Effects of fusicoccin and gibberellic acid on the germination of embryos from dormant barley grains: roles of starch degradation and external pH

In isolated embryos from dormant barley grains, synergistic effects of fusi coccin (FC) and gibberellic acid (GA(3)) were observed on the induction of alpha-amylase mRNA expression. However, no alpha-amylase mRNA expression co uld be induced by both agents in embryos from non-dormant grains. Both ligh t- and electronmicroscopy studies demonstrated that there were large number s of starch granules present in mature embryos (mainly in scutellum) from d ormant barley grains but none or almost none in embryos from non-dormant gr ains. Furthermore, the content of reducing sugars in embryos from dormant g rains was about half of that from non-dormant grains. In contrast to GA(3), FC was able to induce a strong acidification of extracellular pH (pH(e)). Clamping the pH(e) to prevent FC-induced acidification, by using 50 mM MES buffer (pH 5.6), caused an inhibition of GA(3)- or FC-induced alpha-amylase mRNA expression but did not affect the germination of embryos from dormant grains. In addition, in MES buffer, addition of FC or a combination of FC and GA(3) increased the germination rate of embryos isolated from dormant g rains, though large numbers of starch granules were still present in these embryos. Based on these observations, the presence of starch granules and a low reducing sugar level in embryos from dormant grains is not a factor fo r control of grain dormancy and germination.