Auxin involvement in tepal senescence and abscission in Lilium: a tale of two lilies.

Petal wilting and/or abscission terminates the life of the flower. However, how wilting and abscission are coordinated is not fully understood. There is wide variation in the extent to which petals wilt before abscission, even between cultivars of the same species. For example, tepals of Lilium longiflorum wilt substantially, while those of the closely related Lilium longiflorum×Asiatic hybrid (L.A.) abscise turgid. Furthermore, close comparison of petal death in these two Lilium genotypes shows that there is a dramatic fall in fresh weight/dry weight accompanied by a sharp increase in ion leakage in late senescent L. longiflorum tepals, neither of which occur in Lilium L.A. Despite these differences, a putative abscission zone was identified in both lilies, but while the detachment force was reduced to zero in Lilium L.A., wilting of the fused tepals in L. longiflorum occurred before abscission was complete. Abscission is often negatively regulated by auxin, and the possible role of auxin in regulating tepal abscission relative to wilting was tested in the two lilies. There was a dramatic increase in auxin levels with senescence in L. longiflorum but not in Lilium L.A. Fifty auxin-related genes were expressed in early senescent L. longiflorum tepals including 12 ARF-related genes. In Arabidopsis, several ARF genes are involved in the regulation of abscission. Expression of a homologous transcript to Arabidopsis ARF7/19 was 8-fold higher during senescence in L. longiflorum compared with abscising Lilium L.A., suggesting a conserved role for auxin-regulated abscission in monocotyledonous ethylene-insensitive flowers.

Expression and localisation of a senescence-associated KDEL-cysteine protease from Lilium longiflorum tepals.

Senescence is a tightly regulated process and both compartmentalisation and regulated activation of degradative enzymes is critical to avoid premature cellular destruction. Proteolysis is a key process in senescent tissues, linked to disassembly of cellular contents and nutrient remobilisation. Cysteine proteases are responsible for most proteolytic activity in senescent petals, encoded by a gene family comprising both senescence-specific and senescence up-regulated genes. KDEL cysteine proteases are present in senescent petals of several species. Isoforms from endosperm tissue localise to ricinosomes: cytosol acidification following vacuole rupture results in ricinosome rupture and activation of the KDEL proteases from an inactive proform. Here data show that a Lilium longiflorum KDEL protease gene (LlCYP) is transcriptionally up-regulated, and a KDEL cysteine protease antibody reveals post-translational processing in senescent petals. Plants over-expressing LlCYP lacking the KDEL sequence show reduced growth and early senescence. Immunogold staining and confocal analyses indicate that in young tissues the protein is retained in the ER, while during floral senescence it is localised to the vacuole. Our data therefore suggest that the vacuole may be the site of action for at least this KDEL cysteine protease during tepal senescence.

Changes in ultrastructure, protease and caspase-like activities during flower senescence in Lilium longiflorum.

The last phase of flower development is senescence during which nutrients are recycled to developing tissues. The ultimate fate of petal cells is cell death. In this study we used the ethylene-insensitive Lilium longiflorum as a model system to characterize Lily flower senescence from the physiological, biochemical and ultrastructural point of view. Lily flower senescence is highly predictable: it starts three days after flower opening, before visible signs of wilting, and ends with the complete wilting of the corolla within 10 days. The earliest events in L. longiflorum senescence include a fall in fresh and dry weight, fragmentation of nuclear DNA and cellular disruption. Mesophyll cell degradation is associated with vacuole permeabilization and rupture. Protein degradation starts later, coincident with the first visible signs of tepal senescence. A fall in total protein is accompanied by a rise in total proteases, and also by a rise of three classes of caspase-like activity with activities against YVAD, DEVD and VEID. The timing of the appearance of these caspase-like activities argues against their involvement in the regulation of the early stages of senescence, but their possible role in the regulation of the final stages of senescence and cell death is discussed.