Mode of Action of Growth Retarding Chemicals

Increase in Diffusible Auxin after Treatment with Gibberellin

When dwarf pea plants, normal pea plants, and sunflower plants were treated with gibberellin, they yielded 3, 2, and 10 times more auxin, respectively, than untreated plants.

Wound healing in potato tuber tissue: phosphon inhibition of developmental processes requiring protein synthesis

Several aspects of wound healing in tuber tissue of potato (Solanum tuberosum var. Kennebec), known to require protein synthesis, are inhibited by 2,4-dichlorobenzyltributylphosphonium chloride (Phosphon D). Cell division was completely blocked by 60 mum Phosphon and markedly reduced by concentrations as low as 3 mum. When applied at the time of wounding, 0.25mm Phosphon completely prevented the wound-induced respiratory increase. Application at 15 hours after wounding arrested respiration at the rate present at that time. The same concentrations of Phosphon inhibited auxin-induced cell expansion of the tissue, protein synthesis as measured by the incorporation of leucine-(14)C into the trichloroacetic acid-insoluble fraction of tissue disks, and the appearance of wound-induced peroxidase isozymes. None of these inhibitory effects of Phosphon could be prevented or reversed by the application of gibberellic acid. All wound-induced processes inhibited by Phosphon are also inhibited by cycloheximide. It is suggested that inhibitory effects of Phosphon on wound healing in potato and on other developmental processes in excised plant tissues which cannot be reversed by gibberellin are due to interference with protein synthesis.

Rapid Respiratory Changes Due to Red Light or Acetylcholine during the Early Events of Phytochrome-mediated Photomorphogenesis

Two millimeter long secondary root tips of etiolated mung bean (Phaseolus aureus) plants were given 4 minute consecutive treatments of darkness, red light, far red light, and acetylcholine during darkness. We studied the effects of these treatments on exogenous (H(+)) changes, ATP utilization, O(2) uptake, P(1) levels, and ATPase activity. Red light and acetylcholine increased the level of P(1), O(2) uptake, and exogenous H(+), but decreased ATP concentrations. Darkness and far red light caused the amount of ATP to increase and decreased the O(2) uptake and P(1) level. O(2) uptake of both excised root tips and isolated mitochondria was promoted by acetylcholine levels of the same order of magnitude that promoted the other photomimetic phenomena. ADP-O ratios indicated that acetylcholine did not cause an appreciable decrease in ATP synthesis. The total ATPase activity remained constant throughout all treatments. Ouabain caused no adhesion to negatively charged glass in the dark, while the inhibitors valinomycin, atractyloside, digitoxin, gramicidin, and oligomycin caused immediate adhesion. All of the inhibitors prevented release from the glass. In red light ouabain increased adhesion, whereas the other inhibitors caused caused immediate and complete adhesion.These data seem to imply that one of the functions of the phytochrome-mediated response to red light in roots, regulated by acetylcholine, is to cause the rapid utilization of ATP pools; far red light appears to inhibit this utilization.

Antagonism of some gibberellin actions by a substituted pyrimidine

From a comparison of the effects of seven growth retardants and abscisic acid (ABA) on various growth systems, it was found that the gibberellin-regulated growth of lettuce hypocotyls was uniquely inhibited by the growth retardant, alpha-cyclopropyl-alpha-(4-methoxyphenyl)-5-pyrimidine methanol (EL-531). Auxin-regulated growth of coleoptile sections was inhibited by Phosfon and only slightly by EL-531 and Alar. Cytokinin-regulated growth of Xanthium cotyledons showed little or no inhibition by any of the retardants. ABA was inhibitory in all three types of tests. The distinctive effects of EL-531 against gibberellin-stimulated growth and the general ability of gibberellic acid to relieve EL-531 inhibition suggest that this retardant acts in part against the gibberellin-stimulated growth system, but at a locus which discriminates between growth and nongrowth functions of gibberellic acid. It shows little or no antagonism of gibberellin actions which do not involve growth: the barley endosperm test and the Rumex leaf senescence test.

Growth and anthocyanin synthesis in excised Sorghum internodes : I. Effects of growth regulating substances

Ligh-induced anthocyanin synthesis in excised dark-grown internodes of Sorghum was depressed by the addition of auxin to the incubating medium at physiological concentrations. Both IAA and the synthetic auxin, 2,4-D, reduced anthocyanin yield. Similar results were obtained with isolated internode segments and in internodes incubated with coleoptiles (the major source of endogenous auxins) attached. Auxin increased the duration of the lag phase before anthocyanin synthesis began and reduced the rate during the subsequent linear phase. Elongation continued longer with IAA than without it and anthocyanin formation did not begin until extension growth had ceased or was slowing down in both cases; the rate of anthocyanin synthesis in the IAA solution remained depressed compared with that in buffer even after extension growth had ceased in both.At low concentrations IAA stimulated elongation growth without reducing anthocyanin yield and it is unlikely that the effect of IAA on anthocyanin synthesis results from the increased utilisation in growth of substrates needed for anthocyanin formation. The results of reciprocal transfer experiments from dark to light, and vice versa, showed that the action of IAA was associated with its presence in the incubating medium during the irradiation period. If present only in darkness, before or after transfer to light, IAA did not reduce anthocyanin formation; in the former case total yield was increased by IAA as a result of the stimulation of elongation growth, the concentration of anthocyanin remaining unchanged.GA3 also decreased anthocyanin content; the effect was greater in sections incubated with coleoptiles attached and it is possible that GA3 acts by increasing the concentration of endogenous auxins. However, CCC, which has been reported to decrease endogenous auxin levels, also reduced anthocyanin yield.The effect of IAA was not influenced by the presence of ascorbate in the incubating medium, nor did added ascorbate result in the formation of any acylated cyanidin derivative in internodes maintained in darkness.Possible relationships between light-induced anthocyanin formation and the photo-inhibition of elongation are discussed.

Inhibition of apple ripening by succinic Acid, 2,2-dimethyl hydrazide and its reversal by ethylene

Ripening of McIntosh apples (Malus sylvestris L.) was delayed by treatments of succinic acid 2,2-dimethyl hydrazide (B-9) applied 2 weeks after bloom. The extent of retardation was dependent on concentration to 7.4 x 10(-2)m, at which level complete inhibition of ripening occurred under the conditions applied. The onset of the respiratory climacteric and fruit firmness changes were factors used to assess ripening.The inhibitory effect of B-9 was counteracted by 100 ppm of ethylene. This counteraction was independent of B-9 concentration or chronological age of fruit during the period of July 4 to September 20, 1967. The data suggest that B-9 suppresses ethylene biosynthesis or action within the fruit and that this suppression is not related to fruit maturity.

MH and B-nine dependant reversal of CCC induced retardation of early shoot growth in rice

Rice shoot elongation was inhibited by 50% at 0.158 M CCC used as a presowing seed treatment. This inhibition was partially reversed by MH (0.001 M) or B-Nine (0.002 M). The reversing potentiality of MH or B-Nine was not additive. The significance of the result is discussed.

Biosynthesis of (-)-Kaurene in Cell-free Extracts of Immature Pea Seeds

Mevalonate-(14)C was incorporated into (-)-kaurene-(14)C in cell-free extracts of immature pea (Pisum sativum L.) seeds. The identification of (14)C-product as (-)-kaurene was based on: A) comparison with authentic (-)-kaurene on thin-layer and gas-liquid chromatography; and B) oxidation of (14)C-product and (-)-kaurene with osmium tetroxide to form the common derivative kaurane-16,17-diol. The enzyme system is heat labile and is dependent upon ATP and Mg(2+) or Mn(2-), with Mn(2+) being a more effective activator than Mg(2+). The reaction rate was proportional to enzyme concentration in reaction mixtures containing 0.45 to 1.8 mg protein n/ml, and was linear with time through 120 minutes in standard reaction mixtures. Enzyme preparations from immature seeds of tall and dwarf peas appeared to synthesize (-)-kaurene at the same rate. Synthesis of (-)-kaurene was readily inhibited by Amo-1618. (2-Chloroethyl)-trimethylammonium chloride (CCC) also inhibited (-)-kaurene synthesis; however, approximately 1000-fold higher concentrations of CCC were required to evoke the same percentages of inhibition as Amo-1618.

Kinetics of growth retardant and hormone interactions in affecting cucumber hypocotyl elongation

The capacities of indole-3-acetic acid (IAA) and gibberellin A(3) (GA(3)) to counteract the inhibitory effects of (2-chloroethyl) trimethylammonium chloride (CCC), 2-isopropyl-4-dimethylamino-5-methylphenyl-1-piperidinecarboxylate methyl chloride (Amo-1618), and N,N-dimethylaminosuccinamic acid (B-995) on hypocotyl elongation in light-grown cucumber (Cucumis sativus L.) seedlings were investigated. One mug of GA(3) applied to the shoot tip was sufficient to completely nullify the effect of 10 mug of Amo-1618 or 25 mug of B-995 applied simultaneously to the shoot tip, and 10 mug of GA(3) completely counteracted the effect of 10(-3)m CCC added to the root medium. One mug of IAA counteracted the effect of 10(-3)m CCC in the root medium, but IAA did not nullify the action of either Amo-1618 or B-995. Experiments were conducted using 2 growth retardants simultaneously, which indicated that Amo-1618 and CCC inhibit a common process, namely GA biosynthesis, essential to hypocotyl elongation. However, since the effect of CCC was overcome by applications of both GA and IAA, growth retardation resulting from treatment with CCC apparently is not due solely to inhibition of GA biosynthesis. B-995 did not interact additively with either Amo-1618 or CCC, which suggests that B-995 affects a process different from those affected by the other 2 retardants. Thus, while inhibition evoked by B-995 is reversible by applied GA, the action of B-995 does not appear to be inhibition of GA biosynthesis.

Senescence inhibition and respiration induced by growth retardants and N-benzyladenine

Senescence of Grand Rapids leaf lettuce was greatly reduced at 3 storage temperatures by post-harvest treatment with N,N-dimethylaminosuccinamic acid (Alar) and 2-chloroethyltrimethylammonium chloride (CCC), but not with (6)N-benzyladenine (BA). Conversely, Alar and CCC were inactive on broccoli while BA was markedly effective. The deterioration and discoloration of mushrooms was inhibited by Alar with no effect observed from BA or CCC.The inhibition of senescence by BA and the growth retardants was not always associated with a reduction in respiration (O(2) uptake, CO(2) evolution). BA stimulated respiration and hastened senescence in leaf lettuce accompanied by a diametrically opposite effect from Alar and CCC. Thus, BA cannot be considered a universal senescence inhibitor. The variability of senescence responses induced by different chemicals on a variety of plant tissues suggests a dissimilar mode of action or a complex interaction with native growth substances.

[The effect of the "Gibberellin antagonist" (2-chlorethyl)trimethylammonium chloride (CCC) on the rooting of cuttings from twining and non-twining plants]

Root initiation at cuttings is inhibited by application of gibberellin. CCC does not antagonize this inhibition. However, CCC stimulates root initiation at cuttings of three different species of twining plants known to be rich in endogenous gibberellin. CCC seems to be an antagonist for endogenous but not for exogenous gibberellin, which suggests that CCC influences gibberellin biosynthesis.

DAVID POWELL HACKETT MEMORIAL ISSUE: David Powell Hackett 1925-1965

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