Cunxi Wang · Kevan P.C. Croft · David F. Hildebrand

 Auxin [α-naphthaleneacetic acid (NAA) or indole-3-acetic acid] can induce the expression of lipoxygenases (LOXs) in cultured immature zygotic embryo cotyledons of soybean [Glycine max. (L.) Merr]. These auxin-induced LOXs are different from those normally expressed in seeds but have the same isoelectric points (pI) as those found in seedlings. The pIs of the two seedling LOXs were determined to be 5.09 and 5.23. One of the auxin-induced LOXs has the same pI (5.09) and molecular mass (94 kDa) as seedling LOX4. The partial amino acid sequences from the purified NAA-induced pI-5.09 LOX are identical to those of LOX4. RNA protection assays showed that NAA induces the expression of LOX4 and LOX5 mRNAs in cultured embryo cotyledons where they are not normally expressed. Soybean genotypes with a polymorphic variant of LOX4 in hypocotyls showed the same variation as NAA-induced LOXs in the embryo cotyledons. These results demonstrate that the NAA-induced pI-5.09 LOX is seedling LOX4 and also suggest that auxin might be directly or indirectly involved in seedling LOX expression during seed germination. Received: 10 January 2000 / Revision received: 16 June 2000 / Accepted: 29 June 2000     

Somatic embryogenesis in soybean via somatic embryo cycling

Summary The objectives of the present research were: a) to develop an efficient soybean embryogenic regeneration system characterized by a high frequency of explant response and a large number of somatic embryos per explant; b) to evaluate the factors affecting somatic embryogenesis via somatic embryo cycling; and c) to identify the origin of somatic embryos in the system. A highly improved and efficient system for soybean somatic embryogenesis was established using somatic embryo cotyledons and somatic embryo hypocotyl/radicle explants plated on α-naphthaleneacetic acid (NAA) or 2,4-dichlorophenoxyacetic acid (2,4-D) supplemented MS basal media. The system included somatic embryo cycling between liquid and solid medium and it consistently gave rise to a much higher frequency of explant response and a larger number of embryos per responding explant than those obtained from zygotic cotyledon explant tissues. Genotype, differences were observed for response in some of the treatments with cv “Fayette” being more responsive than “J103”. Histological studies revealed that somatic embryos induced in the somatic embryo cycling system originated almost exclusively from epidermal cells on both 2,4-D and NAA inductive media. The cells of the epidermis proliferated to produce somatic embryos directly without an intervening callus phase. A single-cell origin of somatic embryos was observed in cultures on a 40 mg/liter 2,4-D treatment. A large number of responding cells in the epidermis was also observed in the 10 mg/liter NAA treatment. The single-cell origin of somatic embryos from epidermal layers of the explant tissues should facilitate development of an efficient transformation system for soybean.     

The effect of auxin type and concentration on pecan (Carya illinoinensis) somatic embryo morphology and subsequent conversion into plants

Summary Embryogenic cultures were initiated from immature pecan zygotic embryos. Explants were induced for one week on Woody Plant Medium with either -naphthaleneacetic acid or 2,4-dichlorophenoxyacetic acid at 2, 6 or 12 mg/l, then subcultured monthly to fresh basal medium. Observations were made on callus production, embryo formation, and embryo morphology. Somatic embryo morphology and overall callus proliferation were affected by auxin type. Callus proliferation was less extensive and more somatic embryos resembling zygotic embryos were obtained from cultures initiated with -naphthaleneacetic acid than with 2,4-dichlorophenoxyacetic acid. Repetitive somatic embryogenesis was obtained in all auxin treatments. Conversion into plantlets was affected by somatic embryo morphology in that embryos with poorly developed apices exhibited lower percentages of conversion than those with well developed single or multiple apices. Consequently, although more embryos were obtained with 2,4-dichlorophenoxyacetic acid, naphthaleneacetic acid was the superior auxin for production of somatic embryos more likely to convert into plants.Abbreviations BAP 6-benzylaminopurine - IAA indole-3-acetic acid - IBA indole-3-butyric acid - NAA -naphthaleneacetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid - WPM Woody Plant Medium (Lloyd & McCown 1980)     

Regulation of organogenesis and somatic embryogenesis by auxin in melon,Cucumis melo L.

Various tissues of seeds and seedlings of melon were cultured in vitro to study the effects of auxin concentration on organogenesis and embryogenesis. Adventitious shoots and somatic embryos were formed from explants of cotyledons of mature seeds, hypocotyls of seedlings, and leaves and petioles of young plantlets. Expanded cotyledons of seedlings formed only adventitious shoots. All tissues responded similarly to the 2,4-D concentration in the media, that is, adventitious shoots were formed at low concentration, callus proliferated without differentiation at intermediate concentration and somatic embryos were induced at high concentration. Cotyledons of mature seeds formed both adventitious shoots and somatic embryos more efficiently than any other tissues cultured.Effects of three auxins, 2,4-D, NAA and IAA, on organogenesis and embryogenesis were compared using cotyledons of mature seeds. Adventitious shoots were formed at low level of auxins (0 to 0.01 mg/l 2,4-D; 0 to 0.1 mg/l NAA; 0 to 1.0 mg/l IAA), and embryos were formed at high level of auxins (1.0 to 2.0 mg/l 2,4-D; 3.0 to 10.0 mg/l NAA; 20.0 to 100.0 mg/l IAA). IAA gave more efficient shoot formation and embryogenesis than the other auxins.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - NAA -naphthaleneacetic acid - IAA 3indoleacetic acid - BA 6-benzylaminopurine - MS Murashige and Skoog     

A morphological and histological comparison of the initiation and development of pecan (Carya illinoinensis) somatic embryogenic cultures induced with naphthaleneacetic acid or 2,4-dichlorophenoxyacetic acid

Summary Somatic embryos produced in vitro may exhibit structural abnormalities that affect their subsequent germination and conversion into plants. To assess the influence of auxin type on embryo initiation and development, a morphological and histological comparison was made of pecan (Carya illinoinensis) somatic embryogenic cultures induced on media with naphthaleneacetic acid or 2,4-dichlorophenoxyacetic acid (2,4-D), using light and scanning electron microscopy. Both auxins promoted enhanced cell division, particularly in subepidermal cell layers. However, notable differences were observed in mitotic activity, location of embryogenic cell proliferation, epidermal continuity, callus growth, and embryo morphology. Cultures induced on naphthaleneacetic acid had embryogenic regions composed of homogeneous, isodiametric, meristematic cells. Embryos derived from these cultures generally had a normal morphology, were single, and had a discrete apical meristem. In contrast, tissues induced on media with 2,4-D had more intense and heterogeneous regions of cell division. Proliferating cell regions were composed of meristematic cells interspersed with callus and involved more extensive regions of the mesophyll. Marked callus proliferation caused epidermal rupture in some areas. Embryos induced on medium with 2,4-D had a higher incidence of abnormalities that included fasciated, fan-shaped, and tubular embryos. Defined apical meristems were often lacking or partially obliterated due to callus proliferation. The heterogeneous, often intensive proliferation of cells in cultures induced with 2,4-D may interfere with normal patterns of embryo development.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - BAP 6-benzylaminopurine - NAA naphthaleneacetic acid - SEM scanning electron microscopy     

Repetitive somatic embryogenesis from peanut cultures in liquid medium

Summary A regeneration system based on repetitive somatic embryogenesis was developed for peanut (Arachis hypogaea L.). Embryogenic suspension cultures were initiated using individual somatic embryos induced from immature cotyledons cultured on a modified Murashige and Skoog medium containing 40 mg/l 2,4-D for 30 days. After transfer to a modified MS liquid medium, the somatic embryos produced masses of secondary and tertiary embryos which continued to proliferate following manual separation and subculture of the embryogenic clumps. The cultures exhibited exponential growth, and have been maintained for over one year without apparent loss of embryogenic potential. Further embryo development, germination, and conversion were achieved by placing embryo clumps onto hormone-free, solid medium. The inclusion of a desiccation period during embryo development enhanced conversion four-fold. Plants have been established in soil and appear to be phenotypically normal.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - BA 6-benzylaminopurine - MSO Modified Murashige and Skoog basal medium - EM embryogenic masses     

In Plant Protoplasts, the Spontaneous Expression of Defense Reactions and the Responsiveness to Exogenous Elicitors Are under Auxin Control

When auxin was omitted during either the preparation or the culture of tobacco mesophyll protoplasts, as well as during both periods, synthesis of β-glucanase was spontaneously induced. In contrast, when protoplasts were prepared and cultured in the presence of 16 micromolar 1-naphthaleneacetic acid (optimal concentration for protoplast division), the expression of β-glucanase was maintained close to the minimal level observed in tobacco leaves. This inhibitory effect was only promoted by active auxins (1-naphthaleneacetic acid, 2,4-dichlorophenoxyacetic acid, 2,4,5-trichlorophenoxyacetic acid, and 3-indoleacetic acid) but not by inactive auxin analogs. Tobacco protoplasts responded to exogenous elicitors from the cell wall of Phytophthora megasperma glycinea (Pmg) by accumulating β-glucanase in the presence of 16 micromolar 1-naphthaleneacetic acid. At higher auxin concentrations, the elicitor-induced β-glucanase synthesis was inhibited. Naphthaleneacetic acid concentration (3 × 10⁻⁵ molar) required to inhibit by 50% the expression of this defense reaction triggered by a near-optimal elicitor concentration was about 100 times higher than that sufficient to inhibit by 50% the spontaneous expression in nonelicited protoplasts. This is the first demonstration of an auxin-fungal elicitor interaction in the control of a defined defense reaction. The above observations were extended to soybean cell protoplasts. The Pmg elicitor-induced stimulation of the synthesis of pathogenesis related P17 polypeptides and of a 39-kilodalton peptide immunologically related to tobacco β-glucanase was only observed when the spontaneous accumulation of these proteins was inhibited in auxin-treated protoplasts.     

Somatic embryogenesis from immature embryos of redbud (Cercis canadensis)

Somatic embryos developed directly from 96 and 110 day post-anthesis Cercis canadensis L. (redbud) zygotic embryos from one of two trees sampled that were explanted onto modified Schenk and Hildebrandt medium amended with either 1, 2, 3 or 5 mg/1 2,4-D in combination with either 7.6 or 12. 6 mM ammonium ion. Although somatic embryogenesis was expressed on most media, the number of explants that produced somatic embryos and the mean number of embryos formed per explant were greatest on media that contained either 2 or 3 mg/1 2,4-D; 12.6 mM ammonium ion inhibited embryogenesis from 96 day post-anthesis explants. Zygotic embryos explanted 117 days after anthesis produced only callus and roots. Somatic embryos that were bottle-shaped or had distinct cotyledons organized roots on germination media, but only one embryo formed a shoot. No additional development occurred. Histological examination of somatic embryos showed that shoot apical meristems were poorly developed.Abbreviations 2,4-D 2, 4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - BAP 6-benzylaminopurine - FPA Formalin-propionic acid-ethanol (50%)     

Maturation and germination of somatic embryos as affected by sucrose and plant growth regulators in soybeans Glycine gracilis Skvortz and Glycine max (L.) Merr.

The effects of sucrose on maturation and of plant growth regulators on germination of soybean somatic embryos were investigated for the purpose of developing an efficient culture method for plant recovery. Somatic embryos produced on medium with a low sucrose concentration (5 gl^-1), less than 1 mm in length, 0.6 mg in fresh weight, and green in color, were grown for 2 weeks on MS medium containing 5 gl^-1 or 30 gl^-1 sucrose and then for another 5 weeks on MS medium containing 5–90 gl^-1 sucrose. The highest increase in fresh weight of somatic embryos was obtained in the treatment of transferring from 30 gl^-1 sucrose (2 weeks) to 60 gl^-1 (5 weeks). With the increase in fresh weight, the somatic embryos gradually changed color from green to yellow, and finally to white, when they stopped growth. Soybean seed storage proteins (-conglycinin and glycinin) were accumulated in somatic embryos under tissue specific and stage specific control analogous to that in zygotic embryos. Exogenous gibberellic acid was effective in promoting precocious germination of premature soybean somatic embryos, but was not necessary for the germination of mature somatic embryos. The efficiency of somatic embryo germination was as high as 77% from semi-wild soybean and 60–64% from cultivated soybeans, showing that the plant regeneration system developed in this study was efficient and practical.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - BR brassinolide - GA₃ gibberellic acid - IBA indolebutyric acid - NAA -naphthaleneacetic acid - PAGE polyacrylamide gel electrophoresis - SDS Sodium Lauryl Sulfate     

Somatic embryogenesis and plant regeneration from leaf derived callus of winged bean [Psophocarpus tetragonolobus (L.) DC.]

Somatic embryos were obtained from callus cultures derived from leaf explants of the winged bean, Psophocarpus tetragonolobus (L.) DC. Initiation and development of the somatic embryos occurred with a two-step culture method. Callus cultures initiated on MS medium with NAA and BAP, upon transfer to a new medium with IAA and BAP, produced somatic embryos. Maximum embryogenesis of 60% was obtained on induction medium with 0.5 mg/l NAA plus 1.0 mg/l BAP followed by transfer to a secondary medium with 0.1 mg/l IAA and 2.0 mg/l BAP. Optimal embryo germination and plantlet development was achieved on MS medium with 0.2 mg/l BAP plus 0.1 mg/l IBA. The regenerated plants were successfully transferred to glasshouse conditions.Abbreviations MS Murashige and Skoog (1962) medium - 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - IAA Indole-3-acetic acid - IBA Indole-3-butyric acid - BAP 6-benzylaminopurine - KN Kinetin     

Plant regeneration from somatic embryogenic suspension cultures of cotton (Gossypium hirsutum L.)

Maintainable, highly embryogenic suspension cultures of cotton (Gossypium hirsutum L. cv. Coker 310) have been obtained. Callus cultures were initiated from cotyledonary tissues from aseptically-germinated seedlings. To establish the suspension cultures, callus tissue was placed in a liquid medium containing either 0.5 mg/l picloram or 0.1 mg/l 2,4-dichlorophenoxyacetic acid. For proliferation of the embryogenic suspension, 5 mg/l of 2,4-dichlorophenoxyacetic acid was used. Embryo development took place when the embryogenic tissue was transferred to an auxin-free liquid medium containing 15 mM glutamine. Early embryo development was fairly synchronous and large numbers of somatic embryos were produced. Regenerated plants were fertile and smaller than seed-derived plants.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - IAA indole-3-acetic acid     

Embryo culture of Costus speciosus (Koen.) Sm. to regenerate variable diosgenin yielding clones

Mature embryos of Costus speciosus were excised and cultured on Schenk and Hildebrandt's (1972) nutrient medium containing auxins and cytokinins either alone or in combination. Multiple shoots were obtained when kinetin and indole-3-butyric acid were supplemented each at 0.1 mg 1^–1 concentration. Embryo-derived plantlets were multiplied through propagation of rhizomes and the propagules derived from a single embryo were designated as an embryoclone. Twenty such embryo-clones were maintained in the field. Variations in rhizome biomass yield and diosgenin contents of these embryoclones were noted. Thirty-six percent of the embryo-clones studied were high diosgenin yielding types. Diosgenin contents at the intraclonal level were uniform. The in vitro raised plants were morphologically uniform and indistinguishable from their parent.Abbreviations SH Schenk and Hildebrandt (1972) medium - Kn kinetin - BAP 6-benzylaminopurine - 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - IBA indole-3-butyric acid - IAA indole-3-acetic acid - CA casaminoacids (vitamin free) - TLC thin layer chromatography     

Effects of auxins,cytokinins, carbohydrates and amino acids on somatic embryogenesis induction from shoot apices of pea

Studies were conducted to test the effects of various auxins, cytokinins, carbohydrates and amino acids on somatic embryogenesis from shoot apices of pea (Pisum sativum L.) cultured on a sole medium. Picloram (4.5 M) and 4-chlorophenoxyacetic acid (45 M) were the most effective auxins. Addition of cytokinins (benzyladenine, zeatin, kinetin) to auxin-containing medium reduced embryo production. Amino acids (glutamine, alanine, proline) did not improve somatic embryogenesis. Carbohydrate seemed to be a critical factor. Embryogenic efficiency and embryo development were promoted by high carbohydrate concentration. The best results were obtained with fructose (252–504 mM); the number of somatic embryos per cultured explant was 3- to 4-fold higher compared to the control (84 mM sucrose). From these results, an optimized induction medium is proposed.Abbreviations BA benzyladenine - 4-CPA 4-chlorophenoxyacetic acid - 2,4-d 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - 2,4,5-T 2,4,5-trichlorophenoxyacetic acid - MS Murashige & Skoog - EE embryogenic explants - G globular somatic embryos     

Expression of abundant mRNAs during somatic embryogenesis of white spruce [Picea glauca (Moench) Voss]

Embryogenic tissues of white spruce [Picea glauca (Moench) Voss] remain in an early developmental stage while cultured on 2,4-dichlorophenoxyacetic acid and N⁶-benzyladenine, but develop to cotyledonary embryos when these phytohormones are replaced by abscisic acid. Twenty-eight cDNAs were isolated from cotyledonary embryos by differential screening against immature embryo and non-embryonic tissues. Temporal expression patterns of these cDNAs during ABA-stimulated somatic embryo development were observed. This showed that clones could be allocated to various groups, including embryo-abundant, embryo-maturation-induced, and those whose expression was modulated during embryo development, germination or in non-embryogenic tissues. Expression corresponding to these cDNA clones showed that there were various responses to exogenous ABA or polyethylene glycol during a period of 48 h. Analyses of DNA and predicted amino acid sequence revealed that 12 of 28 cDNA clones had no known homologues, while others were predicted to encode different late-embryogenesis-abundant proteins, early methionine-labelled proteins, storage proteins, heat-shock proteins, glycine-rich cell wall protein, metallothionein-like protein and some other metabolic enzymes.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - ABA abscisic acid - BA N⁶-benzyladenine - cDNA complementary deoxyribonucleic acid - Em early methionine-labelled - HSP heat-shock protein - LEA late embryogenesis abundant - PEG polyethylene glycol The authors thank Mr. Terry Bethune for his assistance, and Dr. Larry Pelcher, Mr. Barry Panchuk and Mr. Don Schwab for DNA sequencing and primer synthesis. This is National Research Council of Canada publication number 38929.     

Somatic embryogenesis and plant regeneration from leaf tissue of jojoba

A protocol was developed for the induction, maturation and germination of somatic embryos from leaf tissue of jojoba [Simmondsia chinensis (Link) Schneider]. Explants were placed on their adaxial sides in Petri dishes and maintained in darkness on half-strength Murashige and Skoog basal medium (MS/2). Combinations of 2,4-dichlorophenoxyacetic acid (1.35–4.52 μM) with 6-benzylaminopurine (1.33–4.43μM) and 2 synthetic cytokinins, N-(2-chloro-4pyridyl)-N′-phenylurea (1.21–4.03μM) or (E)-6-[3-(trifluoromethyl)-but-2-enylamino] purine (1.11–3.71μM) resulted in formation of embryogenic cultures and somatic embryos. After two 30-day subcultures, embryogenic cultures were transferred onto MS/2 medium supplemented with different auxins and cytokinins. Somatic embryo maturation, germination and plantlet formation were achieved using 1-naphthaleneacetic acid (3.75μM) or indole-3-butyric acid (3.44μM) in combination with BA (0.44 or 1.33μM) or F3iP (0.37 or 1.11μM). Histology confirmed each stage of development. This revised version was published online in June 2006 with corrections to the Cover Date.     

Somatic embryogenesis of Agave victoria-reginae Moore

Plant regeneration through direct somatic embryogenesis of leaf blade explants from in vitro propagated plants of Agave victoria-reginae Moore, is described. Somatic embryogenesis was evident in a 6-week period on agarsolidified MS medium supplemented with L2 vitamins and 2,4-dichlorophenoxyacetic acid (1,4 µM), and germination of somatic embryos was achieved after 8 weeks on half-strength MS medium and 4 weeks on half-strength SH medium, both lacking growth regulators. Hyperhydricity of somatic embryos and plantlets was reduced by the use of vented culture vessel lids during the last 4 weeks on SH medium. Shoot proliferation was obtained, and hyperhydricity was eliminated on a modified MS medium (with NH₄NO₃ reduced to 5 mN) supplemented with kinetin (4.6 µM) and 1-naphthaleneacetic acid (1.6 µM) and the use of vented culture vessel lids.Abbreviations 2,4-d 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - MS Murashige and Skoog - LOG-1 MS modified medium by Castro-Concha et al. (1990) - L2 Phillips and Collins (1979) vitamins - SH Schenk and Hildebrandt     

<Emphasis Type="Italic">In vitro</Emphasis> plant regeneration via somatic embryogenesis through cell suspension cultures of horsegram [<Emphasis Type="Italic">Macrotyloma uniflorum</Emphasis> (Lam.) verdc.]

Summary In vitro regeneration of plants via somatic embryogenesis through cell suspension culture was achieved in horsegram. Embryogenic calluses were induced on leaf segments on solid Murashige and Skoog (MS) medium with 9.0 μM 2,4-dichlorophenoxyacetic acid (2,4-D). Differentiation of somatic embryos occurred when the embryogenic calluses were transferred to liquid MS medium containing 2,4-D. Maximum frequency (33.2%) of somatic embryos was observed on MS medium supplemented with 7.9 μM 2,4-D. Cotyledonary-torpedo-shaped embryos were transferred to liquid MS medium without growth regulators for maturation and germination. About 5% of the embryos germinated into plants, which grew further on solid MS medium. The plants were hardened and established in soil. Effects of various auxins, cytokinins, carbohydrates, amino acids, and other additives on induction and germination of somatic embryos were also studied. A medium supplemented with 7.9 μM 2,4-D, 3.0% sucrose, 40 mg l⁻¹ L-glutamine, and 1.0 μM abscisic acid was effective to achieve a high frequency of somatic embryo induction, maturation, and further development.     

Direct somatic embryogenesis and plant regeneration from immature embryos of hybrid sunflower (Helianthus annuus L.) on a high sucrose-containing medium

Somatic embryos of sunflower (Helianthus annuus) were obtained by placing immature zygotic embryos on a high sucrose (12%) containing medium. The somatic embryos were first observed 6 days after culture and a callus intermediate was not formed. Histological examination revealed the classical stages of embryo development. The somatic embryos proliferated directly from the surface of the zygotic embryos and germinated after placement on a low sucrose medium. Seeds have been obtained from regenerated plants.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - NAA 1-naphthaleneacetic acid - IAA Indole-3-acetic acid - BAP 6-benzylamino purine. Salaries and research support were provided by State and Federal funds appropriated to OSU-OARDC. Journal Article No. 67–87     

Somatic embryogenesis and plant regeneration inPisum sativum L.

Somatic embryogenesis was induced in immature zygotic embryos of pea (Pisum sativum L.), synthetic auxins α-naphthalene acetic acid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D) and 4-amino-3,5,6-trichloropicolinic acid (picloram, PIC) being used. Only one (line HM-6) of 46 genotypes tested exhibited good potential for somatic embryogenesis. 2,4-D was found as the best somatic embryo inductor. Three different ways of somatic embryo conversion have been described. Plantlets from individual somatic embryos were micropropagated as somaclones and subsequently rooted. A sterile morphological mutant has been found within a group of fertile plants of T₀-generation. Sufficient amount of T₁-seeds is available for somaclonal variation studies.     

Early time course and specificity of auxin effects on turgor movement of the bean pulvinus

Auxin application to the upper side of the pulvinus of primary leaves of Phaseolus vulgaris L. promoted bending away from the place of application. The effect had a latency of less than 20 min and was specifically induced by substances known as active auxins in growth tests (indoleacetic and 1-naphthaleneacetic acid) but not by inactive auxin analogs (2-naphthaleneacetic, 3-indolepropionic and benzoic acid); 2,4-dichlorophenoxyacetic acid, and L-(-)-2,4-dichlorophenoxyisopropionic acid were of intermediate activity. Auxin-promoted bending was reversible and presumably caused by turgor increase in the treated cells.Abbreviations IAA 3-indolacetic acid - NAA naphthylacetic acid