In Situ Abscisic Acid Synthesis

Abstract

When applied to young nondormant embryos of sunflower (Helianthus annus) (7-10 day[s] after pollination [DAP]), abscisic acid (ABA) inhibited germination as long as it was present. However, whatever the dose used and the duration of its application, ABA was unable to induce dormancy because after transfer of treated embryos to control (without ABA) medium, germination occurred. Thereafter, exogenous ABA became effective and allowed the dormancy to develop in 13 and 17 DAP embryos, i.e. in embryos which after isolation were still able to germinate in high percentage. After embryo dormancy was well established (21 DAP), application of fluridone allowed the germination to occur very quickly on control medium. Isolated dormant axes were also induced to germinate by an application of fluridone. Radioimmunological analysis showed that 24 hours after these treatments, endogenous ABA levels were drastically reduced in the axes. When these fluridone-treated embryos were cultured on ABA medium, germination was again inhibited as long as exogenous ABA was present but germination occurred as soon as embryos were transferred to control medium. Such behavior suggested that in situ ABA synthesis is necessary to impose and maintain the embryo dormancy.The involvement ofABA in the initiation ofseed dormancy has been the subject of many recent studies using either mutants deficient in or insensitive to the hormone (4-6) or inhibitors of synthesis, particularly fluridone (3,11,12 embryos isolated at various times after pollination to exogenous ABA. It seemed necessary to distinguish clearly between the physiological consequences of the presence of exogenous ABA in the culture medium and those associated with a genuine induction effect, that is to say those produced by transient application of the hormone and which subsequently persist in its absence. Furthermore, we studied the consequences of an application of fluridone to isolated dormant embryos on their physiological behavior, their endogenous ABA levels and their responsiveness to exogenous ABA. MATERIALS AND METHODS Plant MaterialPlants of sunflower (Helianthus annuus cv Mirasol) were grown in the fields during summer. In September, flowers of male sterile plants were manually pollinated by pollen harvested on small female sterile flowers (seeds generously provided by S.A. Cargill). Flowers were tagged and harvest took place at different times after pollination.For each age, 25 embryos, i.e. a sample considered as sufficient to be representative of the population, were used for in vitro culture and extraction. All the experiments were carried out two or three times in two different years (1989)(1990).

Hormonal Regulation of Dormancy in Developing Sorghum Seeds

“…The effect of inhibiting ABA synthesis in sorghum is consistent with current knowledge about the role of ABA in the onset of dormancy; ABA-deficient mutants Days After Pollination of tomato and Arabidopsis thaliana both have very low levels of dormancy (Karssen et al, 1983;Groot and Karssen, 1992). Lowering ABA levels with fluridone has been shown to reduce dormancy in maize (Fong et al, 1983) and sunflower (Le Page-Degivry et al, 1992) seeds. In addition, changes in dormancy produced by fluridone could, to a certain extent, be related to its effects on embryonic sensitivity to ABA (Fig.…”

“…Tomato (Groot et al, 1991), Arabidopsis (Karssen et al, 1983), and maize (Neill et al, 1987) mutants deficient in ABA synthesis show substantially reduced peak ABA contents and almost no dormancy. Accordingly, the inhibition of ABA synthesis during early development of maize seeds blocks their entrance into dormancy (Fong et al, 1983), and the inclusion of fluridone in the incubation medium stimulates the germination of dormant sunflower embryos (Le Page-Degivry and Garello, 1992). On the other hand, the addition of physiological concentrations of ABA prevents precocious germination, blocks the expression of germination-specific enzymes, and promotes embryonic development (King, 1982;Triplett and Quatrano, 1982;Black, 1983Black, , 1991Quatrano et al, 1983).…”

Role of Abscisic Acid in Seed Dormancy

“…Genetic studies have clearly demonstrated a function for ABA in dormancy imposition during seed development (see above discussion) (Karssen and others 1983;Kermode 1995;Bewley 1997;Phillips and others 1997;Foley 2001). Moreover, de novo synthesis of ABA appears to be necessary for dormancy maintenance after seed imbibition, for example, in seeds of sunflower (Helianthus annuus), barley, beechnut, lettuce (Lactica sativa), and Douglas-fir (Pseudotsuga menziesii) (Wang and others 1995;Bianco and others 1997;Le Page-Degivry and others 1997;Le Page-Degivry and Garello 1992;Yoshioka and others 1998). For instance, when tobacco seeds are imbibed, there is an accumulation of ABA in dormant seeds but not in seeds that have been allowed to after-ripen.…”

Role of Endogenous Abscisic Acid in Potato Microtuber Dormancy

“…The application of carotenoid biosynthesis inhibitors, such as the herbicides norflurazon and FLD (Sanderman and Boger, 1989), often results in a reduction in ABA content as well. FLD treatment results in the precocious gennination of both com and sunflower embryos, and exogenous ABA reverses this effect (Hole et al, 1989;Le Page-Degivry and Garello, 1992).…”