Cytokinin Fluxes during Floral Induction in the Long Day Plant Sinapis alba L.

Abstract

Sinapis alba is a long-day (LD) plant that can be induced to flower by a single LD. A number of changes normally occurring in the meristem of plants subjected to the LD can be produced in short day by a single application of a cytokinin to the apical bud. However, flower buds are not produced indicating that evocation by the cytokinin is only partial. In this work, the cytokinin content of root exudate, obtained under vacuum, and of leaf exudate, obtained by the EDTA-method, has been analyzed comparatively in vegetative and induced plants, using reversedphase HPLC coupled to the Amaranthus bioassay. The results show that, as early as 16 hours after the start of the LD, there is an increase of cytokinin activity in both the root and leaf exudates of induced plants. These ,umol m-2.S -; watering occurred every 2 d with demineralized water and, once a week, with Hoagland solution. Plants were grown for 9 weeks in 8-h SD in which condition they remain strictly vegetative. They were induced to flower by a single 22-h LD and then returned to SD. These plants will be referred to as 'induced' plants in contrast to 'control' plants that were continuously kept in SD.Collection of Root Exudate. Roots of Sinapis do not normally exude when the shoot is excised unless vacuum is applied. This was achieved by fitting a 5 ml disposable syringe with silicon rubber tubing on the cut stump of plants decapitated at the cotyledonary node. By pulling back the piston and maintaining it with a small rod, enough vacuum was provided (about -0.07 to -0.08 MPa) in the syringe to ensure exudation. After 16 h, 3 to 4 ml of exudate were collected per plant. For cytokinin analysis the exudates from 30 plants were pooled. Exudates were immediately frozen after collection and stored at -80°C.Collection of Leaf Exudate. The EDTA method of King and Zeevaart (19) was adapted to Sinapis. The optimum conditions for exudation were determined by measuring the dry weight of exudates obtained from 30 leaves in various conditions. The exudation procedure selected for subsequent analysis was as follows: 200 adult leaves (the first 5 leaves below the half-expanded leaf of 40 plants) were placed together in a 500 ml beaker containing 50 ml of 20 mm EDTA (pH 7.5) after the petioles had been recut under distilled water. The beaker was enclosed in an airtight vessel containing water to ensure maximum RH and to prevent EDTA uptake by the leaves. The leaves were maintained in the EDTA solution until the end of the exudation period.

The frequency of plasmodesmata increases early in the whole shoot apical meristem of Sinapis alba L. during floral transition

“…Plants of Sinapis alba L. (Etablissement Jardins, Nancy, France) were kept vegetative by growing them under 8-h short days (SDs), as described by Lejeune et al (1988). When 2 months old, they were induced to¯ower by a single 22-h LD and then returned to the 8-h SD regime.…”

Dynamics of Cytokinins in Apical Shoot Meristems of a Day-Neutral Tobacco during Floral Transition and Flower Formation1

“…Previously reported results on increased cytokinin levels during flower development (Bernier et al, 1988;Lejeune et al, 1988Lejeune et al, , 1994de Bouillé et al, 1989) in other species pointed to a putative role for cytokinins in floral evocation. However, the decreasing cytokinin levels in tobacco apical meristems at the end of the vegetative phase and the complete disappearance of free cytokinin bases during the reorganization of the meristem prior to flowering (being reported here for the first time to our knowledge), may incite a re-evaluation of the question as to whether cytokinins positively regulate the switch from a vegetative toward a floral meristem.…”

“…Aside from the well-documented stimulatory effect of added cytokinins on growth and differentiation of cultured plant cells, flowering is among the many other developmental processes that cytokinins have been reported to mediate in plants (Mok, 1994). Altered cytokinin concentrations before and after flower induction have been reported for some species (Lejeune et al, 1988(Lejeune et al, , 1994de Bouillé et al, 1989). Intervention in the signal-transduction cascade caused by decreasing the cytokinin sensitivity in Arabidopsis resulted in a pleiotropic effect that included the formation of a single, infertile flower (Deikman and Ulrich, 1995).…”

The shoot apical meristem of Sinapis alba L. expands its central symplasmic field during the floral transition

“…Plants of Sinapis alba L. were grown from seeds (Etablissement Jardins, Nancy, France) and kept vegetative under 8-h short days (SDs), as described by Lejeune et al (1988). When the plants were 2-2.5 months old and approximately 25 cm tall, some of them were induced to flower by exposure to a single 22-h LD, which was followed by return to the regular 8-h SD regime.…”