[Enzyme activities and substrate levels of carbohydrate metabolism in proliferating and suberin synthesizing potato tuber cells]

Isolation of tissue fragments from the potato tuber can initiate either periderm formation including suberin synthesis or cell proliferation without cicatrization effects. TCA-cycle activity has been shown to develop only in causal correlation with suberin synthesis (Lange, 1970). Biochemical pathways of carbohydrate metabolism are analysed by investigating the changing levels of 10 intermediates and the activities of 12 corresponding enzymes. Differences between the metabolic kinetics of the two contrasting types of tissue are discussed as the biochemical background of different respiratory behaviour and different histogenetic development.Glucose and pyruvate as well as all triose- and hexosephosphates investigated except 6-phospho-gluconate generally show an intensive rise in concentration after derepression with subsequent degradation. In several cases not a concomitant rise but rather a contrary drift between the concentration of metabolites and the activity of corresponding enzymes is observed, e.g. phosphoglucomutase/glucose-6-phosphate, enolase/phosphoenolpyruvate. This phenomenon is connected with the occurrence of suberin synthesis and remains totally absent in proliferating tissue.After derepression the pentose phosphate shunt (6-phosphogluconate-dehydrogenase) is strongly activated independently of different histogenetic processes. On the other hand, the glycolytic pathway via fructose-6-phosphate becomes more effective in suberizing tissue, as is indicated by enhanced activity of phosphoglucoisomerase and accumulation of F-6-P.Little or no difference can be found with regard to hexokinase, triosephosphateisomerase, aldolase and pyruvate-kinase; on the other hand suberin formation strongly stimulates phosphoglyceromutase. From the high activity of the TCA-cycle in suberin synthesizing cells it must be concluded that acetyl-CoA is formed at a high rate by oxidative decarboxylation of pyruvate, which leads finally to citrate synthesis. Measurements of different steps of pyruvate metabolism and respiration suggest an inhibition of this pathway in proliferating tissue. Sim a taneously certain compensatory reactions are activated. The activity of glutamaulpyruvate-transaminase increases considerably, whereas it is almost entirely eliteinated in suberin synthesizing cells. Moreover, malic enzyme activity showsmgreater increase in proliferating tissue, and large pools of pyruvate, phospho(enol)-pyruvate, and 2-phospho-glycerate are accumulated. The difference in the glycolytic metabolism of the two tissues suggests a suppression of periderm formation and its substitution by cell proliferation as a result of insufficient production of precursors of suberin biosynthesis such as acetyl-CoA and fatty acids.

[Morphogenetic separation of cell division and suberin biosynthesis in potato tuber tissue by tris-(hydroxymethyl)-aminomethane]

Differential derepression of the genome of potato tuber cells by slicing of the tuber tissue leads to cell divisions. This mitotic activity is totally suppressed by Tris-(hydroxymethyl)-aminomethane (Tris)-buffer widely used in biochemical research. The blockage is reversible if tissue slices are transferred to water in order to wash out the Tris-ions.The actual reason for the inhibition of mitotic activity by Tris is as yet unknown. As a possible mechanism of action an uncoupling of electron transfer from phosphorylation in the mitochondrial respiratory chain is discussed. However, experiments show no difference in O2-absorption between Tris-treated tissue and control in water. Moreover the application of several inhibitors of respiration causes exactly the same effects in both tissues. Amytal (blockage of flavoproteids) has no influence on the respiratory rate at any time. Antimycin A (blockage of electron flow from cytochrome b to cytochrome c) as well as HCN (inhibition of cytochrome-oxidase) inhibit respiration of both tissues during the first 24 hours after derepression. Later on the respiration becomes resistant to both inhibitors. So the quality of respiration is assumed to be the same in mitotic active potato slices as in the Tris-treated tissue.Recent results of biochemical analyses of events in carbohydrate breakdown of the tissues in question point to a differential effect of Tris on several enzymes as possible reason for its inhibitory action.

[Substrate levels, enzymatic activities and genetic regulation after derepression in plant storage tissues]

Differential derepression of the genome of potato tuber cells can be initiated by slicing the tissue into disks. The consequence of this procedure on the cells of the wound surface is dedifferentiation and cell division followed by redifferentiation to a suberized phellem cell. The drift of glucose-, glucose-1-phosphate-, glucose-6-phosphate-, fructose-6-phosphate- and 6-phospho-gluconatelevels has been determined in the derepressed tissue. With the exception of 6-phospho-gluconate all intermediates so far investigated showed a rise in concentration after derepression.

This is interpreted as a consequence of altered enzymic activities which were estimated for phosphoglucomutase, hexokinase, phosphoglucoisomerase, gluco-6-phosphate- and 6-phosphogluconatedehydrogenase. The two dehydrogenases were activated after derepression, the activation represented a de-novo-synthesis, as was demonstrated with the inhibitors Actidione (translation) and p-Fluorophenyl-alanine (protein synthesis in general). Hexokinase and phosphoglucoisomerase were not severely affected by cutting the tissue. Phosphoglucomutase was degrated rapidly, the degradation being dependent on protein synthesis. The importance of an enhanced activity of the pentose phosphate shunt for the stressed cell is emphasized and the possibility of an alteration in the osmotic pressure within the cell and especially in the nucleus — a primary consequence of wounding — as a cause of derepression in potato tuber cells is discussed.

Lag-phases in phytochrome-mediated enzyme synthesis (PAL)

In experiments with the mustard seedling (Sinapis alba L.) it was confirmed that in the case of "secondary irradiation" induction of PAL by phytochrome is a very rapid process. The lag-phase after the onset of light is too brief to be detected. However, the data of other investigators, who found extended "secondary" lag-phases in their experimental material, can be imitated with the mustard seedling. We explain why these investigators were not able to eliminate the "secondary" lag-phase.

[New evidence in favour of the hypothesis of differential gene activation by phytochrome 730]

Phytochrome-mediated anthocyanin synthesis of the mustard seedling (Sinapis alba L.) can be regarded as a prototype of a "positive" photoresponse. "Positive" photoresponses are those which are characterized by an initiation or an increase of biosynthetic or growth processes. The initial lag-phase of this type of photoresponse is relatively long. In the case of anthocyanin synthesis of the mustard seedling it takes about 3 hours after the initial formation of P730, the active phytochrome, until anthocyanin synthesis can be measured (Fig.). This period of time has been called initial (or primary) lag-phase.In earlier papers (e.g. LANGE and MOHR, 1965; MOHR, 1966a, b; SCHOPFER, 1967) we have advanced and supported the hypothesis that the positive photoresponses (in the above sense) of the mustard seedling can be explained by a differential gene activation through P730. Those genes of which the activity can be started or increased by P730 have been called potentially active genes (P730).In order to verify this hypothesis further we have been trying to characterize the processes which occur during the initial lag-phase after the onset of continuous far-red light. There is good evidence (HARTMANN, 1966; CLARKSON and HILLMAN, 1967) that under continuous far-red a low but virtually stationary concentration of the active P730 can be maintained in the seedling's tissues over a period of many hours. Consequently we are dealing with steady state conditions of P730 when we investigate the process of anthocyanin accumulation under continuous far-red.In the present paper we present data which indicate that P730 exerts two functions during the initial lag-phase. First, it eliminates a barrier before the potentially active genes insolfar as it makes these genes accessible for the activating action of P730. Secondly, P730 starts the activity of potentially active genes. To maintain gene activity the continuous presence of P730 is required. On the other hand, it a gene has once been "opened" to the action of P730 it remains easily accessible for the activating action of P730 even in the case when P730 disappeared and gene activity ceased for an extended period of time. After the re-appearance of P730 the gene can be activated almost immediately. These conclusions have been derived from the following facts. 1. The initial lag-phase after the onset of far-red is always in the order of 3 hours (Fig.). If, however, a seedling which was preirradiated with 12 hours of far-red is kept in darkness for an extended period and is then re-irradiated with far-red no lag-phase for the action of the second irradiation can be found even when anthocyanin synthesis had already ceased during the preceding dark period (Fig.).