Low O2 conditions were obtained by flowing N2 through the solution in which the tomato plants (Lycopersicon esculentum Mill cv Heinz 1350) were growing. Time course experiments revealed that low O2 treatments stimulated 1-aminocyclopropane-1-carboxylate (ACC) synthase production in the roots and leaves. After the initiation of low 02 conditions, ACC synthase activity and ACC content in the roots increased and reached a peak after 12 and 20 hours, respectively. The conversion of ACC to ethylene in the roots was inhibited by low levels of 02, and ACC was apparently transported to the leaves where it was converted to ethylene. ACC synthase activity in the leaves was also stimulated by low 02 treatment to the roots, reaching a peak after 24 hours. ACC synthase levels were enhanced by cobalt chloride and aminooxyacetic acid (AOA), although they inhibited ethylene production. Cobalt chloride enhanced ACC synthase only in combination with IOW 02 conditions in the roots. Under aeration, AOA stimulated ACC synthase activity in both the roots and leaves. However, in combination with low 02 conditions, AOA caused a stimulation in ACC synthase activity in the leaves and no effect in the roots.Ethylene production is observed in all higher plants, where it is involved in numerous aspects of plant growth and development but most notably as a response to stress conditions (1).Waterlogging been reported to cause elevated ethylene levels and epinasty in the shoot regardless of whether the condition is imposed by waterlogging or by flushing with N2 (4,12,14). It has been shown that ethylene production by maize (Zea mays L.) roots exposed to 5 kPa oxygen was inhibited by AVG supplemented to the nutrient solution and this also reduced aerenchyma formation (13). This work provided evidence that the stimulation in ethylene production was probably due to an enhancement in ACC synthase and that ethylene was involved the promotion of aerenchyma in adventitious roots of maize. Cohen and Kende, utilizing an in vivo assay for this enzyme (8), showed that ACC synthase levels can be enhanced in deep-water rice internodes (Oryza sativa L.). Because they were unable to detect ACC synthase in homogenates of deepwater rice internodes, they utilized in vivo methods that assayed ACC accumulation in tissue under N2. Their conclusion was that the stimulation of ACC synthase activity by low 02 pressures was one of the first biochemical events leading to internodal growth in deep-water rice. The activity of ACC synthase in homogenates of vegetative tissues is often either much lower than expected or not measurable at all in vitro (8,20). To overcome this problem, we used the method described by Tsai et al. (18) to assay for ACC synthase. In the present study, we surveyed the changes in ACC synthase, ACC, and ethylene in whole tomato plants by flushing the roots with N2 and also evaluated the effects of ethylene biosynthesis inhibitors. MATERIALS AND METHODS Plant PreparationTomato (Lycopersicon esculentum Mill cv Heinz 1350) plants were grown f...
Hypocotyl elongation of pakchoi (Brassica chinensis cv LeiChoi) was stimulated by applying 300 ng of brassinosteroid (2a,3a,22~,23~-tetrahydroxy-24~-methyl-B-homo-7-oxa-5cy-cholestan-6-one, BR) in 1 pL of 50% ethanol to the apex of hypocotyls. BR had its greatest effect on elongation of the apical 3-mm region below the cotyledonary node (75% stimulation) between 6 and 18 h after treatment. Stress/strain (Instron) analysis of this 3-mm region revealed that plastic and elastic components of extension were not significantly different between BR-treated and control seedlings. In pressure-block experiments, the initial rate of relaxation was 2-fold faster in BR-treated plants as compared with controls, whereas after 125 min the total amount of relaxation and the relaxation rate were the same for the two treatments. Osmotic pressure of cell sap expressed from this 3-mm region showed a large decrease (28%) in BR-treated seedlings compared to the controls. We conclude that BR stimulates growth in pakchoi by accelerating the biochemical processes that cause wall relaxation, without inducing a large change in wall mechanical properties.In 1970, brassins were isolated from rape (Brassica napus L.) pollen by Mitchell and coworkers (1970). Brassinolide (2a,3ff,22a,23a-tetrahydroxy-24a-methyl-B-homo-7-oxa5a-cholestan-6-one) was identified as a major biologically active component of brassins (Grove et al., 1979). Brassinolide and its active analogs have been synthesized by Thompson and coworkers (1979).BRs induce elongation of normal and dwarf pea epicotyls, dwarf bean apical segments, mung bean epicotyls, cucumber hypocotyls, Azuki bean epicotyls, and sunflower hypocotyls (Mandava, 1988). In cucumber hypocotyl sections, BR-induced elongation was inhibited in the presence of p-chlorophenoxyisobutyric acid and kinetin (Katsumi, 1985). BR acted synergistically with auxin in elongation (Yopp et al., 1981;Katsumi, 1985) but showed an additive effect with GA3 in cucumber hypocotyl sections (Katsumi, 1985). BR also acted synergistically on auxin-induced ethylene production in etiolated mung bean segments (Arteca et al., 1983). A membrane-bound ATPase inhibitor, dicyclohexylacarbodde, inhibited the BR-or IAA-induced elongation but did not affect GA-induced elongation (Katsumi, 1985). These responses have suggested that BR acts through processes involving auxin. However, northem blot analysis using cDNAs corresponding to auxin-regulated genes as probes has shown that the molecular mechanism of BR-induced elongation is likely to differ from that of auxin-induced elongation in this system (Clouse and Zurek, 1991). In two-dimensional gel analysis of in vitro translated mRNA, BR altered the pattern of gene expression in elongating soybean stem sections in both the presence and absence of added auxin (Clouse and Zurek, 1991).Auxins stimulate growth by increasing stress relaxation of the wall (Cosgrove, 1985), and this effect is usually accompanied by a mechanical weakening of the wall (Cleland, 1984) and enhanced xyloglucan tumover (Labav...
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