Amide-Linked Indoleacetic Acid Conjugates May Control Levels of Indoleacetic Acid in Germinating Seedlings of Phaseolus vulgaris

We have shown that amide-linked IAA (indole-3-acetic acid) conjugates accumulated to high levels during maturation of bean seeds (K. Bialek and J.D. Cohen [1989] Plant Physiol 91: 775-779). In the present study, we were interested in the fate of these and other IAA conjugates during seed germination. The content of amide-linked conjugates of IAA in cotyledons declined dramatically during the first hours of imbibition. The rate of decline slowed markedly during the period of the resumption of axis growth. The level of amide-linked IAA conjugates in cotyledons remained relatively high after almost 1 week of germination. The decline of IAA conjugates in cotyledons was followed by a steady increase in the content of both free and amide-linked IAA in the embryonic axes. Amide-linked IAA conjugates were also present in the axes cultured on agar after the cotyledons were removed, which suggests that de novo production of these IAA conjugates occurs in the axis of germinating bean seedlings. A comparison of relative amounts of free and conjugated IAA in the axes of intact seedlings and axes cultured on agar showed lower levels of free IAA and higher levels of conjugated IAA in much slower growing isolated axes. These results suggest a more general role for IAA conjugates in the control of seedling growth than simply to serve as a seed storage form of auxin.     

Autoinhibition in Polar Transport of Indoleacetic Acid

Autoinhibition in polar transport of 3H-IAA from donor to receiver was demonstrated in pea ( Pisum sativum L. ) and mung bean ( Phaseolus radiatus L. ) using either classic "donor-receiver system" where unlabelled IAA was present in the receiver or using "Y" form explants where tmlabelled IAA in lanolin was applied to the cut surface of a separate side of the explants. Application of unlabelled IAA resulted in accmnulation and high rate of metabolism of SH-IAA in the labelled tissues, and these were further stimulated by prolonging the experimental period.     

Indoleacetic Acid Oxidases in Resting Cereal Grains

Peroxidases oxidizing indoleacetic acid (IAA) are present in the resting grains of barley, rye and wheat. The grains also contain small molecular inhibitors of the enzymes. A partly purified preparation of barley seed proteins was shown to contain at least two IAA oxidases about pI 5 and pI 10, mol. weight 30,000 and with a pH optimum 5.1–5.7. The enzymes require Mn²⁺, 2,4-dichlorophenol and orthophosphate for the maximum activity. Attempts to separate IAA oxidase and peroxidase activities of the enzymes were unsuccessful.     

Evidence for Two Indoleacetic Acid-Induced Growth Responses in the Avena Straight-Growth Indoleacetic Acid Assay

Floating Avena sativa L. cv Victory coleoptile segments were used to determine whether the straight-growth indoleacetic acid (IAA) assay can be reconciled with the Avena curvature assay and the Cholodny-Went theory of photo- and gravitropism. Measurements of segment length after 5 h yield sigmoid-shaped IAA dose-response curves with the growth rate leveling off at 1 [mu]M. However, measurements made at 24 h generate bell-shaped curves with maximal growth being induced by 10 [mu]M IAA. The difference between short- and long-term IAA dose-response curves is not due to IAA degradation; instead, it is the result of two growth responses to IAA. The initial one is rapid, responds to low concentrations of IAA, and lasts for 12 h. The second response is less sensitive to IAA than the first one. It appears after 6 h but is not obvious until the last 12 h of a 24-h incubation. The profile of short-term IAA dose-response curves reflects the initial growth response, whereas that of the 24-h curve is the sum of both growth responses. Linear-linear plots of 5- and 24-h dose-response curves show that coleoptile segment growth rate is proportional to IAA concentration up to 0.3 [mu]M. When the efficiency of IAA action is taken into account, it is found that the most effective IAA concentration for short and long incubations is 0.4 [mu]M. It is concluded that the Avena straight-growth IAA assay is as sensitive as the Avena coleoptile curvature assay, and that it is consistent with the Cholodny-Went theory.     

Transport Gradient of Indoleacetic Acid in Pine Seedlings

Movement of indoleacetic acid (IAA-1-¹⁴C) through 5-min segments from stems and roots of Pinus taeda L., P. jeffreyi Gtrev. and Balf., and P. ponderosa Laws, seedlings was measured. All three species showed the same general pattern of IAA-1-¹⁴C transport. Predominantly downward movement occurred at all points along the stem and root; least IAA-1-¹⁴C transport occurred in segments obtained near the shoot apex; and most occurred in segments obtained near or just below the cotyledons. This pattern of transport gradient is different from that reported in other plants. These results could be interpreted in either of two ways: (1) that transport limitations may control movement of the growth-regulator, or (2) that the transport gradient may be an indication of endogenous growth-regulator levels.     

Polarity of Indoleacetic Acid in young Coleus Stems

Young internodes of Coleus blumei Benth. have long been known for their sizable amount of acropetal indoleacetic acid movement. However, plants of the same clone, under improved growing conditions, now show almost absolute basipetal polarity of ¹⁴C-indoleacetic acid, as measured by liquid scintillation counting of ¹⁴C in the receiver cylinders of agar. The ratio of basipetal to acropetal movement is now as much as 85:1, instead of the 3:1 ratio found years ago under conditions providing slower growth.     

Effect of Indoleacetic Acid and Related Indoles on Lactobacillus sp. Strain 11201 Growth, Indoleacetic Acid Catabolism, and 3-Methylindole Formation

A study was conducted to determine the activity of the 3-methylindole (3MI)-forming enzyme in Lactobacillus sp. strain 11201. Cells were incubated anaerobically with 17 different indolic and aromatic compounds. Indoleacetic acid (IAA), 5-hydroxyindoleacetic acid, 5-methoxy-3-indoleacetic acid, indole-3-pyruvate, or indole-3-propionic acid induced 3MI-forming activity. The highest total enzyme activity induced by IAA was observed in cells incubated with an initial concentration of 1.14 mM IAA. Peak activity of the 3MI-forming enzyme occurred 4 h after bacteria were incubated with either 0.114 or 1.14 mM IAA. Enzyme activity peaked earlier (2 h) and disappeared more rapidly at 5.7 mM IAA than at other concentrations of IAA. The effects of IAA and 3MI on the growth of Lactobacillus sp. strain 11201 and formation of 3MI from IAA also were determined. Bacterial growth and 3MI formation from IAA were reduced in medium containing exogenous 3MI. The growth depression observed in medium containing 5.7 mM IAA appears to be due to the toxicity of 3MI rather than IAA. The formation of 3MI in this ruminal Lactobacillus sp. is mediated by an inducible enzyme, and as 3MI accumulates, bacterial growth and rates of 3MI formation from IAA are reduced.     

Indoleacetic Acid synthesis in soybean cotyledon callus tissue

Growth of an auxin-requiring soybean cotyledon callus tissue (Glycine max L., Merr. var. Acme) was promoted by tryptophan, tryptamine, indole, indoleacetamide and, to a very slight degree, anthranilic acid. When tryptophan-3-¹⁴C was supplied in the growth medium, labeled indoleacetic acid (IAA) was found in both the tissue and the medium. Medium, from which the cells had been removed, was also found to convert labeled tryptophan to IAA. Soybean callus contained 0.044 μmole/g free tryptophan, but this is apparently not available for conversion to IAA. These results suggest that while exogenously supplied trytophan could elevate a specific internal pool where IAA synthesis occurs some of the growth on a tryptophan medium can be accounted for by external conversion.     

Effect of Indoleacetic Acid on the Abscisic Acid Level in Stem Tissue

Excised stem sections from growing plants of Populus tremula L. and Pisum sativum L. including lateral buds were treated with indole-3-acetic acid in a phosphate buffer solution. In control sections the level of the abscisic acid-like inhibitor decreased strongly during 24 h as did the level of the endogenous auxin. Exogenous indoleacetic acid counteracted the decrease in the inhibitor level to a considerable extent. Implications of this auxin effect in relation to apical dominance are discussed.     

Mass Spectra of Indoleacetic Acid and Its Chloroderivatives

In the presence of silver carbonate, primary and secondary alcohols such as γ-hydroxybutyrophenone, 17α-21-dihydroxy-4-pregnene-3,11,20-trione and 11α-hydroxy-17α-methyltestosterone react with alkyl halides in N,N-dimethylformamide under mild reaction conditions to give the corresponding unsymmetrical dialkylcarbonates in good yields.     

Cytokinin-controlled Indoleacetic Acid Oxidase Isoenzymes in Tobacco Callus Cultures

Indoleacetic acid oxidase in tobacco callus tissues (Nicotiana tabacum L., cultivar White Gold) was resolved into seven anionic isoenzymes by polyacrylamide gel disc electrophoresis. Different concentrations of kinetin and zeatin in the presence of indoleacetic acid affected the level of this enzyme, particularly two fast-moving isoenzymes, A₅ and A₆. The optimal concentration of kinetin was 0.2 μm; increasing concentrations above this level progressively lowered the total activity of indoleacetic acid oxidase and repressed the development of isoenzymes A₅ and A₆. Actinomycin D and cycloheximide inhibited the development of these two isoenzymes under the influence of 0.2 μm kinetin, suggesting a requirement for RNA and protein synthesis. The cytokinin-promoted indoleacetic acid oxidase isoenzymes A₅ and A₆ increased with time and paralleled the dry weight increase of tobacco callus tissues, but the total activity of indoleacetic acid oxidase per unit dry weight of tobacco callus varied with time depending on the stage of plant growth.     

Indoleacetic Acid Oxidase Activity in Two High-Peroxidase Tomato Mutants

Homogenates of tomato (Lycopersicon esculentum Mill.) tissues oxidize IAA at a rate proportional to their peroxidase activity. Moreover, isoproteins with peroxidase activity catalyze also the oxidation of IAA. Two monogenic recessive mutations, olivacea and monstrosa, are involved in the control of the peroxidase-IAA oxidase activity. In olivacea leaflets this activity is 15 times that of the normal; all the aerial organs of monstrosa show high activities, particularly the midrib. These high activities depend on the enhancement of the organ-specific isoenzymes and on the derepression of the root isoenzyme band C2. This latter peroxidase-IAA oxidase band is present in olivacea and monstrosa leaflets, in monstrosa stems and, less intensely stained, in monstrosa midribs. The reduction in length and weight of mutant organs and their epinastic and geotropic-like behavior are associated with high peroxidase-IAA oxidase activities and with the derepression of the root-specific band C2. These results suggest an important role of the peroxidase-IAA oxidase system in controlling the metric relationships among different organs of the plant. Olivacea and monstrosa are to be viewed as auxin-related mutations controlling the synthesis of a group of proteins related in function.     

Rhythmicity in the Basipetal Transport of Indoleacetic Acid through Coleoptiles

¹⁴C-Indoleacetic acid was applied to coleoptiles of corn (Zea mays) and oat (Avena sativa). The coleoptiles were detached from the endosperms at 6-minute intervals after indoleacetic acid application, and the radioactivity was determined in successive 2-millimeter regions. The rate (per cent per minute) of basipetal transport of indoleacetic acid is periodic in various regions of the coleoptile, with a period of about 20 minutes. The possible relation of this cyclic phenomenon to other rhythmic processes of similar periodicities is discussed. A distinct acropetal transport (against the concentration gradient) from the subapical region to the apical 2-millimeter region of the coleoptile was detected.