The effect of abscisic acid on the differential expression of α-amylase isozymes in barley aleurone layers.

The treatment of barley aleurone layers with gibberellic acid (GA3) results in the synthesis of two groups of α-amylase isozymes. Addition of abscisic acid (ABA) at the same time as GA3 inhibited the synthesis of both groups of isozymes. However, midcourse ABA addition (12 h or later after GA3) had a more inhibitory effect on the high pI α-amylase group than on the low pI α-amylase group. This midcourse inhibition was detectable within 2 h of ABA addition. Northern analysis results using cDNA probes for the high pI and low pI α-amylase groups paralleled the protein synthesis results for both isozyme groups. High pI α-amylase mRNA levels began to decrease within 2 h of midcourse ABA treatment and were less than 10% of the original level by 4 h. The levels of low pI α-amylase mRNA were decreased less by midcourse ABA addition than were high pI mRNA levels. Cordycepin and cycloheximide blocked the effects of midcourse ABA addition on α-amylase mRNA. These observations indicate that ABA inhibits α-amylase expression at the pretranslational level and that protein and RNA synthesis are required for midcourse ABA action to occur. Our results also show that α-amylase mRNA, which has been thought to be very stable, is degraded after midcourse ABA treatment.