Molecular changes during the expansion and senescence of ethylene -insensitive daylily flowers

The expansion and ethylene-insensitive senescence of daylily flowers occur predictably and quickly, within a single 24 h period. This speed and predictability qualifies daylily as an excellent model for studying floral expansion and ethylene-insensitive flower senescence. While there are no previous reports exploring the molecular basis of flower expansion, a growing body of evidence has implicated the involvement of expansins in the control of the cell elongation of growing stems, leaves, and fruits. Expansins cloned from expanding daylily flowers showed elevated expression levels in sepals, petals, stamens, and carpels prior to and during flower opening. These observations are consistent with the hypothesis that expansins mediate the cell wall changes required for flower expansion.;Gene expression during the 24 hour time course of daylily flower senescence was examined in order to determine if this aspect of terminal plant development, like many others, is accomplished through PCD. Senescence-related genes were identified and cloned, and their expression during daylily flower senescence was examined. Cloning and measured decreases in expression of homologues of DAD genes suggest that daylily flower senescence is analogous to apoptotic events in animals. Aspartic, cysteine, and subtilisin-like serine proteinases found to be expressed during floral senescence could either be hydrolytic proteases required for amino acid remobilization or apoptotic proteases initiating floral PCD. Likewise, protein phosphatases expressed during senescence could be involved in signal transduction pathways responsible for PCD. Increased abundance of endo-1,4-β-glucanase and β-galactosidase transcripts during flower senescence indicates the possible dismantling of cell wall polymers and mobilization of component sugars to other locations in the inflorescence. Expression of metallothioneins and at least one catalase gene suggest that some factors, generally regarded with the delay of senescence, are still being made. Perhaps these protection mechanisms are designed to preserve cellular components during remobilization from oxidative damage inflicted by active oxygen species generated during PCD. Considered collectively, these genes probably represent a small number of the molecular events transpiring during daylily flower senescence, however the patterns displayed by this limited group of genes suggests that the senescence of ethylene-insensitive flowers is an example of programmed cell death.