Developing maize embryos have the capacity to either mature or germinate. Many lines of evidence point to the hormone abscisic acid (ABA) as playing an important role in directing the embryo into maturation. We have been examining embryo maturation and germination in wildtype maize and in viviparous mutants both in planta and in culture to try to dissect the regulatory pathways of this developmental switch and to learn how ABA may control them. We have looked at the polypeptides produced at various stages of embryo development and correlated the appearance of these with the changing levels of embryo ABA. We have also cultured dissected embryos on medium with or without ABA and looked at the polypeptides produced at various stages.
We have found ABA regulated proteins in wildtype maize are first apparent about 18 days after pollination (DAP) (stage 3), a stage when embryo ABA levels become quite high. Synthesis of these proteins can be stimulated in embryos as young as 10 DAP (stage 2) by culturing on 10-5 M ABA. The mutants vp2 and vp5 do not synthesize normal levels of embryo ABA nor do they accumulate these maturation proteins. These mutants germinate precociously on the ear. If cultured with ABA, however, they are inhibited from germinating and the normal spectrum of maturation polypeptides is produced.
When developing embryos are cultured without ABA, they germinate within a few days. The rate of germination varies during early and mid-development. W22 embryos dissected out 18 to 25 DAP and cultured without ABA show a lag of a few days before they stop making maturation proteins and begin to germinate, although the ABA content of these embryos drops very quickly in culture. As shown in Figure 1, embryos of the variety Gaspe Flint behave very differently. At about 18 DAP the germination rate also drops in these embryos, in fact it becomes practically zero, but they do not regain the capacity to germinate precociously until about 40 DAP, long after the endogenous ABA level has declined. If these embryos are prematurely dried and reimbibed, however, they germinate quite synchronously within 24 hours.
The non-germinating embryos of Gaspe Flint are actively synthesizing the proteins characteristic of maturation phase, although the level of ABA in these embryos is negligible. Addition of ABA to the medium does not cause any change in the accumulation of these proteins (Figure 2). It appears that in W22 embryos the induction of maturation proteins and the inhibition of germination caused by ABA is lost quickly when the hormone is removed, but in Gaspe Flint embryos the developmental program initiated by ABA may be stable for a very long period of time.
Chris Holmes-Baker, Timothy Grudt and Carol Rivin
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