THE INTERACTION OF LIGHT AND ABSCISIC-ACID IN THE REGULATION OF PLANTGENE-EXPRESSION

Extended dark treatments of light-grown plants of both Lemna gibba and Arabidopsis thaliana resulted in substantial increases in abscisic ac id (ABA) concentrations. The concentration of ABA could be negatively regulated by phytochrome action in Lemna. As has been noted in other s pecies, ABA treatment reduced Lemna rbcS and Lhcb RNA levels, which ar e positively regulated by phytochrome in many species. In view of thes e observations, the possibility that phytochrome effects on gene expre ssion may be mediated primarily by changes in ABA was tested using a t ransient assay in intact plants. The phytochrome responsiveness of the Lemna Lhcb21 promoter was still apparent in the presence of exogenou s ABA. Additionally, when 2-bp mutations were introduced into this pro moter so that phytochrome responsiveness was lost, a response to exoge nous ABA was still present. We conclude that phytochrome- and ABA-resp onse elements are separable in the Lhcb21 promoter. We tested whether the effects of ABA on RNA abundance could be inhibited by treatment w ith gibberellin and found no evidence for such an inhibition. We have also found that the ABA-responsive Em promoter of wheat can be negativ ely regulated by phytochrome action. It is likely that this regulation is mediated at least in part by phytochrome-induced changes in ABA le vels. Our results demonstrate that it is essential to take into accoun t that dark treatments and the phytochrome system can affect ABA level s when interpreting studies of light-regulated genes.