ABSTRACT
1-Aminocyclopropane-1-carboxylic acid (ACC) is aerobically oxidized in plant tissues to form ethylene by ethylene-forming enzyme (EFE). The effect of substrate (ACC and oxygen) concentrations on ethylene production rate by plant tissues was investigated. The K(m) value for O(2) in ethylene production varied greatly depending on the internal ACC content. When ACC levels in the tissue were low (below its K(m) value), the concentration of O(2) giving half-maximal ethylene production rate ([S](0.5)) ranged between 5 and 7%, and was similar among different tissues. As the concentration of ACC was increased (greater than its K(m) value), [S](0.5) for O(2) decreased markedly. In contrast, the K(m) value for ACC was not much dependent on O(2) concentration, but varied greatly among different plant tissues, ranging from 8 micromolar in apple (Malus sylvestris Mill.) tissue to 120 micromolar in etiolated wheat (Triticum aestivum) leaf. Such a great variation was thought to be due to the different compartmentation of ACC within the cells in different tissues. These kinetic data are consistent with the view that EFE follows an ordered binding mechanism in which EFE binds first to O(2) and then to ACC.
ABSTRACT
While light-grown wheat leaves produced ethylene at a low rate of <0.1 nanomoles per gram per hour and contained 1-aminocyclopropane-1-carboxylic acid (ACC) at low levels of <2.5 nanomoles per gram, etiolated wheat leaves produced ethylene at a rate of 2 nanomoles per gram per hour and accumulated concentrations of ACC at levels of 40 nanomoles per gram. Upon illumination of 8-day-old etiolated wheat seedlings with white light, the ethylene production rate increased initially, due to the activation of ethylene-forming activity, but subsequently declined to a low level (0.1 nanomoles per gram per hour) at the end of the 6-hour illumination. This light-induced decline in ethylene production rate resulted from a decline (more than 35 nanomoles per gram) in ACC level, which was accompanied by a corresponding increase in 1-(malonylamino)cyclopropane-1-carboxylic acid content. These data indicate that illumination promoted ACC malonylation, resulting in reduced ACC level and consequently reduced ethylene production. However, light did not cause any significant increase in the extractable ACC-malonyltransferase activity. The effect of continuous white light on promotion of ACC malonylation was also observed in intermittent white light or red light. A far-red light treatment following red light partially reversed the red light effect, indicating that phytochrome participates in the promotion of ACC malonylation.
ABSTRACT
Since 1-(malonylamino)cyclopropane-1-carboxylic acid (MACC), the major conjugate of 1-aminocyclopropane-1-carboxylic acid (ACC) in plant tissues, is a poor ethylene producer, it is generally thought that MACC is a biologically inactive end product of ACC. In the present study we have shown that the capability of watercress (Nasturtium officinale R. Br) stem sections and tobacco (Nicotiana tabacum L.) leaf discs to convert exogenously applied MACC to ACC increased with increasing MACC concentrations (0.2-5 millimolar) and duration (4-48 hours) of the treatment. The MACC-induced ethylene production was inhibited by CoCl(2) but not by aminoethoxyvinylglycin, suggesting that the ACC formed is derived from the MACC applied, and not from the methionine pathway. This was further confirmed by the observation that radioactive MACC released radioactive ACC and ethylene. A cell-free extract, which catalyzes the conversion of MACC to ACC, was prepared from watercress stems which were preincubated with 1 millimolar MACC for 24 hours. Neither fresh tissues nor aged tissues incubated without external MACC exhibited enzymic activity, confirming the view that the enzyme is induced by MACC. The enzyme had a K(m) of 0.45 millimolar for MACC and showed maximal activity at pH 8.0 in the presence of 1 millimolar MnSO(4). The present study indicates that high MACC levels in the plant tissue can induce to some extent the capability to convert MACC to ACC.
