Temporal Expression Patterns of Alpha-amylase Isozymal Genes in Polysomal and Total RNAs of Germinating Barleys

Abstract

Analyses of alpha-amylase gene expression have focused mainly upon the gibberellin-treated isolated aleurone system. In order to determine how the alpha -amylase isozymes are synthesized in the intact system, high and low pI alpha-amylase transcripts were analyzed in total RNA and polysomal (polyribosomal) messenger RNA from seedlings of the Morex and Himalaya cultivars. Two growing conditions, malting and germination on paper, were compared. Unlike isolated aleurone systems, where high and low pI mRNA levels are roughly equal, the intact seedlings had five- to 10-fold more high pI mRNA than low pI mRNA. This corresponded to the differences in high and low pI enzyme activities observed on isoelectric focusing gels. Both the high and low pI and mRNAs had similar temporal accumulation patterns when analyzed in the total RNA population, but distinctly different patterns were observed in polysomal mRNA. Hihg pI mRNA increased rapidly, reached a peak in 4 to 5 days after the initiation of imbibition, and then declined. Low pI mRNA increased slowly but progressively for at least 6 days. Surprisingly, when gene expression in total RNA reached a peak for either isozymal RNA, only 2·1 to 3·9% of the RNA was bound to polysomes. Gene expression reflected in total RNA from paper-germinated seeds was similar to that in malted seeds, except that peaks in expression occurred 1 day earlier, and the high pI/low pI RNA ratio was lower.