Irradiation with ultraviolet light causes in the hypocotyl of dark-grown gherkin seedlings the partial conversion of transhydroxycinnamic acids to the cis-isomers. The trans-hydroxycinnamic acids inhibit the development of phenylalanine ammonia-lyase activity, and the transformation of these compounds to the much less inhibitory cis-isomers forms a ready explanation for the increase in phenylalanine ammoniaIyase activity in the hvpocotvl of gherkin seedlings irradiated with ultraviolet light. Arguments are advanced that the increase in phenylalanine ammonia-lyase activitv caused by irradiation with blue light is also (at least in part) initiated by trans-cis isomerisation of the hvdroxycinnamic acids.Since Zucker's paper (31) on the induction of PAL2 (EC 4.3. 1 .5) by light in potato tuber tissue the photoinduction of this enzyme has been studied in a great number of other plants, but the mechanism of this process has so far remained obscure (4,22,27,32). Previously (6), I postulated that photoactivation of one or more photoreceptors starts a series of dark reactions that lead to the formation of a factor which promotes PAL synthesis. This concept will probably have to be revised. In more recent investigations (9-14), evidence has been obtained that the PAL level in gherkin hypocotyls is controlled by end products of the shikimic acid pathway: cinnamic acid, p-coumaric, and ferulic acid. and/or glucose esters of the latter two compounds. It appeared that increases in PAL activity similar to those brought about by irradiation were obtained with treatments which caused the release of the cinnamic acid derivatives from the tissue (9) or their conversion into other compounds (13,14). This has led us to the idea that the effect of light on PAL might be due to a temporary diminution in the concentration of (hydroxy)-cinnamic acid(s) in certain cell compartments. The only direct effect of light on these compounds is the trans-cis conversion by the UV part of the spectrum (21). Blue light causes a similar 1 Dedicated to the memory of Milton Zucker who in 1965 was the first to report the photoinduction of phenylalanine ammonialyase. The present author was privileged to have many discussions with him in the years thereafter about the regulatory mechanisms -oncerning this enzyme.2 Abbreviation: PAL: phenylalanine ammonia-lyase.reaction, provided that a suitable photoreceptor is present (25). In this paper, the question will be discussed of whether these reactions play a role in the induction of PAL activity in gherkin hypocotyls. MATERIALS AND METHODSThe experiments were performed with 3-day-old dark-grown gherkin seedlings, Cucumnus sativuis L. cv. 'Venlose niet plekkers," strain Tercken VI (15). The irradiations were carried out at 25 C with UV light of 360 vw/ccrn (Philips HPW, 125 w, Ak,,,x 365 nm) and with blue light of 150 1Aw/cm2 (6). In vivo spectrophotometry of gherkin hypocotyls was carried out in a Cary spectrophotometer with a scattered transmission attachment. In all experiments. one seedling only was ...
1. Exposure of dark-grown gherkin seedlings to blue light causes temporary changes in the level of phenylalanine deaminase (PADAse). Following a time lag of about 90 min the enzyme level increases; about 180 min after the beginning of irradiation it declines again. 2. The light-induced increase in the level of PADAse is due to de-novo enzyme synthesis, since spraying of the plants with cycloheximide prior to the light treatment results in inhibition of the increase in enzyme level. 3. The increase of the enzyme level is a function of light intensity. One of the initial processes involved is the transport of a promotive factor from the cotyledons to the hypocotyl. 4. The decline in the PADAse level which follows the initial increase is probably caused by enhanced inactivation of the enzyme. It can be stopped with cycloheximide, indicating that the inactivation process depends on de-novo protein synthesis. 5. The amount of previously induced PADAse and the capacity of the system to respond to a subsequent light treatment are inversely related. 6. The light-induced changes in PADAse activity are parallel to those in the activity of cinnamic acid hydroxylase, the enzyme responsible for the subsequent step in the synthesis of hydroxycinnamic acids from L-phenylalanine. This result indicates a close connection between the induction of the two enzymes. 7. A hypothesis in advanced which explains the effects of irradiation with blue light ("high-energy" reaction) on phenol synthesis on the basis of enzyme synthesis and enzyme inactivation.
1. Exposure of dark-grown gherkin seedlings to red or far-red light causes changes in the level of phenylalanine deaminase (PADAse) which follow the same time course and show a similar dependence on light intensity as the changes induced in blue light. From this it is concluded that light of different regions of the visible spectrum is initiating the same sequence of dark reactions leading to enhanced production of PADAse, followed by a phase of inactivation of this enzyme and repression of its synthesis. 2. In blue light a higher PADAse level can be obtained than in red or far-red light, and only blue light is capable of inducing PADAse synthesis in excised hypocotyls. This suggests that different pigment systems are involved in the high-energy phenomena in the gherkin seedling. 3. The effects of irradiation sequences with different light qualities and intensities on the accumulation of hydroxycinnamic acids can be explained on the basis of the regulatory mechanisms which control of PADAse level.
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