THE MAIZE (ZEA-MAYS L) CAT1 CATALASE PROMOTER DISPLAYS DIFFERENTIAL BINDING OF NUCLEAR PROTEINS ISOLATED FROM GERMINATED AND DEVELOPING EMBRYOS AND FROM EMBRYOS GROWN IN THE PRESENCE AND ABSENCE OF ABSCISIC-ACID

We previously demonstrated that amounts of Cat1 RNA in developing imma ture maize (Zea mays L.) embryos change in parallel with endogenous ab scisic acid (ABA) content. In excised immature embryos, addition of AB A leads to a large increase in Cat1 RNA accumulation. The Cat1 transcr ipt, however, also accumulates to high amounts in scutella of germinat ing embryos, where ABA content is low and decreasing. Here we show tha t application of ABA to germinated embryos no longer results in the up -regulation of the Cat1 transcript accumulation that is seen during em bryogenesis. This suggests that factors other than ABA control Cat1 ex pression at this developmental stage. Using band-shift and southwester n analyses, we show that the change in sensitivity to ABA is parallele d by changes in nuclear proteins binding to a 28-bp region of the Cat1 promoter in vitro. One protein (CAT1BP-20) shows increased accumulati on in the absence of ABA, suggesting that a repressor-mediated mechani sm accounts for at least a portion of the ABA regulation of Cat1.