Effect of thymidine analogues on reproductive morphogenesis in Arabidopsis thaliana (L.) Heynh

Promotion of flower formation and fruit set in Citrus by antimetabolites of nucleic-acid and protein synthesis

Shoots of the summer flush of Shamouti orange trees, which generally bear only few flowers in the following spring, were caused to produce an abundant number of flowers and eventually many mature fruits by treatments with chloramphenicol-succinate, 5-fluorodeoxyuridine and 5-bromo-3-sec-butyl-6-methyluracil (an herbicide). Increased bloom was accompanied by the shortening of internodes characteristic of flowering branchlets.

[Flower initiation in the long-day plant Hyoscyamus niger by application of 2-thiouracil during short days]

Aqueous solutions of 2-thiouracil (TU) were applied selectively either to the growing point or to the leaves of the long-day plant Hyoscyamus niger in order to determine whether this antimetabolite has an effect on the synthesis of the floral stimulus in the leaves. Applications to the growing point were made by means of a small glass tube covering the shoot apex; application to the leaves was performed by vacuum infiltration. In all experiments all leaves except the three youngest fully expanded leaves and the 8-10 youngest primordia were removed before application. Plants were recorded as having initiated flowers when flower primordia were visible under a dissection microscope 5 weeks after the experiment.TU was inhibitory to photoperiodic induction by long-days of 16 hours when applied to the growing point during the second 8 hours of the daily photoperiod. A concentration of 5·10(-3) M of TU fully suppressed flowering without significant inhibition of leaf primordia increment; however, leaves developing from treated primordia had reduced leaf blades. These results are in agreement with findings already published by other investigators.However, when the leaves were infiltrated by TU, the antimetabolite did not inhibit photoperiodic induction but on the contrary initiated flowering even under short-day conditions. This effect was investigated in more detail by repeated daily infiltrations of TU-solutions in concentrations of 10(-5)-10(-2) M during the second part of an 8 hour photoperiod up to 5 following days. Even after one single infiltration of a 10(-4) M solution 18% of the treated plants were flowering; the percentage of flowering plants increased with increasing concentrations of TU and number of days of application up to approximately 80%. In no case was a flower initiation of 100% obtained. Leaves developing from primordia after infiltration of the leaves with TU have reduced and deformed leaf blades, indicating that TU is transported to the shoot apex to some extent.Some possible explanations of this inductive effect of TU were tested experimentally. Oxygen uptake of the leaves was not decreased and the respiratory quotient was not affected by TU. Photoperiodic induction is not stimulated by low concentrations of TU when applied to the growing point. Infiltration of the leaves by solutions of 2,4-dinitrophenol (10(-4) M) and sodium azide (10(-3) M) had no inductive effect under short-day conditions; a single complete defoliation (except for the 8-10 youngest primordia) is also not inductive. Under short-day conditions additional leaves remaining on the plant that were not infiltrated by TU decreased the percentage of flowering plants but did not fully suppress flower initiation.From these results it is concluded that TU does not act by inhibition of particular metabolic processes concerned in flower initiation or by inhibition of the synthesis of an inhibitor. We suggest that application of TU may lead to synthesis of a floral stimulus in the leaves under short-day conditions also.

[Effect of inhibitors of DNA-, RNA-, and protein synthesis on growth and antheridium formation in Anemia phyllitidis]

A number of inhibitors of DNA-, RNA-, and protein synthesis were applied to prothallia of Anemia phyllitidis, and their effects were investigated with regard to cell division and inhibition of antheridium formation (after induction by gibberellin A3). All compounds tested cause a significant inhibition of growth accompanied by teratologies typical for each inhibitor. Only 5-bromouracil proved to be inactive, even at a concentration of 8×10(-4)m.None of the inhibitors blocked the induction of the biplanar growth form in continuous light.With the exception of 5-bromodeoxyuridine and 5-iododeoxyuridine all the antimetabolites investigated cause a significant lag in antheridium formation. The result of an analysis of this inhibition on the basis of the "critical cell number" (SCHRAUDOLF, 1966a) demonstrates that this time lag is clearly a direct consequence of the retardation of cell division caused by the inhibitors. There is no inhibition of the induction process proper. So therefore it appears highly improbable that antheridium induction in Anemia by gibberellins is based on a specific gene activation in the sense of JACOB and MONOD.The results presented in this paper demonstrate that the production of a time lag in the realisation of a process of differentiation or morphogenesis is not sufficient evidence to permit definite conclusions to be drawn about the basic mechanism of induction; the effect of inhibition of cell division caused by the inhibitors must be excluded or at least taken in to account.The possible function of gibberellins as "realisators" of already present but inactive m-RNA is discussed.5-Bromo- and 5-iododeoxyuridine do not cause a time lag in antheridium formation; both inhibitors, however, cause a deformation of the cell pattern of the antheridia. The degree of this deformation depends on inhibitor concentration as well as on exposure time. Probably a competition exists between these halogensubstituted pyrimidines and thymidine for incorporation into DNA.

Acceleration of flowering of the long-day plant Arabidopsis by 8-azaadenine

The incorporation of 8-azaadenine (2×10(-5)M) to the aseptic culture medium reduced the time required for developing macroscopically visible flower primordia and the number of leaves appearing before the first flower buds to about half under 8 hrs daily illuumination in both wild type and a late monogenic mutant. In the presence of equimolar amount of adenine the flowering was not accelerated by the analog.

Early flowering in Arabidopsis induced by DNA base analogs

Various isogenic lines of Arabidopsis differing in a single factor controlling flower initiation were cultured aseptically on media containing bromodeoxycytidine and bromodeoxyuridine (10(-5) M). The wild type under short-day illumination (8 hours daily) and the late mutant gi (2), in both continuous light and under short day, responded with dramatically earlier flower initiation. Another late mutant (ld) failed to respond. The effect of the DNA base analogs was nullified by the corresponding normal deoxyribonucleosides but not by the corresponding normal ribonucleosides.