Absence Of Exogenous Auxin Research Articles

Low Red Light:Far Red Light Inhibits Branching by Promoting Auxin Signaling

The branching trait is influenced by various environmental signals, including the Red light:Far Red light (R:FR), an indicator of competition and an inhibitor of axillary bud growth. Branch development is influenced by an array of hormones, including auxin which indirectly suppresses bud growth as a consequence of auxin transport and signaling in the main stem. The suppressive effect of auxin sourced from the shoot apex and transported basipetally is a major mechanism contributing to apical dominance, a form of correlative inhibition where superior growing points restrict the growth of those lower on the plant. The current study shows that increased apical dominance is a mechanism that suppresses the branching of plants grown in a low R:FR. The elevated apical dominance was not due to increased levels of the natural auxin IAA, but was associated with enhanced expression of auxin-induced genes both in the presence and absence of exogenous auxin. A direct test of bud sensitivity to auxin confirmed that a low R:FR promoted auxin responsiveness leading to reduced bud growth. Thus, the low R:FR enhancement of auxin sensitivity is a mechanism that contributes to the reduced branching phenotype of plants growing in competitive environments.

Impacts of strigolactone on shoot branching under phosphate starvation in chrysanthemum (Dendranthema grandiflorum cv. Jinba).

Chrysanthemum (Dendranthema grandiflorum cv. Jinba) shoot branching is determined by bud outgrowth during the vegetative growth stage. The degree of axillary bud outgrowth is highly influenced by environmental conditions, such as nutrient availability. Here, we demonstrated that phosphorus (Pi) starvation significantly reduces axillary bud outgrowth in chrysanthemum. A strigolactone (SL) biosynthesis gene, DgCCD7, was isolated and characterized as an ortholog of MAX3/DAD3/RMS5/D17. By using ultra-performance liquid chromatography coupled with mass spectrometry (UPLC-MS), three putative SLs were identified and levels of all three SLs showed strong increase under Pi starvation conditions. Determinations of the distribution of SLs and regulation of DgCCD7/8 in response to Pi changes in root indicate that SL acts systemically. However, temporal expression patterns of biosynthesis and signaling genes in nodes revealed that Pi starvation causes a local response of SL pathway. Treatment of node segments with or without auxin and Pi revealed that in the absence of exogenous auxin, Pi delayed axillary buds outgrowth and up-regulated local SL pathway genes. These data indicated that an auxin-SL regulatory loop responded to Pi starvation for delaying bud outgrowth locally, root biosynthesized SLs were transported acropetally and functioned in shoot branching inhibition under Pi starvation. We proposed that SLs contributed to chrysanthemum shoot branching control in response to Pi-limiting conditions in a systemic way.

Productivity of polyclonal minigarden and rooting of Handroanthus heptaphyllus Mattos minicuttings

Aiming the improvement of the quality of techniques related to the growth of “ipe-roxo” cuttings, this study had as objective to evaluate the productivity of Handroanthus heptaphyllus Mattos ministumps and the rooting capacity of the minicuttings in successive collections in the presence and absence of exogenous auxin. Seedlings were grown in 280 cm3 tubes with selected different sources of seeds. At 141 days after the sowing, seedlings were pruned at 8 cm from the collar with the goal to establish the minigarden. The productivity of ministumps over successive collections as well as the need to apply indol-3-butiric acid on the roots of the minicuttings were evaluated. Eight collects of minicuttings were carried out, at three, five, six, seven, eight, nine, 10 and 11 months after the pruning. In each of those cycles the number of minicuttings were quantified per mini-stump. Before their staking in 180 cm 3 tubes, half of the minicuttings from each cycle were treated with 8000 mg L-1 of Indole-3-Butiric Acid (IBA) (2 seconds). The ministumps tolerated the pruning (with 100% survival); over 70% of the ministumps produced minicuttings along all the time in each collection cycle. A cyclical behavior in the minigarden was observed with higher productivities of the ministumps at 8 and 11 months after pruning of seedlings. The use of IBA stimulated more robust root systems of the cuttings, although this was not a condition for rooting.

Pre and post-severance effects of light quality on carbohydrate dynamics and microcutting adventitious rooting of two Eucalyptus species of contrasting recalcitrance

Adventitious rooting is a complex developmental response affected by genetic and environmental factors. Radiation quality effects on adventitious rooting depend on characteristics such as species, growth stage, irradiance, spectral quality, and time of exposure. Eucalyptus is an essential genus for the paper industry, and high yield plantations depend on adventitious rooting of selected genotypes. This work addressed two hypotheses: (1) radiation quality equally affects adventitious rooting in Eucalyptus species of different recalcitrance; (2) adventitious rooting outcome depends on both donor plant and cutting radiation quality treatments. To that end, the easy-to-root Eucalyptus grandis and the recalcitrant Eucalyptus globulus were evaluated. The effect of white, blue, red and far-red radiation enrichment on microcuttings and donor plants of both species was evaluated in relation to rooting. There was no effect of radiation quality on adventitious rooting of E. grandis or when radiation treatments were applied to E. globulus microcuttings. In contrast, donor plants of E. globulus, grown in medium devoid of sucrose and exposed to far-red radiation, yielded microcuttings showing higher rooting percentage, even in the absence of exogenous auxin in the rooting medium. Sucrose in donor plant medium abolished the positive effect of far-red radiation. An increase in endogenous soluble sugars and starch contents in basal microcuttings was associated with far-red radiation treatment of donor plants. These results underline the importance of appropriate carbohydrate partitioning in donor plants for adventitious rooting of cuttings and provide a basis for understanding and overcoming rooting recalcitrance in E. globulus clones.

Induction of somatic embryogenesis in absence of exogenous auxin in cucumber (Cucumis sativus L.)

Induction of somatic embryogenesis in absence of exogenous auxin can clarify some basic concepts of somatic embryogenesis. Explants from leaf and petiole of Cucumber (Cucumis sativus) were used in this experiment for somatic embryogenesis using three different modified B5 basal media; boron and 2,4-D but no IAA as control (I), boron and IAA but no 2,4-D (II) and no boron, 2,4- D and IAA (III). Leaf explants grown on III produced somatic embryos indicating an induction potential for somatic embryogenesis with an incomplete medium. The endogenous auxin of the leaf explants might have a potential for triggering induction of somatic embryogenesis.

Overexpression of Arabidopsis cyclin D2;1 in cotton results in leaf curling and other plant architectural modifications

The D-class cyclin genes play key roles in controlling the cell cycle in plant development. To evaluate the effects of D-type cyclins on plant architecture, ectopic expression of the ArabidopsiscyclinD2;1 (Arath;CYCD2;1) gene driven by the CaMV35S promoter was investigated in cotton via Agrobacterium-mediated transformation. Northern blot showed the cyclinD2;1 gene was highly expressed in transgenic cotton plants. Phenotype investigation showed that overexpression of Arath;CYCD2;1 led to obvious leaf architecture change: the leaf epidermis of transgenic plants consist of more small cells compared to the wild-type. Mesophyll cells in the inner layers of Arath;CYCD2;1 plants were organized more loosely than those in the inner layers of wild-type plants. Moreover, transgenic plants had darker leaves, more chlorophyll, and a higher rate of photosynthesis than wild-type plants in the field. Tissue cultures indicated that the overexpression of Arath;CYCD2;1 promoted callus formation in the absence of exogenous auxin, but inhibited cell differentiation. The qRT-PCR revealed that several cell cycle–associated genes, particularly the transcript levels of GhRBR and GhCYCD3;1 were regulated by the Arath;CYCD2;1 insertion. The results implied that the cotton plant architecture or cell culture characters could be regulated by ectopic expression of the ArabidopsiscyclinD2;1 gene.

Growth and Anatomical Parameters of Adventitious Roots Formed on Mung Bean Hypocotyls Are Correlated with Galactoglucomannan Oligosaccharides Structure

The effect of galactoglucomannan oligosaccharides (GGMOs) compared with chemically modified oligosaccharides, GGMOs-g (with reduced number of D-galactose side chains) and GGMOs-r (with reduced reducing ends) on mung bean (Vigna radiata (L.) Wilczek) adventitious roots formation, elongation, and anatomical structure have been studied. All types of oligosaccharides influenced adventitious root formation in the same way: stimulation in the absence of exogenous auxin and inhibition in the presence of exogenous auxin. Both reactions are probably related with the presence/content of endogenous auxin in plant cuttings. However, the adventitious root length was inhibited by GGMOs both in the absence as well as in the presence of auxin (IBA or NAA), while GGMOs-g inhibition was significantly weaker compared with GGMOs. GGMOs-r were without significant difference on both processes, compared with GGMOs. GGMOs affected not only the adventitious root length but also their anatomy in dependence on the combination with certain type of auxin. The oligosaccharides influenced cortical cells division, which was reflected in the cortex area and in the root diameter. All processes followed were dependent on oligosaccharides chemical structure. The results suggest also that GGM-derived oligosaccharides may play an important role in adventitious roots elongation but not in their formation.

The cytokinin type-B response regulator PtRR13 is a negative regulator of adventitious root development in Populus.

Adventitious root formation at the base of plant cuttings is an innate de novo organogenesis process that allows massive vegetative propagation of many economically and ecologically important species. The early molecular events following shoot excision are not well understood. Using whole-genome microarrays, we detected significant transcriptome remodeling during 48 h following shoot removal in Populus tremula x Populus alba softwood cuttings in the absence of exogenous auxin, with 27% and 36% of the gene models showing differential abundance between 0 and 6 h and between 6 and 24 h, respectively. During these two time intervals, gene networks involved in protein turnover, protein phosphorylation, molecular transport, and translation were among the most significantly regulated. Transgenic lines expressing a constitutively active form of the Populus type-B cytokinin response regulator PtRR13 (DeltaDDKPtRR13) have a delayed rooting phenotype and cause misregulation of CONTINUOUS VASCULAR RING1, a negative regulator of vascularization; PLEIOTROPIC DRUG RESISTANCE TRANSPORTER9, an auxin efflux transporter; and two APETALA2/ETHYLENE RESPONSE FACTOR genes with sequence similarity to TINY. Inappropriate cytokinin action via DeltaDDKPtRR13 expression appeared to disrupt adventitious root development 24 h after shoot excision, when root founder cells are hypothesized to be sensitive to the negative effects of cytokinin. Our results are consistent with PtRR13 acting downstream of cytokinin to repress adventitious root formation in intact plants, and that reduced cytokinin signaling after shoot excision enables coordinated expression of ethylene, auxin, and vascularization pathways leading to adventitious root development.

Characterization and expression of a Pinus radiata putative ortholog to the Arabidopsis SHORT-ROOT gene

We characterized a Pinus radiata D. Don putative ortholog to the Arabidopsis thaliana (L.) Heynh. SHORT--ROOT gene (AtSHR) and analyzed its expression in different organs during vegetative development and in response to exogenous auxin during adventitious rooting. The predicted protein sequence contained domains characteristic of the GRAS protein family and showed a strong similarity to the SHORT--ROOT (SHR) proteins. Quantitative reverse transcriptase--polymerase chain reaction (qRT-PCR) and in situ hybridization showed that the gene is predominantly expressed in roots, root primordia and in the cambial region of hypocotyl cuttings. Increased mRNA levels were observed, independently of the presence or absence of exogenous auxin, in the cambial region and rooting competent cells of hypocotyl cuttings within the first 24 h of adventitious rooting, before the activation of cell divisions and the organization of the adventitious root meristem. The expression pattern in organs and during adventitious rooting was similar to that of a Pinus radiata SCARECROW-LIKE (PrSCL1) gene, except that PrSCL1 is induced in response to exogenous auxin. Results suggest that the Pinus radiata SHORT-ROOT (PrSHR) gene has a role in root meristem formation and maintenance and in the cambial region of hypocotyl cuttings.

N, N′-bis-(2,3-Methylenedioxyphenyl)urea and N, N′-bis-(3,4-methylenedioxyphenyl)urea enhance adventitious rooting in Pinus radiata and affect expression of genes induced during adventitious rooting in the presence of exogenous auxin

We have analyzed the effect of N, N′-bis-(2,3-methylenedioxyphenyl)urea (2,3-MDPU) and N, N′-bis-(3,4-methylenedioxyphenyl)urea (3,4-MDPU), two symmetrically substituted diphenylurea derivatives with no auxin or cytokinin-like activity, on the rooting capacity of Pinus radiata stem cuttings. Results indicate that both diphenylurea derivatives enhance adventitious rooting in the presence of exogenous auxin (indole-3-butyric acid, IBA), even at low auxin concentration, in rooting-competent cuttings, but have no effect on the adventitious rooting of low or null competent-to-root cuttings. Histological analyses show that, in the simultaneous presence of MDPUs and low concentration of exogenous auxin, adventitious root formation is induced in the cell types that retain intrinsic competence to form adventitious roots in response to auxin. The time course of cellular events leading to root formation and the time of root emergence are closely similar to that observed in cuttings treated only with higher auxin concentration. In addition, the mRNA level of a P. radiata SCARECROW-LIKE gene, which is significantly induced in the presence of the optimal concentration (10 μM) of exogenous auxin needed for cuttings to root, is increased in the presence of MDPUs and low concentration of exogenous auxin (1 μM). The expression of a P. radiata SHORT-ROOT gene in rooting-competent cuttings during adventitious rooting is also affected by the presence of MDPUs when combined with auxin. As MDPUs do not affect the expression of either gene in the absence of exogenous auxin, but only in its presence, we suggest that MDPUs could interact, directly or indirectly, with the auxin-signalling pathways in rooting-competent cuttings during adventitious rooting.

Linked activation of cell division and oxidative stress defense in alfalfa leaf protoplast-derived cells is dependent on exogenous auxin

The activation of cell division and oxidative stress responses has been investigated in the case of leaf protoplast-derived cells. Initiation of protoplast culture was found to be associated with oxidative stress as indicated by the rate of H2O2 release into the medium and/or by catalase and ascorbate peroxidase activities. Both cell division frequency and the above stress-related parameters were dependent on the exogenous auxin (2,4-dichlorophenoxyacetic acid, 2,4-D) concentrations used. In addition, the well known oxidative stress-inducing agent paraquat (1 μM) could promote cell division at suboptimal auxin concentration but not in the absence of exogenous auxin. The H2O2 scavenger dimethylthiourea and the NADPH oxidase inhibitor diphenyleneiodonium inhibited not only the activation of cellular defense reactions but cell division as well. Based on the above experimental observations, it is suggested that exogenous auxin (2,4-D) enhances cellular defense reactions in parallel with cell division activation.

Adventitious root formation in Anacardium occidentale L. in response to phytohormones and removal of roots

Abstract Despite advances in tissue culture techniques, propagation by leafy, softwood cuttings is the preferred, practical system for vegetative reproduction of many tree and shrub species. Species are frequently defined as ‘difficult’- or ‘easy-to-root’ when propagated by conventional cuttings. Speed of rooting is often linked with ease of propagation, and slow-to-root species may be ‘difficult’ precisely because tissues deteriorate prior to the formation of adventitious roots. Even when roots form, limited development of these may impair the establishment of a cutting. In this study we used softwood cuttings of cashew (Anacardium occidentale), a species considered as ‘difficult-to-root’. We aimed to test the hypothesis that speed, and extent of early rooting, is critical in determining success with this species; and that the potential to form adventitious roots will decrease with time in the propagation environment. Using two genotypes, initial rooting rates were examined in the presence or absence of exogenous auxin. In cuttings that formed adventitious roots, either entire roots or root tips were removed, to determine if further root formation/development was feasible. To investigate if subsequent root responses were linked to phytohormone action, a number of cuttings were also treated with either exogenous auxin (indole-3-butyric acid—IBA) or cytokinin (zeatin). Despite the reputation of Anacardium as being ‘difficult-to-root’, we found high rooting rates in two genotypes (AC 10 and CCP 1001). Removing adventitious roots from cuttings and returning them to the propagation environment, resulted in subsequent re-rooting. Indeed, individual cuttings could develop new adventitious roots on four to five separate occasions over a 9 week period. Data showed that rooting potential increased, not decreased with time in the propagation environment and that cutting viability was unaffected. Root expression was faster (8–15 days) after the removal of previous roots compared to when the cuttings were first stuck (21 days). Exposing cuttings to IBA at the time of preparation, improved initial rooting in AC 10, but not in CCP 1001. Application of IBA once roots had formed had little effect on subsequent development, but zeatin reduced root length and promoted root number and dry matter accumulation. These results challenge our hypothesis, and indicate that rooting potential remains high in Anacardium. The precise mechanisms that regulate the number of adventitious roots expressed, remain to be determined. Nevertheless, results indicate that rooting potential can be high in ‘difficult-to-root’ species, and suggest that providing supportive environments is the key to expressing this potential.

Hormone autotrophic growth and differentiation identifies mutant lines of Arabidopsis with altered cytokinin and auxin content or signaling.

We describe mutant tissue lines of Arabidopsis that are able to grow in vitro as callus on hormone-free medium. The 14 lines presented here show different hormone autotrophic differentiation behaviors that can be classified into three categories: (a) forming roots (rooty callus), (b) forming shoots or shoot-like structures (shooty callus), or (c) growing without organ formation (callus). Three fast-growing lines showed altered steady-state mRNA levels of the Cdc2 and CycD3 cell cycle genes. Three of the six rooty callus lines contained about 20- to 30-fold higher levels of auxins than wild-type callus. These and two other lines with normal auxin content showed an increased steady-state level of IAA1 and IAA2 transcripts in the absence of exogenous auxin. Five of the six shooty callus lines had increased steady-state mRNA levels of the CKI1 gene and/or of the homeobox genes KNAT1 and STM, suggesting that the phenotype is linked to altered cytokinin signaling. Also, one cytokinin-overproducing line with only 5% of wild-type cytokinin oxidase activity was identified. These results indicate that screening for hormone-autonomous growth identifies mutants with altered hormone content or signaling.

Expression of the Auxin-Inducible GH3 Promoter/GUS Fusion Gene as a Useful Molecular Marker for Auxin Physiology

Expression of the GH3 promoter/GUS reporter (GH3/ GUS) gene in transgenic tobacco is silent in most vegetative tissues and organs, but can be specifically induced by auxin in all cell types throughout the plant, indicating that auxin is a limiting factor for activation of the GH3/GUS gene. In the absence of exogenous auxin, expression of the GH3/ GUS gene either correlates with physiological events which are presumably mediated by auxin or occurs in cells where relatively high levels of endogenous auxin are expected. For example, the GH3/GUS gene is expressed in dormant lateral buds, tips of adventitious roots, and stem parenchyma cells where adventitious roots are initiating. The GH3/GUS gene is activated at the cut surface nearest the tip of a leaf from which a strip of tissue has been excised, and in the basal end of an excised shoot segment. Also, the GH3/GUS gene is expressed predominantly in the lower side of a gravistimulated shoot and in the dark side of a unilateral light-stimulated shoot. Furthermore, the expression pattern of the GH3/GUS gene is similar to the distribution pattern of 14C-IAA that is applied at the apical end of the gravistimulated shoot. Thus, the results reported here provide additional support for a crucial role of auxin in shoot gravi- and photo-tropisms and adventitious root formation. Furthermore, the data presented in this report also indicate that expression of the GH3/GUS gene is an excellent molecular marker for studying auxin transport and for monitoring changes in either auxin concentration or cellular sensitivity to auxin in planta.

Effect of genotype on rooting of hypocotyls and in vitro-produced shoots of Pinus radiata

Significant genotypic variation at both the family and clone-within-family levels was seen for hypocotyl rooting and the rooting of adventitious shoots produced in vitro for Pinus radiata D. Don. High rooting frequencies for hypocotyls were obtained in the absence of exogenous auxin; auxin greatly stimulated the rooting of adventitious shoots. No correlation was seen between the rooting of hypocotyls and shoots; families whose hypocotyls rooted at high frequencies did not necessarily produce shoots that rooted at high frequency. No correlation was seen between adventitious shoot production and subsequent rooting at either the family or clone level. The lack of a negative correlation indicated that selecting families or clones for high levels of shoot production will not automatically select for low rooting ability, obviating a possible bottleneck for commercial propagation of Pinus radiata. Significant family by replication interaction suggests that rooting protocols could be optimized through manipulations of the rooting environment for each family.

Changes in peroxidase activity, auxin level and ethylene production during root formation by poplar shoots raised in vitro

Shoots of poplar (Populus tremula × P. tremuloides) were multiplied in vitro and rooted on a rooting medium in the presence of NAA. No rooting occurred in the absence of exogenous auxin. A peak of soluble peroxidase activity, which corresponded to a decrease in the free IAA level in the shoots, preceded rooting These events were considered as corresponding to the initiative phase of rooting. They are preceded by a peak in free IAA activity which might initiate the inductive phase of the rooting process. A burst of ethylene production was measured in both rooting and non-rooting shoots, but the ethylene peak from rooting shoots appeared earlier and was higher. The use of ACC indicated that the exogenous auxin might have enhanced ACC-synthetase activity.

MICROPROPAGATION OF THE COLD-HARDY APPLE ROOTSTOCK KSC-3: A MORPHOLOGICAL ANALYSIS

The cold-hardy apple rootstock KSC-3 (Malus × domestica Borkh.) was micropropagated on modified Murashige and Skoog (MS) medium using single node explants as the starting material. Explants grown on media containing BAP (0.5–5.0 mg L−1) and zeatin (5.0 mg L−1) produced multiple shoots. The largest number of shoots (≥ 5.0 mm) were obtained with 1.0 mg L−1 BAP. On the basis of useable shoots (≥ 15.0 mm), however, there was no difference in shoot number between 0.5 and 1.0 mg L−1 BAP. With increasing BAP concentration, shoot length decreased, while stem diameter increased. The longest shoots were obtained with 5.0 mg L−1 zeatin. Rooting was achieved by culturing individual shoots on either full or 1/4 strength MS. Reducing the MS concentration in the absence of exogenous auxin increased the percentage of rooted shoots and the median number of roots/shoot. When auxins were present, reducing the MS concentration had little effect on rooting. NAA, IBA and IAA were effective in promoting root formation while 2,4-D was often inhibitory. Basal callus formation increased when auxins were added to the rooting medium. The presence of 2,4-D or a reduction of the MS concentration to 1/4 strength had a detrimental effect on the shoots.Key words: Micropropagation, KSC-3, Malus, morphology, appleAbbreviations: MS = mineral salts of Murashige and Skoog (1962); BAP = 6-benzylaminopurine; 2iP = N6-(2-isopentyl) adenine; zeatin = 4-hydroxy-3-methyl-trans-2-butenylaminopurine; 2,4-D = 2.4-dichlorophenoxyacetic acid; NAA = 1-naphthaleneacetic acid; IBA = indole-3-butyric acid; IAA = Indole-3-acetic acid.

Evidence for the presence of a native, non-auxinic rooting promoter in avocado (Persea americana Mill.)

The presence of a rooting promoter in paratially purified extracts of avocado (Persea amricana Mill.) organs has been demonstrated using the mung bean rooting bioassay. Extraction with 80% methanol was followed by partition into diethyl ether, paper chromatography (PC) and 3 steps of thin layer chromatography (TLC). The number of roots induced by the rooting promoter in the absence of exogenous auxin was 5 to 7 times higher than that of the water control and 50% higher than by 4-(indol-3-yl) butyric acid (IBA) at its reported optimum concentration. Rooting of tomato, Coleus and young avocado cuttings was also enhanced by the rooting promoter. The rooting promoter was inhibitory in the wheat coleoptile section elongation bioassay for auxins and had slight inhibitory activity in the split pea stem curvature test.

Incorporation of Tritiated Thymidine and Uridine into Adventitious-root Initial Cells of Vigna radiata

Abstract Adventitious roots in mung bean [Vigna radiata (L.) R. Wilcz. cv. Berken] cuttings develop from specific “rooting-zone parenchyma” (R-ZP) cells. Microautoradiography was used to determine the timing of thymidine and uridine incorporation into the R-ZP cells, prior to the first cell division, in the presence or absence of naphthaleneacetic acid (NAA). Since 6-3H-thymidine incorporation reached a maximum between 11 and 14 hours, we suggest that the R-ZP cells were in the G1 phase when the cuttings were taken. Tritiated uridine was incorporated into the RNA of the R-ZP cells 2 hours after the cuttings were placed in the labeled solution. DNA synthesis and cell division of the R-ZP cells occurred along the entire length of the hypocotyl (basal, middle, and top segments), but these initial events were not sufficient to result in the subsequent formation of adventitious roots. NAA promoted adventitious root formation in the cuttings but it had no apparent effect on nucleic acid labeling nor initial cell division of the R-ZP cells. The initial division of the R-ZP cells appears to be a wounding response and occurs in the presence or absence of exogenous auxin.

Avena Coleoptile Elongation: Stimulation by Fluorophenylalanine

A 100 to 150% stimulation of Avena coleoptile segment elongation by the amino acid analogue p-fluorophenylalanine (FPA) has been observed. The effect is reversed by phenylalanine and is not seen with comparable concentrations of sodium fluoride. FPA does not alter elongation of red-irradiated segments. Stimulation by FPA occurs only when the apex is intact and the segments are incubated in the absence of exogenous auxin. In the presence of FPA, (14)C-leucine uptake by coleoptile segments is reduced by 34% and protein synthesis by 42%. When pre-incubated on labeled media and subsequently transferred to unlabeled media, segments fail to incorporate into the protein fraction any of the previously absorbed label. It is therefore difficult to ascertain whether FPA results in a genuine inhibition of protein synthesis in apical coleoptile segments. Possible mechanisms for the action of FPA and its relationship to light dependent elongation are considered.