UP-REGULATION OF A CYSTEINE PROTEASE ACCOMPANIES THE ETHYLENE-INSENSITIVE SENESCENCE OF DAYLILY (HEMEROCALLIS) FLOWERS

The flowers of daylily (Hemerocallis x hybrida cv. Cradle Song) open a t midnight, start to senesce 12 h later, and are completely senescent by the following midnight. Differential screening of a cDNA library co nstructed from tepals of flowers showing incipient senescence revealed 25 clones that were strongly up-regulated in senescent tepals. Re-scr eening and interactive Southern analysis of these clones revealed 3 fa milies of up-regulated clones. Transcripts of one clone, SEN10, were n ot detectable at midnight, but increased dramatically as senescence pr oceeded. The derived amino acid sequence of the full-length cDNA (SEN1 02) has strong homology with cysteine proteases that have been reporte d from other plant tissues. The sequence contains a secretory signal p eptide and a probable prosequence upstream of the mature protein. Amin o acids critical to the active site and structure of cysteine protease s are conserved, and the C-terminus of the polypeptide has a unique pu tative endoplasmic reticulum retention signal -RDEL.