Abscisic Acid Action on Basipetal Auxin Transport

Several experiments have been performed to analyse the ABA effects on the basipetal transport of IAA-2-¹⁴C, using sections of epicotyls prepared from etiolated Lens seedlings. The sections were incubated in an ABA solution or ABA was applied in the donor blocks containing IAA. For each type of assay, the uptake (analyses of the donor blocks) and the movement of IAA-C¹⁴ (analyses of the receiver blocks) were inhibited by ABA. The distribution of continuous decrease of the radioactivity, along the sections' axis, showed a ¹⁴C level from the apical towards the basal segments. ABA caused a decrease in the ¹⁴C concentration for the total sections, but a relative increase for the basal segment. When ABA was applied simultaneously with IAA in the donor blocks, the transport velocity of IAA, through the sections, was not changed significantly, while an ABA pretreatment caused a significant decrease.     

EFFECT OF LIGHT ON AUXIN TRANSPORT AND ELONGATION OF AVENA MESOCOTYL

The present work was undertaken to find if there are relations between light and auxin action on elongation of coleoptilar node and mesocotyl with Avena seedlings. Red light inhibited the elongation of mesocotyl and simultaneously decreased the rate of transport of diffusible auxin through the node. Red light also inhibited the transport of exogenously given IAA through the nodal region. The light inhibition of IAA transport was closely related to the increase of IAA immobilization. As the age proceeds, the ability of IAA immobilization increased with the decrease in the rate of mesocotyl elongation, even if the seedling was grown in complete darkness. The nature of radioactive substances found in the IAA-C¹⁴ treated tissue was examined by paper chromatography. The above results strongly suggested that the increase of IAA immobilization might result in the inhibition of mesocotyl elongation.     

The transport of auxin and regeneration of xylem in okra and pea stems

Internodes were isolated from okra and pea plants grown from seed under controlled conditions. Isolated internodes were treated apically or basally with IAA-C¹⁴ or 2,4-D-C¹⁴ and several vascular bundles were severed in the middle of the internodes. Transport of auxins was basipetally polar, IAA moving with greater facility than 2,4-D. Apically applied auxin stimulated vascular regeneration in the wound area; basally applied auxin did not, but radioactivity from basally applied IAA reached the wound. This suggests that the path of transport or mode of presentation to cells is important unless basally and apically applied auxin are metabolized differently.     

Paper chromatography of auxins and inhibitors in two Nicotiana species and their hybrid

Auxins and auxin inhibitors from tissue extracts of normal Nicotiana plants, Nicotiana glauca, N. langsdorffii and their hybrid (which spontaneously produces tumors) were separated by ascending paper chromatography with n-butanol-distilled water. An Avena curvature test was used for demonstrating growth-promoting and growth-inhibiting substances. IAA could be found in extracts of the parents and the hybrid (R_F 0.75). Hybrid tissue yielded the highest amount (37.1°), N. glauca tissue less (30.8°), and N. langsdorffii tissue the least amount (8.5°) of IAA. A second growth promoter (R_F 0.35) could be separated from the tissue extracts of the parents and the hybrid, but it showed only low activity in the Avena test. Three inhibitors were present in extracts from N. langsdorffii and the hybrid at R_F 0.25, 0.45, and 0.85, whereas N. glauca showed only two of them (R_F 0.25 and 0.85). The inhibitor with an R_F of 0.45 seemed to be identical with the acidic, benzene-insoluble “inhibitor β” of Bennet-Clark and Kefford (1953). The inhibitor (neutral, benzene-soluble) at R_F 0.85 could be found in some tissue extracts of the parents and the hybrid, but showed only little activity in the curvature tests. From neutral and from acidic plant extracts within a pH range of 4.4 to 5.8 a third inhibitor with an R_F of 0.25 could be separated. It seems that the high concentration of natural IAA in the hybrid is regulated by a variety of inhibitors with different specificities in the growth-regulating process. Nicotiana langsdorffii tissue has much less auxin but the same variety of inhibitors as the hybrid, whereas N. glauca tissue contains less auxin than the hybrid and only two of the three inhibitors found in N. langsdorffii and hybrid extracts.     

Further investigations into hormone-directed transport in stems

Summary The effect of IAA and three synthetic auxins on the upward movement of ³²P-orthophosphate and ¹⁴C-sucrose has been investigated in decapitated stems of Phaseolus vulgaris L., Pisum sativum L., Coleus blumci L. and Helianthus annuus L. IAA greatly enhanced the accumulation of ³²P-orthophosphate in Phaseolus vulgaris and Pisum sativum, whereas in C. blumei and H. annuus it did not. 2,4-D like IAA, caused an increase in ¹⁴C-sucrose and ³²P-orthophosphate accumulation in Phaseolus vulgaris but, unlike IAA, caused no increase in Pisum sativum. The downward transport of ¹⁴C-IAA, ¹⁴C-NAA, ¹⁴C-2,4-D and ¹⁴C-2,4,5-T from the decapitated apex was also studied. Results are discussed in terms of current theories of hormone-directed transport.For abbreviations see Table 1.     

POLAR TRANSPORT OF GROWTH-REGULATORS IN PITH AND VASCULAR TISSUES OF COLEUS STEMS

Movement of IAA-C¹⁴ and 2,4-D-C¹⁴ through cylinders of known size and histology was compared using liquid scintillation counting. Both auxins showed strongly polar movement, even through pith parenchyma cut from Coleus internode #5, the youngest internode to have ceased elongation. The polar movement was correlated with sizable elongation of the excised cylinders. Velocities of basipetal movement for a given auxin, as determined by the intercept method, showed small or negligible differences between pith and “corner” cylinders. (Corner cylinders comprised mostly vascular tissue, plus some cortical, pith, and epidermal cells.) For IAA, basipetal velocities ranged from 2.1 to 3.3 mm per hr; for 2,4-D, they were 0.6–0.8. For both auxins there was much more net loss into corner than into pith cylinders, a difference associated with the fact that corner cylinders showed 10 times as many cells in transection. More 2,4-D moved basipetally through corner than through pith cylinders and the reverse was true of IAA. By chromatographic evidence, all the radioactivity in the basal receiving blocks was still associated with the auxin molecules.     

Biosintesi Delle Indolauxine Acide in Plantule di Orzo Marcato con C14: Effetti Dell'irraggiamento Sui Semi (Riassunto)

Abstract

Biosynthesis of acid indolauxins in young barley plants labelled with C¹⁴. Effects of irradiation on seeds. — With this preliminary research we wanted to begin the study of the setting-up of a system in order to mark young plants cultivated in several groups and kept under the same growth conditions. This apparatus has been utilized for the first time to study the irradiation effect of barley seeds on the biosynthesis of the acid indolderivates (PIA, BIA, IAA). The data in literature on the effect of radiations on the indolauxin-synthesis are barely uniform and we can summarize them so: Skoog (1935) has demonstrated that the auxin content of irradiated plants decreases already at very small doses; the results have been reached by the biological test and seem to be barely reproducible.

Afterwards, Gordon (1957) has confirmed the Skoog's data about different species of plants remarking that also with small doses of radiations the quantity of free auxins decreases quickly soon after irradiation. Gordon has also utilized for his experiments the biological test.

The author, supposing that the free auxins of plants were prevalently IAA, has irradiated IAA solutions: the results have pointed out that the molecule of this auxin is not particularly radiosensible. From these results Gordon concluded that in irradiate plants the decrease of IAA was not given to a direct action of irradiations of the molecule, but to a decrease of its biosyntesis. This assumption has also been maintained by Gordon on account of the results of a further experiment: giving triptophane to sprouts of irradiated seedlings of peas the author remarked a lessening of the IAA synthesis as the doses of irradiation increased. He demonstrated, by another experiment made on omogenates, that radiations caused a rent at the enzyme level which turnes the indolacetic aldeid into acid. On the other hand the data obtained by Gordon on the radiosensibility of IAA aqueous solutions are in opposition to the ones got by Skoog by experiments in vitro and by Cervigni and Belli (1962) which confirmed Skoog's data. These A.A. have demonstrated that the results obtained by Gordon are contestable on account of the analytical technique used. It is well known that seed irradiation causes an evident lessening of growth in young plants development. This phenomenon at least on one side might be connected with an alteration of the metabolic system, which causes the growth regulators synthesis, as the indolauxin.

This experiment wants to bring a contribution to the study of this important problem.

In this first experiment barley has been chosen because the irradiation of the seed at the employed doses. (7,5 and 15 Kr of X rays) does not causes a lessening of the germinability, while it does effect the growth.

Young plants derived from irradiated seeds and from the control ones, on the seventh day from the sowing were put into contact with an atmosphere of C¹⁴O₂ for 42 hoūrs. From the experimental data it has resulted that the synthesis of indolderivates acid in young plants originated from irradiated seeds does not decrease. On the contrary it seems that at least as far as the leaves are concerned there is a greater synthesis by increasing the irradiation dose.

If these first results will be confirmed by further experiments, we can think that the lessening of growth caused by irradiation it does not depend on the decreased synthesis of the acid auxins group, as Gordon and others stated, but rather on the alteration of other regulator systems, strictly connected with the plant's development. It cannot also be excluded that the lessening of growth caused by irradiation, may be attributed to a less sensibility of the tissues to the acid indolauxins.     

Studies in Plant Growth Hormones: IV. CHROMATOGRAPHY OF HORMONES AND HORMONE PRECURSORS IN CABBAGE

1. Acid and neutral ethereal fractions and the non-ether-solubleaqueous fraction of an extract of cabbage leaves were chromatographed,and the chromatc-grams assayed using oat coleoptile sections. 2. In the aqueous fraction an acidic precursor of 3-indolylacetonitrile(IAN) was found. When chromatographed in isopropanol/ammonia,the precursor travelled at the position of 3-indolylacetic acid(IAA), but in n-butanol/ammonia it was much closer to the starting-line.IAN is liberated from the precursor under conditions of alkalinehydrolysis including ammoniacal chromatography, and is alsoliberated by heat. Precursor and IAN zones promoted coleoptilegrowth, but the former when sprayed with ferric chloride/perchloricacid or nitrous/nitric acid gave a yellow colour showing thatthere was no free IAN on this part of the chromatogram. Hypothesesto account for this activity are discussed. 3. A neutral inhibitor was present in the aqueous fraction.It is volatile, ether-soluble, and is thought to have been liberatedfrom a water-soluble substance. 4. The neutral fraction, chromatographed in isopropanol/ammonia,contained IAN and a growth promoter at Rf o–o·1:thelatter stimulates cress root growth above that in water. Thispromoter could be formed from a precursor in the aqueous fractionby heat treatment followed by shaking with sodium bicarbonatesolution. It is suggested that this neutral hormone is the accelerator-of Bennet-Clark and Kefford. The data of these workers are analysedto show that this interpretation is consistent with their results. 5. The acid fraction contained IAN but no IAA. The former isthought not to have been liberated from the precursor in thisfraction but to have entered into it from the neutral fractionduring separation with sodium bicarbonate solution. AlthoughIAA may have been absent from the plant material, it is possiblethat any present was destroyed during the process of extraction. 6. Evidence is presented that there are other growth promoterspresent at low concentration in the extract in addition to thosealready mentioned.     

In Vivo Measurement of Indole-3-acetic Acid Decarboxylation in Aging Coleus Petiole Sections

The concentration of indoleacetic acid (IAA) in plant tissues is regulated, in part, by its rate of decarboxylation. However, the commonly used in vitro assays for IAA oxidase may not accurately reflect total in vivo decarboxylation rates. A method for measuring in vivo decarboxylation was utilized in which ¹⁴CO₂ is collected following uptake of [1-¹⁴C]IAA by excised tissue sections. After a 30-minute equilibration period, the evolution of ¹⁴CO₂ was found to follow an approximately linear course with respect to both time and tissue weight.

Decarboxylation rates were measured by this method in petiole sections of the Princeton clone of Coleus blumei Benth. Both the ¹⁴CO₂ evolved per milligram tissue and the percent of [1-¹⁴C]IAA uptake decarboxylated were highest in sections from the youngest petioles tested, and declined in the older tissue. Thin layer chromatography of acetonitrile extracts from the [1-¹⁴C]IAA-treated petioles showed a decreasing amount of free IAA and an increase at the retardation factor of indoleacetylaspartate in the older sections. The decreased decarboxylation rates in the older petioles may be attributable to a generally lower metabolic rate and increased protection of the IAA by conjugation.

     

The Incorporation of Leucine-U-14C into Ipomeamarone

1. It was demonstrated by silica gel thin layer chromatography that leucine-U-¹⁴C was incorporated into furanoterpenes, e. g. ipomeamarone, in sweet potato root tissue infected by Ceratocystis fimbriata.

2. Further proof for ipomeamarone synthesis from leucine-U-¹⁴C was obtained by the constancy of the specific radioactivity of ipomeamarone semicarbazone through repetitive crystallization.

3. The synthetic pathway of ipomeamarone from leucine was found to be connected with the synthetic pathway from acetate at least at some steps.

4. Leucine-U-¹⁴C was incorporated into both saponifiable and non-saponifiable materials in the same way as acetate-2-¹⁴C.     

Effect of Transverse Gravity Stimulation, Gibberellin and Indoleacetie Acid upon Polar Transport of IAAC14 in the Stem of Helianthus annuus

Using IAA^C14, polar transport has been studied in Helianthus annuus shoots in which stem elongation was inhibited by a transverse gravity stimulus induced by horizontal orientation of the plant with daily rotation. Marked inhibition of polar transport of IAA ^C14 occurred in the treated plants. A similar degree of inhibition occurred in the upper and lower halves of non-rotated horizontally trained shoots. Horizontal orientation of stem segments during the transport test had no consistent effect upon IAA transport. Pretreatment of erect plants with gibberellin greatly enhanced IAA^C14 transport and also reduced the inhibitory effect of horizontal orientation. Pretreatment of erect plants with non-radioactive IAA or ethylene inhibited transport of IAA ^C14 and induced the same symptoms in the shoot as the transverse gravity stimulus. The similarities between the response of the auxin transport system to gravity stimulation, IAA and ethylene are discussed.     

Primi Dati Sull'andamento Stagionale delle Temperature del Troneo di Olivo

Abstract

Temperatures recorded in a olive-tree stem in 1961–1962. — In the Botanical Garden of Bari a temperature recorder has been placed, bearing 6 electrothermometers with platinum resistences having a sensitivity of 2,4 mm/C[ddot], with a recording approximation of 0,1 C[ddot].

The olive tree under examination is a grown-up tree, which has been pruned according to the local methode, and it belongs to a local breed named « Ogliarola leccese ».

— Five thermometers have been employed as follows: 1^rst in the air, hanging from the stem, m. 1,30, high; 2^nd into the outer bark, cm, 0,5 deep; 3^rd into the cambium, cm. 1,5 deep; 4^th into the sap-wood, cm. 6 deep; 5^th into the hearth-wood, cm. 22 deep.

— The thermometers were all placed on the east side of the stem.

— The figures and the graphs of the paper show that:

a) The whole stem of the specimen under examination has a mean temperature which is higher than that of the surrounding air.

b) The outer bark gets warmer than the air, during the day-time; during the night it gives up heat outside and inside, namely towards the stem tissues, particularly to the bark-cambium-sapwood zone, and also as far as the hearth wood zone.

c) The whole living zone of the stem (bark-cambium-sapwood) generally keeps temperature values intermediate between those of the outer bark and those of the air. The temperature values vary along the year.

d) In spring the cambium is warmer than the outer bark.

e) In winter cambium and sapwood generally have the same temperature. The temperature values recorded in this season are much lower than those of the outer bark and almost the same of those of the air.

f) The behaviour of temperature in cambium and sapwood is probably ruled by biological factors.

g) The hearth-wood keeps generally warmer than the air and cooler than other outer tissues. Its thermic rhytm is almost reversed in comparison with that of the outer tissues.

Il seems that hearth wood acts as an useful heat collector and distributor towards the living outer tissues during the diurnal and seasonal temperature remissions, at least in mild climates as that of Bari.     

Trichoderma Harzianum WKY1: an indole acetic acid producer for growth improvement and anthracnose disease control in sorghum

The efficiency of two isolates of Trichoderma harzianum (WKY1 and WKY5) as bio-control agents against anthracnose disease in sorghum was investigated. In vitro, T. harzianum WKY1 isolate showed superiority in terms of inhibition of both mycelial growth and spore germination of Colletotrichum sublineolum, the causative agent of sorghum anthracnose, as well as induction of the sorghum seed germination over T. harzianum WKY5 isolate. The culture filtrate of the selected isolate (T. harzianum WKY1) was analysed using GC-MS system to determine their chemical constituents. Twenty-nine components with varied existence percentages were identified. Although T. harzianum WKY1 produced the phytohormone indole-3-acetic acid (IAA) on tryptophan free medium, a marked dependency on tryptophan for the production of IAA was noticed. Nutritional components were optimized for maximizing IAA production using the central composite design. The optimum levels were 1.06, 29.86 and 2.93?g?L^?1 from tryptophan, sucrose and NaNO₃, respectively, with a maximum IAA biosynthesis (138.9?µg?mL^?1) after five days of incubation. Production of IAA in the culture filtrate of T. harzianum WKY1 was qualitatively and quantitatively analysed by LC-MS system using a reference standard of IAA. Under greenhouse conditions, application of T. harzianum WKY1 and/or its filtrate reduced greatly the disease severity as well as improved the plant growth of sorghum. From the present data, we can recommend the application of T. harzianum WKY1 as a dual purpose bio-agent for biological control of anthracnose disease and plant growth promotion.     

Endogenous indoles and the biosynthesis and metabolism of indole-3-acetic acid in cultures of Rhizobium phaseoli

Gas chromatography-mass spectrometric analyses of purified extracts from cultures of Rhizobium phaseoli wild-type strain 8002, grown in a non-tryptophan-supplemented liquid medium, demonstrated the presence of indole-3-acetic acid (IAA), indole-3-ethanol (IEt), indole-3-aldehyde and indole-3-methanol (IM). In metabolism studies with ³H-, ¹⁴C- and ²H-labelled substrates the bacterium was shown to convert tryptophan to IEt, IAA and IM; IEt to IAA and IM; and IAA to IM. Indole-3-acetamide (IAAm) could not be detected as either an endogenous constituent or a metabolite of [³H]tryptophan nor did cultures convert [¹⁴C]IAAm to IAA. Biosynthesis of IAA in R. phaseoli, thus, involves a different pathway from that operating in Pseudomonas savastanio and Agrobacterium tumefaciens-induced crown-gall tumours.Abbreviations IAA indole-3-acetic acid - IAld indole-3-aldehyde - IAAm indole-3-acetamide - IEt indole-3-ethanol - IM indole-3-methanol - HPLC-RC high-performance liquid chromatography-radio counting - GC-MS gas chromatography-mass spectrometry     

The biosynthesis and translocation of 14C-IAA in Ricinus communis

The biosynthesis of ¹⁴C-IAA from ¹⁴C-tryptophan applied to abraded leaves of Ricinus communis and its subsequent export through the phloem were studied. Phloem sap was collected at intervals from incisions made in the stem below the IAA fed leaf. Any upward movement of label through the phloem or downward movement of phloem mobile compounds from leaves above the treated one were restricted by bark-ringing the plants.TLC and HPLC analyses of the collected sap indicate that some conversion of ¹⁴C-tryptophan to ¹⁴C-IAA had occurred. Subsequent GC-MS analysis of the HPLC purified samples of phloem sap revealed high levels of endogenous IAA transported from the fed leaf. The high ratio of unlabelled/labelled IAA in the phloem sap makes unequivocal confirmation by GC-MS of the predicted biosynthesis of ¹⁴C-IAA impossible. It is postulated that IAA is synthesised from tryptophan in mature leaves and exported to developing sink tissues with the flow of photoassimilates in the phloem.     

The biosynthesis of indole-3-acetic acid byFrankia

Summary High perfomance liquid chromatography (HPLC) of the products of [5-³H] tryptophan metabolism byFrankia sp. Avc I1 indicates that small amounts of [³H] indole-3-acetic acid (IAA) are excreted into the growth medium.Frankia has a limited capacity for the catabolism of [2-¹⁴C]IAA and the product that accumulates is different from that detected inRhizobium japonicum cultures following inoculation with [2-¹⁴C]IAA. The data imply that the rate of turnover of IAA is much more rapid inRhizobium thanFrankia and that the two organisms employ different routes for the catabolism of IAA.     

Effects of indole-3-acetic acid on arsenic uptake and antioxidative enzymes in Pteris cretica var. nervosa and Pteris ensiformis

A hydroponic experiment was conducted to investigate the effects of indole-3-acetic acid (IAA) on arsenic (As) uptake and antioxidative enzymes in fronds of Pteris cretica var. nervosa (As hyperaccumulator) and Pteris ensiformis (non-hyperaccumulator). Plants were exposed to 2 mg L^?1 As(III), As(V) or dimethylarsinic acid (DMA) and IAA concentrations for 14 d. The biomass and total As in the plants significantly increased at 30 mg L^?1 IAA. Superoxide dismutase (SOD) activities significantly increased with IAA addition. Catalase (CAT) activities showed a significant increase in P. ensiformis exposed to three As species at 30 or 50 mg L^?1 IAA but varied in P. cretica var. nervosa. Peroxidase (POD) activities were unchanged in P. ensiformis except for a significant decrease at 50 mg L^?1 IAA under As(III) treatment. However, a significant increase was observed in P. cretica var. nervosa at 10 mg L^?1 IAA under As(III) or DMA treatment and at 50 mg L^?1 IAA under As(V) treatment. Under DMA stress, malondialdehyde contents in fronds of P. cretica var. nervosa showed a significant decrease at 10 mg L^?1 IAA but remained unchanged in P. ensiformis. Therefore, IAA enhanced As uptake and frond POD activity in P. cretica var. nervosa under As stress.     

Decarboxylation and transport of auxin in segments of sunflower and cabbage roots

Summary The movement of ¹⁴C from indole-3-acetic acid (IAA) ¹⁴C has been examined in 5 mm root segments of dark-grown seedlings of Helianthus annuus and Brassica oleracea. Contaminants from distilled water, phosphate buffer and the razor-blade cutter increase the decarboxylation of IAA-¹⁴C, and cutting of root segments results in an activation of IAA-destroying enzymes at the cut surfaces. When these sources of errors were eliminated the following was shown: a) Both in sunflower and cabbage there is a slight acropetal flux of ¹⁴C through the root segments into the agar receiver blocks. The amount of ¹⁴C found in the receiver blocks increases with the lenght of the transport period. b) When the root segments, after the transport period, are cut in two equal parts and these assayed separately, the amounts of ¹⁴C in the two parts indicate a greater acropetal than basipetal transport. c) The total radioactivity of the receiver blocks is in part due to IAA-¹⁴C and in part to ¹⁴CO₂, the latter being a result of enzymatic destruction of auxin. d) Addition of ferulic acid, an inhibitor of IAA oxidases, to the receiver blocks markedly inhibits the decarboxylation of IAA-¹⁴C and thus increases the amount transported. This effect is more pronounced after a 20 hr than after a 6 hr transport period.     

The stimulation of cell extension by ethylene and auxin in aquatic plants

Elongation of the shoots of three aquatic plants (Hydrocharis morsus-ranae, Regnellidium diphyllum and Ranunculus sceleratus) is stimulated by treatment with ethylene or IAA. The effects of the two hormones are additive, and experiments with an ethylene biosynthesis inhibitor and silver ions indicate that the mechanisms by which ethylene and IAA stimulate growth may be different. Hydrocharis and Ranunculus leaf discs synthesize [¹⁴C]ethylene from [¹⁴C]methionine, but no [¹⁴C]ethylene is formed by Regnellidium, suggesting the existence of an alternative pathway of ethylene biosynthesis in the fern.Abbreviations IAA Indole-3-acetic acid - RBA 1,2-amino-4-(2-aminoethoxy)-trans-3-butenoic acid     

Presence of indole-3-acetic acid in chloroplasts ofNicotiana tabacum andPinus sylvestris

The compartmentation and metabolism of indole-3-acetic acid (IAA) was examined in protoplasts derived from needles ofPinus sylvestris L., leaves of normal plants ofNicotiana tabacum L., leaves ofN. tabacum plants carrying the T-DNA gene 1 (rG1 plants) and leaves ofN. tabacum plants carrying the T-DNA gene 2 (rG2 plants) by using a rapid cell-fractionation method. In all tissues, 30%–40% of the IAA pool was located in the chloroplast, while the remainder was found in the cytosol. Quantitative analysis of indole-3-ethanol (IEt) showed that in bothPinus andNicotiana the IEt pool was located exclusively in the cytosol. The only plant that contained endogenous indoleacetamide (IAAm) was therG1-mutant ofN. tabacum, expressing theAgrobacterium tumefaciens T-DNA gene 1. Cellular fractionation of protoplasts from this transgenic plant showed that the entire IAAm pool was located in the cytosol. Feeding experiments utilizing [5-³H]tryptophan, [5-³H]IEt, [1′-¹⁴C] and [2′-¹⁴C]IAA demonstrated that the biosynthesis and catabolism of IAA occurred in the cytosol in bothPinus and in the wild type and the different mutants ofNicotiana. Furthermore, the biosynthesis of IAAm in therG1 plants was also shown to be localized in the cytosol.