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 hybridizat ion (SSH). In order to characterize the differential expression of regulato ry genes, the analysis was performed at a very early time point when leaves first differed in their photochemical efficiency (F-v/F-m) and cab transcr ipt levels, but no visible sign of senescence, and no expression of SAG12 c ould be determined. After high-throughput screening, we isolated several di fferentially expressed cDNA clones, including a transcription factor of the WRKY family, WRKY53. All family members contained the WRKY domain, a 60-am ino-acid domain with the conserved WRKYGQK motif at the N-terminal end, tog ether with a novel zinc-finger motif. The mRNA level of WRKY53 increased su bstantially within the rosette leaves of a 6-week-old plant before the expr ession of SAG12 became detectable, was constant in all leaves of a 7-week-o ld plant and decreased again in 8-week-old plants. This indicates that WRKY 53 is expressed at a very early time point of leaf senescence and might the refore play a regulatory role in the early events of leaf senescence.