Metabolism of DL-Tryptophan-3-14C by the Fruitlets of Citrus aurantifolia

Developing lime fruit [Citrus aurantifolia (Christm.) Swingle] was supplied with dl-tryptophan-3-¹⁴C in a special medium. An incubation period of six hours was sufficient for the radioactivity to reach an equilibrium between the fruit tissue and the incubation medium. Analyses of the fruit tissue and the medium for acidic and neutral metabolites of tryptophan indicated the existence of indolic products. The auxin indole-3-acetic acid (IAA) was identified among the products by dry column chromatography and biological assay. Other acidic metabolites included indolepyruvic acid and an unidentified material. Neutral metabolites included indolealdehyde, indoleacetaldehyde, and two unidentified compounds. Biological activity in the Avena curvature test was obtained from extracted compounds which corresponded to IAA and indolepyruvic acid in the acidic fraction and indoleacetaldehyde in the neutral fraction. Radioactive tryptophan was also found in both the acidic and the neutral fractions due to its amphoteric nature. The experiment demonstrated the conversion of tryptophan to its indolic metabolites, including indole-3-acetic acid, in this Citrus tissue.     

Metabolism of Indole-3-Acetic Acid: III. Identification of Metabolites Isolated from Crown Gall Callus Tissue

The metabolism of labeled indole-3-acetic acid (IAA-2-¹⁴C) was investigated in Parthenocissus tricuspidata crown gall callus tissue. After 48 hours incubation, 85 to 90% of the supplied IAA was taken up by the tissue, and of that taken up, about 45% was conjugated with five amino acids. The conjugates found were aspartic and glutamic acid (minor ones) as well as glycine, alanine, and valine (major ones). The last four are being reported for the first time as metabolites of IAA. These conjugates were identified through their chromatographic properties, hydrolysis products, and their mass spectra. The possible significance of these amino acid conjugates is discussed.     

Indoleacetic Acid biosynthesis in Avena coleoptile tips and excised bean shoots

Avena coleoptiles did not elongate when incubated with tryptophan under sterile conditions. Indole, anthranilic acid, and tryptamine promoted elongation. Under the same conditions, the tissue converted tryptophan-¹⁴C to IAA-¹⁴C. More IAA-¹⁴C was produced from indole-¹⁴C than from tryptophan-¹⁴C; however, the free tryptophan content of the tissue was also greatly increased by the indole treatment. Tryptophan-¹⁴C was readily taken up by the tissue but was mainly incorporated into protein and did not increase the free tryptophan level. When bean shoots were labeled with tryptophan-¹⁴C or indole-¹⁴C, the label incorporation into IAA-¹⁴C was very nearly the same. In this tissue the free tryptophan level in the tryptophan-¹⁴C and indole-¹⁴C treatments was also about equal. These results suggest that failure of exogenously supplied tryptophan to promote the elongation of Avena coleoptiles is a result of its predominant incorporation into protein and consequent unavailability for conversion to IAA.     

Indole-3-acetic Acid Synthesis in Tumorous and Nontumorous Species of Nicotiana

The synthesis of indole-3-acetic acid (IAA) in the enzyme extracts of Nicotiana glauca, Nicotiana langsdorffii, their F1 hybrid, their amphidiploid hybrid, and the nontumorous mutant of the hybrid was investigated. Tryptamine, a possible precursor of IAA biosynthesis in Nicotiana tabacum, was not found in the callus tissue of N. glauca, N. langsdorffii, and their F1 hybrid.

In petiole slices, the synthesis of IAA progressively increased during 5 hours of incubation in [¹⁴C]tryptophan. The rate of synthesis was about equal in the hybrid and N. langsdorffii but lower in N. glauca on either a cell or fresh weight basis. It was also found that tryptophan was about 25 times more efficient than tryptamine in promoting synthesis of IAA in petiole slices.

It was found that indoleacetaldehyde oxidase, indoleacetaldehyde reductase, and tryptophan aminotransferase activities were present in all of the species examined; however, tryptophan decarboxylase activity was not found. The tryptophan aminotransferase activity in N. glauca, N. langsdorffii, and the nontumorous mutant required α-ketoglutaric acid and pyridoxal 5-phosphate whereas the addition of pyridoxal 5-phosphate seemed not to increase the enzyme activity in tumor plants.

The tryptophan aminotransferase in the amphidiploid hybrid was partially purified by acetone precipitation. The enzyme activity had a temperature optimum at 49 C and a pH optimum at 8.9. It is suggested that there is an indolepyruvic acid pathway in the synthesis of IAA in the Nicotiana species examined.

     

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     

Induction of indoleacetic Acid synthetases in tobacco pith explants

Formation of indoleacetic acid synthetases in tobacco pith explants was determined by following the growth of tissue cultures under conditions of indole-3-acetic acid (IAA) deprivation and by measuring the enzymatic conversion of tryptophan to IAA in the cultures. The pith explants obtained from the parent plant (Nicotiana glauca) and from basal regions of the tumor-prone hybrid (N. glauca × N. langsdorffii) both show a requirement for exogenous IAA for growth initiation in culture. The parent pith requires the constant presence of added IAA for continued growth, but hybrid pith, after initial treatment with IAA, will grow without further additions. IAA synthetases are detected in the cell homogenates of hybrid pith explants cultured with either continuous or initial IAA addition. These observations indicate that IAA may induce its own production. In contrast, IAA synthetases are not found in the parent pith under comparable culture conditions. Besides IAA, nonhormonal compounds such as indole and tryptophan are also capable of stimulating growth of hybrid pith, possibly through the induction of IAA synthetases needed for IAA formation. Indole and tryptophan are, however, inactive in growth promotion of the parent pith. These results suggest that the genomic expression of IAA synthetase formation is more stringently controlled in N. glauca than in the tumorprone hybrid.     

Ethylene production by callus and suspension cells from cortex tissue of postclimacteric apples

Cortex tissue from postclimacteric `Golden Delicious' apples (Malus domestica, Borkh.) stored at 0 C for 9 months after harvest were induced to form callus in vitro. Cell suspension cultures were subsequently formed from calli. Of five media tested, only the medium of Schenk and Hildebrandt (Can J Bot 1972, 50: 192) and that of Uchimiya and Murashige (Plant Physiol 1974, 54: 936) allowed callus formation. During growth both the callus and cell cultures produced ethylene in a pattern which showed a rapid rise and then a fall as the culture grew. ¹⁴C-Labeled methionine was converted to labeled ethylene by the cell suspension cultures, which also could be inhibited from producing ethylene by a rhizobitoxine analog or free radical scavengers. Ethylene production in these cultures, like that in intact fruit tissue slices, could be stimulated by IAA or suppressed by N⁶-(γ,γ-dimethylallyl) adenosine and GA₃.     

Induced Caulogenesis in Long-term Callus Cultures of Rosrnarinus officinalis L

The callus formed in Rosmarinus officinalis L in association with shoot tip proliferation was isolated and subjected to different treatments for good growth. Two basal media, namely, Murashige and Skoog (MS) and Schenk and Hildebrdndt (SH) and their modifications supplemented with 0.25 mg I^-1 6-benzylaminopurine (BAP), 0.5 mg I^-1 indole-3-acetic acid (IAA) and 1.0 mg I^-1 2,4-dichlorophenoxyacetic acid (2,4-D) were used. Callus in MS medium, was compact and remained fresh and green upto 30 days but grew slowly. Whereas, in SH medium callus growth was rapid but it turned brown within 15 days.The browning of callus could be checked with the addition of 1500 mg I^-1 NH,NO, to the medium, in which callus grew 15 fold in fresh weight during 21 days and remained fresh upto 45 days of incubation.The shoot buds differentiated in this somatic callus with the addition of 0.5 mg I^-1 each of BAP, 2-isopentenyl adenine (2ip), IAA and 10 mg I^-1 gibberellic acid (GA₃), within 15 days of incubation provided the callus remained floating on the liquid medium. Histological investigations revealed both peripheral and occasionally internal differentiation of shoot buds. Differentiated shoot buds were proliferated, rooted and transplanted in the soil.     

ATTEMPTS TO OBTAIN BACTERIA-FREE PLANTS OF PSYCHOTRIA PUNCTATA (RUBIACEAE): GROWTH AND ROOT FORMATION IN CALLUS CULTURES

Attempts were made to obtain bacteria-free plants of Psychotria punctata from tissue cultures. Stem explants and callus derived from them were induced to form roots but failed to form buds on Linsmaier and Skoog medium and 96 chemical modifications of it, including most of those known to induce bud formation in other species. Roots formed with ample IAA (2 mg/liter or more) and a low kinetin concentration (0.25 or 0.50 mg/liter). Adenine inhibited root formation in these media, but tyrosine did not. Tyrosine did lower the percentage of calluses commencing growth. When enzyme-hydrolyzed lactalbumin (1.3 g/liter), kinetin (0.5 mg/liter) and IAA (5 mg/liter) were added to Linsmaier and Skoog medium modified by decreasing inorganic nitrogen and increasing inorganic phosphate, callus grew at the fastest rate observed (increasing threefold in fresh weight in three weeks) and formed numerous roots. This was adopted as the stock callus medium. Casein hydrolysates also stimulated growth but less so than lactalbumin hydrolysate. When lactalbumin hydrolysate or a casein hydrolysate lacking tryptophan was supplied, growth occurred without added auxin if sufficient cytokinin was added. Cytokinin was required at unusually high concentration and was tolerated at still higher concentration. Formation, elongation, and branching of roots persisted on a saturated solution of BA which inhibited callus growth about 70 % and delayed callus senescence. Light caused earlier callus senescence after growth had ceased but did not affect callus growth or root formation. Light-induced senescence was prevented by a high cytokinin concentration.     

Physiology of Hormone Autonomous Tissue Lines Derived From Radiation-Induced Tumors of Arabidopsis thaliana

γ-Radiation-induced tumors of Arabidopsis thaliana L. have been produced as a novel approach to isolation of genes that regulate plant development. Tumors excised from irradiated plants are hormone autonomous in culture and have been maintained on hormone-free medium for up to 4 years. Five tumor tissue lines having different morphologies and growth rates were analyzed for auxin, cytokinin, and 1-aminocyclopropane-1-carboxylic acid (ACC) content, ethylene production, and response to exogenous growth regulators. Normal tissues and two crown gall tissue lines were analyzed for comparison. Rosettes and whole seedlings each contained approximately 30 nanograms· (gram fresh weight)⁻¹ free indoleacetic acid (IAA), 150 nanograms· (gram fresh weight)⁻¹ ester-conjugated IAA, and 10 to 20 micrograms· (gram fresh weight)⁻¹ amide-conjugated IAA. The crown gall lines contained similar amounts of free and ester-conjugated IAA but less amide conjugates. Whereas three of the radiation-induced tumor lines had IAA profiles similar to normal tissues, one line had 10- to 100-fold more free IAA and three- to 10-fold less amide-conjugated IAA. The fifth line had normal free IAA levels but more conjugated IAA than control tissues. Whole seedlings contained approximately 2 nanograms· (gram fresh weight)⁻¹ of both zeatin riboside and isopentenyladenosine. The crown gall lines had 100- to 1000-fold higher levels of each cytokinin. In contrast, the three radiation-induced tumor lines analyzed contained cytokinin levels similar to the control tissue. The radiation-induced tumor tissues produced very little ethylene, although each contained relatively high levels of ACC. Normal callus contained similar amounts of ACC but produced several times more ethylene than the radiation-induced tumor lines. Each of the radiation-induced tumor tissues displayed a unique set of responses to exogenously supplied growth regulators. Only one tumor line showed the same response as normal callus to both auxin and cytokinin feeding. In some cases, one or more tumor lines showed increased sensitivity to certain growth substances. In other cases, growth regulator feeding had no significant effect on tumor tissue growth. Morphology of the radiation-induced tumor tissues generally did not correlate with auxin to cytokinin ratio in the expected manner. The results suggest that a different primary genetic event led to the formation of each tumor and that growth and differentiation in the tumor tissue lines are uncoupled from the normal hormonal controls.     

Role of IAA conjugates in inducing ethylene production by tobacco leaf discs

Indole-3-acetic acid (IAA) labeled in its carboxyl group was metabolized by tobacco leaf discs (Nicotiana tabacum L. cv. Xanthi) into three metabolites, two of which were preliminarily characterized as a peptide and an ester-conjugated IAA. Reapplication of each of the three metabolites (at 10 μM) resulted in a marked stimulation of ethylene production and decarboxylation by the leaf discs. Similarly, these three IAA metab olites could induce elongation of wheat coleoptile segments, which was accompanied by decarboxylation. Both the exogenously supplied esteric and peptidic IAA conjugates were converted by the leaf discs into the same metabolites as free IAA. (1-¹⁴C)IAA, applied to an isolated epidermis tissue, was completely metabolized to the esteric and peptidic IAA conjugates. This epidermis tissue showed much higher ethylene production rates and lower decarboxylation rates than did the whole leaf disc. The results suggest that the participation of IAA conjugates in the regulation of various physiological processes depends on the release of free IAA, which is obtained by enzymatic hydrolysis of the conjugates in the tissue. The present study demonstrates biological activity of endogenous IAA conjugates that were synthesized by tobacco leaf discs in response to exogenously supplied IAA.     

Callus Induction in Small Flowered Willow Herb (<Emphasis Type="Italic">Epilobium parviflorum</Emphasis> Schreb)

In order to determine the most suitable in vitro tissue culture and plant regeneration conditions for the small flowered willow herb (Epilobium parviflorum Schreb), various explants were cultured on semi-solid MS media containing factorial combinations of plant growth regulators. Callus induction from hypocotyl, cotyledon, petiole and leaf explants was achieved on media containing 2,4-dichlorophenoxy acetic acid (2,4-D) and kinetin (KIN). All other growth regulator combinations [□-naphtaleneacetic acid (NAA) ± benzylaminopurine (BAP), NAA ± thidiazuron (TDZ), indol acetic acid (IAA) ± Zeatin (ZEA)] tested failed to respond. The best results with cotyledon- and petiole- derived callus were obtained from MS medium supplemented with 1.0 mg l^?1 2,4-D + 0.1 mg l^?1 KIN and 2.0 mg l^?1 2,4-D + 0.2 mg l^?1 KIN. It was observed that B5 basal medium was more effective than MS basal medium for producing seedling and the most effective seed sterilizing solution was 25 % (v/v) sodium hypochlorite (NaOCl). No plant regeneration was observed in either callus induction or during the subculturing stage. This is the first report on in vitro tissue culture study within the genus Epilobium.     

Stable Isotope Labeling, in Vivo, of d- and l-Tryptophan Pools in Lemna gibba and the Low Incorporation of Label into Indole-3-Acetic Acid

We present evidence that the role of tryptophan and other potential intermediates in the pathways that could lead to indole derivatives needs to be reexamined. Two lines of Lemna gibba were tested for uptake of [¹⁵N-indole]-labeled tryptophan isomers and incorporation of that label into free indole-3-acetic acid (IAA). Both lines required levels of l-[¹⁵N]tryptophan 2 to 3 orders of magnitude over endogenous levels in order to obtain measurable incorporation of label into IAA. Labeled l-tryptophan was extractable from plant tissue after feeding and showed no measurable isomerization into d-tryptophan. d-[¹⁵N]tryptophan supplied to Lemna at rates of approximately 400 times excess of endogenous d-tryptophan levels (to yield an isotopic enrichment equal to that which allowed detection of the incorporation of l-tryptophan into IAA), did not result in measurable incorporation of label into free IAA. These results demonstrate that l-tryptophan is a more direct precursor to IAA than the d isomer and suggest (a) that the availability of tryptophan in vivo is not a limiting factor in the biosynthesis of IAA, thus implying that other regulatory mechanisms are in operation and (b) that l-tryptophan also may not be a primary precursor to IAA in plants.     

The mode of incorporation of 6-benzylaminopurine into tobacco callus transfer ribonucleic Acid: a double labeling determination

Cytokinin-dependent tobacco (Nicotiana tabacum) tissue was grown in the presence of N⁶-benzyladenine (6-benzylaminopurine) labeled in the phenyl ring with ³H and in the 8-position of the purine ring with ¹⁴C. The ³H/¹⁴C ratio of N⁶-benzyladenosine recovered from labeled tobacco callus transfer RNA preparations was compared with the corresponding ratio for N⁶-benzyladenine supplied in the medium. By this means, with suitable controls, the incorporation of the synthetic cytokinin, N⁶-benzyladenine, into the transfer RNA of tobacco callus tissue was shown to involve the intact moiety. The observed level of incorporation was up to one N⁶-benzyladenosine molecule per 10⁴ transfer RNA molecules.     

The role of the conversion of IBA into IAA on root regeneration in apple: introduction of a test system

A test system for root regeneration was developed that consists of stem slices of apple shoots (ca. 0.5 mm thick; fresh weight ca. 1 mg). Roots regenerated synchronously without intervening callus formation and without interference with compounds originating from other parts of the plant. Supply of indolebutyric acid (IBA) or indoleacetic acid (IAA) induced maximally an average of 8 or 4.5 roots per slice, respectively. After uptake of IBA, a high degree of conjugation resulted in a recovery of 2.5% as internal free IBA (ca. 2 times the medium concentration). Due to conversion of absorbed IBA into IAA a fraction of 0.4% was recovered as (physiologically active) free IAA. After incubation on medium with IAA, 0.5% of the absorbed hormone was recovered in the free acid form. No conversion of IAA into IBA was observed. Equimolar contents of internal free IAA after incubation on IBA or IAA resulted for IBA in a higher number of roots than for IAA. This means that IBA may also act via internal free IBA or may synergistically modify the action or endogenous synthesis of IAA.Abbreviations BHT butyl hydroxy toluene - EtOH ethanol - FW fresh weight - HAc acetic acid - HPLC high performance liquid chromatography - IAA indoleacetic acid - IAA^int free (internal) IAA in the tissue derived from IBA or IAA uptake - IBA indolebutyric acid - IBA^int free (internal) IBA in the tissue derived from IBA or IAA uptake - MeOH methanol - PAR photosynthetic active radiation - SEM standard error of the mean - TLC thin layer chromatography - UV ultra violet     

Tryptophan Biosynthesis from Indole-3-Acetic Acid by Anaerobic Bacteria from the Rumen

Microbes in ruminal contents incorporated (14)C into cells when they were incubated in vitro in the presence of [(14)C]carboxyl-labeled indole-3-acetic acid (IAA). Most of the cellular (14)C was found to be in tryptophan from the protein fractions of the cells. Pure cultures of several important ruminal species did not incorporate labeled IAA, but all four strains of Ruminococcus albus tested utilized IAA for tryptophan synthesis. R. albus did not incorporate (14)C into tryptophan during growth in medium containing either labeled serine or labeled shikimic acid. The mechanism of tryptophan biosynthesis from IAA is not known but appears to be different from any described biosynthetic pathway. We propose that a reductive carboxylation, perhaps involving a low-potential electron donor such as ferredoxin, is involved.     

Improvement of shoot proliferation and comparison of secondary metabolites in shoot and callus cultures of <Emphasis Type="Italic">Phlomis armeniaca</Emphasis> by LC-ESI-MS/MS analysis

Phlomis armeniaca Willd. is a medicinal plant in the Lamiaceae family endemic to Turkey. The present study describes efficient plant regeneration and callus induction protocols for P. armeniaca and compares phenolic profiles, total phenol and flavonoid contents, and free radical scavenging activity of in vitro-derived tissues. Stem node explants from germinated seedlings were cultured on Murashige and Skoog medium (MS) supplemented with 75 plant growth regulator (PGR) combinations. The highest shoot number per explant, frequency of shoot proliferation, and frequency of highly proliferated, green, compact callus were obtained on MS medium containing 0.25 mg L^?1 thidiazuron (TDZ) and 0.25 mg L^?1 indole-3-acetic acid (IAA). The best root formation was on MS basal medium (control). Methanol extract of leaves obtained from regenerants contained higher total phenol and flavonoid contents than the callus extract. The callus extract showed stronger free radical scavenging activity than leaves with IC₅₀ [concentration inhibiting 50% of 2,2-diphenyl-1-picrylhydrazyl (DPPH) free radical] values of 4.30 ± 0.08 and 2.21 ± 0.04 mg g^?1 dry weight in leaves and callus, respectively. Apigenin, caffeic acid, p-coumaric acid, luteolin, rutin hydrate, vanillic acid, ferulic acid, salicylic acid, sinapic acid, and chlorogenic acid were detected by liquid chromatography–electrospray ionization multistage tandem mass spectrometry (LC-ESI-MS/MS) analysis in in vitro-grown leaves and callus tissue. Rutin hydrate, p-coumaric acid, and vanillic acid were found at approximately tenfold higher levels in callus than in leaves. This new micropropagation protocol, the first for P. armeniaca, could be used in industrial production for new herbal tea and germplasm conservation.     

Construction of IAA-Deficient Mutants of Pseudomonas moraviensis and Their Comparative Effects with Wild Type Strains as Bio-inoculant on Wheat in Saline Sodic Soil

Tryptophan role in microbial biosynthesis of Indole Acetic Acid (IAA) is very distinct. In present study IAA producing bacteria Pseudomonas moraviensis was applied on wheat for improving growth and physiology; in the presence or absence of L-tryptophan in saline sodic field. Aqueous solution of tryptophan was added to the rhizosphere soil at 10?mg L^?1 with irrigated water. The survival efficiency of P. moraviensis measured in the presence of NaCl and mixture of salts. P. moraviensis increased P, NO₃–N and K contents in soil by 18–35% and further 12–15% increase was recorded in the presence of tryptophan. There were 40–80% increases in indole acetic acid (IAA), abscisic acid (ABA) and gibberellic acid (GA) contents of rhizosphere soil, and 40–45% increase in leaves when tryptophan was added with P. moraviensis. In the second phase, IAA deficient mutants of P. moraviensis were constructed and tested for the conversion of tryptophan to IAA. In transposon mutagenesis, 1800 trans-conjugants were generated and tested for tryptophan conversion. Among these, 11 mutants were selected and inoculated into wheat to compare their growth responses to the wild type. P. moraviensis wild type served as PGPR under salinity, but IAA- deficient mutants of P. moraviensis were unable to produce IAA and halted plant growth.     

Nutrient salts promote light-induced degradation of indole-3-acetic Acid in tissue culture media

The disappearance of indole-3-acetic acid (IAA) from cell-free liquid culture medium was followed in response to nutrient salts found in Murashige-Skoog salt base, light, and pH range of 4 to 7. The loss of IAA was accelerated by light or Murashige-Skoog salts. However, the combination of both light and Murashige-Skoog salts acted synergistically to catalyze the destruction of over 80% of the original IAA within 7 days of continuous incubation. Under these same conditions, the loss of IAA was decreased to approximately 50% by adjusting the initial pH of the medium to 7. Iron was identified as the single major contributor to light-catalyzed destruction of IAA. Removal of nitrates, which represented 87% of the molar salt composition, also reduced the light-catalyzed loss of IAA. Treatments that protected IAA from degradation, such as darkness or removal of iron from the medium, suppressed the growth of muskmelon (Cucumis melo. Naud., var. reticulatus) callus tissue cultured for 30 days. Treatments in the light that rapidly degraded IAA resulted in maximum growth. Consequently, the brief exposure to IAA prior to degradation was apparently sufficient to initiate physiological changes required for growth. Possible approaches to the preservation of IAA during incubation are discussed.     

Effect of Gibberellic Acid and Cycocel on Tryptophan Metabolism and Auxin Destruction in the Sunflower Seedling

A comparative study of tryptophan conversion in different regions of the sunflower seedling indicates that the regions most active in converting tryptophan on a pathway to auxin are the root apical segments and young leaves; next highest in activity is the cotyledonary tissue. The stem apex proper with leaf primordia is less active than the above regions in converting the auxin precursor. Hypocotyl tissue was observed to be least active. Pre-treatment of the apical bud region of the stem with gibberellic acid (GA) gives rise to tryptophan conversion rates which are 2.1 times those in untreated seedlings. The enhanced tryptophan conversion in the apical bud is followed by an increased elongation rate of the 1st internode which is 2.2 times that in the 1st internode of untreated seedlings. Treatment of the seedlings with Cycocel [(2-chloroethyl)trimethylamnionium chloride] does not reduce tryptophan conversion in the apical bud region of the seedling although elongation of the stem is greatly retarded. Indoleacetic acid (IAA) destruction in cell free preparations as well as in whole sections of the elongating region of the seedling stem was studied. IAA-1-¹⁴C destruction rates with the release of ¹⁴CO₂ in whole sections of 1st internode tissue were approximately 3 times those in cell free preparations of the same region. No significant changes in IAA destruction rates in seedlings pre-treated with GA or Cycocel were observed.