Inhibition of the indole-3-acetic acid-induced epinastic curvature in tobacco leaf strips by 2,4-dichlorophenoxyacetic acid

It has been reported that auxin induces an epinastic growth response in plant leaf tissues. Leaf strips of tobacco (Nicotiana tabacum L. 'Bright Yellow 2') were used to study the effects of indole-3-acetic acid (IAA), the principal form of auxin in higher plants, and a synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D), on epinastic leaf curvature. Incubation of leaf strips with 10 micro M IAA resulted in a marked epinastic curvature response. Unexpectedly, 2,4-D showed only a weak IAA-like activity in inducing epinasty. Interestingly, the presence of 2,4-D resulted in inhibition of the IAA-dependent epinastic curvature. In vivo Lineweaver-Burk kinetic analysis clearly indicated that the interaction between IAA and 2,4-D reported here is not a result of competitive inhibition. Using kinetic analysis, it was not possible to determine whether the mode of interaction between IAA and 2,4-D was non-competitive or uncompetitive. 2,4-D inhibits the IAA-dependent epinasty via complex and as yet unidentified mechanisms.     

The Effects of Synthetic Growth-regulator Treatments on the Levels of Free Endogenous Growth-substances in Plants

The possible effects of synthetic auxins and anti-auxins onthe metabolism of indole-3-acetic acid (IAA) in plant tissueshave not been properly studied in the past. For this reasonseedlings of peas, beans, and sunflower have been treated withthe synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D)and two supposed anti-auxins, 2,3,5-tri-iodobenzoic acid (TIBA)and maleic hydrazide (MH), at non-toxic levels sufficient tocause well-marked growth responses. Estimates of the contentof alcohol-extractable growth-substances have subsequently beendetermined, after separation by paper partition chromatography.Although at least six active natural compounds have been indicatedin such extracts, only the effects of treatment on IAA levelshave been followed in detail. 2,4-D treatment of both leaves and roots has no detectable effecton the levels of free endogenous IAA, and it is thereby concludedthat 2,4-D is an auxin in its own right and does not act ongrowth via a disturbance of IAA metabolism. There are indicationsthat considerable amounts of the absorbed 2,4-D are convertedin plant tissues to a neutral detoxication product which iseasily decomposed to liberate 2,4-D during chromatographic analysis. TIBA treatment of pea roots dramatically reduces their freeendogenous IAA content, in some cases to 1/10,000 the normallevel. The implications of these findings are discussed in termsof the physiological and morphological responses of plants toTIBA treatment. There are indications that MH may put up slightly the levelof free endogenous auxin in pea roots but further confirmatorywork is required.     

Timing of the response of coleoptiles to the application and withdrawal of various auxins

Summary A study was made of the time courses of growth promotion and the reversal of growth promotion upon the addition and withdrawal of various auxins. Growth promotion by 1-naphthaleneacetic acid (NAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) occurs more slowly and is less vigorous than growth promotion by the same concentration of indoleacetic acid (IAA).The time required for the reversal of the stimulation of elongation by auxin is many times greater for 2,4-D-stimulated growth than for IAA- or NAA-stimulated growth (80 min vs. about 10 min). This difference appears to be due to the sluggish exit of 2,4-D since (1) experiments with labeled auxins show that 2,4-D moves out of the tissue more slowly than IAA, and (2) it is possible to shorten the time required for a decline in elongation rate after the removal of 2,4-D to 13 min by adding an auxin antagonist (p-chlorophenoxyisobutyric acid).The rapid reversal of the hormonal stimulation of growth is discussed in relation to possible mechanisms of action of auxin.     

Quantitative studies of embryogenesis in normal and 5-methyltryptophan-resistant cell lines of wild carrot

The frequency of embryo formation was determined in normal and 5-methyltryptophan-resistant (5-MT^r) cell lines of wild carrot (Daucus carota L.) grown in the presence or absence of 2-isopentenyladenine (2-ip) and 2,4-dichlorophenoxyacetic acid (2,4-D). 2-ip stimulated the intitation of embryo formation and also accelerated embryo development. 2.4-D inhibited embryo differentiation at several stages: at 0.1 mg/l, it stopped regeneration at the earliest stage, resulting in callus growth instead of embryo formation; at 0.04 mg/l 2,4-D, some globular embryos were produced, but they did not develop into more advanced embryos. Variant cell lines with higher levels of auxin (indole-3-acetic acid, IAA) were used to study the effect of an elevated endogenous concentration of auxin on embryogenesis. IAA at these concentrations suppressed regeneration in the same manner as the exogenous auxin, 2,4-D, did. This result confirms the hypothesis that high levels of IAA are responsible for the suppression of regeneration in the 5-MT^r cell lines.     

Effect of 2,4-Dichlorophenoxyacetic Acid on Growth and on β-Glucan Synthetases of Peroxyacetyl Nitrate Pretreated Avena Coleoptile Sections

Coleoptile sections from Avena sativa L. were exposed to non-lethal concentrations of peroxyacetyl nitrate (PAN). The sections were then incubated in solutions of 50 mM glucose plus 2.5 mM potassium phosphate with various concentrations of 2,4-dichlorophenoxyacetic acid (2,4-D). Growth after 4 hours was measured. A corresponding series of experiments was carried out and the effect of the 2,4-D treatments on enzymes utilizing uridine diphosphate glucose (¹⁴C-glucose) to form glucolipid and β-glucans including cellulose was determined. Growth in the PAN-treated sections was inhibited less at optimal and superoptimal auxin levels than at low auxin levels. Glucolipid synthetase activity was only slightly inhibited by PAN pretreatment and was reduced by increasing levels of auxin. Responses of alkali-soluble glucan and cellulose synthetases were similar to growth in both control and PAN treated tissues. It was concluded that the earlier reported response of cell wall metabolism in vivo probably is due to effects on these enzyme levels.     

Oxidation of NADH by hypocotyl segments from soybean is stimulated by 2,4-D

Sections cut from regions of cell elongation of hypocotyls of dark-grown soybean seedlings oxidized externally supplied NADH as estimated from the decrease in A₃₄₀ measured spectrophotometrically. The oxidation of NADH by 1-cm sections was stimulated 1.5- to 2-fold by 1 μM of the synthetic auxin, 2,4-dichlorophenoxyacetic acid (2,4-D). 2,4-D-Stimulated oxidation of NADH was resistant to cyanide. Stimulations were also given by the naturally occurring auxin, indole-3-acetic acid (IAA) but not by the growth inactive 2,4-D analog 2,3-dichlorophenoxyacetic acid (2,3-D) and the growth inactive β-naphthaleneacetic acid (β-NAA). Since NADH is a membrane impermeant substrate, the findings confirm studies with inside-out and right-side-out vesicles that show the 2,4-D-stimulated NADH oxidase to be located at the external cell surface. Cut surfaces are not responsible for the activity as shown by experiments with lanolin-sealed sections. The external NADH oxidase measurements do not require special equipment and exhibit characteristics normally associated with enzyme-catalyzed reactions.     

The Initiation, Growth, and Characteristics of a Tissue 2

The rate and extent of initiation of callus from potato tuberdiscs depends on the concentrations of auxin and kinetin inthe medium on which they are grown. NAA is the most effectiveauxin, initiating callus at a concentration (0. 01 mg/1) anorder of magnitude lower than for IAA or 2,4-D. There is a week'slag before initiation begins with IAA or 2,4-D. In combinationwith each auxin, kinetin is inhibitory to initiation of callusand its growth on the explant. High-intensity light and lowtemperature are also inhibitory. In isolated callus subcultured so as to prevent dilution ofits accumulated auxin, the only effect of varying exogenousauxin levels is as a progressive inhibition by NAA. If thisdilution is permitted, however, 2,4-D and IAA have an optimumgrowth promoting activity at 1 mg/1, whereas the effect of NAAincreases up to 10 mg/1. The growth of the callus is affectedby agar concentration (1 per cent optimum), and is halted bypH values below 5. The callus grows on various carbon sourcesbut is dependent upon one or more components of N. Z. Amine;it also requires a number of micronutrients. A suspension culture from the callus exhibits the usual growthcurve. The phenolic content follows a pattern different fromthat of growth, protein, and RNA content, and phenolics arerapidly synthesized as active growth ceases. In contrast tothe callus tissue, the suspension culture grows at a wide rangeof pH values and buffers the medium. At low temperatures in the light, potato discs produce greencallus with a chlorophyll content corresponding to that of thediscs from which they grew. The isolated callus tissue doesnot require kinetin and produces and excretes its own cytokinin(s).The amount synthesized varies over the growth cycle.     

Relationship of indole-3-acetic acid and tryptophan concentrations in normal and 5-methyltryptophan-resistant cell lines of wild carrots

A 5-methyltryptophan(5-MT)-resistant cell line of wild carrot (Daucus carota L.), W001, that exhibited auxin-independent callus growth, was found to accumulate indole-3-acetic acid (IAA) and tryptophan (trp). Anthranilate-synthetase activity in W001 cell extract was less sensitive to feedback inhibition by trp than in the original 5-MT-sensitive cell lines. It is hypothesized that the resistant enzyme allowed more trp synthesis and accumulation which, in turn, affected the IAA concentration in the cell. Since carrot cultures cannot regenerate in the presence of exogenous auxin, the elevated IAA concentration in W001 may be responsible for its drastically reduced capacity to regenerate. The relationship between trp and IAA levels was further investigated by examining the effect of 2,4-dichlorophenoxy acetic acid (2,4-D) on the endogenous concentration of trp and IAA. In general, the IAA level was reduced but the trp concentration was elevated when 2,4-D was present in the culture medium.Abbreviations 2,4-D 2,4-dichlorophenoxyacetic acid - IAA indole-3-acetic acid - 5-MT 5-methyltryptophan - 5-MT^r 5-MT-resistant - 5-MT^s 5-MT-sensitive - trp tryptophan     

Genotypic Responses to Auxins in Tissue Cultures of Phaseolus

Responses of seven Phaseolus genotypes to the four auxins picloram, 2,4-D, NAA and 1AA, in tissue cultures, were examined. Callus growth was promoted by picloram and the range of effective concentrations for most genotypes was broad. The auxin 2,4-D also enhanced callus growth, but the range of optimal concentrations was markedly narrower than that of picloram. NAA when supplied at relatively high concentrations gave good growth. IAA was ineffective in supporting callus growth. The differences in 2,4-D concentrations required for optimal growth and the differential responses to low concentrations (0.04–1.25 μM) of picloram between several genotypes tested were large. These genotypic variations in auxin responses were repeatable and may thus reflect genetic differences.     

Effect of 2,4-D on Cell Wall Metabolism of Peroxyacetyl Nitrate Pretreated Avena Coleoptile Sections

Avena coleoptile sections were exposed to nonlethal concentrations of peroxyacetyl nitrate (PAN). The sections were then incubated in solutions of 50 mM glucose plus 2.5 mM poassium phosphate with various concentrations of 2,4-dichlorophenoxycetic acid (2,4-D). Growth after 4 hours was measured. A corresponding series of experiments was carried out with glucose-¹⁴C (U) in the subsequent incubation medium and the effect of the 2,4-D treatments on ¹⁴C incorporation into various cell wall components was determined. Growth in the PAN-treated sections, although still partially inhibited, was greater at auxin levels normally superoptimal for growth than at the former optimum. Incorporation into all cell wall fractions was similar to growth in the case of control treated tissue. Most of the cell wall constituents, but particularly cellulose and less soluble noncellulosic polysaccharides, tended to show higher incorporation at the levels where PAN-treated growth was also higher. It was concluded that effects by PAN on cell wall metabolism in growing tissue are similar to the effects on growth and that the mechanism of alleviation of growth inhibition is probably through decreased inhibition of wall metabolism.     

INDUCTION OF WOUND-VESSEL DIFFERENTIATION IN ISOLATED COLEUS STEM SEGMENTS IN VITRO

Excised internodes and 2-mm-thick transverse stem segments of Coleus blumei were incubated 7 days on media containing 2% sucrose, 1% agar, and various growth substances. Wound-vessel members differentiated in the 2-mm-thick tissue slices incubated on medium containing no exogenous auxin (control). Compared to control slices, the addition to the medium of either IAA (50 or 5 ppm), 2, 4-D (10, 1, or 0.1 ppm), TIBA (50, 5, or 0.5 ppm), or kinetin (50, 5, 0.5 or 0.05 ppm) inhibited wound-vessel differentiation. Simultaneous treatment of tissue slices with IAA and kinetin inhibited wound-vessel differentiation, as did the incubation of tissue slices on medium containing no sucrose. Low concentrations of IAA (0.05 ppm) or 2, 4-D (0.01 ppm) resulted in over a 100% increase in the numbers of wound-vessel members differentiated. These results are interpreted as indicating auxin synthesis by the tissue slices and the participation of auxin as a limiting factor in xylogenesis. The inhibition of wound-vessel differentiation by relatively high concentrations of 2,4-D, TIBA, or kinetin is interpreted as a reflection of the inhibition of polar auxin transport by these substances, and an indication that polar auxin transport enhances xylogenesis.     

Auxin and the response of pea roots to auxin transport inhibitors: morphactin

The auxin transport inhibitor methyl-2-chloro-9-hydroxyfluorene-9-carboxylate (CFM), a morphactin, inhibits negative geotropism, causes cellular swelling, and induces root hair formation in roots of intact Pisum sativum L. seedlings. In excised pea root tips, CFM inhibits elongation more than increase in fresh weight (swell ratio = 1.3 at 20 mum CFM). CFM growth inhibition was expressed in the presence of ethylene. Indoleacetic acid (IAA) prevented the expression of CFM growth inhibition possibly because IAA inhibited the accumulation of CFM into the tissue sections. CFM inhibited the accumulation of IAA and 2,4-dichlorophenoxyacetic acid into excised root tips. Applying Leopold's (1963. Brookhaven Symp. Biol. 16: 218-234) model for polar auxin transport, this result suggests a possible explanation for CFM inhibition of geotropism in pea roots, i.e. disruption of auxin transport by interfering with auxin binding.     

Auxin-resistant mutants of Arabidopsis thaliana with an altered morphology

Summary Mutant lines of Arabidopsis thaliana resistant to the artificial auxin 2,4-dichloro phenoxyacetic acid (2,4-D) were isolated by screening for growth of seedlings in the presence of toxic levels of 2,4-D. Genetic analysis of these resistant lines indicated that 2,4-D resistance is due to a recessive mutation at a locus we have designated Axr-1. Mutant seedlings were resistant to approximately 50-fold higher concentrations of 2,4-D than wild-type and were also resistant to 8-fold higher concentrations of indole-3-acetic acid (IAA) than wild-type. Labelling studies with (¹⁴C)2,4-D suggest that resistance was not due to changes in uptake or metabolism of 2,4-D. In addition to auxin resistance the mutants have a distinct morphological phenotype including alterations of the roots, leaves, and flowers. Genetic evidence indicates that both auxin resistance and the morphological changes are due to the same mutation. Because of the pleiotropic morphological effects of these mutations the Axr-1 gene may code for a function involved in auxin action in all tissues of the plant.     

Control of Internode Length in Pisum sativum (Further Evidence for the Involvement of Indole-3-Acetic Acid)

The effects of altered endogenous indole-3-acetic (IAA) levels on elongation in garden pea (Pisum sativum L.) plants were investigated. The auxin transport inhibitors 2,3,5-triiodobenzoic acid (TIBA) and 9-hydroxyfluorene-9-carboxylic acid (HFCA) were applied to elongating internodes of wild-type and mutant lkb plants. The lkb mutant was included because elongating lkb internodes contained 2- to 3-fold less free IAA than those of the wild type. In the wild type, TIBA reduced both the IAA level and internode elongation below the site of application. Both TIBA and HFCA strongly promoted the elongation of lkb internodes and also raised IAA levels above the application site. The synthetic auxin 2,4-dichlorophenoxyacetic acid (2,4-D) also markedly increased internode elongation in lkb plants and virtually restored petioles and tendrils to their wild-type length. In contrast, treatment of wild-type plants with TIBA, HFCA, or 2,4-D caused little or no increase in elongation above the application site. The ethylene synthesis inhibitor aminoethoxyvinylglycine also increased stem elongation in lkb plants, and combined application of HFCA and aminoethoxy-vinylglycine restored lkb internodes to the wild-type length. It is concluded that the level of IAA in wild-type internodes is necessary for normal elongation, and that the reduced stature of lkb plants is at least partially attributable to a reduction in free IAA level in this mutant.     

Regulation of genes associated with auxin, ethylene and ABA pathways by 2,4-dichlorophenoxyacetic acid in Arabidopsis

The chemical 2,4-dichlorophenoxyacetic acid (2,4-D) regulates plant growth and development and mimics auxins in exhibiting a biphasic mode of action. Although gene regulation in response to the natural auxin indole acetic acid (IAA) has been examined, the molecular mode of action of 2,4-D is poorly understood. Data from biochemical studies, (Grossmann (2000) Mode of action of auxin herbicides: a new ending to a long, drawn out story. Trends Plant Sci 5:506–508) proposed that at high concentrations, auxins and auxinic herbicides induced the plant hormones ethylene and abscisic acid (ABA), leading to inhibited plant growth and senescence. Further, in a recent gene expression study (Raghavan et al. (2005) Effect of herbicidal application of 2,4-dichlorophenoxyacetic acid in Arabidopsis. Funct Integr Genomics 5:4–17), we have confirmed that at high concentrations, 2,4-D induced the expression of the gene NCED1, which encodes 9-cis-epoxycarotenoid dioxygenase, a key regulatory enzyme of ABA biosynthesis. To understand the concentration-dependent mode of action of 2,4-D, we further examined the regulation of whole genome of Arabidopsis in response to a range of 2,4-D concentrations from 0.001 to 1.0 mM, using the ATH1-121501 Arabidopsis whole genome microarray developed by Affymetrix. Results of this study indicated that 2,4-D induced the expression of auxin-response genes (IAA1, IAA13, IAA19) at both auxinic and herbicidal levels of application, whereas the TIR1 and ASK1 genes, which are associated with ubiquitin-mediated auxin signalling, were down-regulated in response to low concentrations of 2,4-D application. It was also observed that in response to low concentrations of 2,4-D, ethylene biosynthesis was induced, as suggested by the up-regulation of genes encoding 1-aminocyclopropane-1-carboxylic acid (ACC) synthase and ACC oxidase. Although genes involved in ethylene biosynthesis were not regulated in response to 0.1 and 1.0 mM 2,4-D, ethylene signalling was induced as indicated by the down-regulation of CTR1 and ERS, both of which play a key role in the ethylene signalling pathway. In response to 1.0 mM 2,4-D, both ABA biosynthesis and signalling were induced, in contrast to the response to lower concentrations of 2,4-D where ABA biosynthesis was suppressed. We present a comprehensive model indicating a molecular mode of action for 2,4-D in Arabidopsis and the effects of this growth regulator on the auxin, ethylene and abscisic acid pathways. Experiment station: Plant Biotechnology Centre, Primary Industries Research Victoria, Department of Primary Industries, La Trobe University, Bundoora, Victoria 3086, and the Victorian Microarray Technology Consortium (VMTC).     

Inhibition of auxin-induced cell expansion in Jerusalem artichoke tuber slices by low concentrations of chloramphenicol

Summary D-threo chloramphenicol (CAP) at 5×10^-5 M, given continuously during a 24-hr aging period and subsequent post-age treatment with 2,4-dichlorophenoxyacetic acid (2,4-D)±kinetin markedly depressed cell expansion in Jerusalem artichoke (Helianthus tuberosus) tuber slices. Both the rate and total amount of expansion were reduced. An inhibitory effect of CAP could be detected at a concentration as low as 6.2×10^-6M with 2,4-D alone and 1.6×10^-6 M with 2,4-D+kinetin. CAP also inhibited if given with 2,4-D to unaged tissue, and partially inhibited growth of aged tissue when supplied only during or only after aging. Expansion was inhibited when IAA was used in place of 2,4-D. Growth of tissue slices free of detectable bacteria was depressed by CAP, eliminating a possible indirect action of the antibiotic through inhibition of beneficial bacteria. CAP also prevented appearance of pink and brown pigments which normally occur in association with auxin-treated tissues. L-threo CAP did not inhibit growth or pigment formation. Cell division in the tuber slices was not inhibited, and was possibly even stimulated, by D-threo CAP, even at a concentration of 2×10^-4 M. It is concluded that the use of CAP for bacterial control in plant cultures can be hazardous and needs careful checking. Presumably the inhibitory action of CAP results from inhibition of growth-dependant protein metabolism in mitochondria and/or plastids which occurs both during aging and post-aging growth. Partial suppression of metabolic changes during aging would maintain the tissue in a state favouring relatively high mitotic activity and slow growth in response to auxin.     

Auxin effects on rooting in pea cuttings

Light-grown stem cuttingss of Pisum sativum L. cv. Weibull's Marma were rooted in a nutrient solution. The presence of 10 μ M indolylacetic acid (IAA) in the solution for 24 h or longer periods decreased the number of roots subsequently formed to about 50% of control, provided IAA was present in the solution during any of the 4 first 24 h periods. Treatment for 6 h or shorter periods caused no or small response. IAA did not appreciably change the time needed for root formation, the time course of root appearance or the pattern of root distribution along the basal internode. IAA at 100 μ M usually increased the number of roots although variable results were obtained with this IAA concentration.
The number of roots was strongly increased by treatment with indolylbutyric acid (IBA) or 2,4-dichlorophenoxyacetic acid (2,4-D). None of these or other synthetic auxins decreased the number of roots in suboptimal concentrations. Experiments with 10 μ M IBA showed that stimulation of rooting was obtained only if the auxin was present in the rooting solution for several days. Simultaneous treatment with IAA decreased the stimulating effect of IBA to some extent, whereas no such response was obtained if IAA was combined with 2,4-D.
IAA applied in lanolin to the stem of intact cuttings decreased the number of roots formed. Decapitation and debudding of the cuttings decreased the number of roots formed. If at least 2 leaves were left this decrease was efficiently counteracted by an optimal IAA dose applied to the upper part of the stem. A five times higher dose was less effective, indicating a negative effect on rooting also by IAA applied to the shoots.     

Effect of diclofop-methyl and 2,4-D on transmembrane proton gradient: a mechanism for their antagonistic interaction

The site of action of the postemergence graminicide, diclofop-methyl (DM), in susceptible plants is possibly the plasmalemma. Indole-acetic acid (IAA)- and fusicoccin (FC)-induced net proton excretion in Avena coleoptiles was inhibited by the free acid, diclofop. However, net proton excretion recovered within 2 h when 2,4-dichlorophenoxy acid (2,4-D) was added simultaneously with diclofop. Diclofop depolarized the membrane potential (Em) within 12 min but the Em recovered within 30 min when diclofop was removed and replaced with either IAA or 2,4-D. The inhibition of IAA-induced coleoptile growth by DM and the membrane effects of its acid, diclofop, were partially reversed by 2,4-D if it was added shortly after treatment of the tissue. These results are consistent with the reversal of DM injury in whole plants with 2,4-D.     

Auxin and Ethylene Regulation of Petiole Epinasty in Two Developmental Mutants of Tomato, diageotropica and Epinastic

The epinastic growth responses of petioles to auxin and ethylene were quantified in two developmental mutants of tomato (Lycopersicon esculentum Mill.). In the wild type parent line, cultivar VFN8, the epinastic response of excised petiole sections was approximately log-linear between 0.1 and 100 micromolar indole-3-acetic acid (IAA) and 2,4-dichlorophenoxyacetic acid (2,4-D) concentrations, with a greater response to 2,4-D at any concentration. When ethylene synthesis was inhibited by aminoethoxyvinylglycine (AVG), epinasty was no longer induced by auxin, but could be restored by the addition of ethylene gas. In the auxin-insensitive mutant, diageotropica (dgt), no epinastic response to IAA was observed at IAA concentrations that effectively induced epinasty in VFN8. In the absence of added IAA, epinastic growth of dgt petioles in 1.3 microliters per liter exogenous ethylene gas was more than double that of VFN8 petioles. IAA had little additional effect in dgt, but promoted epinasty in VFN8. These results confirm that tomato petiole cells respond directly to ethylene and make it unlikely that the differential growth responsible for epinasty results from lateral auxin redistribution. The second mutant, Epinastic (Epi), exhibits constitutively epinasty, cortical swelling, and root branching symptomatic of possible alternation in auxin or ethylene regulation of growth. Only minor quantitative differences were observed between the epinastic responses to auxin and ethylene of VFN8 and Epi. However, in contrast to VFN8, when ethylene synthesis or action was inhibited in Epi, auxin still induced 40 to 50% of the epinastic response observed in the absence of inhibitors. This indicates that the target cells for epinastic growth in Epi are qualitatively different from those of VFN8, having gained the ability to grow differentially in response to auxin alone. The dgt and Epi mutants provide useful systems in which to study the genetic determination of target cell specificity for hormone action.