Planta

Planta (1982)155:231-237

9 Springer-Verlag 1982

Biological activity of gibberellin analogues Ch. Bergner, M. Lischewski, G. Adam, and G. Sembdner Institut ftir Biochemie der Pflanzen, Forschungszentrum ftir Molekularbiologie und Medizin, Akademie der Wissenschaften der DDR, Weinberg 3, DDR-4020 Halle (Saale), German Democratic Republic

Abstract. In order to determine the significance of the C-6 carboxyl group for the biological activity gibberellin A3, 6-epigibberellin A3, 7-norgibberellin A3, 6/~-methyl-7-norgibberellin A3, and 7-homogibberellin A3 were studied using dwarf pea, dwarf maize, dwarf rice, dwarf barley and c~-amylase bioassays. All gibberellin A3(GA3)derivatives tested were considerably less active than GA3. In all bioassays, 6-epi-GA3 showed a low activity of the same order, whereas 6/%methyl-7-nor-GA3 was inactive. Surprisingly, 7-nor-GA3 had some activity in the dwarf rice (root application), dwarf barley, and e-amylase bioassay, in contrary to its low potency in the dwarf pea, dwarf maize, and dwarf rice (micro drop) bioassay. 7-Homo-GA3 was primarily active in the dwarf maize, dwarf barley and dwarf rice bioassay. It also caused antigibberellin effects in dwarf rice. The results demonstrate that the C-6 carboxyl group is not absolutely essential for biological activity of gibberellins. The different activities of 7-nor-GA3 observed in the various test systems may indicate that the C-6 carboxyl group is a structural requirement more for uptake and/or transport processes than for receptor affinity. Key words: - Gibberellin (structure-activity-relation-

ship).

structure-activity considerations are based on data obtained from native gibberellins (e.g., Reeve and Croizer 1975). Thus, the influence on biological activity of those positions naturally non-altered may be of special interest. The 6/?-carboxyl group is present in all native gibberellins, both highly and less active ones. Chemical modifications at this position influence the biological activity considerably. Esterification causes a dramatic drop in activity (Brian et al. 1967; Hiraga et al. 1974). Reduction of this group to aldehyde or alcohol also results in a loss of activity, as shown for gibberellin A3 (GA3) (Adam et al. 1980; Meyer et al. 1982) and GA12 (Hoad et al. 1976). The activity of GA3-7-nitrile was also found to be much lower than that of the parent substance (Adam et al. 1980). Therefore it was concluded (Stoddart and Venis 1980) that a free carboxyl group at 6/?-position is not only an ubiquitous feature of all native gibberellins but also an essential requirement for biological activity. This ionizable function may largely determine partitioning behavior in putative gibberellin-receptor interactions. To verify the postulated role of the 6/~-carboxyl group, we synthesized (Fig. 1) 6-epi-GA3 (II), 7-nor-GA3 (III), 61% methyl-7-nor-GA3 (IV), and 7-homo-GA3 (V). Results obtained from testing these gibberellin analogues in various bioassays are presented here.

Introduction

Materials and methods

Among the fifty-nine naturally occurring gibberellins characterized to date (Yokota and Takahashi 1981; Bearder 1980), a great diversity in the substitution pattern of the gibberellane skeleton has been realized. On the other hand there are also some positions which have remained unchanged. Most of the systematic

Compounds'

Gibberellins 88; Part 87 = Lischewski (1982)

Abbreviation. GA3=gibberellic acid

GA 3 was purified according to Gr~ibner et al. (1975). The following compounds (Fig. 1) were synthesized from GA3 as described earlier. 6-epi-GA3 (II), prepared by oxidation of 6-epi-GA3-aldehyde (Lischewski and Adam 1980a), was separated from the corresponding 6/~-epimer with 4-fold column chromatography on silica gel (Merck, Darmstadt, FRG). It was shown to be free of GA3 by thinlayer chromatography and by constancy of the biological activity after further purification on column chromatography. 7-nor-GA3 (III) see Lischewski (1982); 6/?-methyl-7-nor-GA3 (IV) see

0032-0935/82/0155/0231/$01.40

232

C. Bergner et al. : Biological activity of gibberellin analogues

0

~T~

(.--~-OH

~

CH2

.a-

O /.~-OH =CHz

.o- _T~ '~oo.

-coo.

GA~ 111

s 3

.o

i~

~,~

~ .o"

CH3

C

O (~..~OH H 2

T~ "c.2-eoo. CH3

6~ -methyl7-nor- GA3 [IV)

7 -homo-GA 3 (V)

Fig. 1. Structures of the C-6 modified gibberellin analogues

Dwarf-bartey-bioassay (Hordeum vulgate, mutant 'Dornburg 576" 1). Seeds of this gibberellin-sensitive dwarf mutant were steril-

Lischewski and Adam (1980 b) ; 7-homo-GA3 (V) see Serebryakov et al. (1978). The completed and corrected physical data of compound V are: Amorphous, 1c~]~5+40,8~ (c=0,40; ethanol); IR (Nujol): v..... 3380 and 3450 br. (OH), 1750 (7-1actone-CO), 1720 (acid-CO) and 1660 cm -z (=CH2); 70 eV MS (source 150~ C): m/e 360.1558 (M +, Calc. for C2oH2406 360.1573; 1%), 299.1607 (C19H2303; 25%), 298.1565 (C19H1203; 100%), 280.1454 (C19Hz0Oz; 18%), 240.1139 (C16H1602; 14%), 239.1426 (C17H190 ; 27%), 238.1349 (C17HlsO ; 48%), 221.1318 (C17H17 ; 19%); ~H-NMR (Aceton-D6/TMS): 1.34 (s, 18-H3), 2.50 (d, J = 7Hz, 7-H2), 2.51 (d, J - 1 0 Hz, 5-H), 4.02 (d, J=3.5 Hz, 3-H), 4.86 and 5.17 (m, 17-Hz), 5.81 (dd, J = 9 Hz, J=3.5 Hz, 2-H) and 6.31 ppm (d, J = 9 Hz, l-H).

ized with sodium hypochlorite solution, thoroughly washed in destilled water, and germinated on wet filter paper for 3 d at 20 22 ~ C in the dark. Seedlings with well developed roots and emerging coleoptiles were selected and transferred into small glass jars (30 mm diameter) each containing 2 ml aqueous test solution and seven seedlings. The vessels were put into a moist chamber under fluorescent white light (1,500 lx) at 24 ~ C and after 5 d the length of the primary leaf was measured.

a-Amylase bioassay. Embryo-less, half caryopses of spring wheat (Triticum aestivum cv. 'Hatri', commercially purchased from VEB Saatzucht GDR) were sterilized with 60% ethanol followed by sodium hypochloride solution and thoroughly washed with distilled water. Five half-seeds were slowly shaken in 5 mt aqueous test solution for 48 h at 28 ~ C. The solution was then centrifuged to get a clear supernatant from which 2 ml were mixed with 2 mf

Bioassays The dwarf-rice bioassay (Orzya sativa cv. 'Tan-ginbozu '1) done by applying the test solution via the roots and the dwarf-peabioassay (Pisum sativum cv. ~ 'z) were described earlier

[crn][ Length of the 13 Jr (ast 3 internodes

(Sembdner et al. 1976). The dwarf-rice bioassay (Oryza sativa cv. 'Tanginbozu') done by applying the test solution as a droplet on the coleoptile and the emerging leaf ('micro-drop method') was carried out as described by Murakami (1968).

Dwarf-maize bioassay (Zea mays dwarf mutant dl ~). The technique of Sembdner et al. (1976) was modified in some respects. Seeds were germinated in heat-sterilized sawdust at 25 ~ C for 5 d, selected for uniformity and further cultivated in a special nutrient solution / 5 0 0 g g g-1 Ca (NO3)2x4H20; 140 ~tg g-Z KNO3; 70gg g-1 KC1; 140gg g-1 KH2PO4; 300gg g-1 M g S O 4 x 7 H 2 0 ; 30gg g-~ Fe (III) chelaplex II, 5 ml 1-1 1:25 diluted Hoagland A-Z solution a, five plants per 300 ml/at 24~ C under flourescent white light. After 2 d, the test solution (50 gl/plant) was applied, and after another 6 d the length of the 2nd leaf, including the sheath, was measured.

7-nor- GA3 (lll]

6-epi-G A3 {11)

.-OH CH2

CH;t

.o- T ~ ~ "

CH3

CH]

~

o S ' ~ : "~

1 Source of the seed material see acknowledgements 2 Source of the seed material see Sembdner et al. (1976)

T

1

6-epi- GA 3

i

7-nor-GA3

1 %1

:

x:106 3x1s6

'-s

3x10

'4.

3x10

opptied dosage

'

3x103

'

3xl() 2

'

0,15

IP m~

t]

(an

Fig. 2. Activity of GA3 and its analogues in dwarf pea bioassay (cv ' Meteor')

C. Bergner et al. : Biological activity of gibberellin analogues SSrensen phosphate buffer, pH 7.0. To determine cz-amylaseactivity, 100 mg of a chromogenic substrate prepared according to Heyns and de Moor (1974) was added to each sample. After 15 min incubation at 37~ C and centrifugation, the absorbance (620 nm) of the supernatant was measured with a Spekol (VEB Carl Zeiss Jena, GDR). Mean values obtained from the bioassay data were statistically evaluated by calculating the respective confidence limits on the base of t-distribution ( ~ - 5%).

Results

DwaJf-pea bioassay. A l l m o d i f i c a t i o n s at C-6 considered here result in an o b v i o u s decrease in b i o l o g i c a l activity (Fig. 2). 6 - E p i - G A 3 (II, Fig. 1), s h o w i n g a relative activity o f a b o u t 0.1% ( c o m p a r e d to G A 3 = 100%), was f o u n d to be m o r e active t h a n the o t h e r a n a l o g u e s (III, IV, V). T h e activities o f 7 - h o m o - G A 3 a n d 7 - n o r - G A 3 in d w a r f peas were d e t e c t a b l e only

233 at high d o s a g e s a n d no r e s p o n s e to 6fl-methyl-7-norG A 3 c o u l d be d e m o n s t r a t e d . To study possible interactions between GA3 and its C-6 derivatives ( T a b l e 1), s u b o p t i m a l G A 3 doses were a p p l i e d in c o m b i n a t i o n w i t h t h e respective G A 3 a n a l o g u e . T h e a m o u n t s o f the l a t t e r were c o n s i d e r a b l y h i g h e r t h a n t h a t o f G A 3 , b u t s m a l l e n o u g h to exert o n l y effects l o w e r t h a n t h a t o f the c o r r e s p o n d i n g G A 3 - d o s a g e (see also Fig. 2). I n this w a y it s h o u l d be p o s s i b l e to find o u t a n t a g o n i s t i c influences o f C-6 m o d i f i e d G A 3 - d e r i v a t i v e s o n a G A 3 i n d u c e d response. A s s h o w n in T a b l e 1, no G A 3 - a n a l o g u e signifi c a n t l y interferes w i t h its p a r e n t s u b s t a n c e in this way.

Dwarf rice bioassays. W h e n

s u b s t a n c e s were a p p l i e d via the r o o t s (Fig. 3), 7 - n o r - G A 3 was significantly m o r e active t h a n 6 - e p i - G A 3 a n d 7 - h o m o - G A 3 . U s i n g

Table 1. Interaction between GA3 and its C-6 modified derivatives in dwarf-pea-bioassay (cv. 'Meteor'). Considered combinations can be found at the intersection of the respective line and column. Data are expressed as mean length of the last three internodes (cm) • confidence limits Amount of GA3 (gmol per plant)

Amount of GA3-derivatives (pmol per plant)

0 1.5.10 -s 3.10 5

0

6~epi GA3 6.10 -a

7-nor GA 3 1.5-10 -1

6fl-methyl-7-nor GA3 1.5.10 -1

7-homo-GA3 1.5.10 1

2.7• 5.0• 6.2•

5.0• 6.7 _+0.3 6.2•

3.5_+0.2 4.9• 6.0 •

2.9• 4.6_+0,2 5.6•

4.9_+0.4 5.9 • 6.1+0.1

Length of the

l

16 1/. 12

i/I

10

//./

8 6 42

//•

I _7

+--6

I-5

t _~

10

10

10

10

applied concentration

163[moltll

Fig. 3. Activity of GA3 and its analogues in dwarf rice bioassay (cv. 'Tan-ginbozu', root application)

234

C. Bergner et al. : Biological activity of gibberellin analogues

10 - I

13.0_+ 1.2

---

the micro-drop method (Table 2), the activity of 7-nor-GA3 was much lower. The interactions between GA3 and its C-6 modified derivatives were investigated using the root-application method (Table 3). 6-Epi-GA3 and 6fl-methyl-7-nor-GA3 did not alter the response to GA3 simultaneously applied. With 7-nor GA3 a small additive interaction could be detected. However, 7-homo-GA3 was found to interfere with GA3 action. Despite its remarkable self-activity in growth stimulation it significantly diminished the GA3-induced growth.

1 5 10

14.8-+ 1.6 16.8_+1.1 17.3_+0.9

6.2_+0.4 7.5_+0.5 8.2_+0.7

Dwarf-maize bioassay. Dwarf-maize plants respond

Table 2. Influence of GA a and 7-nor-GA 3 applied via leaf (micro drop method) on growth of rice plants (Oryza sativa, cv. Tan ginbozu). Data are expressed as mean length of the seedlings (cm) _+confidence limits Amount per plant (nmol)

GA3

0 (control) 10 -3 10 -2

7-nor-GA3

4.8 _+0.5 6.3-+0.6 8.3_+1.0

4.8 _ 0.5 -

remarkably to the more polar compounds 6-epi-GA3

Table 3. Interaction between GA3 and its C-6 modified derivatives in dwarf-rice bioassay (cv. ~ root application). Considered combinations can be found at the intersection of the respective line and column. Data are expressed as mean length of the seedling (cm) -+condifence limits Conc. of GAa (mol/l)

Conc. of GA3-derivatives (mot/l)

0 5.10 -7 5.10 6

[cm3 30

6-epi-GA3

7-nor GA3

6/?-methyl-7-nor GA3

7-homo GA3

0

5.10 -5

5.10 -5

5.10 -5

5.10 -5

2.8_+0.3 6.1 _+0.6 8.9-+1.1

4.3_+0.4 6.2-+0.5 8.6-+1.2

6.9_+0.7 7.7___0.9 8.7_+0.8

3.5_+0.2 5.9-+0.4 8.6+1.0

4.0_+0.3 4.9-+0.4 6.8-+0.7

Length of 2nd leaf

28

26

G% l

2,t

22-

20-

18

16

t,t

12

.--t

T

ll--]--'" i"

I

6~-,~thy~-

7 oor

l

Fig. 4. Activity of GA3 and its analogues in dwarf maize (di) bioassay ~

' ~ 3xI0

i-s 3xi0

' c ' d3 3xI0 3xi opplied dosage

' -2 3xi0

0 '15

C. Bergner et al. : Biologicalactivity of gibberellin analogues

13

235

L

seedlings

12 11 10 g B 7

16~

166

167

16s

obsorbance

[rnom]

1(53

i0~

Qpptied concentrotion

Fig. 5. Activity of GA3 and its analogues in dwarf-barleybioassay (mutant 'Dornburg 576')

620nrn

7 6 5 t.

r~

cd

2

~ "

3

io~O Fig. 6. Activity of GA3 and its analogues in c~-amylasebioassay

16~

16~

167

16~

appliedconcentrotion

16s

and 7-homo-GA3. On the Other hand, no effects of 7-nor-GA3 and 6#-methyl-7-nor-GA3 were detectable (Fig. 4).

Dwarf-barley bioassay. The

most active GA3-derivarive in this bioassay was 6-epi-GA3 followed by 7-norGAa and 7-homo-GA3. The differences between these three compounds were not significant at various concentrations. The relative weak response to 6#-methyl7-nor-GA3 can be neglected (Fig. 5).

c~-Amylase-bioassay. In contrary to the other systems, this bioassay is not based on growth stimulation, but

16~[mom]

on enzyme induction. Nevertheless, modification of the C-6 carboxyl group affected biological activity of the GA3-derivatives as drastically as in the growth response tests. The order of activities of the different compounds is comparable to those obtained in the dwarf pea bioassay (Fig. 6). Discussion

The importance of the 6#-carboxyl group for biological activity of gibberellins was examined in previous papers by testing compounds with a masked (Brian et al. 1967; Hiraga et al. 1974; Sembdner et al. 1976)

236

or reduced carboxyl group (Hoad et al. 1976; Adam et al. 1980; Meyer et al. 1982) as well as by testing substances with a nitrogen function at C-7 like GA3nitrile and -amide (Adam et al. 1980). All these experiments gave evidences that this carboxyl group can be considered as an essential requirement for biological activity. However, in these papers, the influence of configuration at C-6 was not regarded. The reduction in biological activity of 6-epi-GA3 in comparison to GA3 may reflect the importance of the/%configuration at this asymmetric center. On the other hand the inversion of the carboxyl group causes specific alterations in the geometry of ring AI This could be demonstrated by N M R data of 6-epi-GA3 (Lischewski and Adam 1980a) which indicate an interaction between the 6 c~-carboxyl group and the 7-1actone function. As shown by Sembdner et al. (1965) and Brian et al. (1967) the lactone ring (C-4~ C-10) plays a remarkable role with regard to the potency of gibberellins. Thus, the relative low activity of 6-epi-GA3 may be due to a complex alteration in the GA3 molecule, only part of which is related to the carboxyl function in discussion. The absence of any activity in 6/~-methyl-7-nor-GA3 is consistent with all considerations, affirming an essential role of the 6fl-carboxyl group and with other experiments demonstrating that progressive reduction of the C-6 carboxyl group parallels a decrease in biological activity in the dwarf pea (Adam et al. 1980; Meyer etal. 1982). 7-Homo-GA3 was shown to possess different potency levels in different bioassay systems: very low in dwarf peas and remarkably higher in dwarf maize. As the modes of application are similar in both cases, the different responses of the test plants may be due to specificity of putative receptor molecules. 7-Nor-GA3 excited in the two bioassay systems based on root application - dwarf rice and dwarf barley - remarkable effects similar to those of moderate active native gibberellins, similar to GA~ and GA9. This finding opposes all theories about the essential role of the 6/3-carboxyl group for biological activity. Application via the leaf in the same species (dwarf rice) or of other test plants (dwarf pea, dwarf maize) caused much smaller responses. It is known that translocation of xenobiotics in higher plants can be modified by the presence or absence of a freeCOOH function. Jacob et al. (1973) demonstrated that molecules bearing a carboxyl group are more effectivly distributed within the plant when they are applied via leaves, in comparison to root application favoring molecules without this function. This can be explained by the mechanism of infiltration into the phloem (Jacob 1981). Therefore, different activities of 7-nor-GA3 may be a result of its different

C. Bergner et al. : Biological activity of gibberellin anaIognes

mobility, depending on the method of application. The relative high activity of this compound in the e-amylase bioassay could reflect the easy uptake and less important transport in the system. Thus, the importance of the C-6 carboxyl group of gibberellins must be considered not only in relation to putative hormone-receptor interactions but also with regard to the mobility of the hormone. Interactions between GA3 and its analogues were studied to determine the antigibberellins, i.e., competitive inhibitors of gibberellin action. The antagonistic effects of 7-homo-GA3 on GA3-stimulated growth of dwarf rice indicate a competition between both compounds for subcellular sites mediating hormonal response. On the other hand, the treatment of the Lineweaver-Burk plot used by several authors to characterize antihormonal action (McRae and Bonner 1953; Jacobson and Corcoran 1977; Iwamura et al. 1979) failed in this case. No linear transformation of the reciprocal values of concentration and growth response could be obtained. If complex systems are used as bioassays to investigate structure-activity relationships, the tested gibberellins can be activated or inactivated by metabolic conversions within the plant tissue (see, e.g., Nash et al. 1978). This possibility must also be considered with the C-6 modified gibberellins presented here. However, the GA3-analogues are chemically stable. The well-known metabolic enzymes are not likely to reconvert the respective derivative into its parent substance. The authors are grateful to Prof. R.P. Pharis (University of CaIgary, Canada) for seeds of Oryza sativa cv. 'Tan-ginbozu' and to Dr. W. Hentrich (Plant Breeding Research Institute Quedlinburg-Dornburg, Agric. Acad. GDR) for seeds of HOrdeum vulgare mut. 'Dornburg 576'. We also thank Dr. D. Kn6fel and Dipl.Chem. R. Kramell for synthesis of chromogenic substrate for eamylase determination.

References Adam, G., Lischewski, M., Voigt, B., Meyer, A., Bergner, Chr., Sembdner, G. (1980) Modifizierung yon Struktur und biologischer Wirkung bei Gibberellinen. In: Wirkungsmechanismen yon Herbiziden und synthetischen Wachstumsregulatoren, pp. 69-75, Schfitte, H.R., ed. Fischer, J e n a Bearder, J.R. (1980) Plant hormones and other growth substances - their background, structures and occurrence. In: Encyclopedia of Plant Physiology, N.S., vol. 9 : Hormonal regulation of development I. Molecular aspects of plant hormones, pp. 9112, MacMillan, J., ed. Springer, Berlin Heidelberg New York Brian, P.W., Grove, J.F., Mulholland, T.P.C. (1967) Relationship between structure and growth promoting activity of the gibberellins and some altied compounds in four test systems. Phytochemistry 6, 1475-1499 Gr~ibner, R., Schneider, G., Sembdner, G. (1975) Fraktionierung von Gibberellinen, Gibberellinkonjugaten und anderen Phytohormonen durch DEAE-Sephadex Chromatographie. J. Chromatogr. 121, 110

C. Bergner et al. : Biological activity of gibberellin analogues Heyns, W., de Moor, V. (1974) 3,17r dehydrogenases in rat erythrocytes. Biochem. Biophys. Acta 358, 1-13 Hiraga, K., Yamane, H., Takahashi, N. (1974) Biological activity of some synthetic gibberellin glncosyl esters. Phytochemistry 13, 2371 2376 Hoad, G.V., Pharis, R.P., Railton, J.D., Durley, R.C. (1976) Activity of the aldehyde and alcohol of gibberellins A~a and AI~, two derivatives of gibberellin A15 and four decomposition products of gibberellin A3 in 13 plant bioassays. Planta 130, 113-120 Iwamura, H., Murakami, S., Koga, J., Matsubara, S., Koshimizu, K. (1979) Quantitative analysis of anticytokinin activity of 4substituted-2-methylthiopyrido/2,3-d/pyrimidines. Phytochemistry 18, 1265-1268 Jacob, F. (1981) Transport and distribution of xenobiotic plant growth regulators. Syrup. on "Mechanism of the assimilate distribution and plant growth regulators" Piestany (CSSR) 1981, (Proc. in press) Jacob, F., Neumann, S., Strobel, U. (1973) Studies on mobility of exogen applied substances in plant. Proc. Res. Inst. of Pomology, Skiernewice, Poland, Series E Nr. 3, pp. 315 330 Jacobson, A., Corcoran, M.R. (1977) Tannins as gibberellin antagonists in the synthesis of c~-amylase and acid phosphatase by barley seeds. Plant Physiol. 59, 129 133 Lischewski, M. (1982) Synthesis of 7-nor-gibberellin-A3. Z. Chem. (in press) Lischewski, M., Adam, G. (1980a) Partialsynthese von 6-Epigibberellin-A 3. Tetrahedron Lett. 1627 Lischewski, M., Adam, G. (1980b) Synthese von 7-desoxygenierten Gibberellin-A~-Verbindungen. Tetrahedron 36, 1237 Meyer, A., Liebisch, H.W., Sembdner, G. (1982) Biologische Aktivitfit C-7 abgewandelter GA3-Analoga; Verteilung und Stoff-

237 wechsel von GA3 und GA3-7-aldehyd in Zwergerbsen. Biochem. Physiol. Pflanzen 177, 75-82 McRae, D.H., Bonner, J. (1953) Chemical structure and antiauxin activity. Physiol. Plant. 6, 485-510 Murakami, Y. (1968) A new rice seedling test for gibberellins 'micro-drop method' and its use for testing extracts of rice and morning glory. Bot. Mag. Tokyo 81, 33 43 Nash, L.J,, Jones, R.L., Stoddart, J.L. (1978) Gibberellin metabolism in excised lettuce hypocotyls: response to GA9 and the conversion of [3H]-GAg. Ptanta 140, 143 150 Reeve, D.R., Croizer, A. (1975) Gibberellin bioassays. In: Gibberellins and plant growth, Krishnamoorthy, N.H., ed. Wiley Eastern Limited, New Delhi Sembdner, G., Schneider, G., Schreiber, K. (1965) Uber die biologische Wirksamkeit einiger Gibberellins/iure-Abbauprodukte (VIII. Mitt. Gibberelline). Planta 66, 65 74 Sembdner, G., Borgmann, E., Schneider, G,, Liebisch, H.W., Miersch, O., Adam, G., Lischewski, M., Schreiber, K. (1976) Biological activity of some conjugated gibberellins. Planta 132, 249-257 Serebryakov, E.P., Lischewski, M., Adam, G. (1978) Partial synthesis of 7-homo-gibberellin-A3. Izv. AN SSSR Ser. Chim. 2 Stoddart, J.L., Venis, M.A. (1980) Molecular and subcellular aspects of hormone action. In : Encyclopedia of Plant Physiology, N.S. vol. 9: Hormonal regulation of development. I. Molecular aspects of plant hormones, pp. 445 501, MacMillan, J., ed. Springer, Berlin Heidelberg New York Yokota, T., Takahashi, N. (1981) Gibberellin A59: A new gibberellin from Canavalia gladiata. Agric. Biol. Chem. 45, 1251-1254 Received 6 February; accepted 24March 1982