Clonal and physiological variation in the cytokinin content of tobacco-cell lines differing in cytokinin requirement and capacity for neoplastic growth

Shemin pathway and peroxidase deficiency in a fully habituated and fully heterotrophic non-organogenic sugarbeet callus: an adaptative strategy or the consequence of modified hormonal balances and sensitivities in these cancerous cells? A review and reassessment

There are many arguments for considering a specific fully habituated (auxin and cytokinin-independent) and fully heterotrophic non-organogenic (HNO) sugarbeet callus cell line as terminating a neoplastic progression, and thus to be made of cancerous cells. The similarities with animal tumour and cancer cells are recalled. All types of habituated tissues examined in the literature share at least three common biochemical characteristics: low apparent peroxidase activity, high content of polyamines (PAs) and low production of ethylene. However, results concerning their auxin and cytokinin levels are not consistent. Peroxidase synthesis in the achlorophyllous HNO callus appears to arise from aminolevulinic acid (ALA) synthesis through the Shemin pathway, commonly used by animals and fungi. This pathway is limited by disturbed nitrogen metabolism that diverts glutamate (directly used for ALA synthesis in green higher plants) from the Kreb's cycle into PA synthesis. There is no argument to suggest that the low ethylene production is caused by a competition with PAs for their common precursor, S-adenosylmethionine. The results we report here indicate modified anabolic and catabolic pathways of auxins and cytokinins but also the possibilities of unusual compounds playing similar roles (dehydrodiconiferyl alcohol glucosides, for instance). A higher turnover of PAs is shown in the HNO callus, which could suggest a role for H2O2 and gamma-aminobutyric acid, products or intermediates in the PA catabolic pathway, as secondary messengers. The habituated cells retain some sensitivity towards exogenous auxins and cytokinins. Their increased sensitivity to PAs and ethylene suggests modified hormonal balances for the control of these actively dividing cells.

Partial auxin deprivation affects endogenous cytokinins in an auxin-dependent, cytokinin-independent tobacco cell strain

The dynamics of individual endogenous cytokinins within the growth cycle (subculture interval) of an auxin-dependent and cytokinin-independent cell suspension culture ofNicotiana tabacum L. (strain VBI-0) were determined using high performance liquid chromatography and radioimmunoassay. In cells grown at an optimum auxin concentration the transient maxima of N(6)-(Δ(2)-isopentenyl)adenine and N(6)-(Δ(2)-isopentenyl)-adenosine correlated with the onset of cell division. Cultivation of the cells in a partially auxin-deprived medium resulted in ca. tenfold increase of all endogenous cytokinins. A very distinct maximum of N(6)-(Δ(2)-sopentenyl) adenine appeared at the beginning of subculture. This indicates that a lack of auxin induced an accumulation of cytokinins predominantly in the form of the free bases, which are physiologically more active than the corresponding ribosides.

Expression of the isopentenyl transferase gene is regulated by auxin in transgenic tobacco tissues

The isopentenyl transferase gene (ipt) from Agrobacterium tumefaciens was isolated and introduced, via a disarmed binary vector, into tobacco using the Agrobacterium tumefaciens-mediated gene transfer system. The expression of the ipt gene was monitored by RNA hybridization, western blotting and cytokinin analysis. The addition of auxin to the media rapidly reduced the level of cytokinins in the transgenic tissues and this was associated with a reduction in IPT mRNA and protein levels. It is concluded that the hormone auxin can regulate expression of a gene involved in biosynthesis of the second hormone cytokinin. Although exogenous benzyladenine did not directly affect ipt gene expression, it did antagonize the effect of auxin on levels of cytokinins and IPT mRNA and protein.

Mass-spectrometric quantitation of cytokinins in tobacco crown-gall tumours induced by mutated octopine Ti plasmids of Agrobacterium tumefaciens

The levels of the major cytokinins, zeatin, zeatin riboside, zeatin riboside-5'-monophosphate and zeatin-7-glucoside were measured in tobacco (Nicotiana tabacum L.) crown-gall tissues carrying insertion and deletion mutations in the T-DNA. Measurements were made by combined gas chromatography-mass spectrometry using selected ion monitoring with (15)N- and (2)H-labelled internal standards. The results demonstrate that, relative to wild-type tumour tissue, cytokinin levels are considerably elevated in tissues lacking functional T-DNA auxin-biosynthetic genes. From a detailed analysis of the major cytokinin metabolites it is concluded that a reduction in the extent of cytokinin degradation via N(6)-side-chain cleavage is an important factor leading to increased cytokinin levels in these tissues.

The effect of auxin concentration on cytokinin stability and metabolism

The stability of [(3)H]zeatin riboside supplied to freshly excised tobacco pith explants was found to be inversely related to α-naphthaleneacetic acid concentration in the incubation medium. At higher concentrations of α-naphthaleneacetic acid greater breakdown of [(3)H]zeatin riboside was indicated by higher levels of degradative metabolites (adenine, adenosine and adenosine nucleotides) formed. This auxin effect on cytokinin metabolism appears to be mediated, at least in part, through cytokinin oxidase. The results of in-vitro assays carried out with partially purified enzyme from corn kernels substantiale this conclusion. These findings are discussed in relation to recent observations of auxin and cytokinin levels in crown-gall tumours with altered morphology.

Hormonal regulation of zeatin-riboside accumulation by cultured tobacco cells

Auxin (11 μM α-naphthaleneacetic acid) and cytokinin (1.4 μM kinetin) regulate cytokinin accumulation by cytokinin-requiring (C(-)) and cytokinin-autotrophic (C(+)) lines of Havana 425 tobacco (Nicotiana tabacum L.) tissues. No trans-zeatin riboside (ZR) (<0.5 pmol·g(-1) fresh weight) was detected in six C(-) and nine C(+) lines grown for 14 d on auxin + cytokinin and auxin medium, respectively. C(+) lines, but not C(-) lines accumulated ZR (1.9-5.1 pmol·g(-1) fresh weight) when incubated on hormone-free medium but both lines accumulated ZR when incubated on kinetin medium. Therefore, it appears that kinetin treatment can induce ZR accumulation and that this accumulation is blocked by auxin treatment. Similar effects were obtained with some lines of cells autotrophic for both auxin and cytokinin. Tobacco plants carrying the dominant Habituated leaf-1 allele (Hl-1) differ from wild-type plants in that leaf-derived tissues in culture exhibit a C(+) phenotype. No differences in ZR content were found in C(+) leaf tissues from Hl-1/Hl-1 plants and C(+) tissues that arise epigenetically in wild-type plants. This indicates that the H-1 allele does not act to induce overproduction of ZR. The Hl-1 allele is known to have oncogenic functions similar to the isopentenyl transferase (ipt) locus of the Ti plasmid. Although Hl-1/Hl-1 cells transformed with Ti plasmids defective at the ipt locus are tumorigenic and hormone-autotrophic in culture, they contain low levels of ZR typical of non-transformed Hl-1/Hl-1 cells. Therefore, the high levels of ZR characteristics of cells transformed with wild-type Ti plasmids are not necessary for expression of the tumor phenotype.

Neoplastic progression in crown gall in tobacco without elevated auxin levels

We have isolated two stable variants from a crown-gall teratoma tissue of tobacco (Nicotiana tabacum L.) transformed by Agrobacterium tumefaciens strain A66, a mutant of the virulent A6 strain containing an insertion sequence in the tumor-inducing (Ti) plasmid at the locus coding for auxin biosynthesis. Normally tobacco cells transformed by strain A66 spontaneously form shoots in culture and will not grow on hormone-free medium unless shoots develop. The variant tissue lines, isolated from the teratoma tissue after prolonged culture in the dark, grew as friable and unorganized tissues on hormone-free growth medium. Growth of the variants was more sensitive to auxin feeding than growth of the parental teratoma line, and the auxin dose-response curves of the variant lines were similar to those obtained with A6-transformed tobacco cells. Southern blot analysis of DNA from the parental teratoma line and one of the variants showed no differences in copy number or organization of the oncogenic DNA sequence (T-DNA) transferred from the bacterium, indicating that the variant phenotype did not result from reversion of the A66 mutation. Radio-immunoassay analysis showed similar levels of indole-3-acetic acid (IAA) in the variants and parental teratoma line (3-50 and 38-42 pmol·(gFW)(-1), respectively), whereas an A6-transformed cell line contained much higher IAA levels (150-1200 pmol·(g FW)(-1)). Low levels of the ethylene precursor 1-aminocyclopropane-1-carboxylic acid in the variants and the parental teratoma line (<5 nmol·(g FW)(-1)) as compared with that found in the A6-transformed line (>100 nmol· (g FW)(-1)) provided additional, indirect evidence for low auxin levels in the variant lines. These results indicate that crown-gall teratoma tissues of tobacco may switch to the unorganized, auxin-sensitive phenotype without an increase in auxin content.