Identification of senescence-associated genes from daylily petals.

The petals of daylily (Hemerocallis hybrid) have a genetically based program that leads to senescence and cell death ca. 24 h after the flower opens. In order to determine the components of this program, six cDNAs, whose levels increase during petal senescence, were isolated and sequenced and designated DSA3, 4, 5, 6, 12 and 15. All six DSAs are members of gene families and all but DSA5 and DSA6 have one to three other very similar genes. GenBank database homology searches indicate that DSA3 is most similar at the amino acid level to an in-chain fatty acid hydroxylase which is bound to cytochrome P450, DSA4 may be an aspartic proteinase, DSA5 is as yet unidentified, DSA6 is a putative S1-type nuclease, DSA12 is very similar to a cytochrome P450-containing allene oxide synthase, and DSA15 may be a fatty acid elongase. Except for DSA12, the genes are expressed at low levels in daylily roots. Levels of the DSA mRNAs in leaves are less than 4% of the maximum detected in petals, and there are no clear differences between younger and older leaves. With the exception of DSA4, accumulation of the DSA mRNAs is increased 3.2 to 43 times by a concentration of abscisic acid that causes premature senescence of the petals. The relationship of the putative DSA gene products to senescence and cell death of daylily petals is discussed.