Klaus Grossmann

Bioregulatory Effects of the Fungicidal Strobilurin Kresoxim‐methyl in Wheat (Triticum aestivum)

Apart from its fungicidal effect, the strobilurin kresoxim-methyl (BAS 490 F) was found to induce physiological and developmental alterations in wheat (Triticum aestivum L.) which are seen in connection with improved yield. In a series of biotests including heterotrophic maize and photoautotrophic algal cell suspensions, duckweed, isolated mustard shoots and germinating cress seeds, kresoxim-methyl showed a similar response pattern to standard auxins (e.g. indol-3-ylacetic acid, IAA; 2-(1-naphthyl)acetic acid, α-NAA). Auxin-like activity of kresoxim-methyl was also found when stem explants of tobacco were cultured on a hormone-free medium. Kresoxim-methyl stimulated shoot formation, particularly at 10 -7 M. The same effect was induced by 10 -8 M IAA. The determination of phytohormone-like substances in shoots of wheat plants foliar-treated with 7 x 10 -4 M kresoxim-methyl revealed only slightly changed levels of endogenous IAA, gibberellins and abscisic acid. In contrast, the contents of dihydrozeatin riboside-type cytokinins increased to 160% of the control, while transzeatin riboside- and isopentenyladenosine-type cytokinins remained nearly unchanged. The most remarkable alterations were the reductions in 1-aminocyclopropane-1-carboxylic acid (ACC) levels and ethylene formation which were demonstrated in intact plants, leaf discs and the shoots of wheat subjected to drought stress. Kresoxim-methyl affected the induction of ACC synthase activity which converts S-adenosyl-methionine to ACC in ethylene biosynthesis. In shoots from foliar-treated wheat plants, 10 -4 M kresoxim-methyl inhibited stress-induced increases in endogenous ACC synthase activity, ACC levels and ethylene formation by approximately 50%. Reductions in ACC synthase activity and ACC levels of 30% were also obtained at low concentrations of α-NAA (10 -6 M). In contrast, ACC synthase activity in vitro was not influenced by adding the compounds. In wheat leaf discs, the inhibiting effect of kresoxim-methyl, α-NAA and IAA on ethylene formation was accompanied by delayed leaf senescence, characterized by reduced chlorophyll loss. However, in contrast to kresoxim-methyl which showed only inhibitory activity on ethylene synthesis over a wide range of concentrations applied, the auxins stimulated ethylene production at high concentrations of about 10 -4 M. The inhibition of ethylene biosynthesis by kresoxim-methyl, together with an increase in endogenous cytokinins could explain the retardation of senescence and the intensified green leaf pigmentation in wheat exposed to this strobilurin.

Regulation of phytohormone levels, leaf senescence and transpiration by the strobilurin kresoxim-methyl in wheat (Triticum aestivum)

Summary Using leaf discs and intact wheat plants (Triticum aestivum L.), the physiological effects of the strobilurin-type fungicide kresoxim-methyl were studied in relation to induced phytohormonal changes. Dose-response experiments revealed that kresoxim-methyl shifted the hormonal balance, favouring cytokinins particularly of the dihydrozeatin riboside-type, as opposed to ethylene and its biosynthetic precursor 1-aminocyclopropane-1-carboxylic acid (ACC). This was closely correlated with delayed leaf senescence. Kresoximmethyl was shown to inhibit the induction of ACC synthase in ethylene formation. In addition, kresoximmethyl caused an up to two-fold increase in endogenous levels of abscisic acid, relative to the control. Concomitantly, the stomatal aperture and water consumption of plants were reduced. It is suggested that kresoxim-methyl changes the hormonal constellation in wheat which leads to delayed leaf senescence and water-conserving effects.

Mediation of Herbicide Effects by Hormone Interactions

Chemical manipulation of the phytohormone system involves the use of herbicides for weed control in modern crop production. In the latter case, only compounds interacting with the auxin system have gained practical importance. Auxin herbicides mimic the overdose effects of indole-3-acetic acid (IAA), the principal natural auxin in higher plants. With their ability to control, particularly, dicotyledonous weeds in cereal crops, the synthetic auxins have been among the most successful herbicides used in agriculture. A newly discovered sequential hormone interaction plays a decisive role in their mode of action. The induction of 1-aminocyclopropane-1-carboxylic acid (ACC) synthase in ethylene biosynthesis is the primary target process, following auxin herbicide signalling. Although the exact molecular target site has yet to be identified, it appears likely to be at the level of auxin receptor(s) for perception or signalling, leading ultimately to species- and organ-specific de novo enzyme synthesis. In sensitive dicots, ethylene causes epinastic growth and tissue swelling. Ethylene also triggers the biosynthesis of abscisic acid (ABA), mainly through the stimulated cleavage of xanthophylls to xanthoxal, catalyzed by 9-cis-epoxycarotenoid dioxygenase (NCED). ABA mediates stomatal closure which limits photosynthetic activity and biomass production, accompanied by an overproduction of reactive oxygen species. Growth inhibition, senescence and tissue decay are the consequences. Recent results suggest that ethylene-triggered ABA is not restricted to the action of auxin herbicides. It may function as a module in the signalling of a variety of stimuli leading to plant growth regulation. An additional phenomenon is caused by the auxin herbicide quinclorac which also controls grass weeds. Here, quinclorac induces the accumulation of phytotoxic levels of cyanide, a co-product of ethylene, which ultimately derives from herbicide-induced ACC synthase activity in the tissue. Phytotropins are a further group of hormone-related compounds which are used as herbicides. They inhibit polar auxin transport by interacting with a regulatory protein, the NPA-binding protein, of the auxin efflux carrier. This causes an abnormal accumulation of IAA and applied synthetic auxins in plant meristems. Growth inhibition, loss of tropic responses and, in combination with auxin herbicides, synergistic effects are the consequences.

Alterations in hormonal and developmental characteristics in transgenic Populus conditioned by the rolC gene from Agrobacterium rhizogenes

Summary One hybrid aspen ( P. tremula L. × P. tremuloides Michx.) clone transgenic for different chimeric gene constructs has been used to determine hormonal levels in several plant tissues. Mainly, transgenic aspen carrying the rolC gene from Agrobacterium rhizogenes under control of the cauliflower-mosaic-virus 35S-promoter and the light inducible rbcS promoter from potato were compared with untransformed aspen. Determination of various hormones in different rolC transgenic plant tissues revealed changes in levels conditioned by 35S-rolC and rbcS-rolC transgenic aspens as compared with controls. These changes could be specific for a woody plant species. Measurements of hormones in predormant buds of 35S-rolC transgenics showed levels of abscisic acid of about half of the content as measured in predormant buds of untransformed controls. Two further determinations during the resting period (at the middle and at the end) confirmed the lower ABA level in 35S-rolC transgenics. In spring, these 35S-rolC transgenics started to flush at least 2 weeks before the control plants. This possibly suggests the involvement of ABA in the process of maintenance of dormancy and break from it in this tree species. Tree-specific developmental characteristics are discussed in the light of varying hormone levels in aspen- Populus , a woody plant model system.

Regulation of endogenous abscisic acid levels and transpiration in oilseed rape by plant growth retardants

Summary Plant growth retardants of the norbornanodiazetine (e.g. tetcyclacis) and triazole (e.g. BAS 111. . W, LAB 150 978) type considerably increased endogenous abscisic acid (ABA) levels in cell suspension cultures, detached leaves and hydroponically grown seedlings of oilseed rape ( Brassica napus ). In detached leaves treated via the vascular system, ABA accumulated in proportion to retardant concentration and was closely correlated with a reduction in transpiration. Thus, the growth retardants appear to influence transpiration by regulating ABA metabolism. In suspension cells and shoots of seedlings, maximal ABA content was achieved 4 and 5 days after treatment, respectively. ABA accumulation may be due to a retardant-caused inhibition of the metabolism of ABA, presumably to phaseic acid, as suggested by [ 14 C] ABA feeding experiments with cell cultures. The initial rise in ABA levels was followed by a sharp decline, an effect that might be explained by a stimulated ABA catabolism and/or by an inhibition of its biosynthesis. In contrast, foliar application of BAS 111. . W to plants at the 4th leaf stage did not change either ABA levels in shoots nor water consumption per shoot fresh or dry weight analysed during 10 days after treatment. In leaves of soil treated plants at the 5th leaf stage ABA levels also remained unaffected or dropped below those of controls, especially at higher BAS 111. . W concentration. It is suggested that the growth retardants regulate ABA metabolism in oilseed rape, dependent on the mode of application and intensity, and duration of action.

Induction of abscisic acid is a common effect of auxin herbicides in susceptible plants

Summary For more than forty years synthetic compounds possessing auxin activity have been among the most successful herbicides used in agriculture. However, in spite of extensive research a consensus on the biochemical basis of their phytotoxicity has yet to emerge. Comparing the effects of several representatives of the known chemical classes of auxin herbicides (e.g. dicamba, 2-methyl-4-chlorophenoxyacetic acid, picloram, quinclorac, quinmerac) and auxins (e.g. α-naphthaleneacetic acid, indole-3-acetic acid) in a variety of plant species, we demonstrate, to our knowledge for the first time, that the induction of ABA is a common effect of auxin-type compounds, which appears to be implicated in their inhibition of growth in susceptible plants.

Plant Growth Retardants as Inhibitors of Ethylene Production

Summary Plant growth retardants of the norbornanodiazetine (tetcyclacis) and triazole (BAS 111 W) types inhibited ethylene production in leaf discs of barley and oilseed rape. The maximal reduction of 70 % was reached 5 h after treatment. The addition of gibberellic acid could not overcome the effects. At concentrations of the retardants up to 3 x 10-4 mol x 1-1 the reduction in ethylene formation was accompanied by increased or nearly constant levels of 1-aminocyclopropane-l-carboxylic acid (ACC) and N-malonylACC in the leaf discs. This suggests an inhibition of the conversion of ACC to ethylene in the biosynthetic pathway. As explanations a direct influence on the ethylene-forming enzyme or an indirect effect via a retardant-caused modification in the cellular membrane properties are discussed.

Influence of the growth retardant tetcyclacis on cell division and cell elongation in plants and cell cultures of sunflower, soybean, and maize.

Taking tetcyclacis, a norbornenodiazentine derivative, as an example, the influence of a growth retardant on the shoot growth of sunflower, soybean, and maize seedlings grown and treated in hydroculture was investigated. In detail, the reduction in the length of various shoot sections {epicotyl, 1st internode, leaf blade) caused by the retardant was studied. At low concentrations of the retardant (lt10(-6) M) the shortening effects are substantially attributable to an influence on cell elongation, whereas cell division is inhibited as the concentration increases (τ10(-6) M). A comparison of the effects of tetcyclacis in cell suspension cultures of appropriate plant species showed that also in this system concentrations τ 10(-6) M inhibited cell division growth, i. e. there is comparability of plant/ cell culture regarding the retardant effect on cell division. In contrast to the intact plants, however, cell elongation appears to be of only subordinate importance for the growth of cell cultures, as it has been shown using parsley cell suspension cultures. It is discussed to what extent influencing the gibberellin or sterol biosynthesis by means of tetcyclacis provides an explanation for the concentration-dependent effect on the cell division and cell elongation processes.

Abscisic Acid is a Causative Factor in the Mode of Action of the Auxinic Herbicide Quinmerac in Cleaver (Galium aparine L.)

Summary Hydroponic treatment of cleaver plants ( Galium aparine L.) with the quinolinecarboxylic acid herbicide quinmerac caused early symptoms, which were elicited within 48 h of quinmerac application (10 -6 to 10 -5 mol · L -1 ) and characterized by stem and leaf epinasty, retardation of root and particularly shoot growth, and reduction in water consumption. The levels of 1-aminocyclopropane-1-carboxylic acid (ACC), ethylene and conjugated ACC began to increase within 3 h and peaked at 24 h, 48 h, and 72 h of treatment, respectively. HCN was increasingly released from Galium shoots into the gas phase after 2 to 3 days of quinmerac application. The concentrations of gibberellins, cytokinins, and 3-indoleacetic acid (IAA) were only slightly changed. Within 24 h of treatment, the respective abscisic acid (ABA) levels in the shoot tissue were elevated up to 20-fold, relative to the control. Application of 10 -4 mol · L -1 IAA increased ACC and ABA contents in Galium shoot and root tissues and induced phenotypic alterations in plants similar to those caused by quinmerac. Quinmerac-induced ABA accumulation was in close correlation with reductions in stomatal aperture, water consumption, photosynthetic CO 2 uptake, and shoot and root fresh weights. Exogenously applied ABA mimicked quinmerac action on these parameters at comparable endogenous ABA concentrations. Additional treatment with the ACC synthesis inhibitor PACME decreased quinmerac-induced ACC formation, ABA accumulation, leaf epinasty and reduction of shoot growth. The effects of quinmerac could partly be restored when plants were additionally treated with ACC. This strongly suggests that ABA, induced ultimately through quinmerac-stimulated ACC synthesis, contributes to the auxinic mode of action underlying the inhibition of growth in cleaver.

Changes in membrane permeability and mineral, phytohormone and polypeptide composition in rice suspension cells during growth and under the influence of the growth retardant tetcyclacis

The plant growth retardant tetcyclacis inhibits cell division growth in rice suspension cultures at concentrations above 10−6 M. Tracer experiments with rice cells revealed that tetcyclacis reduced the incorporation of mevalonic acid into terpenoids after 30 min, the uptake of leucine, uridine and thymidine after 2 h and their incorporation into the corresponding macromolecules after 3–7 h. The changes in membrane permeability concluded to have been caused by an influence on phytosterol biosynthesis are probably also the explanation for alterations of tetcyclacis-treated cells in the content of macro- and microelements.As shown by immunoassay, tetcyclacis did not modify the levels of endogenous gibberellins (Grossmann et al. 1985), cytokinins and indole acetic acid during a growth cycle of 15 d. However, a clear rise in the abscisic acid (ABA) level occurred during the first 5 d of treatment. In untreated cells such a rise coincided only with the aging of the cell culture in the stationary growth phase. Investigations of the cell polypeptide pattern using sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed that the ABA increase following tetcyclacis treatment seems not to be a consequence of advanced cell aging.