Identification and characterization of Arabidopsis indole-3-butyric acid response mutants defective in novel peroxisomal enzymes

Genetic evidence suggests that indole-3-butyric acid (IBA) is converted to the active auxin indole-3-acetic acid (IAA) by removal of two side-chain methylene units in a process similar to fatty acid beta-oxidation. Previous studies implicate peroxisomes as the site of IBA metabolism, although the enzymes that act in this process are still being identified. Here, we describe two IBA-response mutants, ibr1 and ibr10. Like the previously described ibr3 mutant, which disrupts a putative peroxisomal acyl-CoA oxidase/dehydrogenase, ibr1 and ibr10 display normal IAA responses and defective IBA responses. These defects include reduced root elongation inhibition, decreased lateral root initiation, and reduced IBA-responsive gene expression. However, peroxisomal energy-generating pathways necessary during early seedling development are unaffected in the mutants. Positional cloning of the genes responsible for the mutant defects reveals that IBR1 encodes a member of the short-chain dehydrogenase/reductase family and that IBR10 resembles enoyl-CoA hydratases/isomerases. Both enzymes contain C-terminal peroxisomal-targeting signals, consistent with IBA metabolism occurring in peroxisomes. We present a model in which IBR3, IBR10, and IBR1 may act sequentially in peroxisomal IBA beta-oxidation to IAA.     

The rib1 mutant is resistant to indole-3-butyric acid, an endogenous auxin in Arabidopsis

The presence of indole-3-butyric acid (IBA) as an endogenous auxin in Arabidopsis has been recently demonstrated. However, the in vivo role of IBA remains to be elucidated. We present the characterization of a semi-dominant mutant that is affected in its response to IBA, but shows a wild-type response to indole-3-acetic acid (IAA), the predominant and most studied form of auxin. We have named this mutant rib1 for resistant to IBA. Root elongation assays show that rib1 is specifically resistant to IBA, to the synthetic auxin 2,4-dichlorophenoxyacetic acid, and to auxin transport inhibitors. rib1 does not display increased resistance to IAA, to the synthetic auxin naphthalene acetic acid, or to other classes of plant hormones. rib1 individuals also have other root specific phenotypes including a shortened primary root, an increased number of lateral roots, and a more variable response than wild type to a change in gravitational vector. Adult rib1 plants are morphologically indistinguishable from wild-type plants. These phenotypes suggest that rib1 alters IBA activity in the root, thereby affecting root development and response to environmental stimuli. We propose models in which RIB1 has a function in either IBA transport or response. Our experiments also suggest that IBA does not use the same mechanism to exit cells as does IAA and we propose a model for IBA transport.     

IBR5, a dual-specificity phosphatase-like protein modulating auxin and abscisic acid responsiveness in Arabidopsis

Auxin is an important plant hormone that plays significant roles in plant growth and development. Although numerous auxin-response mutants have been identified, auxin signal transduction pathways remain to be fully elucidated. We isolated ibr5 as an Arabidopsis indole-3-butyric acid-response mutant, but it also is less responsive to indole-3-acetic acid, synthetic auxins, auxin transport inhibitors, and the phytohormone abscisic acid. Like certain other auxin-response mutants, ibr5 has a long root and a short hypocotyl when grown in the light. In addition, ibr5 displays aberrant vascular patterning, increased leaf serration, and reduced accumulation of an auxin-inducible reporter. We used positional information to determine that the gene defective in ibr5 encodes an apparent dual-specificity phosphatase. Using immunoblot and promoter-reporter gene analyses, we found that IBR5 is expressed throughout the plant. The identification of IBR5 relatives in other flowering plants suggests that IBR5 function is conserved throughout angiosperms. Our results suggest that IBR5 is a phosphatase that modulates phytohormone signal transduction and support a link between auxin and abscisic acid signaling pathways.     

Multiple facets of Arabidopsis seedling development require indole-3-butyric acid-derived auxin

Levels of auxin, which regulates both cell division and cell elongation in plant development, are controlled by synthesis, inactivation, transport, and the use of storage forms. However, the specific contributions of various inputs to the active auxin pool are not well understood. One auxin precursor is indole-3-butyric acid (IBA), which undergoes peroxisomal β-oxidation to release free indole-3-acetic acid (IAA). We identified ENOYL-COA HYDRATASE2 (ECH2) as an enzyme required for IBA response. Combining the ech2 mutant with previously identified iba response mutants resulted in enhanced IBA resistance, diverse auxin-related developmental defects, decreased auxin-responsive reporter activity in both untreated and auxin-treated seedlings, and decreased free IAA levels. The decreased auxin levels and responsiveness, along with the associated developmental defects, uncover previously unappreciated roles for IBA-derived IAA during seedling development, establish IBA as an important auxin precursor, and suggest that IBA-to-IAA conversion contributes to the positive feedback that maintains root auxin levels.     

IBR3, a novel peroxisomal acyl-CoA dehydrogenase-like protein required for indole-3-butyric acid response

Indole-3-butyric acid (IBA) is an endogenous auxin that acts in Arabidopsis primarily via its conversion to the principal auxin indole-3-acetic acid (IAA). Genetic and biochemical evidence indicates that this conversion is similar to peroxisomal fatty acid β-oxidation, but the specific enzymes catalyzing IBA β-oxidation have not been identified. We identified an IBA-response mutant (ibr3) with decreased responses to the inhibitory effects of IBA on root elongation or the stimulatory effects of IBA on lateral root formation. However, ibr3 mutants respond normally to other forms of auxin, including IAA. The mutant seedlings germinate and develop normally, even in the absence of sucrose, suggesting that fatty acid β-oxidation is unaffected. Additionally, double mutants between ibr3 and acx3, which is defective in an acyl-CoA oxidase acting in fatty acid β-oxidation, have enhanced IBA resistance, consistent with a distinct role for IBR3. Positional cloning revealed that IBR3 encodes a putative acyl-CoA dehydrogenase with a consensus peroxisomal targeting signal. Based on the singular defect of this mutant in responding to IBA, we propose that IBR3 may act directly in the oxidation of IBA to IAA. Electronic supplementary material The online version of this article (doi: ) contains supplementary material, which is available to authorized users.     

Different Behaviour of Indole-3-Acetic Acid and Indole-3-Butyric Acid in Stimulating Lateral Root Development in Rice (Oryza sativa L.)

The plant hormone auxin has been shown to be involved in lateral root development and application of auxins, indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA), increases the number of lateral roots in several plants. We found that the effects of two auxins on lateral root development in the indica rice (Oryza sativa L. cv. IR8) were totally different from each other depending on the application method. When the roots were incubated with an auxin solution, IAA inhibited lateral root development, while IBA was stimulatory. In contrast, when auxin was applied to the shoot, IAA promoted lateral root formation, while IBA did not. The transport of [³H]IAA from shoot to root occurred efficiently (% transported compared to supplied) but that of [³H]IBA did not, which is consistent with the stimulatory effect of IAA on lateral root production when applied to the shoot. The auxin action of IBA has been suggested to be due to its conversion to IAA. However, in rice IAA competitively inhibited the stimulatory effect of IBA on lateral root formation when they were applied to the incubation solution, suggesting that the stimulatory effect of IBA on lateral root development is not through its conversion to IAA.     

Mutations in Arabidopsis acyl-CoA oxidase genes reveal distinct and overlapping roles in beta-oxidation

Indole-3-butyric acid (IBA) is an endogenous auxin used to enhance rooting during propagation. To better understand the role of IBA, we isolated Arabidopsis IBA-response (ibr) mutants that display enhanced root elongation on inhibitory IBA concentrations but maintain wild-type responses to indole-3-acetic acid, the principle active auxin. A subset of ibr mutants remains sensitive to the stimulatory effects of IBA on lateral root initiation. These mutants are not sucrose dependent during early seedling development, indicating that peroxisomal beta-oxidation of seed storage fatty acids is occurring. We used positional cloning to determine that one mutant is defective in ACX1 and two are defective in ACX3, two of the six Arabidopsis fatty acyl-CoA oxidase (ACX) genes. Characterization of T-DNA insertion mutants defective in the other ACX genes revealed reduced IBA responses in a third gene, ACX4. Activity assays demonstrated that mutants defective in ACX1, ACX3, or ACX4 have reduced fatty acyl-CoA oxidase activity on specific substrates. Moreover, acx1 acx2 double mutants display enhanced IBA resistance and are sucrose dependent during seedling development, whereas acx1 acx3 and acx1 acx5 double mutants display enhanced IBA resistance but remain sucrose independent. The inability of ACX1, ACX3, and ACX4 to fully compensate for one another in IBA-mediated root elongation inhibition and the ability of ACX2 and ACX5 to contribute to IBA response suggests that IBA-response defects in acx mutants may reflect indirect blocks in peroxisomal metabolism and IBA beta-oxidation, rather than direct enzymatic activity of ACX isozymes on IBA-CoA.     

The Arabidopsis PLEIOTROPIC DRUG RESISTANCE8/ABCG36 ATP Binding Cassette Transporter Modulates Sensitivity to the Auxin Precursor Indole-3-Butyric Acid

Plants have developed numerous mechanisms to store hormones in inactive but readily available states, enabling rapid responses to environmental changes. The phytohormone auxin has a number of storage precursors, including indole-3-butyric acid (IBA), which is apparently shortened to active indole-3-acetic acid (IAA) in peroxisomes by a process similar to fatty acid β-oxidation. Whereas metabolism of auxin precursors is beginning to be understood, the biological significance of the various precursors is virtually unknown. We identified an Arabidopsis thaliana mutant that specifically restores IBA, but not IAA, responsiveness to auxin signaling mutants. This mutant is defective in PLEIOTROPIC DRUG RESISTANCE8 (PDR8)/PENETRATION3/ABCG36, a plasma membrane–localized ATP binding cassette transporter that has established roles in pathogen responses and cadmium transport. We found that pdr8 mutants display defects in efflux of the auxin precursor IBA and developmental defects in root hair and cotyledon expansion that reveal previously unknown roles for IBA-derived IAA in plant growth and development. Our results are consistent with the possibility that limiting accumulation of the IAA precursor IBA via PDR8-promoted efflux contributes to auxin homeostasis.     

Genetic analysis of indole-3-butyric acid responses in Arabidopsis thaliana reveals four mutant classes

Indole-3-butyric acid (IBA) is widely used in agriculture because it induces rooting. To better understand the in vivo role of this endogenous auxin, we have identified 14 Arabidopsis mutants that are resistant to the inhibitory effects of IBA on root elongation, but that remain sensitive to the more abundant auxin indole-3-acetic acid (IAA). These mutants have defects in various IBA-mediated responses, which allowed us to group them into four phenotypic classes. Developmental defects in the absence of exogenous sucrose suggest that some of these mutants are impaired in peroxisomal fatty acid chain shortening, implying that the conversion of IBA to IAA is also disrupted. Other mutants appear to have normal peroxisomal function; some of these may be defective in IBA transport, signaling, or response. Recombination mapping indicates that these mutants represent at least nine novel loci in Arabidopsis. The gene defective in one of the mutants was identified using a positional approach and encodes PEX5, which acts in the import of most peroxisomal matrix proteins. These results indicate that in Arabidopsis thaliana, IBA acts, at least in part, via its conversion to IAA.     

Hormonal Effects on Growth and Morphology of Normal and Hairy Roots of Hyoscyamus muticus

Treatment of normal and Agrobacterium rhizogenes-transformed root cultures of Hyoscyamus muticus with three different auxins, indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), and naphthaleneacetic acid (NAA), revealed that the response varied considerably among auxins, between transformed and normal roots, and depending on the parameter. In normal roots all three auxins provoked abundant branching, with IBA and NAA being the most effective at 2.5 and 0.5 μm, respectively, whereas IAA was most effective at low concentrations (0.05 and 0.1 μm). In transformed roots exogenously supplied auxins were generally inhibitory or, at best, without effect on growth and branching. Only 0.01 μm IAA significantly enhanced lateral root number, whereas at the higher concentrations IBA, although inhibitory, was the least effective auxin. In both root types IBA had little effect on primary root growth, but normal roots were more sensitive to IAA and NAA. These results suggest a different sensitivity to auxins of normal and transformed roots since there was no significant difference in endogenous free and conjugated IAA content nor in IAA uptake capacity. Ethylene production and biosynthesis were approximately threefold higher in hairy roots, but production could be stimulated up to tenfold that of control levels in normal roots by supplying NAA or 1-aminocyclopropane-1-carboxylic acid (ACC). Treatment with 2.5 μm NAA, but not IAA or IBA, also enhanced ethylene biosynthesis in normal roots but not in transformed ones. ACC and malonyl-1-aminocyclopropane-1-carboxylic acid accumulated to detectable levels only after treatment with an auxin (NAA). Received March 3, 1997; accepted May 28, 1997     

Mutations in an auxin receptor homolog AFB5 and in SGT1b confer resistance to synthetic picolinate auxins and not to 2,4-dichlorophenoxyacetic acid or indole-3-acetic acid in Arabidopsis

Although a wide range of structurally diverse small molecules can act as auxins, it is unclear whether all of these compounds act via the same mechanisms that have been characterized for 2,4-dichlorophenoxyacetic acid (2,4-D) and indole-3-acetic acid (IAA). To address this question, we used a novel member of the picolinate class of synthetic auxins that is structurally distinct from 2,4-D to screen for Arabidopsis (Arabidopsis thaliana) mutants that show chemically selective auxin resistance. We identified seven alleles at two distinct genetic loci that conferred significant resistance to picolinate auxins such as picloram, yet had minimal cross-resistance to 2,4-D or IAA. Double mutants had the same level and selectivity of resistance as single mutants. The sites of the mutations were identified by positional mapping as At4g11260 and At5g49980. At5g49980 is previously uncharacterized and encodes auxin signaling F-box protein 5, one of five homologs of TIR1 in the Arabidopsis genome. TIR1 is the recognition component of the Skp1-cullin-F-box complex associated with the ubiquitin-proteasome pathway involved in auxin signaling and has recently been shown to be a receptor for IAA and 2,4-D. At4g11260 encodes the tetratricopeptide protein SGT1b that has also been associated with Skp1-cullin-F-box-mediated ubiquitination in auxin signaling and other pathways. Complementation of mutant lines with their corresponding wild-type genes restored picolinate auxin sensitivity. These results show that chemical specificity in auxin signaling can be conferred by upstream components of the auxin response pathway. They also demonstrate the utility of genetic screens using structurally diverse chemistries to uncover novel pathway components.     

Comparison of movement and metabolism of indole-3-acetic acid and indole-3-butyric acid in mung bean cuttings

Indole-3-butyric acid (IBA) was much more effective than indole-3-acetic acid (IAA) in inducing adventitious root formation in mung bean ( Vigna radiata L.) cuttings. Prolonging the duration of treatment with both auxins from 24 to 96 h significantly increased the number of roots formed. Labelled IAA and IBA applied to the basal cut surface of the cuttings were transported acropetally. With both auxins, most radioactivity was detected in the hypocotyl, where roots were formed, but relatively more IBA was found in the upper sections of the cuttings. The rate of metabolism of IAA and IBA in these cuttings was similar. Both auxins were metabolized very rapidly and 24 h after application only a small fraction of the radioactivity corresponded to the free auxins. Hydrolysis with 7 M NaOH indicates that conjugation is the major pathway of IAA and IBA metabolism in mung bean tissues. The major conjugate of IAA was identified tentatively as indole-3-acetylaspartic acid, whereas IBA formed at least two major conjugates. The data indicate that the higher root-promoting activity of IBA was not due to a different transport pattern and/or a different rate of conjugation. It is suggested that the IBA conjugates may be a better source of free auxin than those of IAA and this may explain the higher activity of IBA.     

Physiological characteristics of two auxin-resistant mutants of Arabidopsis thaliana,aux-2 and Dwf

Two auxin-resistant mutants of Arabidopsis thaliana L. have been characterized physiologically: aux-2 is a recessive mutation and is unlinked to a dominant mutation, Dwf, which is apparently lethal when homozygous. The progeny of selfed Dwf plants segregate into Dwf (agravitropic) and dwf ⁺ (normal) phenotypes. aux-2 phenotype was indistinguishable from the wild-type on criteria other than resistance to exogenous auxins: 3-fold to 2,4-D and 2-fold to IAA. On the other hand, Dwf plants had a typical dwarf phenotype with single unbranched roots which lacked hairs. Compared to the wild-type, Dwf seedling roots were highly resistant to exogenous auxins: 2000-fold to 2,4-D and 360-fold to IAA. Both aux-2 and Dwf were normal in their response to exogenous ABA. The dwarf phenotype was insensitive to gibberellins but root hair formation was restored by application of auxins.The results indicate that altered auxin phsysiology can lead to agravitropism and dwarfism.Abbrevations ABA Abscisic acid - GA₃ Gibberellic acid - IAA indole-3-acetic acid - 2,4-D 2,4-dichlorophenoxyacetic acid     

Differential responses of primary and lateral roots to indole-3-acetic acid,indole-3-butyric acid,and 1-naphthaleneacetic acid in maize seedlings

The role of auxins on root system architecture was studied by applying indole-3-acetic acid (IAA), indole-3-butyric acid (IBA), and 1-naphthaleneacetic acid (NAA) to maize roots and analysing the main processes involved in root development: primary root (PR) elongation, lateral root (LR) formation, and LR root elongation. We found that these effects were not dependent only on concentration, but also on the type of auxin applied. We also studied temporal changes in auxin inhibition of PR elongation. These temporal changes were analysed calculating the elongation ratio between two consecutive one day periods after auxin application. It was observed that a reduction in root elongation was also dependent on the type of auxin applied and its concentration. The inhibitory effect of IBA and IAA decreased on the second day, and the ratio also increased with the concentration. In contrast, NAA increased root elongation inhibition with time. Indeed, the ratio decreased as the NAA concentration increased. Regarding LR formation, we observed that external auxin increased only LR formation in certain zones of the PR. Finally, comparison of inhibition elongation associated with auxin in the LR and PR clearly demonstrates that PR elongation was more sensitive to auxin than LR elongation.     

Effect of Exogenous Indole-3-Acetic Acid and Indole-3-Butyric Acid on Internal Levels of the Respective Auxins and Their Conjugation with Aspartic Acid during Adventitious Root Formation in Pea Cuttings

The influence of exogenous indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) on the internal levels of these auxins was studied during the first 4 days of adventitious root formation in cuttings of Pisum sativum L. The quantitations were done by high performance liquid chromatography with spectrofluorometric detection. IBA, identified by combined gas chromatography-mass spectrometry (GC-MS), was found to naturally occur in this plant material. The root inducing ability of exogenous IBA was superior to that of IAA. The IAA level in the tissue increased considerably on the first day after application of IAA, but rapidly decreased again, returning to a level twice the control by day 3. The predominant metabolic route was conjugation with aspartic acid, as reflected by the increase in the level of indole-3-acetylaspartic acid. The IBA treatment resulted in increases in the levels of IBA, IAA, and indole-3-acetylaspartic acid. The IAA content rapidly returned to control levels, whereas the IBA level remained high throughout the experimental period. High amounts of indole-3-butyrylaspartic acid were found in the tissue after feeding with IBA. The identity of the conjugate was confirmed by ¹H-nuclear magnetic resonance and GC-MS. IBA was much more stable in solution than IAA. No IAA was detected after 48 hours, whereas 70% IBA was still recovered after this time. The relatively higher root inducing ability of IBA is ascribed to the fact that its level remained elevated longer than that of IAA, even though IBA was metabolized in the tissue. Adventitious root formation is discussed on the basis of these findings.     

The effects of auxin on lateral root initiation and root gravitropism in a lateral rootless mutant Lrt1 of rice (Oryza sativa L.)

Auxins control growth and development in plants, including lateral rootinitiation and root gravity response. However, how endogenous auxin regulatesthese processes is poorly understood. In this study, the effects of auxins onlateral root initiation and root gravity response in rice were investigatedusing a lateral rootless mutant Lrt1, which fails to formlateral roots and shows a reduced root gravity response. Exogenous applicationof IBA to the Lrt1 mutant restored both lateral rootinitiation and root gravitropism. However, application of IAA, a major form ofnatural auxin, restored only root gravitropic response but not lateral rootinitiation. These results suggest that IBA is more effective than IAA in lateralroot formation and that IBA also plays an important role in root gravitropicresponse in rice. The application of NAA restored lateral root initiation, butdid not completely restore root gravitropism. Root elongation assays ofLrt1 displayed resistance to 2,4-D, NAA, IBA, and IAA.This result suggests that the reduced sensitivity to exogenous auxins may be due tothe altered auxin activity in the root, thereby affecting root morphology inLrt1.     

Crosstalk between ABA and auxin signaling pathways in roots of <Emphasis Type="Italic">Arabidopsis thaliana</Emphasis> (L.) Heynh.

Studies of abscisic acid (ABA) and auxin have revealed that these pathways impinge on each other. The Daucus carota (L.) Dc3 promoter: uidA (-glucuronidase: GUS) chimaeric reporter (ProDc3:GUS) is induced by ABA, osmoticum, and the auxin indole-3-acetic acid (IAA) in vegetative tissues of transgenic Arabidopsis thaliana (L.) Heynh. Here, we describe the root tissue-specific expression of ProDc3:GUS in the ABA-insensitive-2 (abi2-1), auxin-insensitive-1 (aux1), auxin-resistant-4 (axr4), and rooty (rty1) mutants of Arabidopsis in response to ABA, IAA and synthetic auxins naphthalene acetic acid (NAA), and 2, 4-(dichlorophenoxy) acetic acid. Quantitative analysis of ProDc3:GUS expression showed that the abi2-1 mutant had reduced GUS activity in response to ABA, IAA, or 2, 4-d, but not to NAA. Similarly, chromogenic staining of ProDc3:GUS activity showed that the aux1 and axr4 mutants gave predictable hypomorphic ProDc3:GUS expression phenotypes in roots treated with IAA or 2, 4-d, but not the diffusible auxin NAA. Likewise the rty mutant, which accumulates auxin, showed elevated ProDc3:GUS expression in the absence or presence of hormones relative to wild type. Interestingly, the aux1 and axr4 mutants showed a hypomorphic effect on ABA-inducible ProDc3:GUS expression, demonstrating that ABA and IAA signaling pathways interact in roots. Possible mechanisms of crosstalk between ABA and auxin signaling are discussed.     

Auxins in the development of an arbuscular mycorrhizal symbiosis in maize

While the levels of free auxins in maize (Zea mays L.) roots during arbuscular mycorrhiza formation have been previously described in detail, conjugates of indole-3-acetic acid (IAA) and indole-3-butyric acid (IBA) with amino acids and sugars were neglected. In this study, we have therefore determined free, ester and amide bound auxins in roots of maize inoculated with Glomus intraradices during early stages of the colonization process. Ester conjugates of IAA and IBA were found only in low amounts and they did not increase in AM colonized roots. The Levels of IAA and IBA amide conjugates increased 20 and 30 days past inoculation (dpi). The formation of free and conjugated IBA but not IAA was systemically induced during AM colonization in leaves of maize plants. This implicated a role for auxin conjugate synthesis and hydrolysis during AM. We have therefore investigated the in vivo metabolism of 3H-labeled IBA by TLC but only slight differences between control and AM-inoculated roots were observed. The activity of auxin conjugate hydrolase activity measured with three different putative substrates showed a decrease in infected roots compared to controls. The fluorinated IBA analog TFIBA inhibited IBA formation in leaves after application to the root system, but was not transported from roots to shoots. AM hyphae were also not able to transport TFIBA. Our results indicate complex control mechanisms to regulate the levels of free and conjugated auxins, which are locally and systemically induced during early stages of the formation of an arbuscular mycorrhizal symbiosis.     

The rib1 mutant of Arabidopsis has alterations in indole-3-butyric acid transport, hypocotyl elongation, and root architecture

Polar transport of the auxin indole-3-butyric acid (IBA) has recently been shown to occur in Arabidopsis (Arabidopis thaliana) seedlings, yet the physiological importance of this process has yet to be fully resolved. Here we describe the first demonstration of altered IBA transport in an Arabidopsis mutant, and show that the resistant to IBA (rib1) mutation results in alterations in growth, development, and response to exogenous auxin consistent with an important physiological role for IBA transport. Both hypocotyl and root IBA basipetal transport are decreased in rib1 and root acropetal IBA transport is increased. While indole-3-acetic acid (IAA) transport levels are not different in rib1 compared to wild type, root acropetal IAA transport is insensitive to the IAA efflux inhibitor naphthylphthalamic acid in rib1, as is the dependent physiological process of lateral root formation. These observed changes in IBA transport are accompanied by altered rib1 phenotypes. Previously, rib1 roots were shown to be less sensitive to growth inhibition by IBA, but to have a wild-type response to IAA in root elongation. rib1 is also less sensitive to IBA in stimulation of lateral root formation and in hypocotyl elongation under most, but not all, light and sucrose conditions. rib1 has wild-type responses to IAA, except under one set of conditions, low light and 1.5% sucrose, in which both hypocotyl elongation and lateral root formation show altered IAA response. Taken together, our results support a model in which endogenous IBA influences wild-type seedling morphology. Modifications in IBA distribution in seedlings affect hypocotyl and root elongation, as well as lateral root formation.