EVIDENCE FOR A SENESCENCE-ASSOCIATED GENE INDUCED BY DARKNESS

A nearly full-length cDNA was isolated from a cDNA library prepared from incipiently senescent radish (Raphanus sativus L.) cotyledons using a previously isolated cDNA clone for darkinducible mRNA as a probe (A Watanabe, N Kawakami, Y Azumi [1989] In Cell Separation in Plants, NATO ASI Series, Vol H35, pp 31-38. Springer-Verlag, Berlin). The clone detected transcripts of 800 bases which increased more than 100-fold after 24 hours of darkness. The transcripts also accumulated under light when plants were exposed to ethylene or heat stress, and 6N-benzyladenine partially repressed its accumulation in the dark. These responses of the gene to physiological stimuli closely paralleled the effects of the stimuli on the progress of senescence of the cotyledons. We have named the gene din 1 (dark inducible gene 1). The cDNA encodes a polypeptide of 20 kilodaltons, and its nucleotide sequence shows a high (49%) similarity to a subfamily of pathogenesis-related proteins of tobacco. The predicted amino acid sequence of the product, however, shows only 20% homology to the pathogenesis-related protein.