Effects of Low O₂ Root Stress on Ethylene Biosynthesis in Tomato Plants (Lycopersicon esculentum Mill cv Heinz 1350)

Abstract

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...

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