The tissue of resting plant storage organs such as carrots, red beets, sugar beets or potato tubers can be activated by slicing into thin disks and incubation of these fragments in a moist atmosphere for different periods of time (“aging”). Activation comprises the turning-on of various genes with subsequent synthesis of transfer-, ribosomal and messenger RNAs and their transport into the cytoplasm. The immediate consequence of all these primary reactions is a vigorous synthesis of various enzymes and structural proteins which enable the cell to greatly enhanced metabolic activities. Thus, degradation of storage polymers and the procession of the resulting products through glycolysis, the pentose phosphate shunt and the shikimateprephenate-pathway and cellulose biosynthesis occur. Deinhibition of the tricarboxylic acid cycle opens the flow of metabolites into fatty acid, phospholipid and steroid biosynthesis, simultaneously providing the respiratory chain with electrons. In spite of functional modifications within the electron transport chain, the enhanced respiration of tissue slices serves as an energy source for the various energy-dependent reactions of the cell such as syntheses and the uptake of solutes. All of these activities accompany a concomitant dedifferentiation process and ultimately lead to renewed redifferentiation of the tissue slice cell.

Metabolism in plant storage tissue slices

Lipids are important constituents of all biological membranes and play a central role in the molecular organization of these membranes. Structural lipids in plant biomembranes are essentially phospholipids, galactolipids, and sterols. In this chapter we shall first review the lipid composition of plant biomembranes; then we shall examine where these lipids come from in plant cells, how they are built, and how they integrate the various biomembranes or cell organelles. However, membrane lipids have not only a structural role. The activities of many membrane-bound enzymes are controlled by the extent and the quality of the lipid environment within the membranes. The last part of this chapter will deal with the results of some recent studies concerning the effect of lipids upon the activities of some membrane-bound enzymes.

Glyco- and Phospholipids of Biomembranes in Higher Plants

The incorporation rates of [1-14C]acetate in vivo into the lipids of different subcellular fractions of potato or cauliflower tissues are consistent with a flow of radioactive lipids from microsomes to mitochondria.

When potato tuber microsomes, containing phospholipids labelled from [1-14C]acetate, [32P]-phosphate or [1-14C]glycerol, are incubated in vitro with unlabelled mitochondria and cytoplasmic supernatant, the radioactivity of the microsomes decreases strongly within 2 hours; the lipids of the mitochondria and those of the supernatant become radioactive, taking up to 40% of the initial radioactivity. The same transfer of labelled lipids is observed when one incubates labelled mitochondria with unlabelled microsomes.

During these experiments, one can observe a marked decrease of the specific radioactivities of the phospholipids present in the initially labelled fractions. This may be explained by a departure of labelled phospholipids from the radioactive fraction towards the other cellular fraction and by an arrival of unlabelled lipids from the initially unlabelled fraction to the radioactive fraction. The results obtained with three different precursors are roughly similar, suggesting that there is an exchange of entire phospholipidic molecules between the cellular fractions. However, the rates of exchange of the lipids labelled from [1-14C]acetate are a little higher than those observed with the other precursors thus suggesting an exchange of free fatty acids in addition to the exchange of phospholipids.

Phosphatidylcholine and phosphatidylethanolamine represent the most important fraction of the exchanged phospholipids.

The cytoplasmic supernatant acts as an intermediate in these exchanges. When a volume of supernatant containing radioactive lipids is incubated with unlabelled mitochondria or unlabelled microsomes, the radioactivity of the supernatant lipids decreases. The lipids of the mitochondrial and microsomal fractions, particularly the phospholipids, become labelled.

The results obtained when mixing cauliflower subcellular fractions in vitro are very similar.

The same flows of lipid exchanges (in vitro) can be observed between potato mitochondria and cauliflower microsomes, or reciprocally. However, a great part of the lipids liberated by the cellular fraction from one species remains blocked in the cytoplasmic supernatant from the other species, which may indicate that the exchange are limited, in this case, by a specific requirement for certain kinds of lipids for the biogenesis of membranes.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1970.tb01002.x

Echanges de lipides entre mitochondries, microsomes et surnageant cytoplasmique de cellules de pomme de terre ou de chou-fleur

1
An active oleyl-coenzyme-A desaturase was associated with a microsomal fraction from “aged” slices of potato tuber; starting from [1-14C]oleyl-coenzyme A as a substrate, in vitro, more than 40% of the final fatty acid radioactivity was recovered in linoleic acid, the most abundant fatty acid of the intracellular membranes of potato tubers.

2
The presence of serum albumin in the reaction mixture was essential for the desaturation reaction and for preventing the degradation of polyunsaturated fatty acids. This stabilization of the desaturase activity by serum albumin allowed a great resistance upon freezing.

3
The microsomal oleyl-coenzyme-A desaturase showed a pH optimum around 7.0–7.2 and a temperature optimum near 25°C. The apparent Km of the desaturase for oleyl-coenzyme A was 25 μM.

4
Oxygen and pyridine nucleotide were required for oleyl-coenzyme A desaturation (NADH or NADPH were found nearly equally effective in the desaturation). Although NADPH was more effective than NADP, NAD was a effective as NADH for desaturation, suggesting that a NADH-regenerating system probably exists in the microsomal suspension.

5
The oleyl-coenzyme-A desaturase was specific in vitro for oleyl-coenzyme A as a substrate; no desaturation was observed with ammonium oleate, which was, however, desaturated in vivo by aged slices of potato tuber.

6
Oleyl-coenzyme A (and the newly labelled linoleic acid molecules) were essentially incorporated into phospholipids and particularly phosphatidylcholine.

7
The oleyl-coenzyme-A desaturase was almost entirely associated with the microsomal membranes, as shown by differential centrifugation and sucrose-gradient separations.

8
The oleyl-coenzyme-A desaturase activity (absent in the freshly cut slices) was induced during the aging of slices; a linear increase with the time of aging was found in many microsomal activities, such as oleyl-coenzyme A desaturase, NADH-cytochrome c reductase and protein synthesis. The inhibition of all these events by cycloheximide, or the partial inhibition by actinomycin D indicated that derepression of the genetic activity and synthesis of new proteins were involved in the induction of the oleyl-coenzyme-A desaturase.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1973.tb02592.x

The oleyl-coenzyme-A desaturase of potato tubers. Enzymatic properties, intracellular localization and induction during "aging" of tuber slices.

Resting plant storage organs, e.g., potato tubers, carrots or sugar beets usually exhibit a very low metabolic activity. Slicing into thin disks and incubation of these tissue fragments in a moist atmosphere, however, leads to a coordinated genetic and metabolic effort to adapt the exposed cells to their new environment. Nearly all reactions occurduring this struggle are dependent on genetic activity: slicing strikingly mimics well known hormone effects in mammalian systems and most certainly evokes an induction similar to that of the hormones (e.g., estrogen on uterine cells, Table VIII). Moreover, the cell controls the magnitude of induced reactions by way of its considerable inductive, repressive or modifying capabilities. However, in sharp contrast to the widely studied hormone systems of the plant and animal kingdoms, plant storage tissue slices and their capabilities have been largely neglected until now or are not known at all. Nevertheless they provide an excellent example of genetic and metabolic regulation occurring side by side and easy-to-handle material for the study of the elementary processes at the gene level following derepression of previously repressed genes in multicellular organisms.

A.Ben Abdelkader

Papers

Biogenese des lipides mitochondriaux au cours de la “survie” (ageing) de disques de parenchyme de tubercule de pomme de terre

Echanges d'acides gras in vitro entre mitochondries, microsomes et surnageants cytoplasmiques de cellules de pommes de terre et de chou-fleur