Revealing programmed cell death events during flower (petal) senescence

Flower petals serve to attract pollinators and inherently have a short life span. The senescence of these plant parts is programmed in the flower's developmental plan. Also at the cellular level, petal cell death is highly programmed going through a number of different phases that culminate in cellular suicide (programmed cell death - PCD). The signalling, biochemical, molecular and gene determinants involved in the regulation of PCD and the morphological characteristics of the process in flower petals have to some extent been described. Still important issues of theoretical and practical significance related to PCD functioning and its contribution to petal deterioration remain unsolved.

This review discusses the occurrence and role of PCD in petal senescence in models of ornamental plants. For comparison, the distinctive and common features of plant and animal PCD types are outlined. The two major plant cell death categories - vacuolar (V) PCD, reminiscent to animal autophagic PCD and apoptosis-like (AL) PCD, sharing features with animal apoptosis - and their ontribution to petal senescence are discussed.

The findings indicate that cellular PCD is tightly connected to petal senescence and support the view that senescence is a specific form of developmental PCD (senescence/PCD), dominated by large scale autophagy and eventual breakdown of the vacuole membrane. Depending on the measured PCD markers, petal cell death is often characterized being either V-PCD or AL-PCD. However, alongside the ongoing V-PCD, in early or late stages of senescence, often AL-PCD-associated features are observed. This indicates that, in senescing petal cells, both PCD pathways operate in parallel and are presumably interconnected. The specific conditions may determine their relative contribution to cell death. The cell death cascade may, in general, start earlier in parenchyma than in epidermal cells. In a fully open, visibly non-senescent flower, a large part of the mesophyll cells may already have died or even disappeared, indicating that petal senescence is well on its way and cannot be reversed. Petal abscission may occur in both non-senescent and senescent petals and its regulation seems independent from petal PCD.