Ausl�sung der Bl�tenbildung bei der Langtagpflanze Hyoscyamus niger im Kurztag durch 2-Thiouracil

Abstract

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.

A complex pattern of changes in polysomal mRNA populations is evident in the leaves of Arabidopsis thaliana (L.) Heynh. during photoperiodic induction of flowering

“…There is experimental support for the hypothesis that changes in gene expression may be a prerequisite not only for the transition of the apical meristem to the reproductive phase (Pierard et al 1980;Lyndon etal. 1983;Yanofsky and Meyerowitz 1990), but also for the initiation of flowering and the production of a stimulating signal in the leaves (Salisbury 1963;Eichhoff and Rau 1969;Kranz 1982;Goto t987). The results obtained from the time-series measurements of leaf mRNA populations in Arabidopsis thaliana performed in this study provide further evidence for this hypothesis.…”

“…Several homeotic genes, which alter floral organ identities, have been successfully isolated from Arabidopsis thaliana (Yanofsky and Meyerowitz 1990). Inhibitor studies in various plants, including Arabidopsis, support the idea that changes in gene expression may be involved also in the induction process in the leaves (Salisbury 1963;Eichhoff and Rau 1969;Kranz 1982;Goto 1987).…”

Changes in the composition of in vitro translated leaf m‐RNA caused by photoperiodic flower induction ofHyoscyamus niger

“…Antimetabolites applied to the growing point prevent the transformation of the vegetative bud towards floral primordia. This led to the conclusion that transcription and translation are necessary for the transition (for refer-ence Bernier 1970), In addition flowering can be induced under strict non-inductive short days by infiltration of leaves with 2-thiouracil, an inhibiting base analogue (Eichhoff and Rau 1969) or by cycloheximide, an inhibitor of translation (Klautke and Rau 1973), Photoregulated gene expression has been assumed (Mohr 1972), and meanwhile proven for several phenomena of photomorphogenesis (for a recent review see Lamb and Lawton 1983) which led to the conclusion that a de novo synthesis of RNA and proteins in the leaves may be a prerequisite for the initiation of flowering as suggested earlier (Hess 1970, Ross 1970, Until now, however, experimental support for this concept has been mostly limited to the above inhibitor studies which lack sufficient specificity to exelude interference with other metabolic processes. On the other hand, attempts to extraet or even elicit a specific leaf protein synthesized during the induetion of reproductive organs are hampered by the abundance of leaf pigments and other secondary metabolic products.…”