A NUCLEAR GENE, ERD1 ENCODING A CHLOROPLAST-TARGETED CLP PROTEASE REGULATORY SUBUNIT HOMOLOG IS NOT ONLY INDUCED BY WATER-STRESS BUT ALSO DEVELOPMENTALLY UP-REGULATED DURING SENESCENCE IN ARABIDOPSIS-THALIANA

A cDNA, ERD1 isolated from one-hour-dehydrated plants of Arabidopsis t haliana L. encodes a putative protein that is similar to the regulator y ATPase subunit (ClpA) of the Clp protease and contains a putative ch loroplast-targeting transit-peptide at the N-terminus. A chimeric gene with the putative plastid-targeting sequence of the erd1 gene fused t o the synthetic green-fluorescent protein (sGFP) gene was constructed and introduced into Arabidopsis protoplasts. The N-terminal region of the ERD1 protein directed the sGFP protein into the plastids of the pr otoplasts, and functioned as a transit peptide. Northern blot analysis indicated that expression of the erd1 gene was induced not only by wa ter stress, such as dehydration and high salinity, but also by natural senescence and dark-induced etiolation. The erd1 gene was not strongl y induced by exogenous abscisic acid. A chimeric gene with the 0.9 kb promoter region of the erd1 gene fused to the beta-glucuronidase (GUS) reporter gene was constructed, and tobacco plants transformed with th e construct. The GUS reporter gene driven by the erd1 promoter was ind uced by dehydration and high salt stress at significant levels in the transgenic plants. The GUS gene was strongly expressed in older leaves without dehydration, and was induced by dark-induced etiolation. Furt hermore, GUS activity was reduced by cytokinin treatment during dark-i nduced etiolation. These results indicate that expression of the erd1 gene is developmentally up-regulated by senescence as well as by water stress.