Characterization of the inhibitory effect of glucocorticoids on the DNA replication of adult rat hepatocytes growing at various cell densities

Dexamethasone inhibited the basal and EGF-stimulated DNA synthesis of adult rat hepatocytes in primary culture. The inhibition was glucocorticoid-specific: It was shown by dexamethasone and hydrocortisone, but not by progesterone, testosterone, or estradiol; and was counteracted by the glucocorticoid antagonist RU-38486 in a concentration-dependent manner. Dexamethasone acted by decreasing the rate of entry into S-phase (kG1/S), while cell cycle parameters were unaffected. The steroid was able to decrease the kG1/S severalfold even when added more than 20 hr after EGF, half-maximal effect occurring 11 hr after the addition of dexamethasone. Densely populated areas were much more sensitive to the inhibition by dexamethasone than sparsely populated areas within the same culture dish: A moderate (10 nM) concentration of dexamethasone nearly abolished the DNA synthesis in densely populated areas of hepatocyte cultures with only marginal effect on sparsely populated cells.     

Successful crossings with early flowering transgenic poplar: interspecific crossings,but not transgenesis,promoted aberrant phenotypes in offspring

In forest tree species, the reproductive phase is reached only after many years or even decades of juvenile growth. Different early flowering systems based on the genetic transfer of heat‐shock promoter driven flowering‐time genes have been proposed for poplar; however, no fertile flowers were reported until now. Here, we studied flower and pollen development in both HSP::AtFT and wild‐type male poplar in detail and developed an optimized heat treatment protocol to obtain fertile HSP::AtFT flowers. Anthers from HSP::AtFT poplar flowers containing fertile pollen grains showed arrested development in stage 12 instead of reaching phase 13 as do wild‐type flowers. Pollen grains could be isolated under the binocular microscope and were used for intra‐ and interspecific crossings with wild‐type poplar. F1‐seedlings segregating the HSP::AtFT gene construct according to Mendelian laws were obtained. A comparison between intra‐ and interspecific crossings revealed that genetic transformation had no detrimental effects on F1‐seedlings. However, interspecific crossings, a broadly accepted breeding method, produced 47% seedlings with an aberrant phenotype. The early flowering system presented in this study opens new possibilities for accelerating breeding of poplar and other forest tree species. Fast breeding and the selection of transgene‐free plants, once the breeding process is concluded, can represent an attractive alternative even under very restrictive regulations.     

A comparative study of seed number, seed size, seedling size and recruitment in grassland plants

In this study we analyse relationships between seed number, seed size, seedling size and recruitment success in grassland plants. The often hypothesised trade-off between seed size and seed number was supported by a cross-species analysis and by an analysis of 35 phylogenetically independent contrasts, derived from a data-set of 72 species. Apart from among-species relatedness, we also controlled for possible confounding effect of plant size that may influence both seed size and seed number. A sowing experiment with 50 species was performed in the field. The seeds were sown in a grassland and subjected to two treatments, disturbance and undisturbed sward. Evidence for seed-limited recruitment was obtained for 45 of the species. Disturbance had a significant, or nearly significant, positive effect on recruitment for 16 of the 45 species. The relative recruitment in undisturbed sward increased with increased seed size, and both recruitment success and seedling size were positively related to seed size. We suggest that a trade-off between competitive ability and number of recruitment opportunities follows from the trade-off between seed size and seed number, through a causal chain from seed size via seedling size to recruitment success. The relationships between seed size, seed number and recruitment may be an important underlying mechanism for abundance and dynamics of plant species in grassland vegetation. This is an example of a direct link between evolutionary life-history theory, and theory of plant community structure.