacetic acid (fi-NAA) were kindly provided by J. R. BxsHo~ from Amehem Products, Inc., Ambler, Pa. p-Chlorophenoxyisobutyric acid (PCIB) was obtained from Calbiochem, Los Angeles. 3-Indole-2-methylacetie acid (meIAA) was synthesized and resolved into the optical isomers n-(q-)-3-indole-2-methylaeetic acid [(-k)-meIAA] and L-(--)-3-indole-2-methylacctic acid [(--)-meIAA] by the procedure of SC~LE~DER (1963). Buffers were prepared according to GOMORI(1955).

b) t?adioactive Compounds. IAA-l-14C (specific activity 10.4 mc mM -1) was obtained from New E @ a n d Nuclear, Boston, Massachusetts, 2,4-diehlorophenoxyacetic aeid-laC (2,4-D-1-14C, spot. act. 12 me relY[-1) from Nuclear Chicago, Ill., and benzoic acid-(earboxyl)-14C from Calbiochem, Los Angeles. Gibberellin A 1 (GA1-3I-I, spec. act. 87 me mM -1) was kindly supplied by H. K ~ D ~ . (synthesis, see KENDE, I967). To assay 14C and 8HI, the agar blocks or the tissue were directly transferred to 10 ml of BRAu solution and counted in a Beckman-CPiK-100 liquid-scintillation counter (efficiency for 14C> 85 %) as described by HI~RTEL and FLOR:r (1968). e) Coleoptiles oI Corn. (Zea mays L., hybrid W F 9 • 38, from Bear Hybrid Corn Co., Decatur, Ill., and sweet corn Golden Bantam 8 Row, from Vaughan's Seed Co., Chicago, Ill.) were used for transport tests as described by HHERTELand FLORu (1968) and for short-term growth assays (see below, f; growth conditions and type of segments as described by EvAns and Hoxa~soN, 1969). d) l~luorometric Assay: Extraction and Partitioning. The low levels of indole and naphthalene compounds that had to be measured necessitated a highly sensitive assay. Therefore, a fluorometrie assay was used (see e.g. STown and SC~ILKV., 1964). Because of the high sensitivity, it was in turn necessary to take special precautions to remove any interfering substances. The glassware was washed thoroughly. The dry Bacto-Agar (Difco) was washed twice with large volumes of chloroform before preparation of donor and receptor blocks. The coleoptile sections were " d r a i n e d " between blank agar blocks for 3 0 ~ 5 rain before being transferred to receptor and donor blocks; strongly fluorescent material seemed to " d r a i n " from the cut surfaces. After the experimental period the tissue extracts could not be assayed, but indole and naphthalene compounds from the donor and receptor blocks could easily be measured fluorometrically after partitioning. Each block (0.5 ml of 1.5% agar) was acidified with 1 ml of 0.1 N HC1 and extracted for 45 min with 3 ml of methylene chloride (CHH2C12,"Analyzed Reagent" from Baker Chem. Co., Phillipsburg, N.J.). The extraction was repeated for 15 rain each with 3 ml and 2 ml of CHIC12 and the organic fractions were pooled; agar and HC1 were discarded. The resulting 8 ml of organic phase were washed with 1 ml of 0.i N HCI which in turn was extracted twice with 1 ml of CHH2C1~. The material in the final ca. 10 ml of CHH~CI2 was then reextracted 3 times with 1 ml of phosphate buffer (0.01 N, p H 7.4); the yield in these 3 ml of buffer was always > 60% for all compounds tested. Extraction and partitioning of IAA-laC and e-NAA-14C showed < 15% loss. The simultaneous extraction of donors and receptors provided a reference level for each experiment since transport will be expressed as "material in receptor/material in donor". e) Fluorometric Assay: Measurement and Evaluation (see U])~NFRIEND, 1962, pp. 160ft., 463, 464). The assays were performed in a 5 ml quartz cuvette in an Aminco-Bowman Spectrofluorometer (from American Instrument Co., Silver Spring, ~r usually with Slit Arrangement 3 (3-2-3-3-2-3) and PM Shutter at 1.0. In the case of the indolic compounds a ca. 2 ml sample of the p H 7.4 buffer extract was assayed directly while for NAA measurements 0.3 ml of 1 M Na2CO s was added to the 3 ml buffer to give a p H of ca. 10. Emission and excitation spectra (Figs. 1 and 2) were found as reported (UD~m~IEND, 1962). For routine

240

R. ItERT~L, M. L. EVA~Xs,A. C. LEOPOLD and It. M. SELL:

assays, excitation was given at 285 m~z. Emission of indoles was quantitatively measured at 360 m~z, and that of the naphthalenes at 335 m~. The background from the empty agar blocks was ca. 0.03 "relative intensity units" ( ~ r.i.u.) on the instrument. A concentration of 10-~ 1~ IAA or mcIAA (corresponding to ca. 5 • 10-~M in the agar block before extraction) yielded 0.27 r.i.u., while 10-TM solution of NAA gave ca. 0.2 r.i.u. Fluorescence of all compounds was linear at least for the concentration range between 10-s and 10-5 M. A modified fluorescence test after KuwAcEx and PROC~AZXA(1964) permitted easy distinction of IAA and meIAA. This test is less sensitive (ca. 0.03 times) but more specific than is direct fluorometry. To 2 ml buffer (pH 7.4) extract, 0.7 ml of H20, 0.75 ml of 40% formaldehyde and 0.55 ml of 70% perchloric acid were added; the mixture was heated in a boiling water bath for 30 min and then excited at 360 m~z at room temperature. The emission peak of the IAA product was at 465 mfz and that of the meIAA derivative appeared at 440 mfz. /) Measurement o] Coleoptile Elongation. Elongation measurements were made by using the high-resolution continuous recording technique of EVANS and RAY (1969). A vertical column of hollow coleoptfle segments was positioned within a specially constructed glass chamber which was then filled with the desired growth medimn. A small weight was placed so that it rested on the uppermost colcoptile segment, and an arc lamp was used to cast a shadow of the weight on thevertical slit in a piece of cardboard. The vertical displacement of the weight by the growing segments below caused the shadow of the weight to move up the slit. This movement was continuously recorded on photographic paper moving horizontally behind the slit. The growth curves shown in Figs. 5 and 6 are direct tracings of such shadowgraphic records, with magnificationfactors and time scales as indicated. The curves are displaced along the ordinate.

C. Results

a) Movement o/ 3-Indoleacetic Acid, 2,4-Dichlorophenoxyaeetic Acid, Benzoic Acid and Gibberellin. The t r a n s p o r t of two a u x i n s a n d two n o n a u x i n s was m e a s u r e d a n d compared. Benzoic acid is chemically related to auxins, b u t i n a c t i v e i n growth tests (Jo~NssoN, 1961); GA 1 is a g r o w t h - p r o m o t i n g substance, b u t of a completely different t y p e (e.g., L~oPoLI), 1964). Coleoptile sections (20 per block; 2 m m long) were exposed to the radioactive c o m p o u n d s a t the donor c o n c e n t r a t i o n of 10-s M. After 90 m i n of basipetal (donor a t the apical end) or acropetal (donor a t the basal end) m o v e m e n t the radioactivities i n receptor, d o n o r a n d tissue were determined. As can be seen from the table, benzoic acid a n d GA 1 do n o t show the t y p i c a l polar m o v e m e n t exhibited b y I A A a n d 2,4-D (see McCREADu a n d JACOBS, 1963; H~gTEL a n d FLOgY, 1968). The d a t a concerning GA1 agree with the findings of CLog (1967). The u p t a k e of benzoic acid was faster t h a n t h a t of the auxins. GA 1 e n t r y into the tissue, however, appears to be v e r y slow, w h e n compared to the a r o m a t i c acids tested. (By e x t e r n a l s t a n d a r d a n d channels ratio i t was assured t h a t this was 9n o t due to a difference i n q u e n c h i n g d u r i n g the aIt assay.)

241

The Specificity of Auxin Transport

Table. Movement o/IAA-14C, 2,4-D-1~C, benzoic acid-14C and gibberellin Az-3H in corn coleoptile sections Transport (90 min) Labeled

Basipetal

compound

Tissue

Acropetal Receptor

Tissue

Receptor

Radioactivity (as % of donor) IAA 2,4-D Benzoic acid GA1

1.43 6.53 20.31 0.69

3.70 1.08 0.17 0.08

0.96 5.60 20.71 0.72

0.03 0.19 0.16 0.10

b) Comparison o] the Movement o/ ~- and fi-Naphthaleneacetic Acid. With respect to growth promoting activity~ ~.-NAA was nearly as effective as I A A while fi-I~AA (where the side chain is shifted by just one position) has only very slight activity (e.g., Jo]~ssoN, 1961; see below, Fig. 5). The movement of ~-NAA in plant tissue is similar in all features to t h a t of I A A (L~oPoLD and LAM, 1961 ; VEv=N, 1967 ; H ~ T ~ L and FLo~u 1968). To test the correlation of growth activity and transportability the movement of fl-NAA was examined in comparison with t h a t of ~-NAA. The excitation and emission spectra from the donors in Fig. l a show t h a t the relative fluorescence was similar for :r and fl-NAA; extraction and partitioning were equally efficient for either. Fig. 1 b documents t h a t no fluorescent material arrived in the receptors when fl-I~AA had been supplied or when ~-NAA had to move acropetMly. BasipetM transport of ~-NAA, however, did occur at a fast rate. (A fluorescent material with an emission peak at ca. 360 m~ appeared several times after acropetal :r transport. I t was not investigated further.) A similar experiment (transport time 150 rain, 20 sec/block, 5 mm long) again resulted in significant amounts of g-I~AA and no fl-NAA in the basal receptors. The uptake was asssayed as the percentage difference of fluorescence at 335 m~ of fresh donors minus t h a t of used donors : it was 62 % for cr and 58 % for fl-NAA. Thus, the difference in uptake cannot account for the great difference in transport of the two compounds. e) Movement o/ the Optical Isomers I)-(+) and L-(--)-3-Indole-2methylacetic Acid. ( + ) - and (--)-meIAA are both active in growth assays, the ( + ) - f o r m more than the (--)-isomer (SCHLEI~DEI% 1963; see Fig. 6, this paper). When the transportability of the two isomers was tested, the emission spectra of receptor extracts shown in Fig. 2 were obtained. No acropetal movement was found; basipetally, ( + ) - m e I A A was transported better than (--)-meIAA but not as well as IAA. The

242

l~. H E E T E L , i~. L . EVANS, A . C. LEOPOLD a n d H . M. SELL:

a) 1.0

Donors Excitation Emission

).8

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Fig. 1 a and b. Transport of c~-NAA and fl-NAA. 30 eoleopti]e sections (2 mm long) transported basipetially or acropetally from donors containing 6 • 10-6 M of a- or fl-NAA for 90 rain. Samples of unused donors and the receptors were extracted and assayed fluorometrically, a Excitation and emission spectra of donor extracts. b Emission spectra of receptor extracts (excitation at 335 m~) after 90 min of transport. Heavy lines represent basipetal, light lines acropetal assembly

time courses of the appearance of the isomers in the receptors, shown in Fig. 3, confirm this finding and may indicate a difference between the velocity of the transport front of (~-)- and (--)-meIAA. Uptake after 3 hr was determined (as above in C, 2) and found to be 44% for IAA, 79% for (~-)-meIAA and 73% for (--)-meIAA. To determine whether the meIAA was converted to IAA before being transported, pooled extracts from 5 receptors 120 m~n after (-]-)-meIAA application and a similar control with IAA were subjected to the modified fluorescence test of KUTXC~K and P~OC~AZKA (1964). The peak of emission from the IAA-derived material was at 460 m~ while with the (~-)-meIAA material it was at 440 my., suggesting that meIAA was transported as such. d) Inhibition o] Auxin Transport by an Antiauxin. PCIB is related chemically to certain auxins; it rapidly and specifically inhibits the effects of auxin as for example the effect on the elongation of coleoptfles ( F o s t e r et al., 1955; EVANS and IIoKANSON, 1969; see this paper, Fig. 6).

243

The Specificity of Auxin Transport 0.3 ,~,,,, I A A

~,~ c '"$

(+)

0.2

L:I z hi I.U nO 0.1

(•

O

I 550

:5 0

1 400

WAVELENGTH

! 450 (m,u)

Fig. 2. Emission spectra from receptor extracts after 60 min of basipetal transport through coleoptile sections (2 mm long, 25 sections/block). The donors contained 10-s M IAA, (+)-meIAA or (--)-meIAA

a)

xJ

b)

x

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90

120

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0 TIME

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(min)

Fig. 3a and b. Time course of movement of (+)-meIAA and (--)-meIAA. a Coleoptile sections (2 mm long; 25/block) were allowed to transport basipetally from donors containing 5 • 10-6M auxin, for the time indicated on the abscissa. b 5-ram-long sections (20/block); otherwise as in a If the site of action and t h a t of transport are related or identical, antiauxins should also inhibit transport. As shown in Fig. 4 a and b, 10 -4 M PCIB, applied through donor and receptor, does reduce the I A A

244

1~. HERTEL, M. L. EVANS, A. C. LEOPOLD and H. M. SELL:

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a) 13. 7=0 = oJ

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Fig. 4 a and b. Effect of I~CIB on auxin movement, a Time course of IAA transport in absence or presence of PCIB (10-a M). Coleoptile sections (5 mm long; 10/block) were pretreatcd • PCIB via an apical and basal agar block for 30 rain; then basipetal transport was allowed to proceed :j= PCIB in the receptors and donors, the latter containing 10-6 M IAA-14C. b The concentration of IAA-ZaC in the donors was varied as indicated on the abscissa, and the time of transport was 30 min in all samples. Other conditions as in a

e x p o r t i n t o t h e r e c e p t o r b y ca. 3 0 - - 5 0 % . I n o t h e r e x p e r i m e n t s , 5 • 10 -5 M P C I B i n h i b i t e d I A A - m o v e m e n t b y ca. 2 0 % , while 5 • 10 -6 M P C I B h a d no significant effect. I A A u p t a k e i n t o t h e tissue was in no case g r e a t l y affected. The effectiveness of t h e a n t i a u x i n , P C I B , in i n h i b i t i n g t h e t r a n s p o r t of a u x i n is consistent w i t h t h e earlier r e p o r t b y Nn~D~RG~G-KAMIE~ a n d LEOPOLD (1959) t h a t t h e c h l o r i n a t e d a n t i a u x i n s , 2,6-diehloropheno x y a c e t i c acid, 2,3,6- a n d 2 , 4 , 6 - t r i c h l o r o p h e n o x y a c e t i c acid, were effective in i n h i b i t i n g t h e t r a n s p o r t of I A A in sunflower h y p o c o t y l sections.

e) Growth Responses. Fig. 5 i l l u s t r a t e s t h e e l o n g a t i o n response of corn coleoptiles to four of t h e s u b s t a n c e s e x a m i n e d in t h e t r a n s p o r t studies (see t h e t a b l e a n d Fig. 1). I A A , 2,4-D a n d ~ - N A A (curves A , B

245

The Specificity of Auxin Transport

2j4-D

t C:(-NA A

~ t l -10

D

~

I

~-NAA I 0

I 10

TIME

I 20

IN

I 30

I 40

I 50

I 60

MINUTES

Fig. 5. Comparison of the growth-promoting properties of IAA, 2,4-D, and ~- and fl-NAA. Growth medium changed from 10-a hi phosphate buffer to the solution indicated, at each arrow. Auxin or analogue concentration was 10-5 hi except in the experiment of curve B, where 2 • 10-6 hi was used. The bracket at the end of each growth curve represents 1 mm elongation and C) all elicit strong growth responses in corn coleoptile sections within 35 rain after the addition of hormone. The apparent differences in the timiug of elongation response to these three hormones have been discussed in an earlier paper (EvAns and HOKA~SON, 1969). I n contrast to these three substances fl-NAA causes practically no increment in the growth rate of the coleoptile segments within the first hour after the addition of the chemical. This indicates that ~-NAA is similar to I A A in its capacity for growth promotion as well as its transportability, fi-NAA is able to enter the tissue as readily as the ~-form, but it is unable to move in the polar transport system in corn coleoptile tissue, and is at the same time very ineffective as a growth promoter. Fig. 6 illustrates the relative growth-promoting capacities of (-~)meIAA and (--)-meIAA. At a concentration of 10-SM, (-~)-meIAA (curve A) is comparable to I A A (Fig. 5, curve A) in its ability to promote elongation. However, (--)-meIAA (curve B) at the same concentration has only about one-haif the growth-promoting effect as I A A or (-~)meIAA. Thus it is clear that in its growth promoting properties, as in the transport studies described above (see Fig. 3), (~-)-meIAA is much more similar to I A A than is the (--)-isomer. 17

Planta (Berl.), Bd. 85

246

R. H~I~TEL, M. L. EVANS, A. C. LEOrOI~D and H. M. SELL:

A

] I mm

o

1

IAA I

-10

I

0

,

I

" 10

~

I

20

~

I

30 TIME

,

I

40

,

I

50 IN

,

I

f

I

,

I

60 70 80 MINUTES

~

1

90

,

I

100

~

I

110

Fig. 6. l~esponse of coleoptile segments to (~-)- or (--)-meIAA and to IAA ~=PCIB. In curves A and B, at each arrow the growth medium was changed from 10-~ M phosphate buffer to a 10-s M auxin solution in buffer of the hormone indicated. In curve C, the growth medium was changed from buffer to 10-6 M IAA at the first arrow, from IAA to IAA -~ 10-~ M PCIB at ~he second arrow, and to 5 • 10-~ IAA ~-10 -~ M PCIB at the third arrow. The brackets at the end of each curve represent 1 mm elongation

Curve C in Fig. 6 d e m o n s t r a t e s t h e s e n s i t i v i t y of I A A - p r o m o t e d g r o w t h to t h e a u x i n a n t a g o n i s t , P C I B (also see EvANs a n d HoxANSOI% 1968). E l o n g a t i o n s u p p o r t e d b y 10 -s M I A A is r a p i d l y a n d s t r o n g l y i n h i b i t e d b y 10 -4 M P C I B . Thus, P C I B a p p e a r s t o be a specific i n h i b i t o r of b o t h a u x i n - p r o m o t e d e l o n g a t i o n a n d a u x i n t r a n s p o r t ( p a r t 4, above).

D. Discussion T h e a u x i n t r a n s p o r t s y s t e m is specific. As far as t e s t e d , all a u x i n s t h a t show t h e t y p i c a l g r o w * h - s t i m u l a t i n g action, are t r a n s p o r t e d in a similar m a n n e r : I A A , ~ - N A A (see LEOPOLD a n d LAM, 1961), 2,4-D (McCREADY, 1963), 2 , 4 , 5 - t r i e h l o r o p h e n o x y a c e t i e a c i d (2,4,5-T) (JAcoBs, 1968), 4-amino-3,5,6-trichloropicolinic acid (I-IoI~TON a n d FL~TCI~R, 1968) a n d m e I A A (these results). On t h e o t h e r h a n d , organic acids t h a t l a c k t h e a u x i n a c t i v i t y in s t i m u l a t i n g growth, such as benzoic a c i d or /~-~qAA, a r e n o t t r a n s p o r t e d even if t h e r e are c h e m i c a l l y v e r y similar to a n a c t i v e auxin. Analogues w i t h lesser g r o w t h - p r o m o t i n g a c t i v i t y a p p e a r t o be less efficiently t r a n s p o r t e d , e.g. ( - - ) - m e I A A as c o m p a r e d t o ( ~ - ) - m e I A A . A n s w e r i n g a question similar to those a s k e d in this p a p e r , JACOBS (1968) r e p o r t s " T h e s y n t h e t i c a u x i n 2,4,5-T, which can

The Specificity of Auxin Transport

247

retard abscission, showed much more polar movement in bean petioles than did 2,4,6-T, an analogue with little auxin activity." Plant hormones of different types, like gibberellins (e.g. CLOR, 1967; this paper, table) or cytokinins (e.g. F o x and WEIs, 1965) do not seem to display the movement typical for auxins (for discussion see also McCREADY, 1966). Evidence for a polar movement of abscisie acid (D6RFFLWO and B61~rOER, 1968) has been found in Coleus tissue. However, polarity of movement does not imply that the substance in question is transported through the auxin-specific sites. Thus, the polar movements of ions like Ca 2+ in roots (EvAns and VAVOEAN, 1966), of the carrier of electric current in Fucus zygotes (JAFfa, 1966) or of sucrose in certain tissues (HxgTT, 1965; ZA~gg and 1V[ITCm~LL, 1967) may well be considered to be through some other systems. The effects of inhibitors on auxin-enhanced elongation and auxin transport show a strong correlation. The anti-auxin PCIB inhibits auxin action specifically and reversibly (EvAns and ttOKA~SO~, 1969), and it decreases IAA movement (see also NIEDERGANG-K~kMIENand LEOPOLD, 1957). The potent transport inhibitor TIBA also inhibits growth at the concentrations commonly used (THIMANN and BONN~R, 1948; KA~DL, 1956). The fact, however, that at very low concentrations TIBA can enhance the IAA-stimulated elongation cautions against a simple interpretation of the inhibitor effects. Comparative studies by KWITT (1966) on the inhibitory effects of substituted benzoic acids on IAA transport and growth may also be relevant to the question of specificity. In analogy to the stereo requirements for attachment of the auxin to its site of action, and on the basis of the effectiveness of analogues of auxin in inhibiting IAA transport, it was suggested that the transport of auxin involved attachment to a specific site (NI~D~gCA~o-KAmEN and LEOPOLD, 1959). The experiments reported here provide evidence of stereo specificity of the auxin transport site for isomers of NAA and for optical isomers of meIAA. The specificities of the sites of auxin action and of auxin transport appear to be similar, or perhaps identical. As long as we do not know the biochemical characteristics of the primary site(s) of auxin action and transport, there are at least the following possible explanations for t h e observed correlation (see HE,TEL and FLogY, 1968): 1. To elicit a growth response, even if the sections are fully immersed in auxin solutions, transport or a connected process may be necessary to bring the auxin to its site of action; 2. the site of action and the site of transport may, by chance and/or selection, have a similar specificity; 3. the non-transportable analogues could be subject to rapid immobilization; 4. a specific step in transport may be identical with the primary action of the auxin. This last possibility - - as well as the first - - is supported by the observation 17"

248

1:~.HERTEL,M. L. EVANS, A. C. LEOPOLDand H. M. SELL:

t h a t diffusible (----transported) r a t h e r t h a n e x t r a c t a b l e a u x i n is correlated with e l o n g a t i o n (e.g. JAco~s, 1950; STr.~V~S a n d BRINGS, 1959). I f the site of a c t i o n a n d the site of t r a n s p o r t are identical, this a u x i n specific s t r u c t u r e , p e r h a p s a macromolecule, should be p a r t of t h e p l a s m a m e m b r a n e , where active t r a n s p o r t processes are likely to occur. The experiments were supported by the U.S. Atomic Energy Commission under Contract No. AT(I1-1)-1338, and by Grant-in-Aid No. 66-12-6 from the Sloan Foundation to Kalamazoo College. We are indebted to Dr. BV~KWK. Z~MERMArCfor suggesting the fluorescence test. We wish to thank R. FLogY and VIexY FAX for their excellent assistance. References

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MSU/AEC Plant Research Laboratory Michigan State University East Lansing, Michigan 48823, U.S.A.

The specificity of the auxin transport system.

In an effort to examine the specificity of the auxin transport system, the movement of a variety of growth substances and of auxin analogues through c...
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