Regulation of Plant Morphology by Growth Retardants

Abstract

The effects of the growth retardants tetcyclacis, a norbornenodiazetine, and LAB 150 978, a dioxanylalkenyl triazole, on seedling growth and endogenous levels of phytohormone-like substances in Glycine max L. cv Maple Arrow were studied. The levels of phytohormone-like substances in the root and in the various shoot tissues were analyzed by immussay. After seed treatment with both compounds, shoot growth was reduced more intensively than root growth. Both compounds decreased, on a fresh weight basis, the amount of various immunoreactive gibberellins when compared with the levels in control plants, especially in the shoot tip. Likewise, the growth retardants lowered the levels of abscisic acid-like material, particularly in the primary leaf, the epicotyl and the root. In contrast, the levels of trans-zeatin-riboside and dihydrozeatin-riboside-type cytokinins were considerably elevated by the growth retardants, mainly in the primary leaf, epicotyl, and hypocotyl. On the other hand the level of isopentenyladenosine-like material was less influenced. In general, the immunoreactive 3-indoleacetic acid content in the different plant parts was changed only slightly. It is assumed that besides their effect on gibberellin content both compounds interfere directly or indirectly with the regulation of the endogenous levels of abscisic acid and cytokinins. This might be seen as an additional mode of action of growth retardants explaining some side effects on developmental processes of treated plants, &g. delayed senescence and enhanced chlorophyll concentration in the leaves.The regulation of plant morphogenesis depends on environmental factors such as light, temperature and nutrient supply, the concentration and interaction of endogenous plant hormones, and upon the changing sensitivity of the tissue to these substances. In addition, many synthetic compounds are known which are able to modify the growth habit of plants when applied exogenously. Among them plant growth retardants influence cell division and cell enlargement in the subapical meristems of the shoot (20,24) probably by interfering with the gibberellin (GA') and sterol biosynthesis (4). Recently, it has been shown for new plant growth retardants of the triazole and norbornenodiazetine type that they inhibit GA biosynthesis in a cell-free system of pumpkin endosperm by blocking the oxidative reactions from ent-kaurene to ent-kaurenoic acid (8, 23). Data have been presented that these compounds reduce the GA content in intact plants, too (12, 32). However, results from cell suspension cultures provide evidence that the norbomenodiazetine tetcyclacis I Abbreviations: GA. = gibberellin A,; ABA = 2-cis-abscisic acid; DZR = dihydrozeatin riboside; IPA = isopentenyladenosine; mAB = monoclonal antibodies; ZR = trans-zeatin riboside. inhibits cell division growth by influencing the sterol biosynthesis (10). This effect is accompanied by alterations in protein biosynthesis and membrane permeability (9, 1 1), which might also be an explanation for reduced levels o...

News from Old Compounds: The Mode of Action of Auxin Herbicides

“…Although ethylene itself is not the primary agent by which auxin herbicides cause phytotoxicity [13], former investigations have made it plausible that enhanced ethylene production is involved in this process (for review see [2,5,6,7]. In a multitude of dicot species and biotypes of differing sensitivity, stimulation of ethylene formation by auxin herbicides has been found to correlate with the induced growth effects, which could be alleviated by adding inhibitors of ethylene biosynthesis [13,14,36,37,38,39]. Accordingly, inactive auxin analogues did not promote ethylene production [5].…”

“…The accumulated ACC appears to be transported to the shoot, where it induces more synthesis of ACC which is converted to ethylene and cyanide. Treatment of isolated shoots with ACC via the vascular system stimulated ACC synthase activity and ethylene and cyanide production [7,37,49,51]. Cyanide was formed as a coproduct of ethylene in the oxidation of ACC, catalyzed by ACC oxidase [14,52].…”

“…Cyanide was formed as a coproduct of ethylene in the oxidation of ACC, catalyzed by ACC oxidase [14,52]. While quincloracinduced increases of ABA and hydrogen peroxide are not enough to elicit herbicidal effects [7], cyanide levels in the grass shoot tissue accumulated according to the herbicide concentration and application time and closely correlated with phytotoxicity [37,51]. In Echinochloa crus-galli, endogenous cyanide accumulated up to 30 μΜ.…”

“…Cyanide is an inhibitor of many important plant enzymes including cytochrome-c oxidase, ribulose-bisphosphate carboxylase and nitrate/nitrite reductase [52]. Therefore, we concluded that the accumulation of cyanide, derived ultimately from quinclorac-induced ACC synthase activity in ethylene biosynthesis, plays a key role in the herbicidal action of quinclorac in sensitive grasses [7,37,51] (Figure 4). In addition, Koo et al [53] suggested that quinclorac inhibits cellulose (cell wall) biosynthesis in susceptible grasses.…”

Características morfológicas de gramas em resposta à aplicação de trinexapac-ethyl

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