Control of Seed Germination by Abscisic Acid

Abstract

Germination of rape (Brassica napus L.) seeds proceeds in two phases, an initial imbibition phase and a subsequent growth phase. The time courses of water uptake, 02 uptake, and ATP accumulation demonstrate that exogenous abscisic acid (ABA, 0.1 millimoles per liter) specifically prevents the embryo from entering the growth phase. The inhibition of water uptake by ABA is a rapid (lag-phase about 1 hour) and fully reversible process which appears to be the cause rather than the result of changes of the energy metabolism. In untreated seeds, an osmotic pressure (polyethylene glycol 6000) of 11 bars is required for a simulation of the ABA effect on water uptake. However, in ABA-treated seeds an osmotic pressure of only 3 bars is sufficient to suppress water uptake. Thus, ABA lowers the ability of the embryo to absorb water under osmotic stress. In a two-factor analysis of the simultaneous action of ABA and osmoticum on germination, a complete synergistic interaction between these factors was found while ABA and cycloheximide exhibit independent (multiplicative) coaction. These results are interpreted in terms of a common controlling point of ABA and osmotic stress in the water relations of germinating seeds.The action mechanism of ABA, a potent physiological inhibitor of seed germination, is not yet understood. In a previous paper (9) we suggested that the hormone induces seed dormancy by restricting water uptake of the imbibed seed during a critical period ofgermination, where an active push ofembryo expansion is needed to support embryo growth. This hypothesis implies that the biochemical changes previously related to the primary action ofABA such as the modification ofnucleic acid or protein synthesis (e.g. 1, 2, 4, 10) are not direct causal links in the action mechanism of ABA but merely later consequences of the growth inhibition through restricted water uptake.In the present communication we attempt to clarify the role of ABA in water uptake of germinating rape seeds. This type of seed is similar to the previously used mustard seed but lacks the mucilaginous seed coat which makes the study of embryo water relations in mustard rather cumbersome. In contrast to mustard, the very thin and brittle testa of rape absorbs only insignificant amounts ofwater and is rapidly ruptured by the swelling embryo soon after imbibition. Rape produces nondormant seeds which require only imbibition at a suitable temperature (e.g. 25C) for rapid germination. There is no indication of physiologically significant levels of endogenous ABA in the mature rape seed. Exogenous ABA inhibits the completion of germination of these seeds in much the same way as described for mustard (9). As outlined in detail in the previous report (9), we define germina-I Supported by the Deutsche Forschungsgemeinschaft (SFB 206 10 x 6 cm). After adding further 5 ml ofliquid, a small meniscus formed around the seeds allowing uniform wetting of the seeds without impairing gas exchange. Germination took place in darkness at 25.0 ± 0.3C. Since germinat...

Emerging and established model systems for endosperm weakening.

“…It was also found that ABA inhibits endosperm rupture, but not testa rupture, of Arabidopsis and lepidium (see Müller et al,Chapter 30,this volume). In agreement with this, ABA also does not inhibit testa rupture of endospermless Brassica seeds (Schopfer and Plachy, 1984). Separate testa rupture and endosperm rupture are an important experimental advantage of tobacco seeds when compared with tomato seeds.…”

“…Although ABA inhibits the embryo growth potential and the transition to postgermination growth, it does not inhibit initial water uptake by imbibition or initial embryo elongation (Liptay and Schopfer, 1983;Schopfer and Plachy, 1984;Homrichhausen et al, 2003;Manz et al, 2005). We measured the regulation of water uptake of germinating tobacco seeds spatially and temporally by in vivo 1 H nuclear magnetic resonance ( 1 H-NMR) microimaging and 1 H-MAS NMR spectroscopy (Manz et al, 2005).…”

Remodeled Respiration in ndufs4 with Low Phosphorylation Efficiency Suppresses Arabidopsis Germination and Growth and Alters Control of Metabolism at Night

“…Early studies of Brassica napus germination and its control by ABA clearly show the correlation between respiration rate, ATP content, and the inhibitory influence of ABA (Schopfer and Plachy, 1984). While the causative factor in these correlations is not known, lowered ATP synthesis in ndufs4 would be consistent with slower germination and higher ABA (that could be overcome by GA-induced germination, as was observed).…”

The Biology of Canadian Weeds. 137. Brassica napus L. and B. rapa L.

“…In Petri dishes, Schopfer and Plachy (1984) demonstrated that below -0.6 MPa, the germination rate and total germination of European winter B. napus diminished rapidly. Similarly, germination of domesticated forage B. napus and B. rapa decreased at soil water potentials of (0.25 and (0.50 MPa at 7.5 o C (Rao and Dao 1987).…”