C. Huault

Atypical metabolisms and biochemical cycles imposing the cancerous state on plant cells

The biological, morphological and biochemical characteristics which define plant cancer cells at the end of a neoplasic progression in the absence of pathogens and which distinguish them from tumorous cells are summarized. Such plant cancer cells have in common with animal cancer cells many metabolic disturbances. The present paper reviews the biochemical changes in nitrogen, carbon, sugar and heme metabolisms which contribute to polyamine (PAs) accumulation. It indicates how these changes are interconnected and even form between each other biochemical cycles which likely maintain these cells in their irreversible state. The role of these cycles in the maintenance of such cells under a probable permanent oxidative stress is debated.

Effect of glutathione on dormancy breakage in barley seeds

A striking relation has been shown between the increase of glutathione levels during dormancy breakage of barley seeds and the induction of germination by exogenous glutathione. These findings suggested that glutathione may play a crucial role in dormancy breakage.

Darkness improves growth and delays necrosis in a nonchlorophyllous habituated sugarbeet callus: Biochemical changes

The transfer of light-cultured green normal (N) and white habituated (HNO) sugarbeet callus to darkness reduced the growth of N callus and improved growth and delayed necrosis in the HNO callus. The decrease of dry matter of N callus under darkness was accompanied by a reduced content of carotenoids and by decreased CO2 fixation, which was compensated by an increased dependency on externally supplied sucrose. The levels of some organic nitrogen compounds such as glutamate, proline, and free polyamines were not affected by transfer to darkness of N or HNO callus. Darkness decreased ethylene emissions in both callus types. In the HNO callus, the sucrose growth dependency and the CO2 fixation were unaffected by darkness. Chlorophylls were absent both in light and darkness, whereas some carotenoids were accumulated in the HNO callus only in dark conditions. In another connection, a significant increase of peroxidase activity, which did not occur in the N callus, was induced by darkness in the HNO callus. A decreased content of thio-barbituric acid (TBA)-reactive substances was measured in the HNO callus transferred to darkness, whereas an increase was noticed in the N callus placed in the same conditions. These metabolic changes and the reduction of cellular damage in darkness revealed light-induced stress reactions leading to necrosis and to reduced growth of HNO callus. It appeared that darkness allowed the HNO callus to avoid the photooxidation stress. Therefore, the favorable effect of darkness on HNO growth might be explained by the suppression of photooxidative damage due to the absence of carotenoids. The higher peroxidase activity in the HNO callus maintained in darkness raised the problem of heme synthesis in this heterotrophic callus.

Disturbed sugar metabolism in a fully habituated nonorganogenic callus of Beta vulgaris (L.)

Habituated (H) nonorganogenic sugarbeet callus was found to exhibit a disturbed sugar metabolism. In contrast to cells from normal (N) callus, H cells accumulate glucose and fructose and show an abnormal high fructose/glucose ratio. Moreover, H cells which have decreased wall components, display lower glycolytic enzyme activities (hexose phosphate isomerase and phosphofructokinase) which is compensated by higher activities of the enzymes of the hexose monophosphate pathway (glucose-6-phosphate dehydrogenase and 6-phosphogluconate dehydrogenase). The disturbed sugar metabolism of the H callus is discussed in relation to a deficiency in H2O2 detoxifying systems.

Fully habituated sugarbeet callus: Under permanent stress?

SummaryA fully habituated nonorganogenic (HNO) sugarbeet callus line, compared to a normal (hormone dependent) one originated from the same plant exhibits many characteristics of a vitrified tissue and several traits common to animal cancer cells. Four types of biochemical or metabolic characteristics of HNO callus [deficiency of tetrapyrrole-containing compounds; lipid (per)oxidation and malondialdehyde formation; high activity of enzyme protective systems; proline, glutamate, and polyamine accumulation] may be interpreted as responses to stress. The deficiency of tetrapyrrole-containing compounds can be considered an indirect protection against activated forms of oxygen as well as the higher activity of the antioxidant defense mechanisms. This fits with suggestions in the literature that the autonomy associated with plant cancer tissue is explained on the basis of antioxidants as stimulators of cell division and corresponding inhibitors of cell differentiation. Such changes occurring in HNO cells may in turn be responsible for a greater absorption and sensitivity to ammonium ions. An altered nitrogen metabolism leads to proline, glutamate, and polyamine accumulation. Lipid peroxidation and malondialdehyde accumulation rapidly occur in this very sensitive HNO callus in a prolonged culture cycle, which might be related to the appearance of necrosis bands. Thus there are arguments allowing us to consider HNO cells as mutants adapted to some stresses, but resulting changes in their structure might have rendered them still more sensitive to other factors.

Effects of water-stress on proline content and ornithine aminotransferase activity of radish cotyledons

Abstract The concentration of proline and the level of ornithine aminotransferase (OAT) activity in light-grown cotyledons of radish (Raphanus sativus cv National) are increased as an inverse function of the relative water content. Gabaculine, which is an irreversible inhibitor of plant OAT, reduced the water-stress induced proline accumulation considerably. It can be assumed that the ornithine pathway contributes via an increase of OAT activity, to proline synthesis, in addition to the glutamate pathway, in water-stressed cotyledons. Drought-induced OAT activity appears to be due to an increase in one of the two OAT isoforms which have been separated by anion-exchange chromatography.

Inhibition of plant ornithine aminotransferase by gabaculine and 4-amino-5-hexynoic acid

Abstract Gabaculine (GAB) and 4-amino-5-hexynoic acid (AHA) have been shown to be enzyme-activated inhibitors of radish ornithine aminotransferase (OAT). The in vitro results were entirely compatible with the in vivo situation. However, it appeared that GAB was a more effective inhibitor than AHA as it was active at a lower concentration (1 μM) leading in vivo to almost complete irreversible inhibition of OAT within a few hours. GAB and AHA will be useful tools to study the physiological consequences of OAT inhibition in plants.

Gabaculine as a tool to investigate the polyamine biosynthesis pathway in habituated callus of Beta vulgaris (L.)

The polyamine content of a habituated callus of Beta vulgaris (L.) is strongly diminished after treatment with gabaculine, a potent inhibitor of ornithine aminotransferase. The inhibitory effect of gabaculine is reversed if ornithine is supplied. This result may indicate that proline catabolism provides ornithine for polyamine synthesis.

Biosynthesis of 5-aminolevulinic Acid via the Shemin Pathway in a Green Sugar Beet Callus

Abstract5-Aminolevulinic acid synthase (ALAS) has been detected in a normal (auxin- and cytokinin-dependent) green sugar beet callus under light and under darkness. ALAS activity was lower when the callus was grown under light. The supply of precursors of the Shemin pathway (glycine and succinate) to dark-grown callus enhanced considerably the capacity of the 5-aminolevulinic acid (ALA) formation. Glutamate, γ-aminobutyrate or α-ketoglutarate also increased ALA accumulation. Such an accumulation was also obtained after inhibition of polyamine synthesis. The results show that glutamate or its derivatives might feed the Shemin pathway in conditions preventing glutamate to be used through the Beale pathway.

Erythromycin as a tool to investigate the tetrapyrrole biosynthetic pathways in habituated and normal sugarbeet calli

Erythromycin (ERT) has been shown to reduce the 5-aminolevulinic acid (ALA) synthesizing capacity of a normal (N) chlorophyllous sugarbeet callus, grown under light, in contrast to a habituated achlorophyllous non-organogenic (HNO) callus of the same species. Similar effects were obtained on total hemes and on catalase which is a hemoprotein used as marker. The effect of ERT, which is an inhibitor of plastid differentiation and of chlorophyll synthesis, was reversed in the N callus by a supply of glycine and succinate. The compounds are the precursors of ALA synthesized through 5-aminolevulinic acid synthase (ALAS) which is implied in the Shemin pathway. The involvement of ALAS appeared to be favoured when plastids were undifferentiated (HNO callus) or when plastids were inefficient (N callus under darkness or under light after ERT treatment).