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.     

Metabolism of Exogenous Indoleacetic Acid to Its Amide Conjugates in Cucumis sativus L

Incubation of hypocotyl segments of light-grown Cucumis sativus L. in 0.1 millimolar 3-indoleacetic acid for 16 hours led to the formation of indoleacetylaspartate and indoleacetylglutamate. There was no evidence for the formation of other conjugates of 3-indoleacetic acid with individual amino acids during the period from 4 to 48 hours of incubation. Indoleacetylglutamate reached its maximum concentration after about 4 hours of incubation and indoleacetylaspartate after about 8 hours. These levels remained unchanged for at least 40 hours. Indoleacetylaspartate caused small increases in cucumber hypocotyl segment growth at high concentrations, 1 millimolar being more effective than 0.1 millimolar.     

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.     

An Interpretation of the Mass Spectra of Salinomycin and Its Derivatives

Salinomycin is a new polyether antibiotic elaborated by Streptomyces albus. An interpretation of the mass spectra of salinomycin and its derivatives has been achieved with the aid of high resolution mass spectrometry along with measurement of the metastable ions. Interesting aspects of the mass spectral data of these compounds are that the characteristic cleavage occurred at the ring C of the tricyclic spiroketal system, and that the sodium salts of salinomycin itself and some derivatives thereof are volatile enough to give distinct spectra which showed fragmentations comparable with those of the free acid and the methyl ester.

An interpretation of the principal fragment ions and their formation mechanism for salinomycin and its derivatives is discussed.     

Indoleacetic Acid and Molecular Differentiation Evidence for Interaction

Proteins of hypocotyls of bean were studied by electrophoresis. Proteins were extracted from hypocotyl segments of various stages of development starting with the relatively undifferentiated hook regions and proceeding by 2 cm segments down the hypocotyl. The proteins were the soluble (pH 7.4), the basic nuclear (histones), acidic ribonuclear and acidic chromosomal. Soluble proteins reflected differentiation of the hypocotyl in that lower hypocotyl segments had more different protein types than did the hook region. Indoleacetic acid (IAA) at 10^?6M when applied to the lower hypocotyl appeared to induce still more different proteins. However, at 10^?3M, IAA appeared to induce molecular dedifferentiation in that hypocotyl protein patterns began to resemble those of the hook. Histones also reflected differentiation, the hook having more histone types than the lower hypocotyl. IAA had no effect on histones. The hook region had two types of acidic chromosomal proteins, the lower hypocotyl one. When lower hypocotyl segments were incubated in 10^?3M IAA, the protein pattern resembled that of the hook in that the second protein normally present in the hook and not in the hypocotyl was in fact induced in the hypocotyl. The hook had two acidic ribonuclear proteins, the lower hypocotyl one. IAA did not affect this protein. These experiments suggest that IAA in some manner regulates molecular (protein) differentiation. It is further suggested that IAA accomplishes this control through the acidic nuclear proteins which are closely associated with genetic material and which reflect differentiation and are also affected by IAA.     

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.     

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.     

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.     

Indoleacetic Acid and the synthesis of glucanases and pectic enzymes

Indoleacetic acid (IAA) and/or inhibitors of DNA, RNA or protein synthesis were added to the apex of decapitated seedlings of Pisum sativum L. var. Alaska. At various times up to 4 days, enzymic protein was extracted from a segment of epicotyl immediately below the apex and assayed for its ability to hydrolyse polysaccharides or their derivatives. With the exception of amylase, the total amounts per segment of all of the tested enzymes increased due to IAA treatment. The development of β-1,4-glucanase (cellulase) activity per unit of protein or fresh weight proceeded according to a typical sigmoid induction curve. Pectinase was formed for about 2 days in control segments and IAA treatment resulted in continued synthesis for at least another 2 days provided cell division took place. β-1,3-glucanase and pectinesterase activities were only enhanced by IAA to the extent that total protein levels increased. Reaction mechanisms for these effects and functions for the enzymes during growth are discussed.     

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.