THE PHYTOCHROME RESPONSE OF THE LEMNA-GIBBA NPR1 GENE IS MEDIATED PRIMARILY THROUGH CHANGES IN ABSCISIC-ACID LEVELS

Two important signaling systems involved in the growth and development of plants, those triggered by the photoreceptor phytochrome and the h ormone abscisic acid (ABA), are involved in the regulation of expressi on of the NPR1 gene of Lemna gibba. We previously demonstrated that ph ytochrome action mediates changes in ABA levels in L. gibba, correlati ng with changes in gene expression evoked by stimulation of the phytoc hrome system. We have now further characterized phytochrome-and ABA-me diated regulation of L. gibba NPR1 gene expression using a transient p article bombardment assay, demonstrating that regulatory elements cont rolling responses to both stimuli reside within 156 nucleotides upstre am of the transcription start. Linker scan (LS) analysis of the region from -156 to -70 was used to identify two specific requisite and nonr edundant cis-acting promoter elements between -143 to -135 (LS2) and - 113 to -101 (LS5). Mutation of either of these elements resulted in a coordinate loss of regulation by phytochrome and ABA. This suggests th at, unlike the L. gibba Lhcb21 promoter, in which phytochrome and ABA regulatory elements are separable, the phytochrome response of the L. gibba NPR1 gene can be attributed to alterations in ABA levels.