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 thalianaL. encodes a putative protein that is similar to the regulatory ATPase subunit (ClpA) of the Clp protease and contains a putative chloroplast‐targeting transit‐peptide at the N‐terminus. A chimeric gene with the putative plastid‐targeting sequence of the erd1gene fused to the synthetic green‐fluorescent protein (sGFP) gene was constructed and introduced into Arabidopsisprotoplasts. The N‐terminal region of the ERD1 protein directed the sGFP protein into the plastids of the protoplasts, and functioned as a transit peptide. Northern blot analysis indicated that expression of the erd1gene was induced not only by water stress, such as dehydration and high salinity, but also by natural senescence and dark‐induced etiolation. The erd1gene was not strongly induced by exogenous abscisic acid. A chimeric gene with the 0.9 kb promoter region of the erd1gene fused to the β‐glucuronidase (GUS) reporter gene was constructed, and tobacco plants transformed with the construct. The GUS reporter gene driven by the erd1promoter was induced 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. Furthermore, GUS activity was reduced by cytokinin treatment during dark‐induced etiolation. These results indicate that expression of the erd1gene is developmentally up‐regulated by senescence as well as by water stress.