A-amylase synthesis

The antagonism of GA action by ABA can be seen in studies of steady-state mRNA accumulation in aleurone cells, a major site of a-amylase synthesis in germinating cereal seeds (Fig. 3). GA treatment promotes the accumulation of high levels of a-amylase, shown by immunoprecipi-tation, while ABA, alone or with GA, completely represses this accumulation. In contrast, ABA clearly promotes the accumulation of a-amylase inhibitor mRNA, as determined by specific immunoprecipi-tation, while GA alone slightly reduces the level of a-amylase inhibitor mRNA. However, GA, together with ABA, does not decrease the level... 

Chandra, G. R. Duynstee, E. E. Methylation of ribonucleic acids and hormone-induced aleurone cells. Biochem. Biophys. Acta., 1971, 232, 514-523. 

More specifically, compounds like podolactone A (Fig. 10.1) inhibit proton efflux from plant cells induced by fusicoccin, without affecting ATP levels.42 The related compound, podolactone E is a strong inhibitor of 6-aminolevulinic acid and chlorophyll synthesis.34 The authors concluded that this was caused by suppression of synthesis of proteins needed in the porphyrin pathway because podolactones also inhibited gibberellic acid-induced a-amylase synthesis in barley embryos. The molecular target site(s) of this class of terpenoid phytotoxins remains to be determined. 

Induction of de novo synthesis of a-amylase by GA in isolated aleurone layers is evident after a lag period of approximately 8 hr following administration of the hormone. In keeping with hormone responses generally, GA must be present continuously if the de novo synthesis of hydrolases is to be sustained. Synthesis of new RNA is essential to the GA-induction of de novo synthesis of hydrolases. Actinomycin D, an inhibitor of RNA synthesis, inhibits the synthesis and release of a-amylase if the inhibitor is presented during the first 7 to 8 hr after treatment. Inhibitors of protein synthesis, such as cycloheximide, also inhibit GA-induction of hydrolases. And, interestingly, abscisic acid, a growth-inhibiting hormone, inhibits GA-induced a-amylase synthesis as well. 

The mechanism of action of abscisic acid (ABA) has been studied to the greatest extent in the barley aleurone system (29), in which ABA counteracts the effect of GA in the induction of hydrolases. This action of ABA has largely been the basis for speculating that ABA may act specifically to inhibit, by some unknown mechanism, DNA-dependent RNA synthesis. Much evidence indicates that ABA acts at the transcriptional level, but it also has been proposed that the inhibition of induction of a-amylase synthesis is caused, at least in part, by an effect on translation because ABA still inhibited the formation of a-amylase at 12 hr when cordycepin (an inhibitor of RNA synthesis) no longer had an effect (30). 

Results with plants show the azalog of indoleacetic acid to be a powerful auxin in Pisum (pea) and Avena (oat) growth.This and the propionic acid isomer also proved to be effective with Partheno-cissus tricuspidata (Boston ivy) tissue. 7-Azatryptophan was found to inhibit a-amylase synthesis in barley endosperm tissue. 

Abscisic acid (ABA) 3-1 was originally detected because of its growth inhibitory properties. It is now known to play an important role in the control of a-amylase synthesis, and regulation of stomatal aperture during water stress. Phaseic acid (PA) 3-3 is an important metabolite of ABA. Over a hundred derivatives of ABA are known, activity correlations have been reviewed, and the difficulty of drawing firm conclusions due to differences in uptake, metabolism and sequestration between the different molecules assayed has been discussed [16-20]. In many correlations, racemates have been used, and it is possible that each enantiomer may be active, have a different type of activity, and/or interfere with the action of the other enantiomer. 

Fig. 2 Growth, 0, a-amylase synthesis, , and glucose consumption for Bacillus licheniformis NCIB 6346 (closed symbols) and the catabolite dere-pressed mutant RMIO (open symbols) Medium was minimal salts (Thiruna-vukkarascu and Priest, 1984) containing glucose (0.2%) and yeast extract (0.05%).

Thirunavukkarasu, M. and Priest, F.G., 1980, Regulation of a-amylase synthesis in Bacillus llcheniformis NCIB 6346, FEMS Microbiol. 

Some workers have claimed that the number of ribosomes present in isolated barley aleurone cells increases in response to GA [35, 65] although others have shown no GA-stimulated synthesis of ribosomal RNA (or transfer RNA) [59, 109]. Since rRNA synthesis can be stopped in GA-treated aleurone layers without significant effect on either a-amylase synthesis or cellular rRNA levels [59] this can be taken as evidence that synthesis of new ribosomes is not... 

The 8-h lag period after GA addition is followed by a 16-(or more) h period of rapid a-amylase synthesis (Fig. 7.1 A). More ultrastructural changes are said to occur in the aleurone cells during this time [66]. These include further proliferation of the RER, distention of the RER cisternae, continued reduction in the size of the aleurone grains, decreases in the number of oil bodies, an increase in the number of plastids, and loss of the phytin globoid. 

Fig. 7.3A and B. The increase with time in (A) the level of translatable messenger RNA for a-amylase, and (B) the increase in the rate of synthesis of the enzyme in vivo in response to GA treatment. U units of a-amylase activity. For (A) poly-(A) RNA was extracted from aleurone layers treated with GA for different time periods, and used to support a-amylase synthesis in vitro. After Higgins etal., 1976 [50]... 

The possibility of post-transcriptional control of enzyme synthesis has been raised by several workers, and it has been noted that methylation of purine residues of tRNA and heavy rRNA is enhanced in GA-treated aleurone tissue [18]. It is not clear how GA might influence this process, or why methylation of certain types of RNA should be essential for a-amylase synthesis. Other studies claiming to show post-transcriptional control by GA [16] have received no support [17]. 

Control of a-Amylase Synthesis by the Products of Enzyme Hydrolysis... 

Attempts have been made to determine why a-amylase synthesis eventually declines in the intact grain (e.g. see the decline in rate of a-amylase synthesis in Figure 7.4A and B after three and two days respectively). One possible reason is that production of the stimulus (i.e. GA) for a-amylase synthesis decreases, perhaps because hormonal synthesis is itself inhibited by the products of starch degradation. Unequivocal evidence supporting this possibility is lacking. In any case, sufficient gibberellin is released into the endosperm during the first one or two days after the start of imbibition to support a-amylase synthesis for at least four to five days thereafter (Fig. 7.4 C). Thus any limitation in GA synthesis after one or two days is unlikely to affect enzyme production. 

Induction of a-amylase synthesis Stimulated Pre-incubation of aleurone tissue with cytokinin enhances subsequent GA stimulation... 

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