ABA-Regulation of Two Classes of Embryo-Specific Sequences in Mature Wheat Embryos

We have previously described the isolation and characterization of ABA-enhanced sequences from developing wheat embryos. Here we use in vivo RNA labeling and the inhibitors alpha-amanitin and cycloheximide to determine the level at which ABA acts to modulate these sequences in cultured mature embryos. Sequences fell into two classes: one, represented by the 7S globulin clone, p511, appears to be regulated at the level of transcription, while the other, represented by the early methionine-labeled polypeptide (E(m))-protein clone, p1015, has an additional posttranscriptional component. In mature embryos cultured in the absence of ABA, mRNA levels of p511 and p1015 declined rapidly until neither was detected at 3 days postimbibition. Levels of p511 increased in mature embryos cultured in the presence of ABA, but remained low in the presence of ABA + alpha-amanitin, suggesting p511 RNA is regulated at the level of transcription. Levels of p1015, in contrast, remained high not only in the presence of ABA, but also in the presence of ABA + alpha-amanitin or alpha-amanitin alone. This suggests p1015 regulation might be at the level of selective RNA stability. Cycloheximide had no detectable effect on ABA-mediated stabilization of p1015, suggesting that newly synthesized proteins are not involved. E(m)-protein synthesis rates closely paralleled E(m) RNA levels, suggesting E(m) expression is not controlled at the level of translation.