Identification of a transcription factor specifically expressed at the onset of leaf senescence

The differential expression of genes was analyzed during leaf senescence in Arabidopsis thaliana (L.) Heynh., using suppression subtractive hybridization (SSH). In order to characterize the differential expression of regulatory genes, the analysis was performed at a very early time point when leaves first differed in their photochemical efficiency (Fv/Fm) and cab transcript levels, but no visible sign of senescence, and no expression of SAG12 could be determined. After high-throughput screening, we isolated several differentially expressed cDNA clones, including a transcription factor of the WRKY family, WRKY53. All family members contained the WRKY domain, a 60-amino-acid domain with the conserved WRKYGQK motif at the N-terminal end, together with a novel zinc-finger motif. The mRNA level of WRKY53 increased substantially within the rosette leaves of a 6-week-old plant before the expression of SAG12 became detectable, was constant in all leaves of a 7-week-old plant and decreased again in 8-week-old plants. This indicates that WRKY53 is expressed at a very early time point of leaf senescence and might therefore play a regulatory role in the early events of leaf senescence.