Floral genes expressed in tomato hypocotyl explants in liquid culture.

This paper confirms, at molecular level, previous data showing that small explants of many plants do form a floral meristem and express specific floral genes after only few days in culture. After 15-20 days of culture, small tomato hypocotyl explants develop differentiated structures often resembling primitive ancestral reproductive organs. Other specific reproductive functions such as chromosomal segregation (somatic meiosis) were also present and demonstrated by means of a cytological and histological analysis. By reverse transcriptase-PCR and in situ hybridization it was found that these structures are indeed able to express flower-specific genes. The TM8 gene, a tomato gene that is expressed very early during floral development, is detectable on the proliferating hypocotyl explants during the first week of culture. The MON9612 gene, which in vivo is expressed only by tomato pistils and ovules, is detectable on the ovulelike structures developed after 20 days of culture. The construction of transgenic tomato plants expressing the GUS gene under the control of the MON9612 promoter allowed us to follow the induction and the expression of this gene during explant proliferation and development of the flowerlike structures. These data confirm the hypothesis that a floral reprogramming can be induced in plant explants as a consequence of wounding and growth factors action. It appears to be an effort to survive stress by means of an unscheduled reproductive program.

On the occurrence of somatic meiosis in embryogenic carrot cell cultures.

During the establishment of an embryogenic cell line from a carrot hypocotyl explant, processes closely resembling meiotic divisions are seen. A microdensitometric analysis revealed that the amount of cellular DNA diminished in the majority of cells to the haploid level. However, the diploid level was re-established in a matter of a few days. The genetic consequences of this segregation were studied by analyzing restriction fragment length polymorphisms (RFLP) and randomly amplified polymorphic DNAs (RAPD). The results showed that the great majority of embryos regenerated from segregants and that different segregants had different genetic constitutions.

Synthesis and toxicity to mammalian cells of the carrot dihydroisocoumarins.

The dihydroisocoumarins (+-)-6-methoxy-8-hydroxy-3-methyl-3,4- dihydroisocoumarin (1), (+-)-6,8-dihydroxy-3-methyl-3,4-dihydroisocoumarin (2), and (+-)-6,8-dimethoxy-3-methyl-3,4-dihydroisocoumarin (3) were synthesized with high yields via metalation of o-methylbenzamides. The toxicity of these compounds and that of (-)-1 extracted from carrot cells were tested, in vitro, on Chinese hamster cells. The toxicity was determined according to the presence or absence of a hydroxyl group in the peri position of the lactonic carbonyl group and according to the stereochemistry of the dihydroisocoumarin.

A carrot cell variant temperature sensitive for somatic embryogenesis reveals a defect in the glycosylation of extracellular proteins.

The temperature-sensitive carrot cell variant ts11c, arrested in somatic embryogenesis after the globular stage, was characterized. The sensitivity to a shift from 24 degrees C (permissive temperature) to 32 degrees C (non-permissive temperature) is greatest at the globular stage of embryogenesis, while cells proliferating in unorganized fashion and plantlets are not affected. Embryogenesis in ts11c is also arrested at the permissive temperature by replacement of conditioned culture medium with fresh medium. The timing of sensitivity of ts11c to medium replacement coincides with the sensitivity to temperature shift. Both sensitivities are recessive in somatic hybrids between ts11c and wild-type cells. Extracellular glycoproteins synthesized by ts11c at the non-permissive temperature contain much less fucose than those synthesized by the wild type. The glycoproteins synthesized by the variant under non-permissive conditions do not accumulate at the periphery of the embryo, as their wild-type counterparts do, but instead show a diffuse distribution throughout the embryo. The defect in ts11c can be fully complemented by the addition of extracellular wild-type proteins. A revertant of ts11c was isolated that simultaneously reacquired temperature insensitivity and normal glycosylation ability. Collectively, these observations indicate that ts11c is not able to perform proper glycosylation at the non-permissive temperature and suggest that the activity of certain extracellular proteins, essential for the transition of globular to heart stage somatic embryos, depends on the correct modification of their oligosaccharide side-chains.

5-azacytidine-induced tumorous transformation and DNA hypomethylation in Nicotiana tissue cultures.

The phenomenon of habituation is considered in plant tissue cultures to be a real process of chemical tumorogenesis; the cultures acquire the capacity of autonomous growth in a hormone-free medium under the influence of a variety of chemical and physical agents. Treatments with 5-azacytidine (AzaC) of in vitro cultured cells of the Nicotiana glauca x N. langsdorffii nontumorous hybrid (NNT) during the culture cycle led to the induction of a habituated phenotype. The repetitive DNA sequences showed a significant lower level of endogenous methylation in the treated cells in comparison with the normal ones. It is worth noting that it was impossible until now to habituate this strain by conventional methods and that the treatments were effective only in the first 5 days of subculturing; various evidence (cytological and biochemical) pointed out a phenomenon of DNA amplification, occurring in the same period. Moreover, analysis of DNA from control and treated cells shows the induction of variations in the endogenous methylation pattern by AzaC in a critical period of cell culture. These results suggest that demethylation can act as a switch from hormone-dependent to autonomous proliferation by activation of genes coding for or regulating the synthesis of growth factors.

DNA methylation of embryogenic carrot cell cultures and its variations as caused by mutation, differentiation, hormones and hypomethylating drugs.

The level of auxin - both natural and synthetic - in the medium has a strong effect on the level of 5-methyl-cytosine in the DNA of carrot cells in culture. This level may vary from approximately 15% to 70% of total cytosine without apparent effects on growth rate and cell morphology. No effect was seen with cytokinin. During somatic embryogenesis, in the absence of hormones, variations were seen in the level of methylation according to a characteristic pattern. If hypomethylation is induced with drugs such as azacytidine, ethionine or ethoxy-carbonyl-pyrimidine, embryogenesis is immediately blocked. A mutant was isolated which is resistant to the action of hypomethylating drugs. It shows variations in the methylation pattern and variations in indole-acetic acid metabolism. In addition its regeneration is often associated with the production of tumors.

Role of permanent dicentric systems in carrot somatic embryogenesis.

A permanent dicentric chromosome system was studied on carrot cultures and regenerated somatic embryos at different stages of development. The large chromosomal variability of the cultures and the presence of the breakage-fusion bridge cycle did not interfere with the initial developmental process up to the seedling stage but subsequent growth proceeded only if healing of the broken ends or dicentric loss had occurred. The behaviour of the dicentric chromosome in culture and during somatic embryogenesis is discussed in relation to chromosomal variability, abnormal development and the somaclonal variation that such mechanisms may generate in regenerated plants.