Barley (Hordeum vulgare 1.) aleurone layers are known to constitutively acidify their surroundings, primarily by L-malic acid release (1. Mikola, M. Virtanen [1980] Plant Physiol 6 6 S-142).Here we demonstrate the antagonistic effeds of the plant hormones gibberellic acid (CA3) and abscisic acid (ABA) on the regulation of extracellular pH (pH.) of barley aleurone layers. We observed a strong correlation between ABA-induced enhancement of extracellular acidification and an ABA-induced increase in imalic acid release. In addition, ABA caused an increase in intracellular L-malate level. CA3 caused a slight decrease in intracellular i-malate level and was able to inhibit the ABA-induced increase in i-malate intracellular concentration and release. In addition, this ABA-induced i-malate release could be completely inhibited by CAI. The ABA-induced release of L-malic acid could not account for the total ABA-induced pH. decrease, suggesting the existence of an additional mechanism involved in the regulation of pH. . It has been reported that ABA induces an intracellular pH (pHi) increase, possibly due to the adivation of plasma membrane proton pumps (R. Van [702][703][704][705][706]. We studied if the effeds of CA3 on i-malate concentration were correlated with changes in pHi and found that CAI caused a pHi decrease and that GA3 and ABA could interfere in the regulation of pHi. In addition, we were able to mimic the effed of both hormones on i-malate release by bringing about artificial pHi changes with the weak acid 5,5-dimethyl-2,4-oxazolidinedione and the weak base methylamine. The physiological meaning of the effeds of CAI and ABA on the regulation of both pH. and pHi during grain germination are discussed.Severa1 processes during barley (Hordeum vulgare L.) grain germination are influenced by pH: a-amylase and severa1 proteases have acidic pH optima and Ca2+ liberation and metabolite uptake by the scutellar epithelium are facilitated by low pH (Hamabata et al., 1988). In addition, the response of barley aleurone layers to GA, a phytohormone known to play an important role in stimulation of grain germination (Akazawa, 1972), is enhanced at low externa1 pH (Sinjorgo et al., 1993). Therefore, changes in pH could be a mechanism by which processes during germination are controlled. 359Barley aleurone layers are generally known to acidify their surroundings, mainly due to a constitutive release of L-malic acid (Mikola and Virtanen, 1980). Macnicol and Jacobsen (1992) reported that during grain maturation the pH of the endosperm decreases. This acidification seems to be brought about by the aleurone and involves malic acid secretion. Both ABA and GA3 have been reported to increase the extracellular acidification of mature barley aleurone layers (Drozdowicz and Jones, 1992). These authors suggested that GA3 stimulates phosphate and organic acid release by the aleurone layers. No stimulation of extracellular acidification was observed when aleurone layers of wheat were treated with GA3 (Hamabata et al., 1988).Since we...
Saccharomyces cerevisiae SU50B and Hansenula polymorpha 8/2, both carrying a multicopy integrated guar a-galactosidase, have been cultivated in continuous cultures, using various mixtures of carbon sources and cultivation conditions. Both S. cerevisiae SU50B and H. polymorpha 8/2 are stable and produce high levels of extracellular ca-galactosidase in continuous cultures for more than 500 h. For these expression systems the strong inducible promoter systems GAL7 and methanol oxidase, respectively, were used. The induction of a-galactosidase synthesis by galactose in SU5OB is limited by the low galactose uptake. Apart from that, at high dilution rates, the glucose repression is substantial, and a maximal expression level of 28.6 mg of extracellular a-galactosidaseg (dry weight) of biomass-' could be obtained. In H. polynorpha, the induction of a-galactosidase synthesis, in addition to methanol oxidase synthesis using formaldehyde, is very effective up to 42 mg of extracellular ar-galactosidase. g (dry weight) of biomass-'. Productivities in terms of specific production rate enable a good comparison with those of other heterologous expression systems in the literature. The productivities found with S. cerevisiae SUSOB and H. polymorpha, 3.25 and 5.5 mg of at-galactosidase. g of biomass-' liter-' h-', respectively, rank among the highest reported in the literature. Enzyme production and secretion in H. polymorpha are more complex. A two-peaked optimum is found in enzyme production. No clear explanation of this phenomenon can be given.
In isolated embryos from dormant barley grains, synergistic effects of fusicoccin (FC) and gibberellic acid (GA3) were observed on the induction of α-amylase mRNA expression. However, no α-amylase mRNA expression could be induced by both agents in embryos from non-dormant grains. Both light- and electron-microscopy studies demonstrated that there were large numbers of starch granules present in mature embryos (mainly in scutellum) from dormant barley grains but none or almost none in embryos from non-dormant grains. Furthermore, the content of reducing sugars in embryos from dormant grains was about half of that from non-dormant grains. In contrast to GA3, FC was able to induce a strong acidification of extracellular pH (pHe). Clamping the pHe to prevent FC-induced acidification, by using 50 mM MES buffer (pH 5.6), caused an inhibition of GA3- or FC-induced α-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 GA3increased the germination rate of embryos isolated from dormant grains, 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 for control of grain dormancy and germination.
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