Submitted to: Journal of the American Society of Brewing Chemists
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/24/1996
Publication Date: N/A
Interpretive Summary: In order for barley and other cereal seeds to germinate and grow properly, they must produce adequate amounts of the enzymes which digest their stored starch. If this process is slowed, the movement of digested stored nutrients to the developing seedling will not allow vigorous establishment of the crop in the field. The same process is important in malting and brewing, where proper amounts of digested starch are essential. The most critical starch-digesting enzymes are two forms of alpha-amylase. These are made by the outer tissue layer of the seed, the aleurone, in response to a hormone made by the germinating embryo. The enzymes are secreted into the internal starch storing organ, the starchy endosperm. This study was conducted to determine whether the production of these enzymes is controlled strictly by an internal mechanism within aleurone cells or whether their production might be altered by the aleurone perceiving signals from the starchy endosperm. The latter model has not previously been postulated to occur in cereal seeds. Evidence was found that the starchy endosperm does in fact alter the production and secretion of one of the alpha-amylase enzymes, the one which is most abundant in developing seedlings. This is important because it opens up another avenue for the improvement of cereal seed germination, malting and enzyme production. Cereal breeders who, at present, cannot explain why certain varieties germinate and malt poorly can possibly improve breeding strategies by selecting for optimal endosperm traits.
Technical Abstract: Studies were conducted on the expression of the high-pI and low-pI alpha- amylase genes in barley to determine whether the starchy endosperm of the germinating seed may influence their expression. It was found that the starchy endosperm may contribute to the pattern of expression seen in normal seedlings by high-pI mRNA accumulation and enhancing secretion of the high-pI isozyme into the starchy endosperm. Detailed isozyme and messenger RNA studies were conducted to analyze temporal expression in 1) intact seedlings, 2) deembryonated half-seeds, with and without gibberellin (GA) hormone treatments, 3) isolated aleurones with GA only and 4) isolated aleurones with GA and a thick homogenate from the starchy endosperms of half-seeds. Half-seeds without GA produced more low-pI than high-pI isozyme, and the ratio of high- to low-pI activity remained constant for six days. When GA was applied, high-pI activity rose quickly and predominated for all six days, reproducing the pattern of intact seedling kernels. Temporal studies of isolated aleurones with and without GA and endosperm homogenate showed that the homogenate prolonged the rise in high-pI mRNA to 30h, whereas the mRNA declined after 12h in unsupplemented aleurones. Also, the high-pI isozyme was secreted into the incubation medium, but in unsupplemented aleurones, only the low-pI isozyme was secreted. It was also discovered that the high-pI isozyme can be differentially destroyed by routine 70 degree heat treatments, whereas the low-pI isozyme is not affected.