Photoinduction of phenylalanine deaminase in gherkin seedlings : III. Effects of excision and irradiation on enzyme development in hypocotyl segments

Antioxidant activity and high-performance liquid chromatographic analysis of phenolic compounds during in vitro callus culture of Plantago ovata Forsk. and effect of exogenous additives on accumulation of phenolic compounds

BACKGROUND

Plantago ovata, commonly called psyllium, is known to be a rich source of polyphenolic compounds. The present study was aimed at determining polyphenol content and studying their antioxidant activities in P. ovata during in vitro callus culture. An attempt was also made to enhance polyphenol content using external additives. The role of PAL gene in polyphenol accumulation was also studied.

RESULTS

The study indicated the presence of significant amounts of polyphenols, including flavonoids, in P. ovata callus. A gradual increase in polyphenol and flavonoid content was observed up to the third passage (63 days) of callus culture, which declined at the next passage. The third-passage callus showed highest antioxidant activity. High-performance liquid chromatographic results indicated the presence of high amounts of gallic acid and rutin in P. ovata calli; however, other polyphenols were also present but to a lesser extent. Additive supplementation was effective in enhancing polyphenol production and in increasing antioxidant activity in P. ovata callus.

CONCLUSION

The present research reported accumulation of polyphenols in callus culture of P. ovata, which could be applied to isolation of polyphenols for various beneficial purposes. It also indicated enhancement in the production of several polyphenols and also an increase in antioxidant activity in the additive-treated callus.

Cytochromes P450 in phenylpropanoid metabolism

The phenylpropanoid pathway controls the synthesis of lignin, flower pigments, signalling molecules, and a large spectrum of compounds involved in plant defence against pathogens and UV light. More than 15 P450-dependent reactions have been characterised in this pathway. Several of these reactions constitute important regulatory branching points. Indirect and direct data indicate that distinct P450s catalyse the different reactions. The coding sequences of two enzymes have recently been determined. They belong to different P450 families, CYP73 and CYP75. The second enzyme of the main pathway, cinnamate 4-hydroxylase (C4H), is the most extensively studied plant P450 and is suspected of playing an essential role in the regulation of the whole pathway.

Light-dark modulation of hydroxycinnamate: CoA ligase activity from stems of Salix babylonica cultivated in vitro

Plantlets of Salix babylonica cultivated in vitro were used to study the regulation of the lignification process. The hydroxycinnamate: CoA ligase activity of the stems was shown to be controlled by light. During photoperiodic cycles, photocontrol also occurred and induced a diurnal oscillation of enzyme activity. The oscillation was maintained in continuous light and to a certain extent in continuous darkness.

Coaction of three factors controlling chlorophyll and anthocyanin synthesis

In a three-factor analysis the rate of chlorophyll a (Chl) accumulation in excised mustard cotyledons was studied as a function of kinetin, light (operating through phytochrome, P fr) and an excision factor. It was found that the three factors operate additively provided that the P fr level is high enough. When the P fr level is below approximately 1 per cent (ϕλ<0.01) the effectiveness of the excision factor decreases while the effect of kinetin remains additive. The observed additivity is explained by a model where the three factors operate independently through a common intermediate (presumably 5-aminolevulinate) in the biosynthetic chain leading to Chl. With regard to the coaction of the excision factor and phytochrome it is concluded that the production of the excision factor requires the operation of phytochrome (even though saturated at a low P fr level) while the action of the excision factor is independent of phytochrome. This conclusion was confirmed by experiments in which the rate of light-mediated anthocyanin synthesis was measured in excised mustard cotyledons. The effect of excision in the case of anthocyanin formation differs kinetically from the effect of excision on Chl formation.

Modulation of L-phenylalanine ammonia-lyase by pathway intermediates in cell suspension cultures of dwarf French bean (Phaseolus vulgaris L.)

The increase in extractable phenylalanine ammonia-lyase (PAL;EC 4.3.1.5.) activity induced in French bean cell suspension cultures in response to treatment with autoclaved ribonuclease A was inhibited by addition of the phenylpropanoid pathway intermediates cinnamic acid, 4-coumaric acid or ferulic acid. The effectiveness of inhibition was in the order cinnamic acid>4-coumaric acid>ferulic acid. Cinnamic acid also inhibited the PAL activity increase induced by dilution of the suspensions into an excess of fresh culture medium. Addition of low concentrations (<10(-5)M) of the pathway intermediates to cultures at the time of application of ribonuclease gave variable responses ranging from inhibition to 30-40% stimulation of the PAL activity measured at 8 h. Following addition of pathway intermediates to cultures 4-5 h after ribonuclease treatment, rapid increases followed by equally rapid declines in PAL activity were observed. The cinnamic acid-stimulated increase in enzyme activity was unaffected by treatment with cycloheximide at a concentration which gave complete inhibition of the ribonuclease-induced response. However, cycloheximide completely abolished the subsequent decline in enzyme activity. Treatment of induced cultures with α-aminooxy-β-phenylpropionic acid (AOPPA) resulted in increased but delayed rates of enzyme appearance when compared to controls not treated with the phenylalanine analogue. The results are discussed in relation to current views on the regulation of enzyme levels in higher plants.

Regulation of induction of phenylalanine ammonia-lyase in suspension cultures of Phaseolus vulgaris

Total RNA was extracted from fast growing suspension cells of bean, the mRNA was translated and the products of protein synthesis analysed by gel electrophoresis. Actinomycin D (20 μg ml(-1)) added to the cultures 12 h before the induction of phenylalanine ammonia-lyase (PAL) activity by naphthylacetic acid (NAA) (1 mg/l) and kinetin (0.2 mg/l) failed to prevent the increased activity of the enzyme usually produced by this ratio of the plant growth hormones. PAL was isolated and purified from suspension cultured bean cells. The purified enzyme ran as a single band on polyacrylamide gel electrophoresis. The protein translated from RNA prepared from induced and non-induced cells was separated by gel electrophoresis and the bands of protein on the gels were compared. There was no evidence for an increase in the amount of PAL synthesised in vitro from the mRNA of induced cells even though these had 5 times the amount of activity of the enzyme compared with that of the non-induced cells. The results indicate that the induction of PAL activity is not immediately preceeded by an increase in the synthesis of PAL-mRNA by the cells. The control of the activity of the enzyme is discussed with respect to this finding.

Superinduction of phenylalanine ammonia-lyase in gherkin hypocotyls caused by the inhibitor, L-alpha-aminooxy-beta-phenylpropionic acid

The extractable activity of L-phenylalanine ammonia-lyase (EC 4.3.1.5) and the concentration of sugar esters of p-coumaric and ferulic acids in the hypocotyls of etiolated gherkin seedlings increase upon irradiation with white light. Treatment of intact seedlings with the phenylalanine ammonia-lyase inhibitors alpha-aminooxyacetic acid and L-alpha-aminooxy-beta-phenylpropionic acid during illumination causes enhanced formation of the lyase and reduces the accumulation of hydroxycinnamic acids. Enzyme activity in excised hypocotyl segments floating on buffer increases in the dark as well as in the light, while hydroxycinnamic acids accumulate only in the light. Phenylalanine ammonia-lyase formation in the segments is inhibited by cinnamic acid and, to a lesser extent, p-coumaric acid, while it is slightly enhanced by caffeic acid and is not affected by ferulic acid. Aminooxyphenylpropionate dramatically promotes phenylalanine ammonia-lyase formation in the segments in darkness and light prevents the accumulation of hydroxycinnamic acids in the light. Aminooxyphenylpropionate does not, however, affect the time course of apparent lyase formation and decay. Cinnamic acid, the product of the lyase reaction, antagonizes the effect of aminooxyphenylpropionate. It is proposed that the reaction product(s) are involved to some extent in the regulation of the pool of active lyase in the hypocotyl tissue.

A specific and reversible macromolecular inhibitor of phenylalanine ammonia-lyase and cinnamic acid-4-hydroxylase in gherkins

A non-dialysable inhibitor of phenylalanine ammonia-lyase (L-phenylalanine ammonia-lyase, EC 4.3.1.5) has been partially purified from dark-grown gherkin hypocotyls. On extraction of tissue it is found both in the soluble (106 000 X g supernatant) and microsomal (106 000 X g pellet) fractions and can be extracted from the microsomal membranes with 10 mM sodium cholate and 1 M KCl. The soluble and microsomal fractions have similar properties, suggesting the presence of the same active component. The inhibitor is small (Mr less than 20 000), thermolabile, sensitive to proteolytic digestion, and apparently hydrophobic. Purification of the inhibitor was achieved by chromatography on DEAE-cellulose by gel filtration on Sephadex G-50. The inhibitor preparations inhibit phenylalanine ammonia-lyase isolated from a number of plant tissues and also cinnamic acid-4-hydroxylase (trans-cinnamate, NADPH:oxygen oxidoreductase (4-hydroxylating), EC 1.14.13.11) from gherkins and peas, but not a wide range of other enzymes. The evidence suggests that inhibition of the two enzymes is due to the same substance, but this has not yet been confirmed. Kinetic experiments show that the inhibitor is competitive with phenylalanine for the lyase and that its association with the lyase is reversible. Further, a mixture of inhibitor and lyase can be separated on non-denaturing polyacrylamide gels without loss of lyase activity. The activities of phenylalanine ammonia-lyase and cinnamic acid 4-hydroxylase are often concurrently regulated and both have regulatory roles in phenol metabolism; it is suggested that the inhibitor may be specifically involved in controlling their activities in vivo.

Phenylalanine ammonia-lyase and cinnamic acid 4-hydroxylase: Product repression of the level of enzyme activity in potato tuber discs

Exogenous supplies of phenylalanne, cinnamic acid and p-coumaric acid can inhibit the appearance of phenylalanine ammonia-lyase (PAL, E.C. 4.3.1.5) activity in potato tuber discs, and exogenous supplies of cinnamic acid and p-coumaric acid can inhibit the appearance of cinnamic acid 4-hydroxylase (CA4H, E.C. 1.14.13.11) activity. The time-courses of the inhibitory effects of (a) exogenous phenylalanine on the appearance of PAL activity and (b) exogenous cinnamic acid on the appearance of CA4H activity suggest that the respective substrates of the two enzymes have no inhibitory effects per se, but rather that metabolic derivatives are the active agents. Accordingly, cinnamic acid and p-coumaric acid, but not phenylalanine can inhibit the initial appearance of PAL activity. Similarly, p-coumaric acid but not cinnamic acid can inhibit the initial appearance of CA4H activity. The inhibitory effects of exogenous cinnamic acid and p-coumaric acid on the initial appearance of PAL activity are independent. These findings are discussed in relation to the control in vivo of the levels of activity of PAL and CA4H.

Regulation of phenylalanine ammonia-lyase synthesis by cinnamic acid. Its implication for the light mediated regulation of the enzyme

1. There is an increase in the extractable acitivity of phenylalanine ammonia-lyase following excision of hypocotyl segments of dark-grown gherkin seedlings This increase is prevented by the reaction product, cinnamin acid. 2. Density-labelling experiments show that cinnamic acid affects the rate of enzyme synthesis. By contrast the regulation of phenylalanine ammonia-lyase activity by light has previously been shown to involve activation of existing inactive enzyme. 3. It is proposed that regulation of synthesis by reaction products represents a mechanism for controlling the size of the pool of phenylalanine ammonia-lyase, and that the activity of this pool is regulated by light.

[The influence of the plasmotype on the regulation of the activity of L-phenylalanine ammonia-lyase (studies on Oenothera forms of the section Raimannia)]

The regulation of the activity of L-phenylalanine ammonia-lyase (PAL) in seedlings of the complex heterocygotes Oenothera berteriana (with the chromosome complexes B and I) and Oenothera odorata (with the complexes v and I) and of their progeny was investigated. The following main results were obtained:The plasmotype (plasmon) has a significant effect upon the amount of increase of the activity of PAL during illumination with continuous far-red light (Figs. 2-5). Combined with the genotypes v.I, B.I and l.v the plasmotype of Oenothera berteriana effects a higher maximal activity after 20 hrs than the plasmotype of Oenothera odorata. This influence of the plasmon is constantly inherited.The influence of the chromosome complexes is characterized by the order I-v-l-B; the I-complex causes the highest, the B-complex the lowest increase in PAL-activity.Cytoplasm and plastids have no apparent influence either on the PAL-activity in darkgrown seedlings or on the amount of increase of PAL-activity which is brought about by wounding (Fig. 7). Therefore it can be concluded that the plasmotype affects very specifically the light-induced PAL-synthesis.There is no evidence so far for the existence of isoenzymes of PAL in the seedlings of Oenothera forms.Because of various correlations between light-induced PAL-activity and anthocyanin formation (Table 2), it can be supposed that the plasmon takes part in the regulation of anthocyanin formation by directing the PAL-activity. By the technique of simultaneous irradiation it is demonstrated that anthocyanin formation under continuous illumination with far-red is mediated by phytochrome (Fig. 6).The possibility of explaining the influence of the plasmotype of Oenothera forms on various phytochrome dependent reactions on the basis of a pleiotropic mechanism is discussed.

Low-temperature effects on phenylalnine ammonia-lyase activity in gherkin seedlings

Treatment at temperatures below 10° causes in both dark-grown and pre-irradiated gherkin seedlings a rise in the activity of the enzyme phenyl-alanine ammonia-lyase (PAL). This increase takes place both in the course of the cold treatment or after transfer to higher temperature; in the latter case, however, it is followed by a decline. The results can be explained, on the basis of evidence obtained previously, that at temperatures above 10° a PAL-inactivating system compensates for PAL synthesis and that the end products (hydroxycinnamic acids) of the reaction catalysed by PAL are involved in the induction and/or functioning of the inactivating system. It is inferred that as a consequence of the low rate of synthesis of these products at lower temperatures newly synthesized PAL is not inactivated and previously synthesized PAL is released from an enzyme-inactivator complex.

Effects of inhibitors of RNA and protein synthesis on bean hypocotyl hook opening and their implications regarding phytochrome action

Inhibitors of protein and RNA synthesis (cycloheximide, puromycin, chloramphenicol, and actinomycin D), as well as Co(++), induce opening of the hypocotyl hook of bean seedlings during the early stage of the opening period both in the darkness and red light. The response is transitory, however, complete straightening of a hook can not be achieved in the presence of these agents. These agents abolish the response of hooks to red illumination. They also block the suppression of hook opening caused by IAA and ethylene. The response and sensitivity to GA are not affected by the inhibitors. Inhibitors of DNA synthesis (FUDR and mitomycin C) have no effect on hook opening. It appears that in this growth response RNA and protein synthesis are more immediately involved in ethylene action than they are in the cell elongation process or the action of GA thereon.The results indicate that phytochrome does not induce hook opening simply by activating genes whose products directly promote growth. It is suggested that the regulation of ethylene formation by light and auxins may be exerted by way of influences on tissue levels of phenolic inhibitors of ethylene biosynthesis.

[The control by phytochrome of phenylalanine ammonia-lyase activity in mustard seedlings (Sinapis alba L.)]

The present paper is a contribution to the "molecular" analysis of photomorphogenesis. L-phenylalanine ammonia-lyase (=PAL) (EC 4.3.1.5) has been used as a model system to demonstrate that enzyme synthesis, enzyme inactivation and gene repression are important in determining the response of a particular enzyme to phytochrome.The level of PAL in the mustard seedling is controlled by Pfr (the active form of phytochrome) in a characteristic manner which is illustrated in Fig. 1. The seedlings were irradiated with continuous standard far-red light. Long time irradiation with far-red will maintain a low but virtually constant level of the effector molecule Pfr in the seedling over an extended period of time. At the moment when the far-red light is turned off the action of Pfr will instantly decrease and will eventually cease probably within the order of an hour (cf. KAROW and MOHR, 1969). The approach followed in the present paper has been to turn off the far-red light after varying periods and follow the enzyme kinetics in darkness (Fig. 2). The main results can be summarized as follows: The far-red kinetics of PAL (Fig. 1) can be explained as the result of three processes, namely, Pfr-mediated enzyme synthesis, inactivation of PAL by an "inactivator", and eventual repression of enzyme synthesis.-During the period 1.5-12 hrs after the onset of far-red only enzyme synthesis occurs. Then enzyme inactivation comes into play while enzyme synthesis continues at a constant rate (Fig. 3). This antagonism of synthesis and inactivation leads to a true steady state which is observed between about 24 and 27 hrs after the onset of far-red. After this period the rate of enzyme synthesis decreases and as a consequence, inactivation dominates. 36 hours after the onset of far-red the Pfr-mediated PAL synthesis is hardly dtectable. The results of "secondary irradiations" with far-red (Fig.4) indicate that the "inactivator" of PAL does not have any direct influence on PAL synthesis. The kinetics in darkness (Fig.1,2) can best be understood by assuming that a certain enzyme level represented by the plateau cannot be overcome in the dark. The "overshoot" response which is obvious in the enzyme kinetics immediately after the cessation of far-red (Fig. 2) cannot be explained readily in molecular terms.